Eyesight

Sight is the most important sense for humans. A person obtains up to 90% of information from the external environment using vision. Vision is a complex process, the essence of which is the reception and processing of signals in the form of photons, therefore we refer to visible light as that which has a wavelength in the range of 400-760 nm. The main organ of vision is the eye. The human eye is a spherical organ with a diameter of about 24 mm.

Basic parts of the eye
The basic parts of the eye include:


 * retina
 * choroid
 * sclera
 * vitreous body (vitreous humor)
 * suspension lens apparatus
 * lens
 * posterior chamber of the eye
 * anterior chamber of the eye
 * pupil
 * iris
 * cornea
 * ciliary body ( corpus ciliare )
 * limbus

The choroid expands into the ciliary body, which includes the smooth muscle musculus ciliaris. The choroid also creates a continuous septum called the iris, which contains 2 main muscles: the sphincter pupillae innervated by the parasympathetic and the dilator pupillae innervated by the sympathetic. In the center of the iris is a circular opening called the pupil.

The lens is located behind the iris, on the edge of which fine fibers ( fibrae zonulares ) are attached, which originates from the ciliary body. The inner layer of the wall of the bulb is formed by the retina. In the region of the posterior pole of the eye, the retina is suddenly narrowed, this entire area is about 3mm in diameter and is called the macula lutea (yellow spot) and the place of sharpest vision. The entire space of the bulb behind the lens is filled by the vitreous humor. Both eye chambers are filled with aqueous humor.

Functions of aqueous humor
The aqueous humor serves to ensure optimal optical properties of the eye. It creates a certain pressure on the vitreous body and the wall of the bulb, thereby creating the correct tone of the eyeball. The pressure of the aqueous humor is referred to as the intraocular pressure and its value is around 15 mmHg. This proper size is maintained by the balance between the formation and outflow of aqueous humor. If there is an increase in the pressure of the aqueous humor, this disease is called glaucoma. Aqueous fluid has a similar composition to blood plasma and circulates constantly, it is formed by the ciliary body and first drains into the posterior chamber of the eye, from where it then passes into the anterior chamber of the eye, drains into Schlemm's canal and then into the venous system of the eye.

Optical system
The light beam must pass through 4 different environments (cornea, aqueous humor, lens and the vitreous body). A real, inverted and reduced image is created on the retina. The CNS itself transforms it into its true form. The total optical power of the eye when looking into the distance is +59D. ​​Light rays emanating from an object located further than 6 m already arrive parallel and intersect in the focal plane. When the eye lens is flattened, it has the smallest optical mass of 19 D. However, if the object were to approach less than 6 m, then the rays would appear as diverging. Based on this, there must be a change in the eye, otherwise the rays would cross behind the retina. Therefore, when looking at close objects, the optical power of the eye increases, and this process is called accommodation.

Close point is the place closest to the eye where we can still see the object clearly. The far point is the farthest point from the eye where we can still see the object clearly, for a legally functioning eye the far point lies at infinity. The accommodation mechanism is relatively simple. The lens is suspended by fibers on the suspension apparatus on the ciliary body, and when looking into the distance, the fibers are tense - the lens is flattened and its power is the smallest. During accommodation, the ciliary muscle contracts - the ciliary body moves forward and inward, which causes the tension of the fibers to relax, the lens to arch and increase its power.

Retina
Axons of glial cells converge to the papilla of the optic nerve ( discus nervi optici ) and leave the eyeball as the optic nerve. There are around 120 million rods and 6 million cones on the retina of one eye. The optic nerve contains 1 million axons. Rod photopigment absorbs light of wavelength 505 nm (blue-green), and cone photopigment absorbs light of wavelength 420 nm (blue), 531 nm (green), and 558 nm (red).

In the area of ​​the central pit, there is the greatest concentration of cones and rods are completely absent. Color vision is best perceived in this place, but at the same time there is lower sensitivity to light. At an angle of 20° from the central pit, there is the greatest concentration of rods and cones are almost completely absent. There is a high sensitivity to light in this place, but only monochrome vision is perceived here. The blind spot is located in the area of ​​the optic nerve papilla and the rods and cones are absent.

Signal transformation
The impact of appropriate light on the disk membrane causes the breakdown of photopigment (rhodopsin, which is composed of 11-cis retinal and opsin) - GP, which has the character of hyperpolarization. In the dark, sodium channels are open and the photoreceptor membrane is depolarized (−30mV). The light then decomposes pigment to 11-cis retinal, one of its intermediates activates transducin (g-protein), which activates photodiesterase. Photodiesterase then breaks down cGMP to GMP, then sodium channels close and hyperpolarize. In the dark, trans-retinal is then regenerated into 11-cis retinal. The alcohol trans-retinol - 11-cis retinol circulates in the blood and it is oxidized to 11-cis retinal.

Defects

 * Myopia (nearsightedness)

The bulb is too long for the optical power of the eye. Rays from a distant object converge in front of the retina. Therefore, the distant point is not at infinity, but at a distance of less than 6 m. Correction using scatterers (concave lens).


 * Hyperopia (farsightedness)

The bulb is too short for its optical power. Rays from a distant object converge behind the retina. So the distant point does not lie at infinity, but behind the retina. Correction using conjunctions.


 * Astigmatism

Under normal circumstances, the cornea is curved equally in all planes. If the cornea is curved more in one of the planes than in the others, it shows greater optical power in this plane. A person with this disease sees an object that is composed of vertical and horizontal lines clearly only horizontally. Even the cornea of ​​a healthy eye is not curved the same everywhere - physiological astigmatism. Correction using a cylindrical lens (with an overlap of 1 D).


 * Presbyopia

This is the loss of the accommodation ability of the lens. It can be corrected with couplings.


 * Black blindness

11-cis retinol is formed from vitamin A metabolites. Its deficiency leads to reduced production and this causes reduced sensitivity to light, especially at low light intensities.


 * Diabetic retinopathy

One of the most common diseases of the retina. Cells around small blood vessels ( pericytes ) form sorbitol when there is an increased need for glucose, which causes swelling and narrowing of vessels and subsequent tissue ischemia. The formation of angiotensin II, which stimulates the formation of VEGF - increased permeability of blood vessels and new formation of blood vessels and bleeding, cloudiness occurs.


 * Central artery occlusion

It leads to the destruction of amacrine, bipolar and ganglion cells and thus to blindness.

Related articles

 * Cataract
 * Glaucoma