Chapter 1 – The Neuron

Figure 1.1

This diagram shows the complete structure of a typical neuron with all major components labeled. Dendrites branch from the cell body in multiple directions, providing extensive surface area for receiving inputs from other neurons. The cell body contains the nucleus and is the metabolic center of the neuron. The axon hillock is labeled at the junction where the cell body transitions into the axon. The axon extends as a single long projection and is covered with myelin segments (shown in gray), with the final segment terminating in the presynaptic terminal. This comprehensive view illustrates how all neuronal components work together: dendrites receive information, the cell body integrates signals, the axon conducts electrical signals, and the presynaptic terminal releases neurotransmitters to communicate with other cells.

Figure 1.2

This diagram highlights the dendritic structure of a neuron, with dendrites shown in green branching from the cell body in a tree-like pattern. The dendrites are labeled and an inset box provides a magnified view of dendritic spines. The inset shows a section of dendrite with small protrusions extending from the dendritic shaft, labeled as “Dendritic Spines.” These spines appear as small knob-like or mushroom-shaped structures projecting from the dendrite surface. Dendritic spines are important sites of synaptic contact and help increase the surface area available for receiving synaptic inputs from other neurons. The main neuron structure shows the typical organization with the dendritic tree, cell body, and myelinated axon.

Figure 1.3

This diagram shows the internal structure of a neuronal cell body (soma) with detailed organelles. The cell body is highlighted in green with an inset box showing the major cellular organelles. The nucleus appears as a large blue oval structure containing the cell’s DNA. Surrounding the nucleus are various organelles including mitochondria (shown as red oval structures), rough endoplasmic reticulum (shown as layered membrane structures), smooth endoplasmic reticulum, and the Golgi apparatus (shown as stacked membrane structures). These organelles are responsible for essential cellular functions including protein synthesis, cellular respiration, and molecular packaging. The neuron also shows typical dendrites and a myelinated axon extending from the cell body.

Figure 1.4

This diagram illustrates the basic structure of a neuron with emphasis on the axon. The neuron shows dendrites extending from the cell body, and a long axon projecting from the axon hillock (labeled at the junction between cell body and axon). The axon extends to the right and is covered with myelin segments (shown in green). The axon hillock is specifically highlighted as the region where the cell body transitions into the axon, which is the site where action potentials are typically initiated. This figure demonstrates the fundamental organization of the axon as a single, long projection that begins at the axon hillock and can extend great distances to reach target cells.

Animation 1.1

The animation illustrates how an action potential moves along a neuron to communicate signals. The visual display shows a complete neuron with branching dendrites, a central cell body, and a long axon extending to the right. A bright green patch highlights the axon hillock at the junction between the cell body and the axon. As the animation plays, a vertical marker representing the action potential originates at the green axon hillock and travels smoothly and continuously from left to right down the entire length of the axonal shaft, demonstrating how the electrical impulse propagates sequentially toward the downstream terminal branches.

Figure 1.5

This diagram shows the structure of a myelinated axon with detailed labeling of myelin and Nodes of Ranvier. The neuron displays typical morphology with dendrites, cell body, and a long axon extending to the right. The axon is covered with segments of myelin sheath (shown in green) with gaps between them labeled as “Nodes of Ranvier.” The myelin appears as discrete segments wrapped around portions of the axon, while the nodes are the uncovered regions of axonal membrane between myelin segments. This structure is essential for saltatory conduction, where action potentials jump from node to node, significantly increasing conduction speed compared to unmyelinated axons.

Figure 1.6

This anatomical diagram illustrates the dramatic variation in axon length using a human body outline. The figure shows a side view of a human silhouette with the brain and spinal cord highlighted. Two types of neurons are contrasted: a short axon interneuron within the spinal cord (shown in an inset box of the spinal cord cross-section) and a long axon that extends from the spinal cord down to the foot. The long axon represents sensory or motor neurons that must travel the entire length of the leg to reach peripheral targets like the toe, demonstrating how axon length is directly related to the distance the neuron must span to connect its input and output regions.

Figure 1.7

This diagram demonstrates the relationship between axon diameter, myelination, and action potential propagation speed. The main neuron shows a typical structure with dendrites, cell body, and axon. An inset box in the upper right displays four cross-sectional views of axons arranged from fastest to slowest conduction speed. The fastest axon (leftmost) shows a large diameter with thick myelin wrapping (multiple concentric circles around a green center). Moving rightward, the axons become progressively smaller in diameter with thinner myelin sheaths, and the rightmost axon shows a very small diameter with no myelin. This illustrates how larger diameter axons with thicker myelin sheaths conduct action potentials faster than smaller, unmyelinated axons.

