19.9: The Retina
The retina is a layer of nervous tissue at the back of the eye that transduces light into neural signals. This process, called phototransduction, is carried out by rod and cone photoreceptor cells in the back of the retina.
Photoreceptors have outer segments with stacks of membranous disks that contain photopigment molecules—such as rhodopsin in rods. The photopigments absorb light, triggering a cascade of molecular events that results in the cell becoming hyperpolarized (with a more negative membrane potential) relative to when it is in the dark. This hyperpolarization decreases neurotransmitter release. Thus, unlike stimuli for most other sensory neurons, light induces a reduction in neurotransmitter release from photoreceptors.
Although rods and cones both detect light, they play distinct roles in vision. Rods are highly sensitive to light, and therefore predominate in low-light conditions, such as at night. Cones are less sensitive and are used for most daytime vision. Cones are densely concentrated in the fovea—a small depression near the center of the retina that contains very few rods—and provide a high level of visual acuity in the area where the eye is focused.
Cones also convey color information, because the different types—S (short), M (medium), and L (long) in humans—maximally absorb different wavelengths of light. This is because different opsin molecules with distinct light absorption properties largely predominate the three cone types, although all opsin varieties are present in each cone. The relative activation of the different types of cones encodes color.
Photoreceptors send visual information to bipolar cells in the middle of the retina, which then synapse onto ganglion cells at the front of the retina. Two additional cell types—horizontal and amacrine cells—mediate lateral interactions between cells at these junctions. Horizontal cells modulate photoreceptor-bipolar synapses, whereas amacrine cells influence bipolar-ganglion synapses. This circuitry allows for the integration of information across wider parts of the retina and enables initial processing of visual information, such as the detection of contrast under varying light conditions.
Visual information then travels down the axons of the ganglion cells, which (along with glial cells) make up the optic nerve at the back of the eye. From the optic nerve, visual information travels to the brain for additional processing and interpretation.