Chapter 22 – Vision: The Retina

Figure 22.1

A cross-sectional illustration of the eye viewed from above, showing the path light travels from the front of the eye to the back. At the front, the cornea forms the outer curved surface. Behind it, the iris (shown in blue) surrounds the pupil (shown in gray), the opening that allows light through. The lens sits directly behind the pupil and iris as a clear oval structure. The retina is drawn as a red line running along the entire back interior surface of the eye, curving from the top around to the bottom. A small dark red segment on the retina, roughly in line with the pupil, marks the fovea, the region of highest visual acuity. Where the retina ends at the back of the eye, a tan-colored structure labeled the optic disc marks the point where the optic nerve exits the eye and travels toward the brain.

Figure 22.2

An illustration of the five retinal cell types arranged in three parallel vertical columns, showing their layered organization and synaptic connections. At the top of each column, a photoreceptor (shown in magenta with a striped outer segment, resembling a rod) extends downward to a cell body, then narrows into a synaptic terminal. The three photoreceptor terminals connect to a middle row of cells: gray horizontal cells with branching processes that span laterally across the photoreceptor terminals, and green bipolar cells that extend straight down from each photoreceptor synapse. The bipolar cells continue downward to a lower synaptic layer, where blue amacrine cells branch laterally, similar to the horizontal cells above. Below this layer, three orange ganglion cells receive the bipolar cell input and send long processes off the bottom of the image, representing the axons that form the optic nerve. The image illustrates that horizontal cells connect laterally at the photoreceptor-to-bipolar cell synapse, while amacrine cells connect laterally at the bipolar-to-ganglion cell synapse.

Figure 22.3

An illustration showing the eye on the left with a yellow arrow labeled “Light” entering through the pupil and traveling to a small boxed region at the back of the retina. This region is enlarged in an inset on the right, showing three parallel columns of ganglion cells (orange, left), bipolar cells (green, middle), and photoreceptors (magenta, right), connected by horizontal and amacrine cells at their respective synapses. Below the inset, two arrows point in opposite directions: a yellow arrow labeled “Direction of Light” points right, toward the photoreceptors, while a black arrow labeled “Direction of Neuronal Communication” points left, toward the ganglion cells. The figure illustrates that light must pass through the ganglion and bipolar cell layers before reaching the photoreceptors, while the resulting neural signal travels in the reverse direction, from photoreceptors back toward the ganglion cells.

Figure 22.4

A side-by-side illustration comparing the shapes of a rod and a cone photoreceptor. The rod, shown in magenta on the left, has a long, cylindrical outer segment with uniform width and horizontal striping, connected to a small oval cell body and a thin synaptic terminal. The cone, shown in blue on the right, has a shorter outer segment that tapers to a point, also with horizontal striping, connected to a similarly shaped cell body and terminal. The comparison highlights the difference in outer segment shape: rods have a uniform, elongated cylinder, while cones have a tapered, cone-shaped structure, consistent with their names.

Figure 22.5

An illustration of the back surface of the retina viewed as a circle, densely packed with small circles representing photoreceptors. Most of the surface is filled with magenta circles labeled “Rods” in the legend. A cluster of blue circles with a hatched pattern, labeled “Cones” in the legend, is concentrated in a star-shaped patch to the right of center, marking the fovea. A solid pink circular gap with no photoreceptors sits to the left of the fovea, marking the optic disc, where the axons of the ganglion cells exit the eye. The figure illustrates that cones are concentrated almost exclusively at the fovea, rods cover the remainder of the retinal surface, and no photoreceptors are present at the optic disc.

Figure 22.6

Two side-by-side panels, each showing a photoreceptor illustration paired with a membrane potential graph and glutamate release indicators. In panel A, the photoreceptor moves from dark to light: the graph shows the membrane potential starting at a depolarized level (labeled “Dark,” filled circle) and dropping to a more hyperpolarized level (labeled “Light,” open circle) over time. Below the photoreceptor, three small dots represent a low level of glutamate release in the light condition. In panel B, the photoreceptor moves from light to dark: the graph shows the membrane potential starting at a hyperpolarized level (labeled “Light,” open circle) and rising to a more depolarized level (labeled “Dark,” filled circle) over time. Below the photoreceptor, six small dots represent a higher level of glutamate release in the dark condition. Together, the panels show that photoreceptors hyperpolarize and release less glutamate when moving into light, and depolarize and release more glutamate when moving into dark.

Figure 22.7

An illustration of the photoreceptor membrane in the dark, magnified from a small boxed region near the top of the outer segment shown at the far left. Within the membrane, an opsin protein (lavender) sits inactive, associated with a G-protein transducin, shown as three subunits: Gα (light blue) bound to GDP (green), and a Gβγ complex (dark blue). To the right, a brown phosphodiesterase (PDE) protein sits embedded in the membrane, also inactive. Green triangles representing cGMP molecules are scattered nearby in high numbers. On the far right, two open cation channels, shown in blue, allow sodium (dark blue circle) and calcium (purple circle) to flow into the cell, indicated by inward-pointing arrows. Small green triangles bound to each channel represent cGMP holding the channels open. The figure illustrates that in the dark, high cGMP levels keep the channels open, allowing sodium and calcium influx that depolarizes the photoreceptor, while the opsin, transducin, and PDE remain inactive.

Animation 22.1

The animation illustrates phototransduction across a photoreceptor membrane. Initially, a purple opsin protein sits linked to transducin beside an inactive brown PDE protein and two blue cation channels held open by green triangular cyclic GMP molecules, with sodium and calcium spheres floating outside the cell. When a yellow light arrow strikes opsin, it turns yellow, causing the cyan Gα subunit of transducin to slide across the membrane and activate PDE. Active PDE converts the triangular cyclic GMP molecules into oval GMP molecules. As cyclic GMP detaches, the blue channels snap shut, trapping sodium and calcium ions outside the cell and stopping positive charge influx

Figure 22.8

An illustration comparing an OFF bipolar cell synapse in the dark and in the light. On the left, two membrane potential graphs stacked vertically show the photoreceptor and OFF bipolar cell both hyperpolarizing in the light (transitioning from a filled “Dark” circle to an open “Light” circle on each graph). To the right, a blue photoreceptor terminal synapses onto a green OFF bipolar cell terminal, magnified in a two-panel inset. In the “In the dark” panel, several cyan star shapes representing glutamate surround an ionotropic receptor channel, with one glutamate molecule bound to the channel and a sodium ion flowing into the cell through an open channel, indicated by a downward arrow. In the “In the light” panel, no glutamate stars are present, and the channel is shown closed with no ion flow. The figure illustrates that in the dark, glutamate binds ionotropic receptors on the OFF bipolar cell, opening sodium channels and depolarizing the cell, while in the light, the absence of glutamate closes the channels and hyperpolarizes the cell.

Figure 22.9

An illustration comparing an ON bipolar cell synapse in the dark and in the light, structured like Figure 22.8. On the left, two membrane potential graphs show the photoreceptor hyperpolarizing in the light while the ON bipolar cell depolarizes in the light (its graph rises from a “Light” open circle baseline to a “Dark” filled circle level — opposite in direction to the OFF bipolar cell graph). To the right, a blue photoreceptor terminal synapses onto a green ON bipolar cell terminal, magnified in a two-panel inset. In the “In the dark” panel, several cyan glutamate stars surround a hexagonal metabotropic receptor (mGluR), with one glutamate bound and an associated G-protein (Gα bound to GTP) below it; a separate cation channel is shown closed. In the “In the light” panel, no glutamate is present, the mGluR is unbound, its G-protein subunits (Gα-GDP and Gβγ) are shown separated and inactive, and the cation channel is open, allowing sodium and calcium  influx. The figure illustrates that in the dark, glutamate activates metabotropic receptors that close cation channels and hyperpolarize the ON bipolar cell, while in the light, the absence of glutamate leaves the channels open and depolarizes the cell.

Figure 22.10

An illustration comparing the full signaling pathway from photoreceptor to ganglion cell for the OFF and ON pathways side by side. On the left side, a photoreceptor, OFF bipolar cell, and OFF-center ganglion cell are shown top to bottom, each paired with a membrane potential graph. The photoreceptor and OFF bipolar cell graphs both show hyperpolarization moving from dark to light. The OFF-center ganglion cell graph shows a train of action potentials that is dense before the light transition and sparse after, indicating a decreased firing rate in the light. On the right side, the same arrangement is shown for the ON pathway: the photoreceptor again hyperpolarizes in light, but the ON bipolar cell graph shows depolarization, and the ON-center ganglion cell graph shows a sparse train of action potentials before the light transition and a dense train after, indicating an increased firing rate in the light. The figure illustrates that a single change from dark to light produces opposite firing rate changes in OFF-center and ON-center ganglion cells.

Figure 22.11

An illustration with two parts. On the left, a pink circle represents the retinal surface, with the fovea marked near the center. Three small circles of increasing size — labeled Cell 1, Cell 2, and Cell 3 — are positioned near the fovea and toward the periphery, representing receptive field size at each location; Cell 1’s field is smallest, Cell 2’s is intermediate, and Cell 3’s is largest. On the right, three corresponding columns show the convergence of photoreceptors onto bipolar and ganglion cells at each location. In Cell 1’s column (near the fovea), a single photoreceptor synapses onto a single bipolar cell, which synapses onto a single ganglion cell. In Cell 2’s column, four photoreceptors converge onto two bipolar cells, which converge onto one ganglion cell. In Cell 3’s column (peripheral retina), nine photoreceptors converge onto three bipolar cells, which converge onto one ganglion cell. The figure illustrates that receptive field size increases with the degree of convergence from photoreceptors to bipolar cells to ganglion cells, and that convergence is lowest near the fovea and highest in the periphery.

Figure 22.12

An illustration showing a receptive field transitioning from dark to light in its center. Two circular receptive field diagrams are shown side by side: on the left, the center is filled black (dark) with a light gray surround; on the right, the center is filled white (light) with the same gray surround, illustrating the change. Below each receptive field, a blue center photoreceptor connects downward to a green ON bipolar cell, flanked by lighter, grayed-out photoreceptors and horizontal cells representing the surround (not active in this panel). A red arrow points downward from the center photoreceptor to the ON bipolar cell, indicating direct synaptic communication. On the right, two membrane potential graphs show the center photoreceptor hyperpolarizing (moving from a “Dark” filled circle to a “Light” open circle) and the ON bipolar cell depolarizing in response. The figure illustrates that light falling on the center of an ON bipolar cell’s receptive field causes the center photoreceptor to hyperpolarize, which depolarizes the ON bipolar cell through direct synaptic communication.