Figure 1.8

This simplified diagram shows the basic synaptic contact between two neurons. A presynaptic cell is shown on the left making synaptic contact with a postsynaptic cell on the right. The postsynaptic neuron displays typical neuronal features including dendrites, cell body, and a myelinated axon with gray myelin segments. The synapse is highlighted with green coloring at the contact point, emphasizing where the presynaptic terminal forms synaptic connections with the postsynaptic cell membrane. This illustrates the fundamental principle of neuronal communication through synaptic contacts.

Figure 1.9

This diagram illustrates the synaptic relationship between neurons, showing a presynaptic cell and postsynaptic cell. The presynaptic neuron is shown on the left with its axon terminal (highlighted in green) making synaptic contact with the dendrites and cell body of the postsynaptic neuron on the right. The presynaptic terminal contains small green structures representing synaptic vesicles or neurotransmitter release sites. The postsynaptic neuron shows typical neuronal morphology with branching dendrites and a myelinated axon (shown with gray myelin segments). This illustrates the directional nature of synaptic transmission, where the presynaptic cell releases neurotransmitters that act upon the postsynaptic cell.

Figure 1.10

This diagram shows different types of neuron structures illustrating the variability in neuronal morphology. The top row displays two examples: on the left, a multipolar neuron with extensive dendritic branching (shown in brown/orange) extending from a central cell body (black circle), and a single axon (blue) projecting downward with terminal branches. On the right is a bipolar neuron with dendrites extending upward and a single axon extending downward. The bottom row shows additional examples including a neuron with highly elaborate dendritic arborization resembling a tree-like structure on the left, and a simpler unipolar neuron structure on the right. All neurons demonstrate the common structural components of dendrites, cell body, and axon, but with dramatically different branching patterns and overall morphology depending on their location and function in the nervous system.

Chapter 2 – Ion Movement

Figure 2.1.

This diagram illustrates the molecular structure of the neuronal membrane’s phospholipid bilayer. The membrane is composed of two layers of phospholipid molecules arranged with their hydrophilic heads (shown as circular structures) facing outward toward both the extracellular and intracellular solutions, and their hydrophobic tails (shown as straight lines) pointing inward toward the center of the membrane. The top layer shows phospholipid molecules with their hydrophilic heads oriented toward the extracellular solution, while the bottom layer has hydrophilic heads facing the intracellular solution. The hydrophobic tails of both layers meet in the middle, creating a barrier that prevents water and water-soluble molecules like ions from freely crossing the membrane. This arrangement is fundamental to the membrane’s selective permeability properties.

Figure 2.2

This diagram shows the phospholipid bilayer with embedded ion channels that allow specific ions to cross the membrane. The membrane structure shows the same phospholipid arrangement as the previous figure, but now includes various types of ion channels spanning the width of the membrane. Blue dotted channels represent sodium channels, green striped channels represent potassium channels, and yellow solid channels represent chloride channels. These protein channels provide pathways for ions to move across the membrane when open, bypassing the impermeable phospholipid bilayer. The channels are shown as large protein structures that completely span from the extracellular solution to the intracellular solution, creating selective pores for ion movement.

Animation 2.1

The animation demonstrates how ions cross a cell membrane using specialized channel proteins. A horizontal membrane separates an upper extracellular solution from a lower intracellular solution. Spanning across this barrier are three types of closed, color-coded channel proteins. Floating nearby are matching colored spheres representing different ions: blue sodium, green potassium, and yellow chloride. As the sequence continues, the closed channels shift open simultaneously. Individual blue sodium spheres immediately begin migrating down through the blue channels, green potassium spheres move through the green channels, and a yellow chloride sphere passes through the yellow channel, demonstrating that ions move across the membrane passively by matching exclusively with their specific protein pathways.

Animation 2.2

The animation presents a side-by-side comparison of the two distinct physical forces that dictate electrochemical movement. The left half of the screen, titled Concentration Gradient, features a membrane embedded with ion channels. Initially, a large cluster of positive spheres is tightly packed in the space above the membrane, with only a single sphere resting below. When the animation plays, the spheres flow downward through the channels, moving from a crowded region of high concentration to an area of low concentration until they are evenly distributed. The right half of the screen, titled Electrical Gradient, features two parallel plates labeled with a positive sign at the top and a negative sign at the bottom. A loose row of positive spheres sits between them. Once activated, all the positive spheres are repelled downward away from the positive upper plate and cluster tightly against the negative lower plate, illustrating how charge attraction and repulsion drive physical movement.

Animation 2.3

The animation illustrates the dynamic nature of electrochemical equilibrium across a membrane barrier. A horizontal cell membrane spans the center of the frame, embedded with open ion channels. Positive spheres are distributed on both sides, with a higher initial concentration present in the upper compartment compared to the lower compartment. As the sequence progresses, positive spheres cross the membrane in both directions simultaneously. For every positive sphere that drifts downward through a channel following its concentration gradient, another positive sphere moves upward through a channel driven by electrical forces, demonstrating that equilibrium is a steady dynamic balance where the net exchange rate of ions across the barrier equals zero.

