Our sense organs give us information about the outside world and our own body. Yet "seeing" and "hearing" are completed not in the eye or ear but in the cerebral cortex. The exam often covers the structure of the eye and ear, the path of light and sound, vision defects and their correction, and the parts of an analyser.
§1. The analyser
An analyser (I. P. Pavlov’s term) is a system that receives and analyses stimuli. It has three parts:
Peripheral part — the receptors (for example, rods and cones of the retina).
Conducting part — nerve pathways (optic nerve, auditory nerve).
Central part — the relevant area of the cortex (vision — occipital lobe, hearing — temporal lobe).
Sensation arises in the cerebral cortex. Even with a healthy eye, a person cannot see if the occipital lobe is damaged.
Receptors are highly sensitive only to their own (adequate) stimulus: the eye to light, the ear to sound. With prolonged stimulation sensitivity falls — adaptation; for example, after a while we stop noticing a smell in a room.
§2. The visual analyser
Accessory structures: eyebrows, eyelids and eyelashes, tear glands (moisten and wash the eye and contain an antimicrobial substance), and the muscles that move the eyeball.
The eyeball has three coats:
Sclera (fibrous coat) — outer and tough; its transparent front part is the cornea.
Choroid (vascular coat) — rich in blood vessels; its front parts are the ciliary body and the iris (gives eye colour). The opening in the centre of the iris is the pupil; it constricts in bright light and dilates in the dark.
Retina — the inner coat, containing the receptors:
rods — vision in dim light (twilight), no colour; they contain the pigment rhodopsin, which needs vitamin A to form. Lack of it causes "night blindness";
cones — daylight and colour vision.
The yellow spot (fovea) has the densest cones and the sharpest vision; the blind spot is where the optic nerve leaves, with no receptors.
Optical (light-refracting) system: cornea → fluid of the anterior chamber → pupil → lens (biconvex) → vitreous body → retina. The image on the retina is reduced, inverted and real; the brain perceives it the right way up.
Accommodation is the change in the lens’s curvature by the ciliary muscle: when looking at a near object the lens becomes more convex, at a distant one flatter.
Vision defects:
Short-sightedness (myopia) — distant objects are blurred; the eyeball is too long or the lens too convex, so the image is focused in front of the retina. Corrected with concave (diverging) lenses.
Long-sightedness (hypermetropia) — near objects are blurred; the image is focused behind the retina. Corrected with convex (converging) lenses. With age the lens loses elasticity and long-sightedness increases.
Worked example
Through which structures of the eye, and in what order, does light pass? Where does the impulse go from the receptors?
Optic nerve → diencephalon (thalamus) → occipital lobe of the cortex.
Answer: in that order; the visual sensation arises in the occipital lobe.
§3. The auditory and vestibular analysers
Outer ear — the pinna and the external ear canal (with glands that secrete earwax). The eardrum lies between the outer and middle ear.
Middle ear — an air-filled cavity with three ossicles: the hammer (malleus), anvil (incus) and stirrup (stapes), the smallest bones in the body. They amplify vibrations and pass them to the oval window of the inner ear. The Eustachian tube connects the middle ear to the nasopharynx and equalises pressure on both sides of the eardrum.
Inner ear — a fluid-filled labyrinth in the temporal bone. The cochlea contains the organ of Corti with the hearing receptors (hair cells).
Path of sound: pinna → ear canal → eardrum → hammer → anvil → stirrup → oval window → fluid of the cochlea → organ of Corti → auditory nerve → temporal lobe of the cortex.
The vestibular apparatus (organ of balance) is also in the inner ear: the sacs of the vestibule (sense body position and linear acceleration) and three mutually perpendicular semicircular canals (sense rotation). Rocking can cause "motion sickness" — nausea.
Humans hear sounds from roughly 16–20 to 20,000 hertz. Loud noise damages hearing; cleaning the ears with sharp objects can injure the eardrum.
Worked example
When a plane lands, your ears feel "blocked", and swallowing clears them. Why?
Outside pressure changes quickly while pressure in the middle ear stays the same, so the eardrum is pushed to one side.
Swallowing opens the Eustachian tube, letting air into or out of the middle ear.
Answer: the pressure equalises and the eardrum vibrates normally again.
§4. Smell, taste, skin and muscle sense
Smell — olfactory cells lie in the mucous membrane of the upper nasal cavity; odour substances are sensed after dissolving in the mucus. The centre is on the inner surface of the temporal lobe.
Taste — taste buds in the papillae of the tongue; the basic tastes are sweet, salty, sour, bitter and umami. Taste and smell work together — with a cold, food seems "tasteless".
Skin senses — receptors for touch, pressure, warmth, cold and pain. Pain is a protective signal.
Muscle sense — receptors in muscles and tendons let us know the position of body parts even with eyes closed.
§5. Spectacle lenses, key numbers and care of the sense organs
The strength of a spectacle lens is given by its optical power, measured in dioptres (D):
D = 1 / F, where F is the focal length of the lens in metres. A converging (convex) lens has D > 0; a diverging (concave) lens has D < 0.
That is why a prescription for short sight shows “minus” dioptres, and one for long sight or age-related long sight shows “plus” dioptres.
Some important numbers:
the retina has about 120 million rods and 6–7 million cones; rods are more common towards the edge of the retina, while cones are concentrated in the yellow spot;
there are three types of cone (sensitive to red, green and blue-violet light); if one type does not work, a person cannot tell certain colours apart — colour blindness (it is X-linked, so it is more common in men);
dark adaptation (the build-up of rhodopsin in rods) takes several tens of minutes — so when we walk from a bright room into a dark one, at first we see nothing.
Care of the eyes: hold a book about 30–35 cm from the eyes, let the light fall from the left, rest the eyes every 30–40 minutes (look into the distance), do not read lying down or in a moving vehicle, and do not rub the eyes with dirty hands.
Care of hearing: loudness is measured in decibels (dB). Ordinary conversation is about 60 dB; prolonged noise above 85 dB (a noisy workshop, loud headphones) damages the hair cells of the organ of Corti, and they do not recover. Inflammation of the nasopharynx can spread through the Eustachian tube to the middle ear and cause otitis.
Worked example
A patient’s spectacles are −2.5 D. What kind of lens is this, what is its focal length, and what defect does it correct?
D < 0 → a diverging (concave) lens.
F = 1 / D = 1 / (−2.5) = −0.4 m (−40 cm).
A concave lens moves the image from in front of the retina onto the retina.
Answer: a concave lens, F = −0.4 m; it corrects short sight (myopia).
Check yourself
Which receptors provide vision in the dark, and which vitamin do they need?
Where is the image focused in myopia, and what lens is needed?
Why is there no vision at the blind spot?
Name the ossicles of the middle ear in order.
Where are the hearing receptors located?
Which structures sense rotation?
A converging lens has a focal length of 0.5 m. What is its optical power?
What is the inability to tell colours apart called, and why is it more common in men?
Common mistakes
Writing that rods detect colour — cones do.
Correcting myopia with convex lenses — it needs concave ones.
Confusing the blind spot with the yellow spot.
Placing the hearing receptors in the middle ear — they are in the inner ear.
Saying the Eustachian tube connects to the outer ear — it links the middle ear with the nasopharynx.
Locating the visual centre in the temporal lobe — it is in the occipital lobe.