Figure 22.13

An illustration paralleling Figure 21.12 but showing light falling on the surround instead of the center. Two circular receptive field diagrams are shown side by side: on the left, the entire field is black; on the right, the surround has turned white while the center ring remains gray, indicating light in the surround only. Below, three blue photoreceptors are shown, with the outer two representing the surround and the center one representing the direct input to the green ON bipolar cell. Gray horizontal cells sit between the photoreceptor terminals. Red arrows trace an indirect path: downward from a surround photoreceptor to a horizontal cell, then upward from the horizontal cell to the center photoreceptor, then downward from the center photoreceptor to the ON bipolar cell. On the right, four membrane potential graphs show: the surround photoreceptor hyperpolarizing in light, the horizontal cell also hyperpolarizing, the center photoreceptor depolarizing (despite no direct light change), and the ON bipolar cell hyperpolarizing as a result. The figure illustrates that light in the receptive field surround produces the opposite effect on the ON bipolar cell compared to light in the center, through an indirect pathway involving the horizontal cell.

Figure 22.14

An illustration of three adjacent receptive fields (A, B, and C) responding to an edge of light. On the left, all three fields are shown fully dark. On the right, an edge of light has moved in from the right side, covering the surround of field B and the center of field C, while field A remains unaffected. Below, three blue photoreceptors connect to three green bipolar cells (labeled A, B, C), with gray horizontal cells linking adjacent photoreceptor terminals. To the right, two sets of graphs compare direct and indirect effects. Under “Direct effects, Cell C,” a graph shows Cell C’s photoreceptor depolarizing in response to light directly hitting its receptive field center. Under “Indirect effect, Cell B,” two graphs are compared: one labeled “Cell B” with no lighting change shows a flat membrane potential line, while a second graph labeled “Cell B” with a dashed and solid line comparison shows Cell B hyperpolarizing further due to horizontal cell input, even though no light is directly falling on Cell B’s center. A note states this additional hyperpolarization is caused by horizontal cell action rather than a direct light change, representing lateral inhibition. The figure illustrates that an edge of light produces a direct depolarizing effect on the bipolar cell whose center it covers, while simultaneously producing an indirect hyperpolarizing effect on the neighboring bipolar cell whose surround it covers, widening the membrane potential difference between the two cells at the edge.

Figure 22.15

A three-dimensional illustration of a cluster of photoreceptors viewed from the side, resembling a dense bundle of cylindrical outer segments. Most of the photoreceptors are shown in gray, with a large group in the upper portion of the image highlighted in magenta. Two white concentric ovals are overlaid on this magenta region: a larger oval enclosing a broad group of magenta photoreceptors, and a smaller oval nested inside it enclosing a lighter-colored subset. The image illustrates that a receptive field exists as a two-dimensional area across the retinal surface, with the inner oval representing the receptive field center and the outer oval representing the surround, each encompassing multiple photoreceptors rather than a single cell.

Chapter 23 – Vision: Central Processing

Figure 23.1

An illustration showing how the two eyes together create the full visual field. Two large ovals labeled “Full visual field” overlap slightly at the bottom, outlined in a thick black border. Below them, two smaller circles labeled “Left eye” and “Right eye” represent the retinas, each with a thick black arc marking a portion of its surface. Thin gray lines connect points on each retina to points along the border of the full visual field ovals, showing that the extent of each retina’s black arc corresponds to the boundary of the combined visual field above. The figure illustrates that the two eyes working together define the full visual field, the entire visual space visible without moving the head or eyes.

Figure 23.2

An illustration comparing the visual field seen by each eye individually. Two pairs of overlapping ovals are shown side by side. On the left, under the label “Left eye visual field,” one oval is shaded green and outlined with a thick border, representing the portion of the full visual field visible to the left eye; a thin, unshaded oval behind it represents the full visual field for reference. Below, two circles labeled L and R represent the retinas, with the left retina’s temporal (outer) portion marked in green and its nasal (inner) portion marked in blue, connected by lines to the shaded region above. On the right, under the label “Right eye visual field,” the same layout is mirrored, with the right eye’s visible field shaded blue and its retina’s temporal portion marked in blue. The figure illustrates that each eye alone views a large but incomplete portion of the full visual field, missing the far periphery on its nasal side.

Figure 23.3

An illustration comparing the retinal origins of the left and right hemifields. Two pairs of overlapping ovals are shown side by side. On the left, under the label “Left hemifield,” the left half of the combined visual field is shaded orange. Below, four circles labeled L and R (left and right eye) show the nasal and temporal retina in each eye, colored orange and blue respectively; lines trace from the orange-labeled nasal retina of the left eye and the orange-labeled temporal retina of the right eye up to the shaded left hemifield, showing that both regions contribute to viewing it. On the right, under the label “Right hemifield,” the same layout is mirrored in blue, with the nasal retina of the right eye and the temporal retina of the left eye contributing to the right hemifield. The figure illustrates that each hemifield is viewed using the nasal retina of one eye combined with the temporal retina of the other eye.

Figure 23.4

An illustration comparing monocular and binocular regions of the visual field. On the left, under the label “Monocular visual fields,” two crescent-shaped regions are shaded magenta at the outer edges of two overlapping ovals, representing the peripheral areas visible to only one eye. On the right, under the label “Binocular visual field,” the central region where the two ovals overlap is shaded yellow, representing the area visible to both eyes simultaneously. The figure illustrates that peripheral vision is monocular, while central vision is binocular, due to the degree of overlap between the two eyes’ individual visual fields.

Figure 23.5

An illustration showing the retinal ganglion cell axon pathway from the eyes to the brain. At the top, two ovals divided into orange (left half) and blue (right half) represent the full visual field. Below, two circles labeled L and R represent the retinas, each with a nasal (inner) and temporal (outer) portion outlined and colored to match the visual field regions they view. From each retina, blue and orange lines representing axon bundles converge and cross at a point labeled “Optic chiasm,” continuing downward as the “Optic tract” after being labeled “Optic nerve” before the crossing. At the chiasm, the axons from the nasal retina of each eye cross to the opposite side, while axons from the temporal retina continue on the same side. The figure illustrates that after the optic chiasm, each optic tract carries information exclusively from the contralateral visual hemifield.

Figure 23.6

A horizontal section of the brain viewed from above, shown as a tan outline of the cortical surface. Two colored pathways are traced through the interior of the brain: a blue pathway on the left side and an orange pathway on the right side, each entering from the posterior-medial region and curving forward before turning back toward the posterior pole of the brain. Three arrows label key points along the pathway: “Optic tract entering from the optic chiasm” at the entry point, “Lateral geniculate nucleus of the thalamus” at a small gray oval structure where the pathway synapses partway along its course, and “Primary visual cortex” at the posterior end where the pathway terminates. The figure illustrates that the optic tract projects to the lateral geniculate nucleus and then continues to the primary visual cortex in the occipital lobe.

Figure 23.7

A flowchart showing the full visual pathway as a linear sequence. Five boxes connected by arrows read, in order: “Photoreceptor,” “Bipolar cell,” “Ganglion cell,” “Lateral geniculate nucleus,” and “Primary visual cortex.” A bracket beneath the first three boxes is labeled “Retina,” indicating that photoreceptors, bipolar cells, and ganglion cells are all retinal components. Above the arrow connecting “Ganglion cell” to “Lateral geniculate nucleus,” a label reads “Via optic nerve, optic chiasm, optic tract,” specifying the route ganglion cell axons take to reach the thalamus. Below “Lateral geniculate nucleus” is the label “Thalamus,” and below “Primary visual cortex” are the labels “Occipital lobe” and “Cerebral cortex.”

Figure 23.8

An illustration comparing how circular thalamic receptive fields combine to form linear cortical receptive fields, with two examples showing different orientations. In the top example, three circular thalamic neurons are arranged horizontally in a row, each connected by a curved line to a single oval-shaped visual cortex neuron; the resulting receptive field, shown as a star burst pattern beneath the thalamic circles, is horizontal. In the bottom example, the same three thalamic neurons are arranged vertically in a column, again converging on a single visual cortex neuron, producing a vertical receptive field pattern. The figure illustrates that the spatial arrangement of converging thalamic receptive fields determines the orientation of the resulting cortical receptive field.

Figure 23.9

A diagram comparing line orientation to neuronal firing rate. Along the top, five thick black lines are shown at different orientations, from left to right: vertical, diagonal (upper-left to lower-right), horizontal, diagonal (upper-right to lower-left), and vertical again. Below each line, a horizontal axis labeled “Firing rate of visual cortex neuron” shows a burst of vertical tick marks representing action potentials, aligned beneath each line orientation. The two vertical line orientations show a moderate number of tick marks, the horizontal line shows the highest number of tick marks, and the two diagonal orientations show few or no tick marks. The figure illustrates that a visual cortex neuron’s firing rate depends on how closely a line’s orientation matches the neuron’s preferred orientation, in this example horizontal.

Figure 23.10

A side view of the brain with the primary visual cortex shaded orange at the posterior pole. Two white arrows outlined in black trace the two processing streams. One arrow, labeled “Dorsal stream to parietal lobe,” curves upward and forward from the primary visual cortex into a pink-shaded region covering the parietal lobe. The other arrow, labeled “Ventral stream to temporal lobe,” curves forward along the lower brain surface into a green-shaded region covering the temporal lobe. The cerebellum is visible below, unshaded and cross-hatched, and the frontal lobe is left unshaded. The figure illustrates that visual information leaving the primary visual cortex splits into two parallel streams, one ascending to the parietal lobe and one extending forward into the temporal lobe.

Figure 23.11

A side view of the brain, colored identically to Figure 23.10, with the primary visual cortex shaded orange and the dorsal stream region shaded pink in the parietal lobe. A small dark brown circular region labeled “V5 / Area MT” is marked at the boundary between the primary visual cortex and the parietal lobe, with an arrow pointing to its location. The figure illustrates that area MT sits at an early point along the dorsal stream, positioned between the primary visual cortex and the rest of the parietal lobe.

Figure 23.12

A side view of the brain, colored identically to Figure 23.10, with the ventral stream region shaded green across the temporal lobe. Within this region, two areas are outlined with dashed borders and labeled with arrows: “Inferior temporal,” a large dashed region spanning the lower temporal lobe, and “Fusiform gyrus,” a smaller dashed region near the ventral, posterior edge of the temporal lobe close to the cerebellum. A separate small dashed circle labeled “V4” is marked near the boundary of the primary visual cortex and temporal lobe. The figure illustrates the progression of ventral stream processing, from area V4 near the primary visual cortex, through the inferior temporal lobe, to the fusiform gyrus.

