Pigment epithelium and photoreceptors
The retina is true nervous tissue, unlike the connective tissue of the vascular and fibrous tunics. Its outermost layer, bordering the choroid and lying on the basal lamina, forms the pigment epithelium. These cells use their processes to surround the dendrites of light-sensitive neurons (one epitheliocyte serves from 3–7 up to 30–40 dendrites). Due to melanin granules, they absorb 85–90% of excess light, preventing light scattering. They also phagocytize worn-out cellular components and transport vitamin A for the synthesis of rhodopsin and iodopsin.
The first level of visual signal transmission is represented by light-sensitive (photoreceptor) neurons. The cell consists of three parts:
- Dendrite (peripheral part): includes inner and outer segments. The outer segment faces the pigment layer.
- Neuron body (nucleus-containing part).
- Axon (central part): forms a synapse with the second-level cells.
Based on dendrite shape, there are cone neurons (responsible for color daylight vision, concentrated in the fovea centralis, or macula lutea) and rod neurons (responsible for black-and-white scotopic vision, prevailing in the periphery).
Locally associative neurons (second level)
Middle-level cells interconnect retinal neurons. They are divided into three functional types:
- Bipolar neurons. They provide vertical impulse transmission to ganglion cells. Rods are characterized by the principle of convergence: several receptors contact a single bipolar cell, and several bipolar cells contact a single ganglion cell. For cones, a "one-to-one" direct connection or even direct contact with the ganglion bypassing the bipolar cell is possible.
- Horizontal neurons. They provide lateral connections. Their dendrites receive a signal from one group of photoreceptors, and their axons transmit it to another group. This triggers the process of lateral inhibition (suppression of neighboring activity) to sharply increase the contrast of the visual image.
- Amacrine neurons. They 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 main task is to collect impulses and transmit them to the brain. The axons of these cells head toward the blind spot (optic disc). An important detail: before exiting the eye, these axons lack a myelin sheath, which ensures the optical transparency of nerve fibers for the passage of light.
The space between nerve cells is filled with glia, which perform a supporting and trophic function. The most numerous are Müller cells (radial gliocytes). These are long supporting cells penetrating the retina perpendicular to the layers. Their processes form two key structures:
- External limiting membrane (between the nucleus-containing parts of receptors and their dendrites).
- Internal limiting membrane (at the border of the retina and the vitreous body).
Briefly: answers to topical questions
What ten histological layers are distinguished in the retina of the eye?
Ten histological layers are distinguished in the retina of the eye, located from the choroid to the vitreous body in the following order:
- Layer of pigment epithelium — the outermost.
- Layer of rods and cones — the photosensory layer.
- Outer limiting membrane.
- Outer nuclear layer — contains photoreceptor nuclei.
- Outer plexiform layer — the synaptic zone.
- Inner nuclear layer — contains the nuclei of bipolar, horizontal, and amacrine cells.
- Inner plexiform layer — the synaptic zone.
- Ganglion cell layer — bodies of ganglion cells.
- Nerve fiber layer — axons of ganglion cells.
- Internal limiting membrane.
In which layer of the retina are the bodies of bipolar, horizontal, and amacrine neurons located?
The bodies of bipolar, horizontal, and amacrine neurons are located in the inner nuclear layer of the retina. Functionally, these cells belong to locally associative neurons that provide connections between other nerve cells. In addition to them, the inner nuclear layer also contains the nuclei of radial gliocytes (Müller cells).
What specific visual pigments are contained in rods and cones?
The receptor cells of the retina contain various specific visual pigments, which are in an inactive form in the dark.
| Receptor type | Visual pigment | Function |
|---|---|---|
| Bacilli | Rhodopsin (visual purple) | Scotopic vision |
| Cones | Iodopsin (cone opsin) | Photopic vision |
Both pigments consist of retinal (a vitamin A derivative) and a protein part, opsin. Iodopsin differs in its protein part and is subdivided into three types that absorb blue, green, or light waves.
Remembering the path of the vertical impulse is simple: FBR (Photoreceptors → Bipolar → Ganglion neurons).
Class and exam questions
Why are there far fewer ganglion neurons in the retina than photoreceptor neurons?
This is due to the principle of convergence: the signal from multiple rod neurons converges onto a single bipolar cell, and from several bipolar cells onto a single ganglion neuron.
What function is performed by the absence of myelin on ganglion cell axons?
The absence of a myelin sheath until the axons exit the eyeball makes nerve fibers transparent, which prevents light scattering.
What forms the limiting membranes of the retina?
The internal and external limiting membranes are formed by the processes of Müller cells (radial gliocytes), which function as a supporting framework.
What else to study on this topic
- Morphological differences between the outer and inner segments of rods and cones.
- Trophic function of the pigment epithelium and vitamin A metabolism in rhodopsin synthesis.
- Mechanisms of lateral inhibition: interaction of horizontal and amacrine cells.
- Topographic distribution of receptors: retinal periphery versus the fovea centralis.
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