Chapter 3 – Membrane Potential

Figure 3.1

This diagram illustrates the experimental setup for measuring membrane potential in a neuron. A neuron is shown with typical morphology including dendrites, cell body, and myelinated axon. Two electrodes are positioned to measure the voltage difference across the membrane: a reference electrode placed in the extracellular solution (labeled and shown penetrating from above) and a recording electrode inserted directly into the cell body (also labeled). The wavy line above represents the extracellular solution. The membrane potential is calculated as the voltage difference between these two electrode locations, with the recording electrode measuring the intracellular voltage and the reference electrode providing the extracellular baseline.

Figure 3.2

This figure shows two graphs illustrating changes in membrane potential over time. The left graph demonstrates depolarization (decrease in membrane potential), where the membrane potential starts at -65 mV and increases toward 0 mV, shown as an upward curve. The right graph shows hyperpolarization (increase in membrane potential), where the membrane potential starts at -65 mV and becomes more negative, shown as a downward curve. Both graphs have time in milliseconds on the x-axis and membrane potential in millivolts on the y-axis. The terminology clarifies that depolarization represents a decrease in membrane potential (movement toward 0) while hyperpolarization represents an increase in membrane potential (movement away from 0).

Figure 3.3

This diagram shows the charge distribution across a neuronal membrane at rest. The neuron displays typical morphology with dendrites and cell body. Two electrodes (reference and recording) are positioned as in the membrane potential measurement setup. Around the cell body, small circles with positive (+) and negative (-) symbols illustrate the charge distribution. The positive charges (shown in orange/red circles) are concentrated outside the cell, while negative charges (shown in blue circles) are concentrated inside the cell. This uneven charge distribution creates the negative resting membrane potential, with the inside of the neuron being more negatively charged relative to the outside.

Figure 3.4

This diagram shows the distribution of specific ions across the neuronal membrane at rest. The phospholipid bilayer is shown with embedded ion channels (blue dotted for sodium, green striped for potassium, yellow solid for chloride). In the extracellular solution, there are high concentrations of sodium ions (Na+, blue circles), calcium ions (Ca2+, purple circles), and chloride ions (Cl-, yellow circles). In the intracellular solution, there are high concentrations of potassium ions (K+, green circles) and anions (A-, gray circles representing negatively charged proteins and organic molecules). This uneven distribution of ions across the membrane is fundamental to establishing the electrochemical gradients that drive ion movement and maintain the resting membrane potential.

Figure 3.5

This diagram illustrates the electrochemical gradients acting on sodium and potassium ions at rest. The membrane setup is similar to previous figures, showing the phospholipid bilayer with ion channels and the characteristic ion distribution. On the right side, arrows indicate the direction of electrochemical gradients: a blue downward arrow labeled “Sodium” shows that sodium’s electrochemical gradient drives it into the cell, while a green upward arrow labeled “Potassium” shows that potassium’s electrochemical gradient drives it out of the cell. These opposing gradients reflect the combination of concentration and electrical forces acting on each ion type when the membrane is permeable to them.

Animation 3.1

The animation illustrates how concentration and electrical gradients interact dynamically to determine the equilibrium potential of sodium. A horizontal membrane separates an upper extracellular space with a high concentration of blue sodium spheres from a lower intracellular space with a low concentration of sodium. Open blue sodium channels span the membrane. Two dynamic vertical arrows track the driving forces: a large, static downward arrow represents the concentration gradient, while an adjacent arrow tracks the electrical gradient. Initially, the electrical gradient arrow points downward, showing both forces pulling sodium into the cell. As blue sodium spheres cross inward through the open channels, the electrical gradient arrow shortens, flips, and begins pointing upward to represent a growing outward electrical repulsion. The movement stabilizes when the upward electrical gradient arrow grows to match the exact size of the downward concentration gradient arrow, demonstrating that the opposing gradients are perfectly balanced at the equilibrium potential of sodium.

Figure 3.6

This figure consists of two panels (A and B) showing ion movement relative to equilibrium potentials. Panel A shows a membrane at -70 mV with sodium channels open. Sodium ions flow into the cell (shown by downward arrows) because the cell’s membrane potential is more negative than sodium’s equilibrium potential of +60 mV. A graph shows the membrane potential moving upward (toward +60 mV) as indicated by a red line. Panel B shows the same membrane at -70 mV but with chloride channels open. Chloride ions flow out of the cell (shown by upward arrows) because the cell’s membrane potential is more negative than chloride’s equilibrium potential of -65 mV. The graph shows membrane potential moving upward toward -65 mV. Both panels demonstrate how ions move in directions that drive the membrane potential toward their respective equilibrium potentials.