Figure 23.13

Two coronal brain sections stacked vertically, each preceded by a small side-view brain outline with a dashed vertical line marking the section plane, positioned at the temporal lobe. In the top section, two small orange regions are visible bilaterally in the medial temporal lobe, labeled “Amygdala” with arrows. In the bottom section, taken at a slightly more posterior plane, two purple, curled regions are visible bilaterally in the medial temporal lobe, labeled “Hippocampus” with arrows. The figure illustrates that both the amygdala and hippocampus are located deep within the medial temporal lobe, near the pathway of the ventral visual stream.

Figure 23.14

A mid-sagittal section of the brain showing two parallel pathways from the retina. A solid red line labeled “From retina” enters from the lower left, passing through a green-shaded region labeled “Thalamus” near the center of the brain. From the thalamus, a solid red arrow continues down to a small orange region labeled “Amygdala (in temporal lobe).” A separate dashed red arrow branches from the thalamus and curves posteriorly toward the occipital lobe, representing the longer pathway to the visual cortex and ventral stream. The figure illustrates that the amygdala receives a direct projection from the thalamus that bypasses the visual cortex, in addition to the longer route through the ventral stream.

Figure 23.15

A horizontal section of the brain, shown as a tan outline, with a small side-view brain outline and dashed line indicating the section plane at the level of the midbrain and thalamus. Within the section, four structures are colored and labeled with arrows: a small pink region labeled “Suprachiasmatic nucleus (Hypothalamus)” near the midline, anterior to the other structures; a green oval labeled “Lateral geniculate nucleus (Thalamus)” lateral to the suprachiasmatic nucleus; a small green region labeled “Pretectum” near the midline, posterior to the thalamus; and a blue band labeled “Superior colliculus” at the midline, posterior to the pretectum, near the primary visual cortex label at the back of the section. The figure illustrates that retinal ganglion cell axons project to multiple brain targets beyond the lateral geniculate nucleus, including the suprachiasmatic nucleus, pretectum, and superior colliculus.

Chapter 24 – Somatosensory Systems

Figure 24.1

A cross-sectional illustration of the spinal cord with a primary afferent fiber traced in red. The gray butterfly-shaped central region represents spinal cord gray matter, with the dorsal horn labeled at its upper points. Outside the cord on the right side, a red line traces the path of a primary afferent fiber: it originates outside the spinal cord, passes through a gray circle labeled “Dorsal root ganglion,” where the somatosensory cell body is located, then continues through the “Dorsal root” into the spinal cord, terminating near the dorsal horn. A branch of the red line curves upward into the “Dorsal column,” a white matter region at the top of the cord, with an arrow labeled “Ascending fibers” showing this branch traveling toward the brain. On the outer right edge, a “Spinal nerve” is labeled where the dorsal root and a separate “Ventral root” converge. The figure illustrates that the primary afferent fiber’s cell body sits in the dorsal root ganglion outside the cord, with its axon either synapsing locally near the dorsal horn or ascending to the brain via the dorsal column.

Figure 24.2

An illustration comparing the four types of primary afferent axons by diameter and myelination, arranged left to right from fastest to slowest. Each axon is shown in cross-section as a green circle (the axon) surrounded by concentric black rings (myelin). The leftmost axon, labeled “Group I,” is the largest, with the thickest myelin sheath, under the heading “Fastest.” Moving right, “Aβ” is somewhat smaller with slightly less myelin, “Aδ” is much smaller with a thin myelin layer, and the rightmost axon, labeled “C,” is the smallest with no myelin, under the heading “Slowest.” Below each axon, a “Sensation” row lists the modality each axon type conveys: Group I carries proprioception, Aβ carries touch, Aδ carries pain and temperature, and C carries pain, temperature, and itch. The figure illustrates that axon diameter and myelination decrease together from Group I to C fibers, and that conduction speed decreases accordingly.

Figure 24.3

An illustration comparing dermatome regions on a human body outline to their corresponding spinal cord segments. On the left, a front-facing body silhouette is divided into colored regions: green covers the head, neck, shoulders, and arms; purple covers the chest and abdomen in horizontal bands; blue covers the hips and legs in vertical bands; and orange marks a small region at the groin and heels. On the right, a side view of the brain and spinal cord shows the cord divided into matching colored segments from top to bottom: green segments labeled “Cervical (C1–C8),” purple segments labeled “Thoracic (T1–T12),” blue segments labeled “Lumbar (L1–L5),” and orange segments labeled “Sacral (S1–S5).” The figure illustrates that each spinal segment innervates a specific band of skin, with cervical segments mapping to the head, neck, and arms, thoracic segments to the trunk, lumbar segments to the lower body and legs, and sacral segments to the groin and posterior legs.

Chapter 25 – Touch: The Skin

Figure 25.1

A cross-sectional illustration of the skin showing the location of four mechanoreceptor types across three tissue layers, labeled from top to bottom as epidermis (pink), dermis (gray), and subcutaneous layer (dark gray, textured). Near the top of the epidermis, several blue, clustered Merkel cells sit close to the skin surface within finger-like epidermal ridges that dip into the dermis. Just below the epidermis, in the upper dermis, oval blue-gray Meissner corpuscles are positioned near the same ridges. Deeper in the dermis, two green, kidney-bean-shaped Ruffini endings sit farther from the surface, and at the bottom of the dermis, near the subcutaneous layer, a yellow, layered Pacinian corpuscle is shown with a striped, onion-like internal structure. Dashed lines connect each receptor to its associated nerve fiber. The figure illustrates that Merkel cells and Meissner corpuscles sit close to the skin surface, while Ruffini endings and Pacinian corpuscles are located deeper in the dermis, corresponding to their different roles in touch perception.

Figure 25.2

Two side-by-side panels showing a block of skin with a Pacinian corpuscle and a membrane potential recording. In panel A, labeled “No stimulation of receptive field,” two yellow, irregularly shaped regions on the skin surface are labeled “Receptive fields,” and no probe touches the skin; the corresponding graph, labeled “Few (baseline) action potentials,” shows only two spikes over the recording period. In panel B, labeled “Stimulation of receptive field,” a brown cylindrical probe presses down into one of the yellow receptive field regions; the corresponding graph, labeled “Increased action potential firing,” shows five closely spaced spikes over the same time period. In both panels, a dashed line traces from the Pacinian corpuscle, located deep in the skin block, to a recording electrode. The figure illustrates that touching a mechanoreceptor’s receptive field increases its firing rate above the low baseline rate present without stimulation.

Figure 25.3

A three-dimensional block of skin showing four mechanoreceptor types and their relative receptive field sizes on the skin surface. At the top surface, labeled “Receptive fields on skin surface,” five shapes of varying size represent each receptor’s receptive field: a large yellow shape, a medium green shape, a small purple dot, a small red dot, and a small light blue oval, with arrows pointing from two of the labels down to their corresponding surface regions. Below, within the skin layers, the receptors themselves are labeled: a yellow Pacinian corpuscle and green Ruffini ending sit deep in the dermis, corresponding to the two largest surface regions, while blue-gray Meissner corpuscles and reddish Merkel cells sit near the epidermis, corresponding to the smallest surface regions. The figure illustrates that receptors located deeper in the skin, Pacinian corpuscles and Ruffini endings, have larger receptive fields than the more superficial Meissner corpuscles and Merkel cells.

Figure 25.4

Two side-by-side blocks of skin comparing receptor density and receptive field size at two body locations. On the left, labeled “High receptor density, Small receptive fields, Example: Hand,” six small blue-gray circles of varying tiny sizes are scattered across the skin surface, with correspondingly small Meissner corpuscle clusters shown in the dermis below, closely spaced together. On the right, labeled “Low receptor density, Large receptive fields, Example: Back,” three larger blue-gray shapes are spread across the skin surface with more space between them, corresponding to fewer, more widely spaced Meissner corpuscle clusters in the dermis below. The figure illustrates that a higher density of receptors, as found in the hand, produces smaller receptive fields, while a lower density, as found in the back, produces larger receptive fields.

Figure 25.5

Two side-by-side blocks of skin, each showing a two-pronged caliper touching the skin surface. On the left, labeled “Caliper points in same receptive field, Perceive one stimulus,” both caliper points press into a single large yellow receptive field region, with one Meissner corpuscle cluster shown below in the dermis. On the right, labeled “Caliper point in different receptive fields, Perceive two stimuli,” the two caliper points press into two separate, smaller receptive field regions, with two distinct Meissner corpuscle clusters shown below. The figure illustrates that whether two caliper points are perceived as one stimulus or two depends on whether they fall within the same receptive field or activate two separate receptive fields.

Animation 25.1

The animation tracks action potential firing in a slowly adapting mechanoreceptor. A 3D skin cross-section on the left displays a superficial green receptor connected to a recording electrode, while traces on the right monitor stimulus pressure and firing rate. When a wooden probe applies a weak pressure, the pressure trace steps up, and a steady, continuous stream of action potential spikes fires on the bottom trace. When pressure steps higher to strong, the firing rate accelerates to a faster, continuous stream of spikes that persists without stopping for the entire duration of the applied stimulus.

Animation 25.2

The animation tracks action potential firing in a rapidly adapting mechanoreceptor. A 3D skin cross-section on the left shows a deep yellow concentric Pacinian corpuscle wired to a recording electrode, while traces on the right plot stimulus pressure and firing rate. When a probe applies weak pressure, the pressure trace steps up, and the bottom trace displays a brief, temporary burst of action potential spikes before falling silent. When pressure steps up to strong, another short burst of spikes occurs only during the transition, after which firing stops completely despite sustained pressure.

Animation 25.3

The animation demonstrates two mechanical gating mechanisms for ion channels across a lipid membrane, with blue sodium spheres and purple calcium spheres floating in the upper extracellular space. On the left, horizontal arrows labeled “Stretch” pull directly on the membrane bilayer, widening a central blue channel pore and allowing sodium and calcium ions to flow downward into the intracellular space. On the right, horizontal arrows labeled “Pull” tug on a brown extracellular protein link and a pink intracellular protein link, mechanically pulling a second blue channel open to allow identical sodium and calcium influx.

