Pigment Epithelium and Photoreceptors
The retina is true nervous tissue, unlike the connective tissue of the vascular coat and the fibrous layer (sclera). Its outermost layer, bordering the choroid and resting on the basal lamina, forms the pigment epithelium. These cells use their processes to wrap around the dendrites of light-sensitive neurons (one epithelial cell serves from 3–7 up to 30–40 dendrites). Containing melanin granules, they absorb 85–90% of excess light, preventing light scatter. They also phagocytose worn-out cellular components and transport vitamin A for the synthesis of rhodopsin and iodopsin.
The first level of visual signal transmission consists of light-sensitive (photoreceptor) neurons. Each cell consists of three parts:
- Dendrite (peripheral part): includes inner and outer segments. The outer segment faces the pigment layer.
- Neuronal cell body (nuclear region).
- Axon (central part): forms a synapse with the second-level cells.
- Based on dendrite shape, cone neurons (responsible for color daylight vision, concentrated in the macula lutea) and rod neurons (responsible for black-and-white scotopic/dim-light vision, dominating in the periphery) are distinguished.
Intercalated Neurons (Second Level)
The intermediate layer cells interconnect retinal neurons. They are divided into three functional types:
- Bipolar neurons. Provide vertical impulse transmission to ganglion cells. Rods exhibit convergence: multiple receptors contact a single bipolar cell, and multiple bipolar cells contact a single ganglion cell. Cones may have a direct "one-to-one" connection or even direct contact with a ganglion cell bypassing the bipolar cell.
- Horizontal neurons. Provide lateral connections. Their dendrites receive signals from one group of photoreceptors, and their axons transmit them to another group. This initiates lateral inhibition (suppression of neighboring activity) to sharply enhance the contrast of the visual image.
- Amacrine neurons. Perform a function similar to horizontal cells, but operate at the level of bipolar neuron axons, blocking impulse transmission along neighboring pathways.
Ganglion Neurons and Glial Apparatus
The third level is represented by ganglion neurons. This is the least numerous cellular layer of the retina (the result of massive convergence). Their primary task is to collect impulses and transmit them to the brain. The axons of these cells travel toward the blind spot (optic disc). Crucially, before exiting the eye, these axons lack a myelin sheath, ensuring the optical transparency of the nerve fibers for light passage.
The space between nerve cells is filled by glia, which perform supportive and trophic functions. The most numerous are Müller cells (radial gliocytes). These are long supporting cells that traverse the retina perpendicular to the layers. Their processes form two key structures:
- Outer limiting membrane (between the nuclear regions of the receptors and their dendrites).
- Inner limiting membrane (at the boundary between the retina and the vitreous body).