Without the complicated system of organs that perceive movement, orientation and rotation, balance would be impossible. The system through intricate, interlinked mechanisms, involving the eyes, ears, sensory nerves and muscles, enables accurate control of posture and most movements. That part of the inner ear that is responsible for our appreciation of movement and position in space is known as the ‘non-auditory labyrinth’ and consists of the semicircular canals, the utricle and the saccule.
The semicircular canals
The three semicircular canals, containing a fluid called the endolymph, lie in three planes. Two of them arch upwards in the forwards and sideways planes whereas the third lies horizontally. Each canal expands into a duct that contains an area of specialized haircalled the crista. The ends of the hairs are embedded in a small gelatinous structure called the cupola, which almost fills the cavity of the ampulla. When movement of the endolymph occurs within the canal, the cupola is displaced and the hairs are bent, giving rise to nerve impulses from the at their base.
What causes the endolymph to move is movement of the head. This effect may be better understood by likening the skull to a glass of water standing on a record player’s turntable. When the record deck is started the water will tend to remain still in the glass for a short period because it has its own inertia. The water will then slosh back in the glass, causing a differential between the movement of the glass and the water. If the inside of the glass were lined with hairs, like the hairs in the crista, the hairs would be bent backwards by the sloshing water. This simple example demonstrates only movements in the horizontal plane and by analogy therefore refers only to the horizontal semicircular canals, although a similar mechanism will apply to the other canals in response to rotation in different directions. The three directions are the same as those that a ship can pass through: pitch, roll and yaw. Pitch is an up and down motion from front to back, roll is rotation around its long axis and yaw is when the ship moves from side to side.
Returning to the record player analogy, if the glass is allowed to rotate for a while, the water in it will eventually rotate at the same speed as the glass and there will no longer be any movement differential between the two. If the turntable is stopped, the water, again because of inertia, will continue to move although the glass is now still. The imaginary hairs will again be bent, this time in the direction of the original rotation. The analogy is important as it shows that the organs of balance can register an apparent movement when the original impetus stops. This fools the brain and explains why we feel giddy when we stop being spun round and step off a roundabout.
Eyes, fast forward and slow back, known as nystagmus, and is a reflex action. If an incorrect signal is received from the semicircular canals when our head is actually at rest , then the reflex will operate and nystagmus occurs. This movement of the eyes makes the environment appear to spin and the result is vertigo, or giddiness.
On board ship in a rough sea sensations arrive at the brain in confusing quantity. Pressure on the feet and the position of the joints is changing constantly. The horizon is swinging up and down, and messages from the inner ear are varying from second to second. All these discordant impulses arriving at the brain centres cannot be properly processed and will give rise to the extremely unpleasant effects of seasickness. On the other hand, deprivation of any of the normal sensations, as in the weightlessness of outer space, or in the dark silent world of the deep-sea diver, may also give rise to disorientation and vertigo. Why some people are more prone to motion sickness than others is not known for certain, but it is thought that people can get used to unusual movements. Dancers, sailors and astronauts are all, after long training, able to tolerate any but the most abnormal stimulation of the inner ear.
The utricle and saccule
The utricle and saccule also each contain a patch of hair-bearing epithelium, the macule, and the ends of the hairs are again embedded in a gelatinous mass. This mass is different from the cupola of the semicircular canals, however, because it contains a number of chalky crystals, called the otoliths. These are relatively heavier than the endolymph so that the pull of gravity on the otoliths during movement in a straight line, or acceleration in a straight line, called linear acceleration, in any direction, distorts the hair cells and gives rise to nerve impulses at their base. In the utricle, the macule is in the horizontal plane whereas that of the saccule is in the vertical plane, and the different sets of nerve impulses from the macules of each side are correlated in the brain to give us our sense of position and orientation.
The vestibular nerve
The nerve fibres from the sensory hair cells in the crista of the semicircular canals join the macule of the utricle and saccule are collected together to form the vestibular nerve. This joins the cochlear nerve in the internal auditory canal where the combined nerve is known as the VHIth cranial or auditory nerve. The auditory nerve leaves the petrous bone and passes into the brain-stem where it divides into separate vestibular and cochlear pathways. The central connections of the vestibular nerve are complex, and nerve fibres go to many of the vital centres of the brainstem, mid-brain and forebrain. Three main ones are: 1. nerve centres of the Illrd, Ivth and Vlth cranial nerves, governing the movement of the eyes; 2. that part of the spinal cord responsible for muscle reflexes and muscle tone; 3. the cerebellum, which co-ordinates and adjusts movements of the limbs and the body as a whole.