Chapter 26 – Vision: Central Processing

Figure 26.1

Two panels comparing a dorsal column neuron’s response with and without center stimulation. In both panels, three colored mechanoreceptors (purple A, blue B, green C) sit above a blue star-shaped dorsal column neuron labeled E, each connected by a line with a plus sign indicating excitatory synapses. A dashed oval around the receptors represents the dorsal column neuron’s overall receptive field, with a smaller solid oval marking receptor B’s individual receptive field at its center. In panel A, no probe touches the skin, and the firing rate box shows a moderate, evenly spaced train of spikes for neuron E. In panel B, a brown probe presses into the center of the receptive field (over receptor B), and the firing rate box shows a denser train of spikes for neuron E. The figure illustrates that direct excitatory input from receptors in the receptive field center increases the dorsal column neuron’s firing rate.

Figure 26.2

Two panels comparing dorsal column neuron responses to surround stimulation. In both panels, three receptors (purple A, blue B, green C) connect with plus signs to three dorsal column neurons (purple D, blue E, green F), and a red inhibitory interneuron sits between receptor A and neuron E, connected by a minus sign. In panel A, no probe touches the skin, and the firing rate box shows similar baseline spike trains for D, E, and F. In panel B, a probe presses into receptor A’s receptive field, which lies in the surround of neuron E’s receptive field (indicated by the dashed oval) but in the center of neuron D’s receptive field. The firing rate box shows an increased spike train for D, a decreased spike train for E, and an unchanged baseline for F. The figure illustrates that stimulating a receptor in the surround of one neuron’s receptive field decreases that neuron’s firing rate via an inhibitory interneuron, while simultaneously increasing the firing rate of the adjacent neuron for which that same receptor is in the center.

Figure 26.3

A diagram showing three receptors (purple A, blue B, green C) connected to three dorsal column neurons (purple D, blue E, green F) via both direct excitatory connections (plus signs) and indirect inhibitory connections through two red interneurons (minus signs). A probe presses into receptor B’s receptive field, which is the center of neuron E’s receptive field and the surround of neurons D’s and F’s receptive fields. The firing rate box shows an increased spike train for E, and decreased spike trains for D and F relative to baseline. To the right, two graphs compare stimulus strength across the same region of skin: “Actual stimulus strength” shows a smooth, symmetric bump, while “Perceived stimulus strength” shows a sharper, narrower peak flanked by dips below baseline on either side. The figure illustrates that lateral inhibition sharpens the perceived edge of a stimulus by combining increased firing at the point of contact with decreased firing in the surrounding region.

Figure 26.4

An illustration tracing the dorsal column–medial lemniscus pathway from the body to the cortex, using a human silhouette and cross-sections at four labeled levels: A (lumbar spinal cord), B (cervical spinal cord), C (caudal medulla), and D (somatosensory cortex). At level A, a sensory neuron’s peripheral branch is shown entering the lumbar spinal cord via the dorsal root ganglion, with its central branch ascending ipsilaterally, labeled “stays ipsilateral.” At level B, a second sensory neuron from the upper body enters the cervical spinal cord, also ascending ipsilaterally. At level C, both axon populations reach the caudal medulla, where they synapse in the dorsal column nuclei: the gracile nucleus (lower body) and cuneate nucleus (upper body), both labeled with arrows. Second-order axons cross the midline here, labeled “decussation,” and continue as the medial lemniscus. At level D, a coronal brain section shows the medial lemniscus terminating in the ventral posterolateral nucleus of the thalamus, with third-order neurons projecting to the somatosensory cortex. The figure illustrates that lower and upper body sensory information travels ipsilaterally up the spinal cord, crosses the midline in the medulla, and then ascends via the medial lemniscus to the thalamus and cortex.

Figure 26.5

An illustration tracing the trigeminal pathway from the face to the cortex, using a head silhouette and cross-sections at two labeled levels: A (pons, decussation) and B (somatosensory cortex). At level A, a sensory neuron is shown entering the brainstem via the trigeminal nerve (cranial nerve V), with its cell body in the trigeminal ganglion located just outside the pons. The peripheral branch travels to the face, and the central branch synapses on the trigeminal nucleus within the pons. A second-order neuron crosses the midline, labeled “decussation.” At level B, a coronal brain section shows this projection terminating in the ventral posteromedial nucleus of the thalamus, with a third-order neuron projecting to the somatosensory cortex. The figure illustrates that facial sensory information reaches the brainstem via the trigeminal nerve, crosses the midline in the pons, and ascends through the thalamus to the cortex.

Figure 26.6

A flowchart comparing the two somatosensory pathways as parallel sequences. The top row, for the body and neck, reads: “Periphery, Body & neck” (entering “via spinal nerve”) to “Dorsal root ganglion” to “Dorsal column nuclei” (entering “via dorsal column”) to “Ventral posterolateral nucleus” (entering “via medial lemniscus”) to “Primary somatosensory cortex,” with “Spinal cord,” “Brainstem,” “Thalamus,” and “Parietal lobe / Cerebral cortex” labeled beneath the corresponding boxes. The bottom row, for the face and head, reads: “Periphery, Face & head” (entering “via cranial nerve V”) to “Trigeminal ganglion” to “Trigeminal nucleus” to “Ventral posteromedial nucleus” to “Primary somatosensory cortex,” with a bracket grouping the ganglion and nucleus under “Brainstem,” and “Thalamus” and “Parietal lobe / Cerebral cortex” labeled beneath the final two boxes. The figure illustrates that both pathways follow the same overall structure of periphery to spinal cord or brainstem to thalamus to cortex, differing in their specific relay nuclei and cranial versus spinal nerve entry points.

Figure 26.7

A cross-sectional illustration of the postcentral gyrus and posterior parietal cortex, shown as a folded strip of cortex with the central sulcus labeled at the left edge. Moving from the central sulcus toward the right, the postcentral gyrus is divided into four dark maroon regions labeled 3a, 3b, 1, and 2, with 3a and 3b positioned in the sulcus wall closest to the central sulcus, and 1 and 2 sitting along the crown and far wall of the gyrus. To the right, a lighter purple region labeled “Posterior parietal cortex” is divided into two subregions, 5 and 7. The figure illustrates that the primary somatosensory cortex is organized into four sequential processing areas, with the posterior parietal cortex as a further downstream region.

Figure 26.8

An outline illustration of the somatosensory cortex drawn in the recognizable shape of a sensory homunculus, viewed in cross-section. The outline’s contours form identifiable body-part shapes along its length: finger-like projections at the upper portion, a hand-like shape below them, and a face-in-profile shape near the lower left, all forming the boundary of a single unbroken region labeled “Somatosensory Cortex.” No internal divisions, colors, or individual body-region labels are shown within the outline in this version of the figure.

Figure 26.9

A side view of the brain with three regions highlighted along the parietal lobe. A dark maroon band, labeled “Postcentral gyrus, Primary somatosensory cortex,” runs vertically just posterior to an unlabeled central sulcus. A lighter purple region, labeled “Posterior parietal cortex,” sits posterior to the postcentral gyrus in the superior parietal lobe. A small pink region, labeled “Secondary somatosensory cortex,” is located inferior to the postcentral gyrus, near the lateral fissure. The figure illustrates the relative locations of the primary somatosensory cortex, posterior parietal cortex, and secondary somatosensory cortex within the parietal lobe.

Figure 26.10

Two panels comparing a somatotopic map of the fingers before and after amputation. In panel A, a strip of somatosensory cortex is divided into five colored regions (1 through 5, in yellow, gray, blue, green, and magenta), each corresponding to a differently colored finger on a hand diagram below, with digit 3 (blue) mapped between digits 2 and 4. In panel B, following loss of digit 3, the cortical strip is redrawn with only four regions; the space previously occupied by digit 3’s blue region has been divided between the neighboring gray (digit 2) and green (digit 4) regions, and the hand diagram below shows only digits 1, 2, 4, and 5. The figure illustrates that after loss of a digit, the cortical territory that once represented it is not left unused but is instead reassigned to represent the neighboring digits.

Chapter 27 – Pain

Figure 27.1

A cross-sectional illustration of the skin showing free nerve endings in the epidermis, each ending in small branching structures near the skin surface. Two axon types are traced from these nerve endings down through the dermis to a single point at the right edge of the image: a thin solid line labeled “C fiber” and a dashed line labeled “Aδ fiber,” both converging on the same location. Free nerve endings appear at multiple points across the epidermis, each connecting to one of the two fiber types below. The figure illustrates that nociceptors are branching, bare nerve endings in the skin, and that their signals travel to the spinal cord via either unmyelinated C fibers or lightly myelinated Aδ fibers.

Figure 27.2

A diagram comparing Aδ and C fiber structure and function. At the top, two green circles represent axon cross-sections: the left one, labeled “Aδ,” is surrounded by a thin double-line ring labeled “Myelin,” while the right one, labeled “C,” has no surrounding ring. Below, a table lists “Myelination” as “Thin” for Aδ and “None” for C, and “Sensation” as “First pain” for Aδ and “Second pain” for C. At the bottom, a graph plots pain intensity over time following an injury: a sharp, narrow spike appears early, aligned under “First pain,” followed by a broader, lower, more gradual rise and fall aligned under “Second pain.” The figure illustrates that the thin myelination of Aδ fibers allows faster conduction and the sharp, immediate first pain sensation, while unmyelinated C fibers conduct more slowly and produce the delayed, longer-lasting second pain sensation.

Figure 27.3

Illustration of two nociceptor terminal diagrams shown side by side, each depicting a different pain transduction pathway. In the left panel, two arrows labeled “Mechanical force” and “Pressure” point down into a channel labeled Piezo, positioned in the cell membrane at the top of the terminal. Below the channel, sodium (Na+, dark blue dot) and calcium (Ca2+, purple dot) are shown entering the cell. Further down, voltage-gated sodium channels on both sides of the tapering terminal allow additional sodium entry, shown as a cluster of dark blue dots. An arrow at the bottom points downward, labeled Aδ-I fiber. In the right panel, arrows labeled “Noxious heat” and “Capsaicin” (the latter marked with a tan hexagon icon) point down into a channel labeled TRPV1. Below the channel, sodium and calcium are again shown entering the cell, followed by voltage-gated sodium channels and a cluster of entering sodium ions. An arrow at the bottom points downward, labeled Aδ-II fiber.

Figure 27.4

A two-panel illustration. Panel A shows a body outline with a nociceptor fiber traced from the hand, up the arm, and into the spinal cord, with three stacked spinal cord cross-sections at the right showing the fiber’s cell body in the dorsal root ganglion and its central branch entering the cord, then splitting into ascending and descending branches (shown as a dotted line spanning the three segments) that terminate in the dorsal horn (blue) at each level, marked with small purple synaptic terminals. Panel B is a magnified single spinal cord cross-section with a circled inset at the dorsal horn labeled “Lissauer’s tract,” showing the small purple synaptic terminal region, with an arrow also pointing to the underlying blue “Dorsal horn.” The figure illustrates that nociceptor axons entering the spinal cord branch and travel up and down several segments within Lissauer’s tract before terminating in the dorsal horn.

