Photoreceptors

General structural plan of a photoreceptor

Both rod and cone neurosensory neurons consist of three main functional parts:

• Peripheral part — a modified dendrite that directly captures light. It is always divided into an outer and an inner segment, between which lies a narrow connecting region called the cilium (cilium).
• Nucleus-containing part — a cell expansion housing the nucleus.
• Central part — an axon responsible for synaptic transmission of the nerve impulse to retinal bipolar and horizontal cells.

Ultrastructure and pigments of the outer segments

Outer segments are the photosensitive "antennas" of the cells, but rods and cones have different structures.

Rods. The outer segment is cylindrical and contains a stack of approximately 1,000 flat, isolated membrane discs. Rhodopsin — a visual pigment consisting of a protein and a non-protein moiety (retinal, an oxidized form of vitamin A) — is densely embedded in their membranes. The discs are continuously renewed: at night, new ones are formed at the base of the segment by invagination of the plasmalemma (about 100 per day), while in the morning, old discs at the apex are shed and phagocytosed by the retinal pigment epithelium cells.

Cones. The outer segment is formed not by discs, but by half-discs. These are deep invaginations of the plasma membrane that do not pinch off completely, so their internal space communicates directly with the extracellular environment. The visual pigment of cones is iodopsin. It also contains retinal, but differs from rhodopsin in its specific protein part (opsin). The connecting cilium in cones is noticeably shorter, and the inner segment forms special finger-like processes embracing the base of the outer segment. In the very inner segment of a cone, a large lipid droplet surrounded by a mitochondrial ellipsoid is prominent.

Physiology of excitation and ion exchange

Photoreceptor function is based on an ionic balance unusual for nerve cells.

At rest (in complete darkness), sodium channels in the plasmalemma of the outer segment remain open. Na⁺ ions continuously enter, diffuse into the inner segment, and are actively pumped out there by a powerful Na⁺,K⁺-pump. In the extracellular environment, sodium returns to the outer segment, completing the cycle. The huge energy expenditure for pump operation is met by an accumulation of mitochondria in the inner segment.

When a light quantum hits the pigment, the following occurs:

1. The spatial structure of the pigment changes.
2. A biochemical cascade of reactions is triggered.
3. Sodium channels in the outer segment close.
4. The transmembrane potential sharply increases, resulting in hyperpolarization.
5. The hyperpolarization wave reaches the synapse, triggering mediator release and further signal transmission.

Color vision and genetics

The brain's perception of color shades is based on analyzing the ratio of excitation of three types of cones, each synthesizing only one type of pigment:

• Blue (S-cones): capture short wavelengths with a peak at 420 nm. In the area of best vision (fovea centralis), their proportion is only about 3%.
• Green: medium-wave receptors (peak at 530 nm).
• Red: long-wave receptors (peak at 625 nm).

Genes located on different chromosomes are responsible for pigment production. The rod rhodopsin gene is on chromosome 7, and the "blue" pigment gene is on chromosome 3. The genes for the "green" and "red" pigments are localized on the X chromosome. For this reason, hereditary color vision defects — color blindness — are sex-linked. Impaired perception of red color is called protanopia, and green color is called deuteranopia.

Adaptation to light level

The retina flexibly adjusts its sensitivity by changing the state of the pigment and mechanical movements within the pigment epithelium.

• Dark adaptation: The pigment fully regenerates into the unexcited state. Melanosomes (melanin granules) inside the pigment epithelial cells flow from the processes into the cell body. The processes become transparent, allowing even single photons of light to reach the rods unhindered.
• Light adaptation: In bright light, the fraction of unexcited pigment drops. Melanosomes migrate back into the processes of the epithelial cells. These pigment-filled processes densely surround the photoreceptors up to the level of their nuclei, acting as an optical filter. Melanin screens and absorbs up to 85–90% of light, reliably protecting neurons from light overexcitation.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Sodium ion cycle and energy supply of the inner segment
• Mechanism of rod membrane disc renewal
• The role of pigment epithelium melanosomes in light and dark adaptation
• Structure of visual pigments: rhodopsin and iodopsin
• Morphological differences of the connecting apparatus of rods and cones

Other topics in "Sensory organs: organs of vision and olfaction"

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