A thin layer of neural tissue lining the back of the eye. Part of the CENTRAL NERVOUS SYSTEM, it is displaced into the eye during development. The retina contains two types of photoreceptors—RODS which mediate dim light vision, and CONES which are responsible for colour vision—and four major types of neurons—HORIZONTAL, BIPOLAR, amacrine and GANGLION CELLS. One major type of GLIAL CELL (MÜLLER CELL) is present in the retina. The retinal cells are organized into three cellular (nuclear) layers separated by two synaptic (PLEXIFORM) layers. Virtually all of the retinal synapses occur within the two plexiform layers and all visual information crosses at least two synapses, one in the outer plexiform layer, the other in the inner plexiform layer before it leaves the eye via the axons of the ganglion cells. Each PHOTORECEPTOR passes on the visual signal to the second-order horizontal and bipolar cells in the outer plexiform layer, whereas the bipolar cell terminals pass on the visual signal to the third-order amacrine and ganglion cells in the inner plexiform layer.
In the outer plexiform layer, visual information is first separated into on- and off-bipolar cell channels. If on-bipolar cells are blocked pharmacologically, animals cannot see objects brighter than the background whereas they can see objects darker than the background. Furthermore, a spatial type analysis occurs in the outer plexiform layer such that both on-and off-bipolar cells demonstrate an antagonistic centre-surround RECEPTIVE FIELD organization, termed a CONTRAST-SENSITIVE RECEPTIVE FIELD. The centre response reflects direct photoreceptor-bipolar cell synaptic interactions, whereas the antagonistic surround response is mediated via the horizontal cells. Contrast-sensitive receptive fields play an important role in the detection of light-dark borders and can explain the MACH BAND PHENOMENON. In animals with good COLOUR VISION, evidence of the first stages of colour processing is observed in the receptive field properties of the bipolar cells. Centre responses of the bipolar cell receptive fields are maximally responsive to light of one wavelength whereas antagonistic surround responses are maximally responsive to another wavelength. Such a receptive field organization is termed colour opponent.
The inner plexiform layer is concerned more with the temporal aspects of the visual stimulus. Many amacrine cells respond transiently to retinal illumination and are highly responsive to moving stimuli. Many ganglion cell re sponses reflect primarily outer plexiform layer processing—these cells respond in a sustained fashion to appropriately positioned stimuli on the retina and show a centre-surround receptive field organization. Some of these ganglion cell receptive fields are also colour opponent. Other ganglion cells reflect inner plexiform layer processing—the cells give transient responses to static spots of light projected onto the retina but respond vigorously to moving stimuli. These cells may or may not show a centre surround receptive field organization. Furthermore, some of the movement-sensitive ganglion cells in non-primate species show directional properties. They respond vigorously to spots of light moving in one direction (preferred direction) but are inhibited by light spots moving in the other (null) direction.
Most retinas have one rod type but multiple cone types that absorb light maximally from different regions of the spectrum (for instance red, green or blue). The ability to discriminate colour relates to the light-absorbing properties of the different cone types. Within the outer segment region of the photoreceptor cells are light-sensitive molecules, the visual pigments. Red-sensitive cones contain a visual pigment that absorbs red light maximally, green cones a visual pigment that absorbs green light maximally, and so on. Loss of one or another of the cone types leads to COLOUR BLINDNESS; red blind individuals have no red-sensitive cones; green blind individuals lack green-sensitive cones, etc. Receptors, horizontal and bipolar cells respond to retinal illumination with sustained graded electrical responses—they do not generate action potentials as do amacrine and ganglion cells. These outer retinal cells also generally respond to light with hyperpolarizing (see HYPERPOLARIZATION) electrical potentials, again in contrast to amacrine and ganglion cells which generate both depolarizing (see DEPOLARIZATION) and hyperpolarizing electrical signals.
Photoreceptor and bipolar cells release L-GLUTAMATE at their synapses whereas horizontal and amacrine cells release mainly inhibitory neurotransmitters, namely GABA and GLYCINE. The retina, like other regions of the brain, contains a number of monoamines (see MONOAMINE) and NEUROPEPTIDES. These substances are found mainly in amacrine or amacrine-like cells and are thought to play a role in NEURO-MODULATION in the retina. The role of dopamine is best understood; it has been shown to modify the properties of both chemical and electrical synapses within the retina. Dopamine does this by interacting with receptors linked to the enzyme ADENYLATE CYCLASE, resulting in the synthesis of cyclic AMP. PROTEIN KINASE A (PKA) is ultimately activated and phosphorylation of MEMBRANE CHANNELS at the synapses occurs modifying their properties. This results in a modification of the retinal circuitry appropriate to the light-dark conditions.