Chapter 4 – The Membrane at Rest

Figure 4.1

This diagram illustrates a neuronal membrane at rest showing the uneven distribution of ions across the phospholipid bilayer. The extracellular solution (top) contains higher concentrations of sodium ions (Na+, shown in blue circles), calcium ions (Ca2+, shown in purple circles), and chloride ions (Cl-, shown in yellow circles). The intracellular solution (bottom) contains higher concentrations of potassium ions (K+, shown in green circles) and anions (A-, shown in gray circles). Embedded in the membrane are various leak ion channels: sodium leak channels (blue dotted channels), potassium leak channels (green striped channels), and chloride leak channels (yellow solid channels). The diagram shows that significantly more potassium leak channels are open compared to sodium leak channels, which explains why the membrane at rest is most permeable to potassium. This differential permeability, combined with the concentration gradients, contributes to the negative resting membrane potential of approximately -65 mV.

Figure 4.2

This diagram shows the same neuronal membrane setup as the previous image but with fewer leak channels illustrated to emphasize the key concept of differential permeability. The extracellular solution (top) shows sodium ions (Na+, blue circles), calcium ions (Ca2+, purple circles), and chloride ions (Cl-, yellow circles), while the intracellular solution (bottom) contains potassium ions (K+, green circles) and anions (A-, gray circles). The membrane contains the same types of leak channels: sodium leak channels (blue dotted), potassium leak channels (green striped), and chloride leak channels (yellow solid). This simplified view clearly demonstrates that at rest, there are more open potassium leak channels than sodium leak channels, which is the primary reason why the membrane has higher permeability to potassium. This differential permeability drives potassium efflux, making the inside of the cell more negative relative to the outside.

Animation 4.1

The animation illustrates the directional movement of potassium ions when a neuron is at rest. A horizontal membrane separates an upper extracellular space from a lower intracellular space, embedded with open green potassium leak channels. A large green arrow points straight up to represent that the net electrochemical gradient drives potassium out of the cell. As the sequence plays, green potassium spheres move continuously upward from the intracellular space through the open green channels to enter the extracellular space, demonstrating how positive charge leaves the cell interior during the resting state.

Animation 4.2

The animation maps out the simultaneous baseline movement of multiple ion varieties across a resting neuronal membrane. The horizontal layout displays an extracellular compartment at the top and an intracellular compartment at the bottom, separated by a row of leak channels. As the video plays, specific channels are highlighted sequentially to draw focus. A yellow channel opens to allow a yellow chloride sphere to slowly slip across the lipid barrier. Immediately afterward, another channel opens to show a blue sodium sphere drifting from the high-concentration extracellular zone down into the cell interior, demonstrating how minor sodium and chloride leaks occur alongside baseline potassium currents.

Animation 4.3

The animation details the chemical mechanics of the sodium-potassium pump during active transport. A horizontal cell membrane features an active transport pump protein complex at the center, initially open exclusively toward the lower intracellular space. Three blue sodium spheres enter the open inner cavity of the pump and lock into place. An ATP molecule then approaches the pump, binding to deliver a single phosphate group. This investment of energy causes the pump to change shape, closing its internal space to the bottom and opening wide toward the upper extracellular space, which prompts the three blue sodium spheres to eject upward out of the cell. While still open to the top, two green potassium spheres enter the upper cavity and bind. The phosphate group is then released, causing the pump protein to reset back to its original configuration—closing off the top and opening toward the bottom to release the two green potassium spheres into the cell interior against their concentration gradients.

Chapter 5 – Postsynaptic Potentials

Animation 5.1

The animation illustrates how ion flow changes the membrane potential of a postsynaptic neuron. The main frame displays a complete neuron receiving an external stimulus at its receptive dendrites on the left. An inset box focuses on a tiny section of a dendritic branch, revealing a microscopic view of a cell membrane embedded with ion channels. Each time the stimulus strikes the dendrite, channels open, and individual blue sodium spheres pass directly downward from the extracellular space into the cell interior, demonstrating how synaptic activation drives localized ionic currents inside the postsynaptic cell.

Animation 5.2

The animation tracks the generation of an excitatory postsynaptic potential. A horizontal cell membrane separates a high concentration of blue sodium spheres in the upper extracellular space from the lower intracellular space. When the sequence begins, specialized ligand-gated channels open, and multiple blue sodium spheres rush downward into the intracellular compartment. An inset graph monitors the membrane potential, tracking a temporary upward wave that depolarizes the cell closer to threshold before curving back down to rest as the channels close.