Figure 27.5

An illustration tracing the spinothalamic pathway from the body to the cortex, using a human silhouette and cross-sections at four labeled levels: A (lumbar spinal cord), B (cervical spinal cord), C (caudal medulla), and D (somatosensory cortex). At levels A and B, nociceptor fibers are shown entering the spinal cord via the dorsal root ganglion and immediately crossing the midline within the same spinal segment, labeled “input decussates” at both levels. The crossed fibers ascend together as the spinothalamic tract, shown continuing through the caudal medulla at level C. At level D, a coronal brain section shows the spinothalamic tract terminating in the ventral posterolateral nucleus of the thalamus, with third-order neurons projecting to the somatosensory cortex. The figure illustrates that, unlike the touch pathway, spinothalamic pain fibers decussate immediately upon entering the spinal cord rather than crossing later in the medulla.

Figure 27.6

An illustration tracing the trigeminothalamic pathway from the face to the cortex, using a head silhouette and cross-sections at three labeled levels: A (caudal medulla), B (pons), and C (somatosensory cortex). Nociceptor fibers are shown entering the brainstem via the trigeminal nerve (cranial nerve V), with the cell body in the trigeminal ganglion located just outside the pons at level B. The fiber descends within the brainstem to the spinal trigeminal nucleus at level A in the caudal medulla, where it synapses; the second-order neuron then ascends and crosses the midline before reaching level C, a coronal brain section showing termination in the ventral posteromedial nucleus of the thalamus and a third-order projection to the somatosensory cortex. The figure illustrates that facial pain information enters at the pons but descends to the medulla to synapse before ascending again to the thalamus and cortex.

Figure 27.7

A flowchart comparing the spinothalamic and trigeminothalamic pathways as parallel sequences. The top row, for the body and neck, reads: “Periphery, Body & neck” (entering “via spinal nerve”) to “Dorsal root ganglion” to “Dorsal horn” (entering “via spinothalamic tract”) to “Ventral posterior lateral nucleus” to “Primary somatosensory cortex,” with “Spinal cord,” “Spinal cord,” “Thalamus,” and “Parietal lobe / Cerebral cortex” labeled beneath the corresponding boxes. The bottom row, for the face and head, reads: “Periphery, Face & head” (entering “via cranial nerve V”) to “Trigeminal ganglion” to “Spinal trigeminal nucleus” to “Ventral posterior medial nucleus” to “Primary somatosensory cortex,” with a bracket grouping the ganglion and nucleus under “Brainstem,” and “Thalamus” and “Parietal lobe / Cerebral cortex” labeled beneath the final two boxes. The figure illustrates that both pain pathways follow the same overall structure of periphery to spinal cord or brainstem to thalamus to cortex, differing in their specific relay nuclei and point of decussation.

Figure 27.8

An illustration of a small patch of tissue with a central red, jagged burst labeled “Tissue damage.” Three arrows point downward from the damage site, labeled “ATP,” “Prostaglandins,” and “H+.” To the left, a teal, irregularly shaped mast cell releases an arrow labeled “Histamine.” To the right, a purple, irregularly shaped macrophage sits nearby. Below the tissue damage site, a branching nociceptor fiber extends downward and to the right, labeled “To spinal cord,” with one branch curving left toward a dark red tube labeled “Blood vessel” and releasing an arrow labeled “Substance P” toward it. The figure illustrates that tissue damage triggers release of inflammatory chemicals from the damaged tissue and nearby immune cells, while the nociceptor itself releases substance P onto local blood vessels, together producing the inflammatory response that sensitizes nearby nociceptors.

Figure 27.9

An illustration of the spinal cord dorsal horn, shown as a wedge-shaped gray region, with two afferent fibers entering from the upper left. An orange fiber labeled “Aα or Aβ fiber mechanoreceptor” and ending in a star shape labeled M enters above a second fiber labeled “C fiber nociceptor” and ending in a star shape labeled N. Within the dorsal horn, the mechanoreceptor (M) makes an excitatory connection (+) onto a black inhibitory interneuron and also continues to a pathway labeled “to dorsal column.” The inhibitory interneuron makes an inhibitory connection (−) onto the nociceptor terminal (N) and an excitatory connection (+) onto a green projection neuron. The nociceptor (N) also makes a direct excitatory connection (+) onto the same projection neuron, which sends output labeled “to spinothalamic tract.” A legend on the right identifies the nociceptor, mechanoreceptor, inhibitory interneuron, and projection neuron symbols. The figure illustrates that mechanoreceptor input activates an inhibitory interneuron that suppresses the nociceptor’s excitatory drive onto the projection neuron, reducing the pain signal sent to the brain.

Figure 27.10

An illustration of the spinal cord dorsal horn, structured similarly to Figure 27.9. A C fiber nociceptor, labeled N, enters from the left. From above, a dashed line labeled “from medulla” descends into the dorsal horn, connecting to a purple star shape labeled P (periaqueductal gray neuron) and an orange star shape labeled M (serotonin- or norepinephrine-releasing medulla neuron). Within the dorsal horn, the medulla neuron (M) makes excitatory connections (+) onto two blue star shapes labeled E (enkephalin neurons). Each enkephalin neuron makes inhibitory connections (−) onto the nociceptor (N) and onto a green projection neuron. The nociceptor also makes a direct excitatory connection (+) onto the projection neuron, which sends output labeled “to spinothalamic tract.” A legend on the right identifies all cell types and their symbols. The figure illustrates that descending input from the medulla, driven by the periaqueductal gray, activates enkephalin-releasing interneurons that inhibit both the nociceptor terminal and the projection neuron, reducing pain signal transmission.

Figure 27.11

A horizontal bar illustrating a temperature scale from 0°C to 50°C, shaded with a color gradient from blue on the left through white near the middle to red on the right, with tick marks and labels at 0°C, 8°C, 18°C, 28°C, 43°C, and 50°C. Above the bar, horizontal lines with arrows indicate the approximate temperature range detected by each channel. GluK2 is shown with a leftward-pointing arrow extending from about 18°C off the left edge of the scale, indicating sensitivity to noxious cold. TRPM8 spans from 8°C to 28°C, the range associated with non-noxious cool sensation. TRPV3/TRPV4 spans from 28°C to 43°C, the range associated with non-noxious warm sensation. TRPV1 is shown with a rightward-pointing arrow extending from 43°C off the right edge of the scale, indicating sensitivity to noxious heat.

Figure 27.12

Illustration of a pruriceptor terminal depicting two itch signaling pathways side by side. At the upper left, a mast cell, drawn as a teal irregular shape with a central nucleus, releases histamine, shown with an arrow pointing to an H1 receptor in the cell membrane. At the upper right, a non-histamine pruritogen, drawn as a tan oval icon, binds an Mrgpr receptor in the membrane. Below the H1 receptor, an arrow leads to an open TRPV1 channel, with sodium (Na+, dark blue dot) and calcium (Ca2+, purple dot) entering the cell. Below the Mrgpr receptor, an arrow leads to an open TRPA1 channel, with sodium (Na+, dark blue dot) entering the cell. Further down, voltage-gated sodium channels on both sides of the tapering terminal allow additional sodium entry, shown as a cluster of dark blue dots. An arrow at the bottom of the cell points downward, labeled C fiber.

Chapter 28 – Proprioception

Figure 28.1

This illustration depicts the structure of a muscle spindle embedded within skeletal muscle and its sensory and motor innervation. The spindle is drawn as a vertical, elongated structure surrounded by a connective tissue capsule, situated among the long, striped extrafusal muscle fibers that make up the bulk of the surrounding muscle. Inside the capsule are several thinner intrafusal muscle fibers, each showing a banded central region. Two types of sensory afferent axons innervate the intrafusal fibers. Group Ia afferent axons, shown in black, wrap tightly around the very center of each intrafusal fiber in a spiral pattern. Group II afferent axons, shown in magenta, form branched, spray-like endings on the intrafusal fibers slightly to either side of the Ia endings, flanking the central region rather than wrapping around it. Gamma motor neurons, shown in green, innervate the intrafusal fibers at their poles, the contractile ends located outside the central sensory region where the Ia and Group II afferents terminate.

Figure 28.2

A detailed anatomical diagram of a Golgi tendon organ (GTO) proprioceptive receptor located at the muscle-tendon junction. The illustration shows an elongated, spindle-shaped structure positioned between muscle fibers on the left (depicted as pink cylindrical structures) and tendon on the right (shown as parallel gray striations). The central portion of the GTO contains an intricate network of intertwined black collagen fibers and red sensory axon terminals that weave throughout the structure in a complex, braided pattern. A single red “Group Ib afferent axon” emerges from the top of the GTO, carrying sensory information about muscle tension to the central nervous system. The diagram includes a legend at the bottom identifying red structures as “Axon” and black structures as “Collagen fibers.” Arrows point to and label the “Muscle fibers” on the left and “Tendon” on the right, showing the GTO’s strategic position to detect tension generated during muscle contraction.

Figure 28.3

Two spinal cord cross-sections side by side, labeled “Cervical” and “Thoracic.” In the cervical section, two teal-colored regions are labeled with arrows: the “Dorsal spinocerebellar tract,” positioned at the dorsolateral edge of the white matter, and the “Ventral spinocerebellar tract,” positioned just below it at the lateral edge. In the thoracic section, a small red dot near the base of the dorsal horn is labeled “Clarke’s nucleus” with an arrow. The figure illustrates the relative locations of the dorsal and ventral spinocerebellar tracts at the cervical level and Clarke’s nucleus at the thoracic level, the spinal segment where proprioceptive afferents from the lower body synapse before ascending.

Figure 28.4

A flowchart tracing proprioceptive pathways from the lower body, split into two branches shown in black and orange. Starting at “Thoracic spinal cord,” a black line rises from the “Dorsal root ganglion” to “Clarke’s nucleus.” From Clarke’s nucleus, the black “Dorsal spinocerebellar tract” ascends directly to the “Cerebellum,” while a second black branch continues up to the level of the “Medulla,” labeled “Near gracile nucleus,” crosses the “Midline” (marked by a vertical dashed line), and ascends through the “Pons” as the “Medial lemniscus” to the “VPL Thalamus.” Separately, an orange line rises from the “Dorsal horn” and crosses the midline immediately as the “Ventral spinocerebellar tract,” ascending to the level of the pons, where it is labeled as decussating again (shown by the orange arrow crossing back to the same side) before terminating in the “Cerebellum.” The figure illustrates that lower limb proprioceptive information reaches the cerebellum via two routes—one that stays ipsilateral throughout (dorsal spinocerebellar tract) and one that crosses the midline twice (ventral spinocerebellar tract)—while a separate branch of the dorsal pathway crosses once in the medulla to reach the thalamus and cortex.

