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Retina

Retina

For medical students2 min readUpdated 2026-10-10

The retina is the peripheral part of the visual analyzer, which can functionally be considered an extension of the brain. Its primary role is to capture light, convert light energy into nerve impulses, and transmit these signals through a chain of neurons to the visual cortex.

Pigment epitheliumLocated behind the photoreceptors, absorbs light rays, and reduces scattering.
Blind spotThe optic disc area where photoreceptors are completely absent.
MaculaThe central region of the retina responsible for maximum visual acuity.
ConvergenceThe pooling of impulses from multiple rods to achieve high sensitivity in the dark.

Optic Disc (Blind Spot)

Two structures on the fundus hold special clinical significance. The first is the optic disc. In this region, retinal nerve fibers converge to form the optic nerve.

Anatomically, the optic disc is located 2.5–3 mm medial to the posterior pole of the eye and 0.5–1 mm inferior to it. Visually, it appears as a round or slightly vertically oval structure measuring about 1.5–1.7 mm in diameter. At the very center of the disc lies a small depression, the physiological cup, which accommodates the entry of the central retinal artery and vein. A crucial physiological feature of this area is the complete absence of photoreceptors. Any light falling on this zone cannot be perceived, which is why it is known as the physiological blind spot.

Macula Lutea and Central Vision

The second key structure is the macula lutea (macula), which represents the central region of the retina. The macula is located on the temporal side, about 4 mm from the optic disc, and has a diameter of approximately 5.5 mm. In its center lies the fovea centralis featuring a tiny pit, the foveola, with a diameter of only about 0.2 mm.

Histologically, the macula is unique:

The macula is responsible for maximum visual acuity. In the foveola, convergence is completely absent: a signal from a single cone is transmitted to a single bipolar cell, and from there strictly to a single ganglion cell (a direct 1:1 pathway). Thanks to this individual transmission, every point of the image is perceived separately, providing the brain with a sharp and clear picture.

Peripheral Retina and Scotopic Vision

The remaining area beyond the macula constitutes the peripheral retina. As it approaches the ora serrata, the capacity for sharp vision declines. This zone is dominated by rods, which reach their maximum density at a distance of 20° from the center of the macula.

Unlike cones, the rod system operates via high (broad) convergence. This means that impulses from a vast number of receptors (up to 500 rods) converge via bipolar cells onto a single ganglion cell. As a result, weak signals undergo spatial summation. Additionally, rods contain a significantly higher concentration of visual pigment.

Together, these factors provide the peripheral retina with exceptional sensitivity to dim light, enabling twilight vision (scotopic vision). However, because of signal pooling, fine details are lost, rendering peripheral vision low in acuity.

Mnemonic

Cones mean Central concentration and Clarity (1:1 pathway). Rods mean Periphery and Periods of dim light (sensitivity via signal summation).

Frequently asked questions

What are the cellular layers of the retina?

The human retina comprises ten layers, arranged from outside to inside in the following order:

  • Pigment epithelium layer — borders the choroid.
  • Layer of rods and cones — contains the dendrites of photoreceptor neurons.
  • Outer limiting membrane — formed by the processes of Müller glial cells.
  • Outer nuclear layer — contains the cell bodies of photoreceptors.
  • Outer plexiform layer — site of synapses between photoreceptors and association neurons.
  • Inner nuclear layer — contains the cell bodies of association neurons and glial cells.
  • Inner plexiform layer — site of synaptic contacts between inner neurons.
  • Ganglion cell layer — contains the cell bodies of retinal ganglion cells.
  • Nerve fiber layer — axons of ganglion cells.
  • Inner limiting membrane — forms the inner surface of the retina.
Which visual pigments are found in rods and cones?

Retinal photoreceptor cells contain various visual pigments composed of retinal (a vitamin A aldehyde) and the glycoprotein opsin.

  • Rhodopsin — the visual pigment of rods (absorption maximum at ~500 nm).
  • Iodopsin (cone opsin) — the visual pigment of cones.

Based on their protein moiety, iodopsins are divided into three types:

Iodopsin TypeAbsorbed WavelengthsWavelength
Type IBlue (short-wavelength)420 nm
Type IIGreen (medium-wavelength)530 nm
Type IIIRed (long-wavelength)625 nm

In the dark, pigments remain in an inactive conformation; exposure to light triggers their dissociation into active components.

What is the molecular mechanism of phototransduction in retinal receptors?

Photoreception begins when the absorption of a light quantum alters the pigment's structure, causing retinal to dissociate from opsin. This event triggers a biochemical cascade that leads to the closure of sodium channels in the plasma membrane of the outer segment. Consequently, the transmembrane potential increases, resulting in hyperpolarization. When this hyperpolarization wave reaches the synapse, it decreases neurotransmitter release, modulating the activity of downstream neurons (bipolar and horizontal cells).

What is the total number of rods and cones in the human retina?

The number of photoreceptor cells varies significantly by type:

  • Rod neurosensory cells — number approximately 130 million. They are distributed primarily across the peripheral retina and mediate scotopic vision.
  • Cone cells — are roughly 20 times less numerous than rods. Approximately 7 million cones are concentrated within the macula lutea.
Why is the optic disc physiologically a blind spot?

This region serves as the convergence site for nerve fibers and retinal blood vessels, meaning it completely lacks light-sensitive receptors.

How does signal transmission differ between the macula and the periphery?

In the macula, there is no convergence (one cone connects to one ganglion cell), yielding maximum detail. In the periphery, impulses from hundreds of rods converge onto a single ganglion cell, providing high sensitivity at the expense of visual acuity.

What is the function of the retinal pigment epithelium?

The retinal pigment epithelium lies immediately behind the photoreceptor layer; it absorbs light rays and minimizes light scattering inside the eye.

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