Animation 5.3

The animation maps out the production of an inhibitory postsynaptic potential via chloride influx. A cell membrane divides an upper extracellular environment containing a high concentration of yellow chloride spheres from a lower intracellular environment. When the animation plays, specialized ligand-gated channels open, and several yellow chloride spheres migrate downward into the cell interior. An inset graph monitors the membrane potential, tracking a temporary downward wave that hyperpolarizes the cell farther away from threshold.

Animation 5.4

The animation illustrates a steady-state inhibitory condition where the membrane potential already sits at the ion’s equilibrium value. Yellow chloride spheres are distributed on both sides of a horizontal cell membrane. When the chloride channels open, individual yellow chloride spheres slowly swap places across the barrier, crossing in both directions at an equal rate. An inset graph monitors the membrane voltage, which remains completely flat and horizontal, demonstrating that opening ion channels at equilibrium yields no net voltage change.

Animation 5.5

The animation shows an inhibitory depolarization event that occurs when the starting resting potential is more negative than the chloride equilibrium potential. A horizontal membrane contains closed chloride channels, with yellow chloride spheres situated on both sides. When the channels shift open, yellow chloride spheres cross upward from the intracellular space out into the extracellular space. As these negative ions exit the cell, an inset graph records a minor upward depolarization wave, demonstrating that chloride movement can cause a temporary upward voltage shift if the cell’s starting voltage is below the ion’s equilibrium point.

Animation 5.6

The animation illustrates the temporal summation of multiple excitatory inputs. Blue sodium spheres hover in the extracellular space above a membrane containing sodium channels. An inset graph plots voltage over time against a designated threshold marker. A stimulus indicator flashes, prompting the channels to open briefly and release blue sodium spheres downward, creating a small upward voltage wave. Before this initial wave can return to rest, the stimulus flashes two more times in rapid succession, causing additional waves of sodium ions to enter. Each new influx adds directly onto the preceding voltage wave, driving the graph line upward in a staircase pattern until it successfully crosses the threshold line.

Figure 5.1

This figure illustrates different types of excitatory postsynaptic potential (EPSP) summation. On the left is a diagram of a postsynaptic neuron with four myelinated axon inputs (Input 1-4) making synaptic contact with the dendrites and cell body. Three panels on the right show different stimulation patterns and their effects on membrane potential. The first panel “EPSP Alone” shows Input 1 providing a single excitatory stimulus (blue vertical line), resulting in a small depolarization from -65 mV. The second panel “Temporal Summation” shows Input 1 providing multiple rapid stimuli in succession (three blue vertical lines), resulting in progressively larger depolarizations that build upon each other. The third panel “Spatial Summation” shows all four inputs (1-4) providing simultaneous excitatory stimuli, resulting in a large depolarization that is the sum of all individual EPSPs.

Figure 5.2

This figure demonstrates the interaction between excitatory and inhibitory postsynaptic potentials. On the left is a diagram of a postsynaptic neuron receiving input from three myelinated axons (Input 1-3). Two panels on the right compare different stimulation scenarios. The left panel “EPSP Alone” shows Input 1 providing an excitatory stimulus (blue vertical line), resulting in a depolarization from -65 mV toward -50 mV. The right panel “EPSP & IPSP Summation” shows simultaneous stimulation with Input 1 providing an excitatory stimulus (blue line) and Input 3 providing an inhibitory stimulus (red vertical line). The resulting membrane potential change is much smaller (red trace), demonstrating how inhibitory inputs can counteract or reduce the effects of excitatory inputs when they occur simultaneously.

Animation 5.7

The animation illustrates the cancellation of opposing postsynaptic potentials. Blue sodium spheres and yellow chloride spheres populate the upper extracellular space above a membrane. When a stimulus arrives, sodium channels and chloride channels open at the exact same moment, causing blue sodium spheres and yellow chloride spheres to pour down into the intracellular space together. Because the positive charges and negative charges enter concurrently, they counteract one another, and an inset voltage graph shows a flat line with no net change from the resting potential.

Chapter 6 – Action Potentials

Figure 6.1

This graph shows the characteristic shape of an action potential plotted as membrane potential (in mV) versus time. The trace begins at the resting membrane potential (indicated by a dotted horizontal line at approximately -65 mV). The membrane potential shows small initial fluctuations representing EPSPs that summate to reach threshold. Once threshold is reached, there is a rapid depolarization (rising phase) that shoots up to approximately +30 mV, followed by an equally rapid repolarization (falling phase) that returns toward rest. The trace then dips below the resting potential (undershoot) before finally returning to the resting membrane potential. The entire event occurs over just a few milliseconds, demonstrating the brief but dramatic nature of the action potential.

