General Structure of a Photoreceptor
Both rod and cone neurosensory cells consist of three main functional regions:
- Peripheral part — a modified dendrite that directly captures light. It is always divided into an outer and an inner segment, connected by a narrow connecting stalk (cilium).
- Nucleus-containing region — the cell expansion housing the nucleus.
- Central part — the axon responsible for synaptic transmission of the nerve impulse to retinal bipolar and horizontal cells.
Ultrastructure and Pigments of the Outer Segments
The outer segments act as the light-sensitive "antennas" of the cells, but their structure differs between rods and cones.
Rods. The outer segment is cylindrical and contains a stack of approximately 1,000 flat, isolated membrane discs. Rhodopsin, a visual pigment composed of a protein moiety (opsin) and a non-protein chromophore (retinal, an oxidized form of vitamin A), is densely embedded in their membranes. These discs are continuously renewed: at night, new discs form at the base of the segment via plasma membrane invagination (about 100 per day), while in the morning, old discs at the tip are shed and phagocytosed by the retinal pigment epithelium.
Cones. The outer segment is formed not by closed discs, but by half-discs (lamellae). These are deep invaginations of the plasma membrane that do not fully pinch off, so their interior 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 moiety (opsin). The connecting cilium in cones is noticeably shorter, and the inner segment forms specialized finger-like processes embracing the base of the outer segment. A prominent lipid droplet, surrounded by a mitochondrial ellipsoid, is a characteristic feature of the cone inner segment.
Physiology of Excitation and Ion Exchange
Photoreceptor function relies on an ion balance that is unusual for nerve cells.
At rest (in complete darkness), sodium channels in the plasma membrane of the outer segment remain open. Na⁺ ions continuously enter the cell, diffuse into the inner segment, and are actively pumped back out by a powerful Na⁺,K⁺-ATPase pump. Sodium returns to the outer segment via the extracellular space, completing the cycle. The massive energy expenditure required to drive these pumps is met by a dense accumulation of mitochondria in the inner segment.
When a light quantum strikes the pigment:
- The spatial structure of the pigment changes.
- A biochemical cascade of reactions is triggered.
- Sodium channels in the outer segment close.
- The transmembrane potential increases sharply, resulting in hyperpolarization.
- The wave of hyperpolarization reaches the synapse, decreasing neurotransmitter release and transmitting the signal downstream.
Color Vision and Genetics
Color perception by the brain relies on analyzing the relative activation of three types of cones, each synthesizing only one specific pigment:
- Blue (S-cones): absorb short wavelengths with a peak at ~420 nm. In the region of highest visual acuity (fovea centralis), they account for only about 3% of cones.
- Green (M-cones): medium-wavelength receptors (peak at ~530 nm).
- Red (L-cones): long-wavelength receptors (peak at ~562–525 nm).
The pigments are encoded by genes located on different chromosomes. The rod rhodopsin gene is on chromosome 3, the "blue" pigment gene on chromosome 7, while the "green" and "red" pigment genes are localized on the X chromosome. Consequently, inherited color vision deficiencies (color blindness) are X-linked. Red color blindness is called protanopia, and green color blindness deuteranopia.
Light and Dark Adaptation
The retina flexibly adjusts its sensitivity by altering pigment states and through mechanical movements within the retinal pigment epithelium.
- Dark adaptation: Pigment fully regenerates to its unbleached state. Melanosomes (melanin granules) within the retinal pigment epithelial cells flow from the cell processes back 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 proportion of unbleached pigment drops. Melanosomes migrate back into the epithelial cell processes. These pigment-filled processes closely surround the photoreceptors up to the level of their nuclei, acting as an optical filter. Melanin screens and absorbs up to 85–90% of the light, protecting the neurons from phototoxicity and overexcitation.