Figure 28.5

A flowchart structured identically to Figure 28.4 but for the upper limbs, tracing two branches from the “Cervical spinal cord.” A black line rises from the “Dorsal root ganglion” to the “External cuneate nucleus.” From there, the black “Cuneocerebellar tract” ascends directly to the “Cerebellum,” while a second branch continues to the level of the “Medulla,” crosses the “Midline,” and ascends through the “Pons” as the “Medial lemniscus” to the “VPL Thalamus.” Separately, an orange line rises from the “Dorsal horn” and crosses the midline immediately as the “Rostral spinocerebellar tract,” ascending to the pons, where it decussates again (shown by the orange arrow crossing back) before terminating in the “Cerebellum.” The figure illustrates that upper limb proprioceptive information follows the same dual-pathway organization as the lower limb, substituting the external cuneate nucleus for Clark’s nucleus and the cuneocerebellar and rostral spinocerebellar tracts for their lower-limb counterparts.

Figure 28.6

A flowchart tracing proprioceptive information from the face and head. Starting at “Periphery, Face & head,” a line enters “via cranial nerve V” to the “Mesencephalic trigeminal nucleus,” labeled as located in the “Brainstem.” From there, the pathway splits into two arrows: one leading directly to the “Cerebellum,” and the other leading to the “Ventral posteromedial nucleus,” labeled “Thalamus,” which continues to the “Primary somatosensory cortex,” labeled “Parietal lobe, Cerebral cortex.” The figure illustrates that facial and head proprioceptive information reaches the mesencephalic trigeminal nucleus in the brainstem and then splits into a pathway to the cerebellum for unconscious coordination and a pathway to the thalamus and cortex for conscious proprioception.

Chapter 29 – Auditory: The Ear

Figure 29.1

An illustration of the outer ear shown in cross-section alongside the visible pinna. On the left, the pinna is drawn as the tan, curved cartilage structure of the visible ear, labeled with an arrow. From the pinna, a pink tube representing the auditory canal extends inward toward the skull, ending at a blue oval labeled the tympanic membrane, which marks the boundary with the middle and inner ear structures shown in gray beyond it. A bracket beneath the pinna, auditory canal, and tympanic membrane labels this entire region as the “Outer ear.” The figure illustrates that the outer ear consists of the pinna, which collects sound waves, and the auditory canal, which channels those waves inward to vibrate the tympanic membrane at the boundary with the middle ear.

Figure 29.2

An illustration of the middle ear, magnified from a boxed region within a smaller overview of the full ear on the left. Within the magnified view, the tan tympanic membrane is shown as a large blue oval on the left, connected to the first of three small tan ossicles: the malleus, which attaches directly to the membrane. The malleus connects to the incus, which connects to the stapes, the smallest of the three bones, attached to a blue structure labeled the oval window on the right. Two small striped muscles are shown attached to the ossicles: the tensor tympani muscle, connected near the malleus, and the stapedius muscle, connected near the stapes. A bracket beneath the magnified region labels the entire structure as the “Middle ear.” The figure illustrates that the three ossicles form a mechanical chain linking the tympanic membrane to the oval window, with two associated muscles capable of dampening ossicle movement.

Figure 29.3

An illustration of the inner ear, magnified from a boxed region within a smaller overview of the full ear on the left, showing the coiled cochlea. The magnified cross-section reveals the layered internal structure of the cochlea: the scala vestibuli (upper chamber) and scala tympani (lower chamber) curve around a central region containing the scala media and basilar membrane. Within this central region, a blue structure labeled the tectorial membrane sits above an orange structure labeled the Organ of Corti, which contains a cluster of purple cells labeled outer hair cells and a single purple cell labeled inner hair cells. The figure illustrates that the cochlea’s three fluid-filled chambers surround the Organ of Corti, the structure on the basilar membrane where inner and outer hair cells convert basilar membrane vibration into neural signals

Figure 29.4

An illustration of the cochlea unrolled into a schematic spiral, viewed from above, with three concentric rings representing the scala vestibuli (inner ring), the scala media and basilar membrane together (middle ring), and the scala tympani (outer ring). At the outer edge of the spiral, near the base, the oval window and round window are labeled, with frequency markers of 15000 Hz and 5000 Hz placed along the basilar membrane close to this base region. Moving inward along the spiral toward the apex at the center, frequency markers decrease to 700 Hz and then 200 Hz. An inset near the base shows an exaggerated wave pattern labeled “Vibration in basilar membrane,” illustrating the traveling wave that moves along the membrane. The figure illustrates that the basilar membrane is tonotopically organized, with high-frequency sounds producing maximal vibration near the base of the cochlea and low-frequency sounds producing maximal vibration near the apex.

Figure 29.5

An illustration of the Organ of Corti, magnified from a boxed region within a schematic cross-section of the cochlea on the left. In the magnified view, the orange Organ of Corti sits atop the tan basilar membrane at the bottom of the image. Three purple oval cells labeled outer hair cells sit on the left side of the Organ of Corti, each with small stereocilia projecting from their tops toward the blue, wing-shaped tectorial membrane above. A single purple oval cell labeled inner hair cells sits separately on the right, its stereocilia also contacting the tectorial membrane. The figure illustrates that both inner and outer hair cells project stereocilia into contact with the overlying tectorial membrane, positioning them to be mechanically stimulated as the basilar membrane vibrates relative to the tectorial membrane above.

Figure 29.6

Two side-by-side illustrations of an inner hair cell comparing stereocilia bending in opposite directions. In the left panel, labeled “Depolarization,” the stereocilia bend toward the tallest cilium, and two green, striped mechanically-gated channels at the tips of the shorter stereocilia are shown open, with green potassium (K+) ions flowing into the cell. Below, two pink calcium channels on the cell body open, with purple calcium (Ca2+) ions flowing in, and small circles representing neurotransmitter vesicles release their contents toward an afferent axon at the base of the cell, labeled “Neurotransmitter release.” In the right panel, labeled “No depolarization,” the stereocilia bend away from the tallest cilium, the mechanically-gated channels are shown closed, no potassium enters, the calcium channels remain closed, and no neurotransmitter is released onto the afferent axon. The figure illustrates that the direction of stereocilia bending determines whether mechanically-gated potassium channels open, and that channel opening is required to trigger the depolarization, calcium influx, and neurotransmitter release that signal the afferent axon.

Figure 29.7

An illustration of a single inner hair cell showing the full depolarization and repolarization cycle. At the top, stereocilia project into a region labeled “Endolymph, High K+ concentration, Low Na+ concentration,” with green potassium ions entering through open channels at the stereocilia tips, arrows pointing down into the cell body, labeled “Depolarization.” On the left side of the cell body, a pink calcium channel allows calcium (Ca2+) ions to enter. On the right side of the cell body, a green potassium channel allows potassium ions to exit into a region labeled “Perilymph, Low K+ concentration, High Na+ concentration,” with an arrow labeled “Repolarization” pointing to this outward flow. An afferent axon is shown at the base of the cell. The figure illustrates that inner hair cells reverse the typical direction of potassium flow seen in most neurons: potassium enters the cell at the stereocilia from the high-potassium endolymph to drive depolarization, then exits at the cell body into the low-potassium perilymph to drive repolarization.

Chapter 30 – Auditory: Central Processing

Figure 30.1

An illustration tracing the central auditory pathway from the ear to the cortex through five labeled levels: A (medulla), B (pons), C (pons), D (midbrain), and E (cortex). At level A, a small ear and cochlea icon on the lower left connects to the “Cochlear nucleus,” shown as a red dot in the medulla. From there, black lines ascend bilaterally to level B, where they reach the “Superior olive,” marked with orange dots on both sides of the pons. Orange dotted lines continue upward from the superior olive to level C, converging with black lines at the “Nucleus of the lateral lemniscus,” marked with purple dots. From level C, purple and orange lines continue to level D, the midbrain, converging at the “Inferior colliculus,” marked with red dots on both sides. From the inferior colliculus, black lines continue to level E, a coronal section of the cortex, first reaching the “Medial geniculate nucleus of the thalamus” and then projecting further out to the “Auditory cortex” at the lateral edges of both hemispheres. The figure illustrates that auditory information ascends through a series of brainstem and midbrain relay nuclei with extensive bilateral crossing before reaching the thalamus and primary auditory cortex.

Figure 30.2

A six-panel illustration in two columns (B and C) with a shared top row (A) showing anatomical location. In panel A, two brainstem cross-sections show the superior olive circled on the left and, on the right, the medial superior olive specifically highlighted with lines tracing input from both ears. In column B, three panels show a sound source positioned to the left of the head. In the top panel, a bell icon sends arrows of different lengths to each ear, with a shorter arrow to the left ear. In the middle panel, a red action potential trace appears at the left cochlea while the signal is still traveling (dashed arrow) toward the right cochlea, shown reaching a row of three circles representing MSO neurons at different, staggered times. In the bottom panel, both red and orange action potential traces converge on the same MSO neuron (the bottom circle, marked with a burst symbol), indicating coincidence detection at that neuron for a left-sided sound. In column C, the same three-panel sequence shows a sound source directly ahead, with solid arrows of equal length to both ears, action potentials arriving at both cochleae simultaneously, and coincidence detected at a different MSO neuron (the middle circle) due to the equal path lengths. The figure illustrates that MSO neurons are arranged so that each responds maximally to input from both ears when a specific relative delay between them is met, allowing different neurons to signal different sound locations based on interaural time difference.

Figure 30.3

An illustration of a single brainstem cross-section with an ear and cochlea icon on each side, each connected by a line curving down into the brainstem. On the left side, the line terminates at a blue, S-shaped structure labeled the “Lateral superior olive,” with a plus sign marking an excitatory synapse. A red triangular structure labeled the “Medial nucleus of the trapezoid body” sits near the midline, receiving input from the right ear’s line and sending an inhibitory connection (marked with a minus symbol, implied by the red inhibitory color) to the left lateral superior olive. On the right side, the mirrored structure shows the right ear’s line terminating with a plus sign at the right lateral superior olive, while the left ear’s line crosses to activate the medial nucleus of the trapezoid body near the midline before inhibiting the right lateral superior olive. The figure illustrates that each lateral superior olive receives direct excitatory input from the ipsilateral ear and indirect inhibitory input from the contralateral ear via the medial nucleus of the trapezoid body, allowing it to compare intensity between the two ears.