Animation 6.1

The animation illustrates how an action potential moves along a neuron to communicate signals. The visual display shows a complete neuron with branching dendrites, a central cell body, and a long axon extending to the right. A bright green patch highlights the axon hillock at the junction between the cell body and the axon. As the animation plays, a vertical marker representing the action potential originates at the green axon hillock and travels smoothly and continuously from left to right down the entire length of the axonal shaft, demonstrating how the electrical impulse propagates sequentially toward the downstream terminal branches.

Figure 6.2

This diagram illustrates the locations of voltage-gated ion channels along a neuron. The neuron displays typical morphology with dendrites, cell body, axon hillock (labeled), myelinated axon segments, and presynaptic terminal (labeled). Red arrows point to specific locations where voltage-gated ion channels are concentrated: at the axon hillock, at several points along the myelinated axon (likely at Nodes of Ranvier), and at the presynaptic terminal. These locations are critical for action potential initiation, propagation, and neurotransmitter release. The diagram emphasizes that voltage-gated channels are not uniformly distributed throughout the neuron but are strategically located at key sites necessary for electrical signaling.

Animation 6.2

The animation demonstrates how voltage changes gate the opening of specialized channels across a horizontal membrane. Initially, blue sodium channels and green potassium channels remain closed. An inset graph monitors the membrane potential as it rises continuously in a staircase pattern toward a designated threshold marker. The moment the voltage line crosses this threshold, lightning bolt symbols flash against the channels, prompting the closed blue sodium channels to shift open, demonstrating that reaching threshold is the direct trigger that activates voltage-gated pathways.

Figure 6.3

This detailed graph shows the labeled phases of an action potential. The trace starts with small depolarizations (EPSPs) that reach threshold (marked by a dotted horizontal line). The rising phase is labeled as the rapid depolarization from threshold to the peak. The overshoot is labeled at the peak where the membrane potential becomes positive (approximately +30 mV). The falling phase is labeled as the rapid repolarization back toward rest. The undershoot (or afterhyperpolarization) is labeled where the membrane potential becomes more negative than the resting potential before finally returning to rest. This comprehensive labeling helps identify each distinct phase of the action potential and its relationship to the resting membrane potential and threshold.

Animation 6.3

The animation details the ionic movement that drives the rapid upward swing of an action potential. A cell membrane contains voltage-gated sodium channels, with blue sodium spheres clustered in the extracellular environment above. An inset graph tracks the membrane voltage over time. When a lightning bolt strikes the channel to signify it reaching threshold, the sodium channel immediately shifts open. Blue sodium spheres rapidly rush downward through the open pore into the intracellular space, and the inset graph records a sharp, near-vertical climbing line, demonstrating how a massive sodium influx produces the rising phase of the action potential.

Animation 6.4

The animation illustrates the structural changes and ionic currents that reverse the membrane potential during an action potential. A membrane features a blue sodium channel and a green potassium channel next to green potassium spheres in the intracellular space. An inset graph displays a voltage line at its peak. As a timer tracks a 1 millisecond delay, a purple ball swings into place from the intracellular side to plug the bottom of the sodium channel pore, inactivating it and stopping sodium entry. Simultaneously, the green potassium channel shifts open, allowing green potassium spheres to rush rapidly upward out of the cell. As these positive charges leave, the inset graph plots a sharp downward line, demonstrating how sodium inactivation combined with potassium efflux creates the falling phase.

Animation 6.5

The animation shows the ion movement responsible for hyperpolarizing a neuron below its normal resting level. A cell membrane contains a green potassium channel that remains wide open, while an adjacent sodium channel is closed. Green potassium spheres continue to flow upward from the intracellular space out into the extracellular environment. An inset graph tracks the membrane potential as it dips below the normal resting membrane potential line, curving downward to approach the potassium equilibrium potential, illustrating how prolonged potassium efflux creates the action potential undershoot.

Animation 6.6

The animation tracks how a neuron restores its baseline ion distribution and resting electrical state after an action potential. A membrane features a sodium-potassium pump protein alongside a closed channel. An inset graph shows the membrane potential starting at a hyperpolarized low point. Driven by ATP energy, the sodium-potassium pump actively cycles, pulling blue sodium spheres out of the cell interior and pumping green potassium spheres back into the cell against their concentration gradients. As the pump works, the green potassium channels close completely, and the inset graph line rises smoothly back up to settle exactly at the flat resting membrane potential line.

Animation 6.7

The animation illustrates how an action potential propagates along a myelinated axon via saltatory conduction. A long horizontal axon is segmentally wrapped in gray myelin sheaths, leaving uninsulated gaps exposed. Insets reveal microscopic views of the membrane at these gaps. As the action potential travels, it moves step-by-step from one exposed gap to the next. At the active gap, voltage-gated sodium channels open and a large cluster of blue sodium spheres rushes down into the cell to create a localized rising phase. This internal positive charge spreads forward through the axon, depolarizing the adjacent downstream gap to threshold, which triggers its channels to open and repeat the influx, demonstrating how the electrical signal jumps sequentially down a myelinated fiber.

