Anatomy and physiology of the human eye
Neurologist or neurosurgeon at eye examination can directly observe the vascular and nervous tissue. Eyeball is the only one organ that due to its anatomy allows the doctor to see the nerve and vessels, without executing it and no pre-cuts or punctures healthy tissues of the patient.
Eye - an organ of vision. The eyeball designed to focus light on a highly sensitive neuronal membrane - the retina. Going into the eyeball, light first passes through the cornea, aqueous humor, lens and vitreous body, then crosses the transparent retinal layers and reaches photoreceptors in the outer nuclear layer.
External and internal anatomy of the human eyeball in the section.
Amount of light entering the lens required to produce an image on the retina, is controlled by the diaphragm. Performs the role of such a diaphragm iris. Aperture of the iris - the pupil - may narrow or expand with the help of special muscles of the iris. Caught on the retina photoreceptor cells captured light fundus. These nerve cells in the retina called rods and cones. The rods and cones of the retina contain the visual pigment. This visual pigment allows them to capture the luminous flux, consisting of photons. Physiological reaction occurs neural excitation and inhibition in complex synaptic levels of retinal cells. This allows you to evaluate the properties of light entering the eye to spatial position, the light and the spectral and temporal features.
Rods and cones located in the retina, different in function. Rods capture light of low intensity (scotopic vision) and do not participate in the determination of color. Cones react to light of high intensity (photopic vision). The cones also have good resolution and color vision to participate. Cones galore located in the center of the retina in the macular area, which consists of the central fossa and the smallest circular dimples. The fovea is a distance of 3 mm from the edge toward the temple of the optic nerve. At this point, note the great acuity (normally 20/20). Visual acuity decreases sharply in paramacular zone, where the number of cones is becoming a much smaller. In the human retina number of rods outnumber cones (100 million rods, cones 60 million). Rods missing in fovea, their concentration reaches a peak at a distance of 20 ° from the fovea, gradually decreasing toward the periphery.
External and internal structure of the human eyeball (side sectional view).
The distribution of ganglion cells has the same character as that of the cones. In the fovea one ganglion cell through diaxon forms a bond with one cones 1:1 ratio maximizes resolution. Primary processing of visual information takes place in the retina, and then it transmitted in the form of electrical impulses from ganglion cells in their nerve fibers in the optic nerve to the lateral geniculate body of the brain. After synaptic switching fibers pass through the geniculate occipital way to the visual centers in the cortex of the occipital lobe of the brain.
Vision divided into shaped, color and light perception.
In clinical practice, shaped vision is evaluated using visual acuity, tests on the functioning of the macula, and it should be part of a full medical examination, regardless of whether or not there are relevant complaints. Snellen (1862) located at a distance of 6 m from the patient. Sivtsev table used to determine the visual acuity in the USSR, at a distance of 5 m from the patient.
Snellen test card consists of letters of different sizes. The distance by which each dimension reduced by the angle 5° indicated on the tableside. Patient-corrected refractive error during the study must wear glasses. Normal vision was 20/20. If the patient can only read the letters to the line 20/30 acuity defined as 20/30. If the patient is unable to discern the greatest letter E on the top line, it must be land closer to the table, thus changing the distance. Visual acuity defined as 10/400, if the patient is able to identify the letter at a distance of 3 meters away from the table.
Using the Snellen test card (located at a distance of 6 m from the patient) or Sivtsev table (located at a distance of 5 m from the patient) define human visual acuity.
If the patient cannot read a line of 20/30, it is necessary to explore stenopeic vision. Through stenopeic hole, passed a narrow beam of light rays, the patient with a secondary decrease in impairment of refraction must read line to 20/20. If this visual acuity does not intensify, should look for another cause of its decline, for example blurred eye media, stains or optic nerve damage.
Visual acuity, corrected by glasses or contact lenses only to 20/200 or less on both sides, as well as concentric narrowing of the visual field to 10 ° in the U.S. officially consider blindness, such a patient should be registered with the Society of the Blind in the community.
Often visual impairment characterized by acquired defects of color perception. For example, in some cases, macular lesions (due to intoxication or degenerative causes) or optic nerve (multiple sclerosis, toxins, drugs, malnutrition, tobacco-alcohol amblyopia) patients do not distinguish between red and green colors, although white color normally perceive.
For the study of color vision is most commonly used polychromatic Ishihara test. Polychromatic Ishihara test help identify visual defects on the red and green colors, shapes and Hardy Rand and Rittler (HRR) that reveal the lesions of perception of red and green and blue and yellow colors. To work in some specialties person needs fully preserved color vision. It is also possible hereditary blindness in red, green and other colors (color blindness).
When macular lesions resulting from intoxication or degeneration patients do not distinguish between red and green, but the white color while normally perceive.
Human light perception evaluated by examining the visual fields. The study process called perimetry visual fields. Changes in the visual fields indicate the portion of the optic tract lesion on the retina (the optic nerve) to the visual cortex. Most convenient for the study of the visual fields method kinetic perimetry (hemispherical perimeter or Goldmann visual field). He consists in moving object in the view and setting points of equal sensitivity in the two fields. Any patient with perimetry beeps when it sees an object indicates when he disappears and then reappears when. Thus, the circuit can be composed of fields of view of a patient with a precise indication of defects from the periphery to the central fixation point. You can also compare the peripheral field of view of the patient and the doctor.
Visual field examination reveals its defects and their location during the diagnostic procedure perimetry.
Field of vision considered normal if a person sees an object for diagnostics field of view during perimetry:
- 90° — when perimetry from the temple;
- 50° — when perimetry from the nose;
- 50° — perimetry to the upward;
- 65° — perimetry to the bottom.
This visual impairment may occur in patients as each individually and in combination. Such visual impairment in patients may result from changes in the refractive (refractive) properties of transparent media of the eye, retinal lesions of the optic nerve and other parts of the brain with which they are associated.
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