21 Vision: The Retina

Casey Henley

Vision begins with the process of phototransduction in the retina, where light energy is converted into neural signals. Specialized cells and circuits within the retina transform this visual information before sending it to the brain for further processing.

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Anatomy of the Retina

The front of the eye consists of the cornea, pupil, iris, and lens. The cornea is the transparent, external part of the eye. It covers the pupil and the iris and is the first location of light refraction. The pupil is the opening in the iris that allows light to enter the eye. The iris is the colored portion of the eye that surrounds the pupil and along with local muscles can control the size of the pupil to allow for an appropriate amount of light to enter the eye. The lens is located behind the pupil and iris. The lens refracts light to focus images on the retina. Proper focusing requires the lens to stretch or relax, a process called accommodation.

The retina is the light-sensitive region in the back of the eye where the photoreceptors, the specialized cells that respond to light, are located. The retina covers the entire back portion of the eye, so it’s shaped like a bowl. In the middle of the bowl is the fovea, the region of highest visual acuity, meaning the area that can form the sharpest images. The optic nerve projects to the brain from the back of the eye, carrying information from the retinal cells. Where the optic nerve leaves, there are no photoreceptors since the axons from the neurons are coming together. This region is called the optic disc and is the location of the blind spot in our visual field.

A cross-sectional diagram of the eye showing how light passes through the cornea, pupil, and lens before reaching the retina, with the fovea and optic disc marked at the back of the eye. Link to detailed alternative text in caption.
Figure 21. 1. Cross section of the eye. The cornea is the transparent outer surface and the first point of light refraction. The pupil (gray region) is the opening in the iris (blue region) that controls how much light enters the eye. The lens sits behind the pupil and iris and further refracts light to focus it on the retina. The retina (red line) is located along the back of the eye. The fovea (dark red section) is a small portion of the retina where visual acuity is highest, and the optic disc is located where the optic nerve (tan region) leaves the eye, creating a blind spot with no photoreceptors. ‘Eye Anatomy’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Retinal Cells

There are five primary types of cells in the retina

In addition to the photoreceptors, there are four other cell types in the retina. The photoreceptors synapse on bipolar cells, and the bipolar cells synapse on the ganglion cells. Horizontal and amacrine cells allow for communication laterally between the neurons.

A diagram of the five retinal cell types — photoreceptors, horizontal cells, bipolar cells, amacrine cells, and ganglion cells — showing how horizontal and amacrine cells provide lateral communication at each synaptic layer. Link to detailed alternative text in caption.
Figure 21.2. There are five cell types in the retina. The photoreceptors synapse on bipolar cells, and the bipolar cells synapse on ganglion cells. The horizontal cells allow for communication between photoreceptors by interacting with the photoreceptor-bipolar cell synapse, and the amacrine cells allow for communication between bipolar cells by interacting at the bipolar cell-ganglion cell synapse. ‘Retinal Neurons’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Direction of Information

When light enters the eye and strikes the retina, it must pass through all the neuronal cell layers before reaching and activating the photoreceptors. The photoreceptors then initiate the synaptic communication back toward the ganglion cells.

A diagram showing that light travels through the ganglion and bipolar cell layers to reach the photoreceptors at the back of the retina, while the resulting neuronal signal travels in the opposite direction, from photoreceptors toward the ganglion cells. Link to detailed alternative text in caption.
Figure 21.3. When light enters the eye, it must pass through the ganglion and bipolar cell layers before reaching the photoreceptors. The neuronal communication travels in the opposite direction from the photoreceptors toward the ganglion cells. ‘Light in the Retina’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Receptors

The photoreceptors are the specialized receptors that respond to light. There are two types of photoreceptors: rods and cones. Rods are more sensitive to light, making them primarily responsible for vision in low-lighting conditions like at night. Cones are less sensitive to light and are most active in daylight conditions. The cones are also responsible for color vision.

An illustration comparing the physical structure of a rod and a cone photoreceptor, showing the rod's long cylindrical outer segment and the cone's shorter, tapered outer segment. Link to detailed alternative text in caption.
Figure 21.4. Rods and cones differ in the shape of their outer segments. Rods have a long, cylindrical outer segment, while cones have a shorter, tapered outer segment. This structural difference underlies their separate roles in visual processing, with rods specialized for low-light vision and cones specialized for daylight and color vision. ‘Rod and Cone’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Receptor Density

In addition to having different visual functions, the rods and cones are also distributed across the retina in different densities. The cones are primarily found in the fovea, the region of the retina with the highest visual acuity. The remainder of the retina is predominantly rods. The region of the optic disc has no photoreceptors because the axons of the ganglion cells are leaving the retina and forming the optic nerve.