Figure 6.4

A comparative diagram showing three different types of axons with varying diameters and myelination. The largest axon (top) has a wide diameter and thick myelin sheaths with regular Nodes of Ranvier, labeled as having fast action potential conduction speed. The middle axon has moderate diameter with thinner myelin sheaths, labeled as having intermediate conduction speed. The smallest axon (bottom) has a narrow diameter with no myelin sheath present (unmyelinated), labeled as having slow conduction speed. Each axon is drawn to scale to emphasize the size differences. The myelin sheaths are shown as white or light-colored wrappings around the axons, with their thickness proportional to axon diameter. Voltage-gated channels may be indicated at the nodes of the myelinated axons and distributed along the entire length of the unmyelinated axon. The diagram illustrates the positive correlation between axon diameter, degree of myelination, and conduction velocity.

Animation 6.8

A top-and-bottom comparison illustrates how myelination changes the speed of action potential propagation along an axon. The top pathway features an unmyelinated axon, where an action potential triggers voltage-gated channels to open sequentially in a continuous, slow wave from left to right along every continuous patch of membrane. The bottom pathway features a myelinated axon insulated by gray myelin segments. In this fiber, the action potential skips the insulated zones entirely and leaps rapidly from one exposed node to the next. The comparison demonstrates that myelination significantly increases propagation speed, as the signal on the bottom reaches the end of the axon far ahead of the unmyelinated wave on the top.

Figure 6.5

This diagram shows the molecular mechanism underlying unidirectional action potential propagation along a myelinated axon. Three panels represent different states during propagation: “At Rest,” “Falling Phase,” and “Rising Phase.” Each panel shows a segment of axon with myelin and insets depicting the status of voltage-gated sodium (blue dotted) and potassium (green striped) channels. At rest, channels are closed. During the falling phase, sodium channels are inactivated (shown in purple) while potassium channels are open. During the rising phase, sodium channels are open and active. The diagram shows how the refractory state of sodium channels in the “falling phase” region prevents the action potential from propagating backward, while the “rising phase” region continues forward propagation.

Figure 6.6

This figure demonstrates how neurons encode stimulus strength through action potential frequency. Panel A shows a weak stimulus (depicted as a black bar) that generates only two action potentials during the stimulus period. Panel B shows a strong stimulus (same duration but presumably higher intensity) that generates six action potentials in rapid succession during the stimulus period. Both panels show identical action potential shapes and amplitudes, emphasizing that individual action potentials do not change in magnitude. Instead, stronger stimuli are encoded by higher frequencies of action potential firing. The resting membrane potential is marked by dotted lines in both traces.

Figure 6.7

This figure illustrates the absolute and relative refractory periods during action potentials. Panel A shows the absolute refractory period (highlighted in blue) spanning the rising and falling phases when voltage-gated sodium channels are either open or inactivated. During this time, no additional action potentials can be generated regardless of stimulus strength. The membrane diagram shows sodium channels in their inactivated state (purple). Panel B shows the relative refractory period (highlighted in green) during the undershoot phase when sodium channels have recovered but potassium channels remain open. During this time, action potentials can be generated but require stronger-than-normal stimuli to overcome the hyperpolarization caused by open potassium channels.

Figure 6.8

This figure compares action potentials under normal and altered extracellular conditions using two panels. Panel A labeled “Control” shows three normal action potentials with typical characteristics: rapid rising phases reaching approximately +30 mV, sharp peaks, fast falling phases, and brief undershoots before returning to the resting membrane potential (marked by a dotted line). Panel B labeled “Low Extracellular Sodium” shows the effect of reduced external sodium concentration on action potential shape. These action potentials have noticeably slower rising phases, reduced peak amplitudes (reaching only about -10 mV instead of +30 mV), and overall smaller magnitude changes. The timing between action potentials appears similar, but the individual action potentials are dramatically altered in both speed and amplitude due to the weakened sodium electrochemical gradient.

Chapter 7 – Voltage Clamp

Figure 7.1

This diagram shows the initial preparation for a voltage clamp experiment in two panels. Panel A displays a complete neuron with dendrites, cell body, and myelinated axon. A rectangular box highlights a segment of the axon that will be removed for experimentation. Panel B shows the isolated axon segment placed in an experimental chamber containing physiological extracellular solution (represented by the wavy line above the axon). The isolated axon maintains its cylindrical structure and contains all the voltage-gated ion channels necessary for studying action potential mechanisms. This preparation allows researchers to study ion channel function in a controlled environment where the axon segment retains the same ion concentrations and electrochemical gradients as in the intact neuron.

