Optic disc (Blind spot)
Two structures of particular clinical significance are distinguished on the fundus. The first of these is the optic disc. In this area, the nerve fibers of the retina converge to form the optic nerve.
Anatomically, the 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 is a round or slightly vertically elongated formation with a diameter of about 1.5–1.7 mm. In the very center of the disc, there is a small depression intended for the passage of the central retinal artery and central vein. A crucial physiological feature of this region is the complete absence of photoreceptors. Any light falling on this area is not perceived, which is why it is called the physiological blind spot.
Retinal spot (Macula) and central vision
The second key structure is the macula (macula), which represents the central region. The macula is located on the temporal side, 4 mm from the optic disc, and has a diameter of about 5.5 mm. In its center lies the fovea centralis with a miniature pit (foveola) measuring only about 0.2 mm in diameter.
Histologically, the macula is unique:
- Blood vessels are completely absent in it.
- The number of nerve fibers is kept to a minimum.
- The receptor apparatus is represented exclusively by cones (about 7 million).
It is the macula that is responsible for maximum visual acuity. In the fovea, the phenomenon of convergence is completely absent: a signal from a single cone is transmitted to a single bipolar cell, and from it strictly to a single ganglion cell (1:1 direct connection). Thanks to this individual transmission, each point of the image is perceived separately, and the brain receives a clear and sharp picture.
Peripheral retina and scotopic vision
The remaining space outside the macula belongs to the peripheral part of the retina. As it approaches the ora serrata (ora serrata), the ability to see clearly decreases. This zone is dominated by rods, the maximum density of which is recorded at a distance of 20° from the center of the macula.
Unlike cones, the rod apparatus operates on the principle of 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 nerve cell. As a result, weak signals are summed up. Additionally, rods contain significantly more visual pigment.
Both factors provide the retinal periphery with the highest sensitivity to dim light, allowing us to see in twilight (scotopic vision). However, due to the mass pooling of signals, fine details are lost, which is why peripheral vision has low acuity.
Briefly: answers to topical questions
What cellular layers are distinguished in the structure of the retina?
There are ten layers in the human retina, arranged from outside inward in the following order.
- Layer of pigment epithelium — 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 light-sensitive cells.
- Outer plexiform layer — the region of contacts between photoreceptors and local association neurons.
- Inner nuclear layer — contains the cell bodies of local association neurons and gliocytes.
- Inner plexiform layer — the region of contact between neurons.
- Ganglion cell layer — contains the cell bodies of ganglion neurons.
- Layer of nerve fibers — axons of ganglion cells.
- Inner limiting membrane — on the inner surface of the retina.
What visual pigments are contained in rods and cones?
The receptor cells of the retina contain various visual pigments consisting of retinal 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 the nature of the protein moiety, iodopsin is subdivided into three types:
| Type of iodopsin | Absorbed waves | Wavelength |
|---|---|---|
| Type I | Blue (short-wavelength) | 420 nm |
| Type II | Green (medium-wavelength) | 530 nm |
| Type III | Red (long-wavelength) | 625 nm |
In the dark, pigments are in an inactive form, and under the influence of light, they dissociate into an active form.
What is the molecular mechanism of phototransduction in retinal receptors?
The molecular mechanism of photoreception begins with the absorption of a light quantum causing a change in the pigment structure, and retinal dissociates from opsin. This event triggers a cascade of reactions leading to the closure of sodium channels in the plasmalemma of the outer segment. As a result, an increase in transmembrane potential—hyperpolarization—occurs. When the hyperpolarization wave reaches the synapse, a neurotransmitter is released, which excites the neurons of the next link (bipolar and horizontal cells).
What is the total number of rods and cones in the human retina?
The number of light-sensitive cells in the retina varies significantly depending on their type.
- Rod neurosensory cells — their number is about 130 million. They are distributed predominantly in the periphery of the retina and are responsible for scotopic vision.
- Cone cells — they are 20 times fewer in number than rods. It is known that in the region of the macula, where they are concentrated, there are about 7 million cones.
Cones mean Central concentration and Clarity (1:1 connection). Rods mean Periphery and Dim light (sensitivity due to signal summation).
Class and exam questions
Why is the optic disc physiologically a blind spot?
This area represents the convergence point of nerve fibers and the passage of central vessels, which is why it completely lacks light-sensitive elements (receptors).
How does the impulse transmission pathway differ between the macula and the periphery?
There is no convergence in the macula lutea (one cone is connected to one ganglion cell), which provides maximum detail. In the periphery, impulses from hundreds of rods converge onto a single ganglion cell, ensuring high sensitivity at the expense of clarity.
What is the function of the retinal pigment epithelium?
The pigment epithelium lies immediately behind the receptor layer; it absorbs light rays and reduces their scattering within the eye.
What else to study on this topic
- Mechanism of conversion of light energy into a nerve impulse
- Biochemical composition of visual pigments in rods and cones
- Detailed histological structure of all retinal layers
- Features of impulse transmission along the neuronal chain to the cortical center
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