A diagram of photoreceptor distribution across the retina, showing cones concentrated at the fovea, rods covering the rest of the retinal surface, and no photoreceptors at the optic disc. Link to detailed alternative text in caption.
Figure 21.5. Rods and cones are distributed across the retina in different densities. Cones are located at the fovea. Rods are located everywhere else. The optic disc lacks all photoreceptors since the optic nerve fibers are exiting the eye at this location. ‘Retinal Receptor Density’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Phototransduction

Photoreceptors hyperpolarize in response to light, do not fire action potentials, and release glutamate

The photoreceptors are responsible for sensory transduction in the visual system, converting light into electrical signals in the neurons. To examine the function of the photoreceptors, we will A) focus on black and white light (not color vision) and B) assume the cells are moving from either an area of dark to an area of light or vice versa.

Photoreceptors do not fire action potentials; they respond to light changes with graded receptor potentials (depolarization or hyperpolarization). Despite this, the photoreceptors still release glutamate onto the bipolar cells. The amount of glutamate released changes along with the membrane potential, so a hyperpolarization will lead to less glutamate being released. Photoreceptors hyperpolarize in light and depolarize in dark. In the graphs used in this lesson, the starting membrane potential will depend on the initial lighting condition.

Two graphs showing that photoreceptors hyperpolarize and release less glutamate when moving from dark to light, and depolarize and release more glutamate when moving from light to dark. Link to detailed alternative text in caption.
Figure 21.6. Photoreceptors respond with graded potentials when moving from light to dark or vice versa. A) When moving from dark to light, the photoreceptor will hyperpolarize, and glutamate release will decrease. B) When moving from light to dark, the photoreceptor will depolarize, and glutamate release will increase. ‘Photoreceptor Receptor Potentials’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

In the dark, the photoreceptor has a membrane potential that is more depolarized than the “typical” neuron we examined in previous chapters; the photoreceptor membrane potential is approximately -40 mV. Photoreceptors have open cation channels that allow the influx of sodium and calcium in the dark. These channels are gated by the presence of cyclic GMP (cGMP), a molecule important in second-messenger cascades that is present in the photoreceptor in the dark.

A diagram of the photoreceptor membrane in the dark, showing that high cGMP levels keep cation channels open, allowing sodium and calcium influx that depolarizes the cell while opsin, transducin, and PDE remain inactive. Link to detailed alternative text in caption.
Figure 21.7. In the dark, the photoreceptor is depolarized due to an influx of sodium and calcium through open ion channels that are gated by cGMP. The photoreceptor has high levels of cGMP when it is in the dark. Additionally, the opsin proteins, the G-protein transducin, and phosphodiesterase (PDE) are all inactivated. ‘Retinal Dark Current’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

When the photoreceptor moves into the light, the cell hyperpolarizes. Light enters the eye, reaches the photoreceptors, and causes a conformational change in a special protein called an opsin. This change activates a G-protein called transducin, which then activates a protein called phosphodiesterase (PDE). PDE breaks down cGMP to GMP, and the cGMP-gated cation channels that were open in the dark close. The decrease in cation flow into the cell causes the photoreceptor to hyperpolarize.

 

Animation 21.1. Light reaching the photoreceptor causes a conformational change in the opsin protein, which activates the G-protein transducin. Transducin activates phosphodiesterase (PDE), which converts cGMP to GMP. Without cGMP, the cation channels close, stopping the influx of positive ions. This results in a hyperpolarization of the cell. ‘Phototransduction’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Transmission of Information within Retina

Photoreceptors synapse onto bipolar cells in the retina. There are two types of bipolar cells: OFF and ON. These cells respond in opposite ways to the glutamate released by the photoreceptors because they express different glutamate receptors. Like photoreceptors, the bipolar cells do not fire action potentials and only respond with graded postsynaptic potentials.

OFF bipolar cells depolarize in the dark; ON bipolar cells depolarize in the light

OFF Bipolar Cells

In OFF bipolar cells, the glutamate released by the photoreceptor is excitatory. OFF bipolar cells express ionotropic glutamate receptors. In the dark, glutamate released by the photoreceptor activates the ionotropic receptors, and sodium can flow into the cell, depolarizing the membrane potential. In the light, the absence of glutamate causes the ionotropic receptors to close, preventing sodium influx, hyperpolarizing the membrane potential.