Figure 7.2

This diagram illustrates the electrode configuration for measuring membrane potential in the isolated axon preparation. The cylindrical axon segment is surrounded by extracellular solution (wavy line). Two electrodes are positioned for measurement: a reference electrode placed in the extracellular solution and a recording electrode inserted into the axon interior. These electrodes are connected to measuring equipment that calculates the voltage difference between the intracellular and extracellular compartments. This setup represents the initial step in voltage clamp methodology, establishing baseline membrane potential measurements before experimental manipulation begins.

Figure 7.3

This diagram outlines the four sequential steps of the voltage clamp control system using three electrodes. The isolated axon is equipped with a reference electrode in the extracellular solution, a recording electrode inside the axon, and a current-passing electrode also inserted into the axon. The process flows as follows: first, measure the membrane potential using the recording and reference electrodes; second, set the desired membrane potential value; third, compare the measured potential with the desired potential; and fourth, make the measured membrane potential equal to the desired potential by injecting current through the current-passing electrode. This system enables researchers to maintain constant membrane potential while studying ion channel behavior.

Figure 7.4

This diagram emphasizes the continuous, cyclical nature of voltage clamp control by showing the same three-electrode setup surrounded by directional arrows forming a closed loop. The process flows clockwise through the four steps: measure membrane potential → set desired membrane potential → compare measured with desired → inject corrective current → return to measure membrane potential. The arrows indicate that this cycle repeats continuously throughout the experiment, ensuring that any changes in membrane potential due to ion channel activity are immediately detected and corrected by appropriate current injection, thereby maintaining a constant voltage across the membrane despite ongoing ionic currents.

Figure 7.5

This diagram shows the initial state of a voltage clamp experiment, displaying the isolated axon with reference and recording electrodes in position. Red text indicates “Cell at rest at -65 mV,” showing the axon’s resting membrane potential measurement before any experimental manipulation. This represents the baseline condition where the axon exhibits its natural resting potential due to the normal distribution of ions and leak channel activity. The setup shows only the measurement electrodes since no current injection is required at this stage—the system is simply recording the inherent electrical state of the axon membrane.

Figure 7.6

This diagram shows the experimental setup after researchers have established the target membrane potential for the voltage clamp experiment. The axon shows “Cell at rest at -65 mV” in red text, indicating the current measured potential, while “Desired potential: 0 mV” appears above in red, showing the experimental target voltage. The reference and recording electrodes continue measuring the membrane potential, while the system prepares to begin current injection that will bring the membrane potential from its resting value of -65 mV to the desired experimental value of 0 mV. This step represents the transition from passive measurement to active voltage control.

Figure 7.7

This diagram illustrates the comparison phase where the voltage clamp system evaluates the difference between actual and desired membrane potentials. The setup shows “Cell at rest at -65 mV” and “Desired potential: 0 mV” with red text stating “Actual potential too low; must depolarize.” This represents the system’s assessment that the current membrane potential is more negative than the target value, requiring depolarization to reach the desired potential. The comparison determines what type of corrective action (current injection) will be needed to achieve the experimental membrane potential, setting the stage for active voltage control.

Figure 7.8

This final diagram demonstrates the voltage clamp system actively maintaining the desired membrane potential through current injection. The current-passing electrode is shown injecting positive current into the axon (indicated by “Inject positive current into the cell”). A voltage trace graph on the right shows the membrane potential recording, with “Current turned on” marking when the potential rapidly jumps from -65 mV to 0 mV and remains there. This illustrates successful voltage clamping, where the equipment maintains membrane potential at the desired value through continuous current injection that compensates for any ion flow through voltage-gated channels activated by the depolarization.

Animation 7.1

The animation demonstrates how the voltage clamp technique reveals ionic currents across an axon membrane. An initial baseline shows a flat resting membrane potential on an upper inset graph and a flat baseline on a lower ion flow graph. When the clamp system passes current to forcefully step and hold the membrane potential above threshold, an adjacent blue sodium channel is triggered to shift open. Blue sodium spheres immediately rush downward from the extracellular space into the axon interior. The lower graph tracks this initial movement by plotting a prominent downward curve that drops below the baseline, demonstrating how the experimental voltage step captures a transient inward sodium current.

Animation 7.2

The animation tracks the subsequent phase of current changes measured under a sustained voltage clamp. The upper inset graph shows the membrane potential continuously held steady at a depolarized level above threshold. As time passes, a purple ball inactivates the blue sodium channel, stopping further entry of sodium. Simultaneously, an adjacent green potassium channel opens, allowing green potassium spheres to rush rapidly upward out of the axon interior. The lower ion flow graph plots this movement, showing the line rise back up through baseline and ascend into a high plateau, demonstrating how the sustained voltage hold captures a delayed, continuous outward potassium current.