Sense Organs: Eye, Ear, Nose, Tongue and Skin

The sense organs are the parts of the body that receive information from the surroundings and send it to the brain as nerve impulses. There are five of them in the human body: the eye for sight, the ear for hearing and balance, the nose for smell, the tongue for taste and the skin for touch, pressure, heat, cold and pain. Each organ carries a particular kind of receptor, and in every case the organ only collects the stimulus; the sensation itself is produced in the brain. Questions from this topic are among the easiest marks in the general science paper, because they turn on names of parts and their functions rather than on calculation.

The eye

The human eyeball has three coats. The sclera is the tough white outer coat that protects the eye, and its transparent front part is the cornea, where most of the bending of light takes place. The choroid is the middle coat, dark and rich in blood vessels, and its front part forms the iris, the coloured ring that controls the size of the pupil and so the amount of light entering the eye. The pupil is only the hole in the middle of the iris. Behind it lies the crystalline lens, a transparent biconvex structure held by ciliary muscles that change its curvature to focus on near and distant objects, an adjustment called accommodation. The space between the cornea and the lens holds a watery aqueous humour, and the larger chamber behind the lens holds the jelly-like vitreous humour.

The retina is the innermost, light sensitive layer, and it carries two kinds of photoreceptor cells. Rods contain the pigment rhodopsin, also called visual purple, work in dim light and give vision in black and white. Cones contain iodopsin, work in bright light and give colour vision; they are of three kinds, sensitive to red, green and blue. The yellow spot, or macula, has the densest packing of cones and is the point of sharpest vision, while the blind spot is the place where the optic nerve leaves the eyeball and has no rods or cones at all, so an image falling there is not seen. The image formed on the retina is real and inverted, and the brain interprets it as upright.

Common defects of vision

The ear

The ear is both the organ of hearing and the organ of balance, and it has three parts. The outer ear is made of the pinna, which collects sound, the auditory canal and the eardrum or tympanic membrane, which vibrates when sound strikes it. The middle ear is an air filled cavity holding the three smallest bones of the body, the malleus or hammer, the incus or anvil and the stapes or stirrup; the stapes is the smallest bone in the human body. These bones carry the vibration from the eardrum to the inner ear and magnify it. The Eustachian tube runs from the middle ear to the pharynx and keeps the air pressure on the two sides of the eardrum equal, which is why the ears pop in an aeroplane or a lift.

The inner ear has two working parts. The cochlea is a coiled, fluid filled tube that contains the organ of Corti, where vibrations are turned into nerve impulses and carried to the brain by the auditory nerve; this is the part that hears. The three semicircular canals, set at right angles to one another, along with the utricle and saccule, sense the position and movement of the head and maintain balance. A healthy human ear can hear sounds roughly between twenty hertz and twenty thousand hertz.

The nose and the tongue

Smell and taste are chemical senses: the receptor is stimulated by molecules dissolved in moisture rather than by light or pressure. The olfactory receptors lie in a patch of epithelium in the roof of the nasal cavity, and they are the most sensitive of the human receptors, able to detect very small quantities of a substance. The tongue carries taste buds, mostly on small projections called papillae, and the receptors in them are the gustatory receptors. Five basic tastes are recognised: sweet, sour, salty, bitter and umami, the savoury taste. The old diagram that puts each taste in a separate zone of the tongue is not correct; all five tastes can be detected over the whole surface where taste buds occur. Much of what is called the flavour of food is actually smell, which is why food seems tasteless when the nose is blocked by a cold.

The skin

The skin is the largest organ of the human body and the organ of touch. It has an outer epidermis, which has no blood vessels and whose surface cells are dead and keratinised, and a thicker dermis below it, which carries blood vessels, nerve endings, hair follicles, sweat glands and sebaceous glands. Different receptors handle different sensations: Meissner's corpuscles respond to light touch, Pacinian corpuscles to deep pressure and vibration, separate endings to heat and cold, and free nerve endings to pain. The pigment melanin, made by cells called melanocytes, gives the skin its colour and shields it from ultraviolet rays. The skin also regulates body temperature through sweat and the blood vessels of the dermis, and it makes vitamin D when sunlight falls on it.