A diagram showing that glutamate released by the photoreceptor in the dark opens ionotropic receptors on OFF bipolar cells, allowing sodium influx and depolarization, while the absence of glutamate in the light closes the channels and hyperpolarizes the cell. Link to detailed alternative text in caption.
Figure 21.8. Photoreceptors hyperpolarize in light and decrease the amount of released glutamate. Glutamate is excitatory in OFF bipolar cells, opening ionotropic receptors and allowing sodium influx. In the dark, the OFF bipolar cells are depolarized, and in the light the OFF bipolar cells are hyperpolarized. ‘OFF Bipolar Cells’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

ON Bipolar Cells

In ON bipolar cells, the glutamate released by the photoreceptor is inhibitory. ON bipolar cells express metabotropic glutamate receptors. In the dark, glutamate released by the photoreceptor activates the metabotropic receptors, and the G-proteins close cation channels in the membrane, stopping the influx of sodium and calcium, hyperpolarizing the membrane potential. In the light, the absence of glutamate results in the ion channels being open and allowing cation influx, depolarizing the membrane potential.

A diagram showing that glutamate released by the photoreceptor in the dark activates metabotropic receptors that close cation channels on ON bipolar cells, hyperpolarizing the cell, while the absence of glutamate in the light leaves the channels open and depolarizes the cell. Link to detailed alternative text in caption.
Figure 21.9. Photoreceptors hyperpolarize in light and decrease the amount of released glutamate. Glutamate is inhibitory in ON bipolar cells, activating metabotropic receptors, which closes cation channels. In the dark, the ON bipolar cells are hyperpolarized, and in the light the ON bipolar cells are depolarized. ‘ON Bipolar Cells’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Ganglion Cells

OFF-center ganglion cells increase firing rate in the dark; ON-center ganglion cells increase firing rate in the light

OFF and ON bipolar cells synapse on OFF-center and ON-center ganglion cells, respectively. Ganglion cells are the only cell type to send information out of the retina, and they are also the only cell that fires action potentials. The ganglion cells fire in all lighting conditions, but it is the relative firing rate that encodes information about light. A move from dark to light will cause OFF-center ganglion cells to decrease their firing rate and ON-center ganglion cells to increase their firing rate.

A diagram comparing the OFF and ON pathways, showing that a move from dark to light decreases the firing rate of OFF-center ganglion cells but increases the firing rate of ON-center ganglion cells. Link to detailed alternative text in caption.
Figure 21.10. A move from dark to light will hyperpolarize all photoreceptors. OFF bipolar cells will also hyperpolarize in light, which will lead to a decreased firing rate in OFF-center ganglion cells. ON bipolar cells will depolarize in light, which will lead to an increased firing rate in ON-center ganglion cells. ‘Retinal Ganglion Cells’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Receptive Fields

Each bipolar and ganglion cell responds to light stimulus in a specific area of the retina. This region of retina is the cell’s receptive field. Receptive fields in the retina are circular.

Size of the receptive field can vary. The fovea has smaller receptive fields than the peripheral retina. The size depends on the number of photoreceptors that synapse on a given bipolar cell and the number of bipolar cells that synapse on a given ganglion cell, also called the amount of convergence.

A diagram showing that ganglion cell receptive field size increases with the degree of photoreceptor convergence, from minimal convergence near the fovea to substantial convergence in the peripheral retina. Link to detailed alternative text in caption.
Figure 21.11. Ganglion receptive field sizes can vary depending on location of the bipolar and ganglion cells and the amount of convergence onto those cells. When the photoreceptors are in or near the fovea (Cell 1), the receptive fields are small. In the fovea, each bipolar cell receives input from only one photoreceptor and then synapses on only one ganglion cell. Toward the periphery (Cells 2 and 3), more photoreceptors synapse on each bipolar cell, and more bipolar cells synapse on each ganglion cell, making the surface area of the receptive field larger. ‘Retinal Receptive Fields’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Receptive Field Example

Let’s use an example of an ON bipolar cell to look at the structure of receptive fields in the retina. The bipolar and ganglion cell receptive fields are divided into two regions: the center and the surround. The center of the receptive field is a result of direct innervation between the photoreceptors, bipolar cells, and ganglion cells. If a light spot covers the center of the receptive field, the ON bipolar cell would depolarize, as discussed above; the light hits the photoreceptor, causing it to hyperpolarize and decrease glutamate release. Less glutamate leads to less inhibition of the ON bipolar cell, and it depolarizes.