Figure 30.4

A flowchart showing convergence at the inferior colliculus. Three boxes on the left — “Cochlear nucleus,” “Superior olive,” and “Nucleus of the lateral lemniscus” — each send an arrow inward to a central box labeled “Inferior colliculus.” From the inferior colliculus, two arrows branch outward: one continuing to “Medial geniculate nucleus” at the far right, and another branching down to “Superior colliculus.” Labels beneath the flowchart identify the anatomical level of each stage: “Brainstem” beneath the three input structures, “Midbrain” beneath both the inferior and superior colliculus, and “Thalamus” beneath the medial geniculate nucleus. The figure illustrates that the inferior colliculus serves as a convergence point for three separate brainstem auditory structures before relaying integrated information onward to both the superior colliculus and the thalamus.

Figure 30.5

A side view of the brain with the lateral fissure labeled and indicated by an arrow along the lower temporal lobe. Within the fissure, a green region labeled “Primary auditory cortex” occupies the folded cortical surface, largely hidden within the fissure itself. Surrounding the primary auditory cortex, an orange region labeled “Secondary auditory cortex” extends along the adjacent temporal lobe surface, both above and lateral to the primary region. The figure illustrates that the primary auditory cortex sits largely within the lateral fissure, with secondary auditory cortex bordering it on the exposed temporal lobe surface.

Chapter 31 – Vestibular System

Figure 31.1

An illustration of the vestibular apparatus, with the cochlea shown at the right for orientation. On the left, three semicircular canal loops are labeled with arrows: “Anterior (superior)” at the top, “Posterior” in the middle, and “Horizontal (lateral)” at the bottom, grouped under a bracket labeled “Semicircular canals.” At the base of the canals, where they converge near the cochlea, two thin lines point to this junction region, labeled “Ampullae.” The figure illustrates that the three semicircular canals are oriented in different planes, each with an ampulla at its base, positioned adjacent to the cochlea within the inner ear.

Figure 31.2

Two side-by-side illustrations of a single ampulla, magnified from a small boxed region on a semicircular canal shown at the far left. In the left panel, labeled “At rest,” a purple, cone-shaped cupula extends from the floor of the ampulla to its ceiling, with tan hair cells at its base sending stereocilia upward into the cupula; orange vestibular nerve fibers extend from the base of the hair cells, and pale green shading fills the surrounding chamber, labeled “Endolymph.” In the right panel, labeled “Head rotation to the left,” the cupula is shown bent to one side, with curved arrows indicating “Endolymph flow” moving in the direction opposite the bend, and a callout labeled “Bending hair bundles” pointing to the deflected stereocilia at the cupula’s base. The figure illustrates that head rotation causes the endolymph to lag behind due to inertia, deflecting the cupula and bending the hair cell stereocilia embedded within it.

Figure 31.3

Two illustrations side by side. On the left, the vestibular apparatus is shown with the two otolith organs labeled under the bracket “Otolith organs”: a purple pouch labeled “Utricle” and a blue pouch labeled “Saccule,” both positioned near the base of the semicircular canals and adjacent to the cochlea. On the right, a magnified view sets the saccule and utricle within a three-dimensional box outlining the x, y, and z axes, showing their orientation relative to one another: the saccule sits oriented vertically along one wall of the box, while the utricle lies oriented horizontally along the floor. Each organ has a curving line of small black arrows labeled “Striola” running along its length, with arrows pointing outward in different directions along the curve to indicate the varying orientation of hair cells across each organ. The figure illustrates the location of the utricle and saccule within the inner ear and shows that the vertically oriented saccule and horizontally oriented utricle each contain hair cells arranged in multiple directions along the striola.

Figure 31.4

Two illustrations of the sensory epithelium (macula) side by side. On the left, labeled “At rest,” a bracket labeled “Macula” spans four blue hair cells with stereocilia projecting upward into an overlying gray otoconia layer that lies flat and level above them; arrows label the “Kinocilium” (the tallest cilium in each bundle), “Striola,” “Otoconia,” “Hair cells,” and “Vestibular nerve fibers” extending from the base of each cell. On the right, labeled “Head tilt forward,” the otoconia layer is shown shifted and tilted relative to the hair cells beneath it, causing the stereocilia bundles to bend in different directions depending on their position: the left two hair cells have their stereocilia bent toward the kinocilium, labeled “Toward kinocilium,” while the right two have their stereocilia bent away from the kinocilium, labeled “Away from kinocilium.” The figure illustrates that head tilt shifts the heavier otolithic membrane relative to the underlying hair cells, bending stereocilia bundles in different directions depending on each hair cell’s orientation.

Figure 31.5

An illustration of three hair cells side by side, each with a bundle of stereocilia at a different position, paired with a shared membrane potential trace above and a nerve firing trace below. The left hair cell, labeled “At rest,” has its stereocilia in a neutral, upright position, aligned under a flat segment of the membrane potential trace, with a moderate, evenly spaced firing rate below. The middle hair cell, labeled “Toward kinocilium,” has its stereocilia bent to one side, aligned under a raised segment of the trace labeled “Depolarization,” with a denser firing rate below. The right hair cell, labeled “Away from kinocilium,” has its stereocilia bent to the opposite side, aligned under a lowered segment of the trace labeled “Hyperpolarization,” with a sparser firing rate below. The figure illustrates that stereocilia bending toward the kinocilium depolarizes the hair cell and increases nerve firing above baseline, while bending away from the kinocilium hyperpolarizes the cell and decreases firing below baseline.

Figure 31.6

An illustration of the inner ear on the left, connected by a purple line to the brainstem on the right. The purple line originates from the semicircular canals and cochlea region, converges at a purple oval labeled “Scarpa’s ganglion,” and continues as a single line labeled “Vestibular nerve” to the brainstem, where it terminates at two purple-shaded regions on either side of the brainstem’s dorsal surface. The figure illustrates that the cell bodies of primary vestibular afferent neurons are located in Scarpa’s ganglion, outside the brainstem, with central processes traveling via the vestibular nerve to terminate in the brainstem.

Figure 31.7

Two illustrations side by side. On the left, a magnified cross-section of the brainstem, shown within a small overview inset, displays four colored regions labeled with arrows: the teal “Lateral nucleus” and tan “Superior nucleus” sit adjacent to each other at the top, with the purple “Medial nucleus” and blue “Inferior nucleus” stacked below, all grouped under the label “Vestibular nuclei.” On the right, small illustrations of a single semicircular canal ampulla and a section of otolith organ sensory epithelium are shown above a repeated, larger version of the four-colored vestibular nuclei complex. Colored lines trace projections from the semicircular canals (orange) and otolith organs (teal) to different combinations of the four nuclei, with orange lines reaching the superior, lateral, and medial nuclei, and teal lines reaching the lateral, medial, and inferior nuclei. The figure illustrates that the semicircular canals and otolith organs send overlapping but distinct projections to the four vestibular nuclei, with the superior nucleus receiving canal input, the lateral nucleus receiving mainly otolith input, and the medial and inferior nuclei receiving input from both.

Figure 31.8

A circuit diagram showing the horizontal vestibulo-ocular reflex following a leftward head turn. At the bottom left, a curved arrow around a small eye-like icon labeled “Left horizontal semicircular canal” indicates the direction of rotation, with a blue arrow marked “+” leading to a circle labeled “Vestibular nucleus” on the left side of the diagram. From this nucleus, a blue excitatory arrow crosses the midline (marked by a vertical dashed line) to a circle labeled “Abducens nucleus” on the right, while a red dashed inhibitory line leads to a circle labeled “Abducens nucleus” on the left. The right abducens nucleus sends a blue excitatory arrow directly up to the right eye, labeled “Excited,” and a second blue excitatory arrow crossing back over the midline via a path labeled “Left medial longitudinal fasciculus” to the left oculomotor nucleus, which then excites the left eye’s medial muscle. Simultaneously, the left abducens nucleus, receiving inhibitory input, sends gray “Decreased activity” arrows to the left eye, labeled “Inhibited,” and to the right oculomotor nucleus, reducing its output to the right eye’s medial muscle. At the top, curved arrows above the eyes labeled “Head turns” and “Eyes turn” show the head moving left while both eyes rotate right. A legend identifies blue arrows as excitatory projections, red dashed lines as inhibitory projections, and gray arrows as decreased activity. The figure illustrates that a leftward head turn activates the left vestibular nucleus, which excites contralateral eye muscles to turn both eyes right while inhibiting ipsilateral eye muscles, producing coordinated compensatory eye movement opposite the head turn.

Figure 31.9

An illustration tracing two descending pathways from the vestibular nuclei through the spinal cord, shown at three levels: cervical, thoracic, and lumbar. At the top, the brainstem shows the “Vestibular nuclei,” from which an orange dashed line and a yellow dashed line both descend. The orange line, labeled “Medial vestibulospinal tract,” descends bilaterally and terminates at the cervical spinal cord level, marked with red dots on both sides of the cord. The yellow line, labeled “Lateral vestibulospinal tract,” descends ipsilaterally and continues through the cervical, thoracic, and lumbar levels, marked with red dots at each level on the same side. The figure illustrates that the medial vestibulospinal tract projects bilaterally but only as far as the cervical spinal cord, controlling neck and shoulder muscles, while the lateral vestibulospinal tract projects ipsilaterally down the full length of the spinal cord to control postural muscle tone throughout the body.

Chapter 32 – Taste

Figure 32.1

Three-panel illustration progressing from whole tongue to cellular detail. The left panel shows the dorsal surface of an open mouth and tongue, dotted with small bumps called papillae, concentrated toward the back and sides. A boxed region on the tongue surface indicates the area magnified in the middle panel. The middle panel shows a cross-section through a papilla, a fold in the tongue surface containing two taste buds, each drawn as an oval structure with lines converging to a bundle of afferent taste axons exiting the base of the papilla. Boxed regions within the taste buds indicate the areas magnified in the right panel. The right panel shows a single taste bud composed of several elongated taste cells arranged side by side, along with two darker, kidney bean-shaped basal cells positioned toward the base. Each taste cell narrows at its outer end into microvilli, which project into a shared opening at the top of the bud labeled the taste pore. At the base of the taste bud, thin processes from each taste cell converge into a bundle labeled afferent taste axons.