A diagram showing that light in the center of an ON bipolar cell's receptive field causes the center photoreceptor to hyperpolarize and, through direct synaptic communication, the ON bipolar cell to depolarize. Link to detailed alternative text in caption.
Figure 21.12. A photoreceptor in the center of an ON bipolar cell’s receptive field moves from dark to light. The photoreceptor will hyperpolarize, and the ON bipolar cell will depolarize. The red arrows show the direct synaptic communication from photoreceptor to ON bipolar cell. ‘ON Bipolar Cell – Light in Center’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

The surround portion of the receptive field is a result of indirect communication among the retinal neurons via horizontal and amacrine cells. The surround also has an opposing effect on the bipolar or ganglion cell compared to the effect of the center region. If a light spot covers the surround portion, the ON bipolar cell would respond by hyperpolarizing. The light would cause the photoreceptor in the surround to hyperpolarize. This would cause the horizontal cell to also hyperpolarize. Horizontal cells have inhibitory synaptic effects, so a hyperpolarization in the horizontal cell would lead to a depolarization in the center photoreceptor. The center photoreceptor would then cause a hyperpolarization in the ON bipolar cell. These effects mimic those seen when the center is in dark. So even though the center photoreceptor is not directly experiencing a change in lighting conditions, the neurons respond as if they were moving toward dark.

A diagram showing that light in the surround of an ON bipolar cell's receptive field hyperpolarizes the bipolar cell through an indirect pathway involving horizontal cell communication with the center photoreceptor. Link to detailed alternative text in caption.
Figure 21.13. A photoreceptor in the surround of an ON bipolar cell’s receptive field moves from dark to light. The photoreceptor will hyperpolarize, and the postsynaptic horizontal cell will hyperpolarize. This will cause the center photoreceptor to depolarize, and the ON bipolar cell to hyperpolarize. The red arrows show the indirect synaptic communication between the surround photoreceptor and the ON bipolar cell. The surround photoreceptor synapses on the horizontal cell, which synapses on the center photoreceptor, which synapses on the bipolar cell. ‘ON Bipolar Cell – Light in Surround’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Lateral Inhibition

Lateral inhibition is critical for enhancing the perception of edges and borders

The center-surround structure of the receptive field is critical for lateral inhibition to occur. Lateral inhibition is the ability of the sensory systems to enhance the perception of edges of stimuli. It is important to note that the photoreceptors that are in the surround of one bipolar cell would also be in the center of a different bipolar cell. This leads to a direct synaptic effect on one bipolar cell while also having an indirect effect on another bipolar cell.

A diagram of lateral inhibition showing that an edge of light directly depolarizes the bipolar cell at its center while indirectly hyperpolarizing the neighboring bipolar cell through horizontal cell input, widening the membrane potential difference across the edge. Link to detailed alternative text in caption.
Figure 21.14. An edge of a light stimulus moves into the receptive field surround of ON bipolar cell B. This edge is also falling on the receptive field center of ON bipolar cell C. The light will cause bipolar cell C to depolarize because of the direct synapse with the photoreceptor. The light will also cause bipolar cell B to hyperpolarize because of the indirect synapses through the horizontal cell. This hyperpolarization causes a larger membrane potential difference between cells B and C that would occur if the horizontal cells were absent. The larger membrane potential difference between the cells will lead to an enhancement in the perception between the dark and light side of the edge. ‘Lateral Inhibition’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Although some of the images used here will simplify the receptive field to one cell in the center and a couple in the surround, it is important to remember that photoreceptors cover the entire surface of the retina, and the receptive field is two-dimensional. Depending on the level of convergence on the bipolar and ganglion cells, receptive fields can contain many photoreceptors.

A three-dimensional diagram of the retinal surface showing that a receptive field's center and surround each span a two-dimensional area encompassing multiple photoreceptors. Link to detailed alternative text in caption.
Figure 21.15. The receptive fields exist in two-dimensions along the surface of the retina. Depending on the location of the receptive field, and the amount of convergence that occurs at the bipolar or ganglion cell, the receptive field may contain many photoreceptors. ‘Retinal Surface’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Conclusion

The retina performs substantial neural processing before visual information reaches the brain. Photoreceptors, bipolar cells, and ganglion cells work together to encode features like light intensity, edges, and contrast before the signal ever leaves the eye.

Key Takeaways

  • Photoreceptors and bipolar cells do not fire action potentials
  • Photoreceptors hyperpolarize in the light
  • ON bipolar cells express inhibitory metabotropic glutamate receptors
  • OFF bipolar cells express excitatory ionotropic glutamate receptors
  • Receptive fields are circular, have a center and a surround, and vary in size
  • Receptive field structure allows for lateral inhibition to occur

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Vision: The Retina Copyright © 2021 by Casey Henley is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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