Figure 32.2

Illustration of an open mouth showing the tongue divided into color-coded regions indicating relative taste sensitivity. A green region across the back of the tongue is labeled bitter. Blue regions along both sides of the tongue are labeled sour. An orange region covering the front tip of the tongue is labeled sweet, salt, and umami. The center of the tongue is left uncolored.

Figure 32.3

Illustration of a bitter taste receptor cell. At the top, a bitter molecule (black hexagon) binds one of several G-protein coupled receptors from the T2R family, each drawn as a colored wedge shape with an attached G-protein subunit. The activated receptor signals to phospholipase C (brown shape), which generates IP3. IP3 acts on a calcium store (oval structure containing purple dots), releasing calcium. The released calcium (Ca2+) opens a calcium-activated ion channel (blue rectangle) on the side of the cell, allowing sodium (Na+) entry and contributing to depolarization, shown with a downward arrow. At the base of the cell, depolarization opens ATP-permeable channels (orange striped rectangles), releasing ATP (yellow pentagons) into the synapse onto the afferent axon, drawn as a cup-shaped terminal below the cell.

Figure 32.4

Illustration of a sweet taste receptor cell, structured identically to the bitter taste transduction pathway. At the top, sweet molecules (tan pentagons) bind G-protein coupled receptor dimers composed of two associated proteins, T1R2 (purple) and T1R3 (teal), each paired with a G-protein subunit. Receptor activation signals through phospholipase C (brown shape) to generate IP3, which triggers calcium release from intracellular calcium stores (oval structure with purple dots). The released calcium opens a calcium-activated ion channel (blue rectangle), allowing sodium (Na+) influx and depolarization. At the base of the cell, depolarization opens ATP-permeable channels (orange striped rectangles), releasing ATP (yellow pentagons) onto the afferent axon.

Figure 32.5

Illustration of an umami taste receptor cell, structured identically to the sweet and bitter taste transduction pathways. At the top, umami molecules (tan pentagons) bind G-protein coupled receptor dimers composed of T1R1 (purple) and T1R3 (teal) subunits, each paired with a G-protein subunit. Receptor activation signals through phospholipase C (brown shape) to generate IP3, which triggers calcium release from intracellular calcium stores (oval structure with purple dots). The released calcium opens a calcium-activated ion channel (blue rectangle), allowing sodium (Na+) influx and depolarization. At the base of the cell, depolarization opens ATP-permeable channels (orange striped rectangles), releasing ATP (yellow pentagons) onto the afferent axon.

Figure 32.6

Illustration of a salt taste receptor cell. At the top, two epithelial sodium channels (gray-blue rectangles) allow sodium (Na+) from food to enter the cell, producing depolarization, shown with a downward arrow. Below this point, on both sides of the cell, depolarization opens voltage-gated sodium channels (blue rectangles), allowing further Na+ entry, and voltage-gated calcium channels (pink rectangles), allowing calcium (Ca2+) entry. The rising calcium triggers release of serotonin-containing vesicles (yellow starburst shapes within circles) into the synapse, acting on the afferent axon, drawn as a cup-shaped terminal below the cell.

Figure 32.7

Illustration of a sour taste receptor cell. At the top, two proton-selective ion channels (tan rectangles) allow hydrogen ions (H+) to enter the cell. Each proton then blocks an adjacent potassium channel (green rectangle), shown by an inhibitory line, preventing potassium (K+) efflux. The combined effect of proton entry and blocked potassium efflux produces depolarization, shown with a downward arrow. Below this point, on both sides of the cell, depolarization opens voltage-gated sodium channels (blue rectangles), allowing Na+ entry, and voltage-gated calcium channels (pink rectangles), allowing Ca2+ entry. The rising calcium triggers release of serotonin-containing vesicles (yellow starburst shapes within circles) into the synapse, acting on the afferent axon.

Figure 32.8

Two-panel illustration comparing labeled line and population coding in taste cells. Top panel, labeled line coding: a taste bud diagram with three cells (Cell 1, Cell 2, Cell 3) indicated by leader lines pointing to different cells within the bud. To the right, three membrane potential traces are plotted against exposure to sugar (sweet), quinine (bitter), and MSG (umami). Cell 1 shows a depolarizing peak only during quinine exposure, Cell 2 shows a peak only during sugar exposure, and Cell 3 shows a peak only during MSG exposure — each cell responds to a single tastant. Bottom panel, population coding: a similar taste bud diagram with two cells (Cell 4, Cell 5) indicated by leader lines. To the right, membrane potential traces for each cell are plotted against exposure to NaCl (salt) and HCl (acid). Both Cell 4 and Cell 5 show depolarizing peaks during both NaCl and HCl exposure, illustrating that each cell responds to more than one tastant.

Figure 32.9

Illustration of a head shown in profile, with the nasal and oral cavities shaded pink. Arrows label the nasal cavity at the top, the palate forming the roof of the mouth, the tongue within the oral cavity, the pharynx at the back of the throat, and the epiglottis, a flap-like structure below the pharynx near the entrance to the airway.

Figure 32.10

Two-panel illustration of the taste pathway. The left panel shows a head in profile with the brain visible and the tongue/throat region shaded purple. Cranial nerves VII, IX, and X are labeled, with lines tracing from the tongue and throat into the brainstem and converging on the nucleus of the solitary tract, labeled and shaded blue at the top of the brainstem. The right panel shows a coronal section through the brainstem and brain. At the bottom, the medulla is shown with the nucleus of the solitary tract labeled and shaded blue on both sides. A line projects upward from the nucleus of the solitary tract to a red dot in the thalamus, labeled ventral posterior medial nucleus of the thalamus, and continues to a second red dot in the overlying cerebral cortex, labeled gustatory cortex.

Figure 32.11

Flow chart illustrating the taste pathway in four sequential steps. Tongue and throat, at left, connects via cranial nerves VII, IX, and X to the nucleus of the solitary tract, labeled below as located in the brainstem. An arrow leads to the ventral posterior medial nucleus, labeled below as located in the thalamus. A final arrow leads to the primary gustatory cortex, labeled below as located at the border of the frontal and temporal lobes in the cerebral cortex.

Chapter 33 – Olfaction

Figure 33.1

Two-panel illustration of nose and olfactory bulb anatomy, shown in profile. The left panel shows a cross-section through the head with the nasal cavity labeled as a large open space, bounded below by the palate and above by the cribriform plate, a thin bone. Above the cribriform plate, the olfactory bulb sits just beneath the brain. A thin yellow line labeled olfactory nerve runs from the floor of the cribriform plate up into the olfactory bulb. A boxed region at the junction of the nasal cavity, cribriform plate, and olfactory bulb indicates the area magnified in the right panel. The right panel shows this region enlarged: a pink-shaded strip labeled olfactory epithelium lines the floor beneath the cribriform plate, with short hair-like projections extending upward from it. These projections converge into bundles that pass through openings in the cribriform plate and terminate in the yellow-shaded olfactory bulb above, labeled with arrows for olfactory nerve and olfactory bulb.

Figure 33.2

Cross-sectional illustration of the olfactory epithelium, organized in layers from top to bottom. At the top, a tan band represents the cribriform plate, with two arrows above it labeled “To olfactory bulb.” Below the cribriform plate, a layer of purple triangular cells is labeled basal cells. Extending through and below the basal cell layer are elongated green olfactory receptor neurons, each with a cell body containing a dark green nucleus and a single process that projects upward through the cribriform plate and another that projects downward. Gray oval supporting cells are interspersed among the green receptor neurons, filling the blue-shaded middle layer. At the bottom of the receptor neurons, the processes narrow and terminate in a pink-shaded mucus layer, where short hair-like olfactory cilia branch out. Small dots labeled odorants are scattered in the tissue below the mucus layer.

Figure 33.3

Illustration of a segment of neuron membrane, shown as a phospholipid bilayer, depicting the olfactory transduction cascade from left to right. At the left, an odorant (white pentagon) binds an odorant receptor (purple hexagon) associated with a G-protein made of alpha, beta, and gamma subunits. An arrow leads to activated adenylyl cyclase (brown/yellow shape), which converts ATP (black diamonds) into cAMP (black triangles). cAMP molecules act on a cAMP-gated cation channel (purple striped channel) in the membrane, allowing influx of sodium (Na+) and calcium (Ca2+), shown with an arrow into the cell. The entering calcium is shown acting on a separate calcium-gated chloride channel (yellow striped channel), causing efflux of chloride (Cl-) out of the cell. Both the cation influx and the chloride efflux are labeled with arrows converging on the word depolarization at the bottom right.

Figure 33.4

Grid illustration showing population coding of odorants. Four colored badge-shaped icons across the top row, representing four distinct olfactory receptors, are colored blue, orange, cyan, and brown. Four rows below represent four different odorants, each shown as a colored, jagged-edged icon (magenta, green, gray, and purple). Green checkmarks fill the grid cells where a given odorant activates a given receptor. The magenta odorant activates the blue, cyan, and brown receptors. The green odorant activates the orange and cyan receptors. The gray odorant activates the orange, cyan, and brown receptors. The purple odorant activates the blue and orange receptors. No two odorants produce an identical pattern of activated receptors.

Figure 33.5

Illustration showing convergence of olfactory receptor neuron axons onto glomeruli in the olfactory bulb. In the lower portion, a cross-section of the olfactory epithelium shows individual olfactory receptor neurons, each labeled A, B, or C to indicate which odorant receptor type it expresses, colored green (A), orange (B), or blue (C) accordingly. Neurons of the same letter/color are scattered among neurons of other types across the epithelium rather than being grouped together. Each neuron’s axon passes upward through the cribriform plate. In the olfactory bulb above, axons are shown converging by color: all green (A) axons meet at one glomerulus, all orange (B) axons meet at a second glomerulus, and all blue (C) axons meet at a third glomerulus, each glomerulus depicted as a dashed circle. Within each glomerulus, the converging axons synapse onto the dendrites of two mitral cells matching that glomerulus’s color. The mitral cell axons extend across the top of the olfactory bulb and bundle together at the right side, labeled olfactory tract.

Figure 33.6

Flow chart illustrating the central olfactory pathway. At the left, olfactory receptors connect via cranial nerve I to the olfactory bulb. From the olfactory bulb, four arrows branch out to four regions labeled olfactory tubercle, pyriform cortex, amygdala, and entorhinal cortex, grouped together and labeled olfactory cortex below. A bracket connects these four regions to four further targets: orbitofrontal cortex, thalamus, hippocampus, and hypothalamus.