The semicircular canals, utricle and saccule gather information about movements and position. But other sense organs play a large part in the perception of movement and maintenance of balance. Of these the eyes are probably the most important. Our sense of vision is constantly monitoring the environment to estimate our position and movement by reference to surroundings. Also, sensation from the skin of the feet when standing, the hands, the buttocks when sitting, and messages from the muscles, tendons and joints are all co-ordinated by the brain to give us an overall impression of our orientation and movement. Should any of these mechanisms fail we would lose balance and our co-ordination would fail also. When the head is rotated the eyes attempt to retain a ‘fix’ on the environment and will therefore hold one object in view, remain fixed there as rotation continues and then jump forward to focus on another object. This gives rise to a rhythmic movement of the
Smell and taste
The senses of smell and taste, although anatomically quite separate, are nevertheless closely associated with each other. They are both stimulated by the presence of chemical substances either in the air or in our food, and they both play an important part in starting the digestive processes – the smell or the first taste of a palatable food is sufficient to increase the flow of saliva, the secretion of insulin and digestive enzymes, and the movement of the muscles lining the stomach and intestines.
In terms of sensitivity, the range of sensations detectable by the nose is far more extensive and sophisticated than the taste buds, which can detect only four basic ‘tastes’.
Our appreciation of the flavour of food, and our ability to differentiate between the flavours of different foods, is the result of a combination of the sensations of taste and smell, together with sensations of texture and temperature from the teeth, tongue and cheek lining. If we have a cold and our sense of smell isthen food ‘loses its flavour’. Also, people wearing dentures for the first time often complain that food has become insipid and many foods, delicious when hot, may be quite unpalatable when cold. The sense of smell may also be of use to warn us of potential dangers in the environment such as infected food or escaping gas. The smelling sense of the dog is 260 times more sensitive than that of humans. In many animals smell plays an important part in reproduction, usually by the secretion of odour-producing chemicals called pheromones. These are used in the detection, recognition, attraction or stimulation of a sexual partner and if one accepts the claims of some perfume, after-shave and body-oil manufacturers it is possible that this mechanism has not been entirely lost in humans.
One interesting phenomenon linked with the sense of smell is the fact that occasionally certain smells can evoke very strong memories. These memories are normally associated with either childhood, or times when the emotions were strongly aroused. This is thought to occur because the part of the brain associated with smell perception is relatively ‘primitive’ in evolutionary terms, and this has led researchers to conclude that smell may be closely linked to areas of the brain involved in emotion.
The olfactory membrane
Located in the upper part of the nasal cavity is a small area of sensory tissue or epithelium called the olfactory membrane. It consists of some 10 to 20 million smell-detecting cells in a supporting layer of mucus-secreting cells and glands. These are the olfactory cells and they combine the function of both sensory receptor and nerve cell. They have slim outgrowths from their surfaces which project through the surface membrane into the air that enters the nose. Nerve fibres from these olfactory cells collect together into a number of small bundles which pass through small holes in the bony roof of the nose directly to the olfactory bulbs of the brain. The olfactory bulbs are in turn connected with the olfactory cortex on the inside of the temporal lobes of the brain and extend to the neighbouring cortex.
A great many substances give off particles that can be detected – ‘smelled’ – by the olfactory cells, but the precise means whereby the olfactory cells are able to detect and recognize the vast number of different odours with which we are familiar is not yet clear. Scientists think it probably involves recognition of differences in the density of air. Existing research indicates that there are about seven primary sensations of smell.
The taste buds
Our sense of taste arises from taste buds. In children these occur mainly on the tongue but to some extent on the palate, the lining of the cheeks and the upper throat. Their numbers and distribution decrease with increasing age until in the adult there are only 9,000 taste buds which are largely confined to the tip, edges and back of the tongue.
Each taste bud in the human adult consists of a small oval chamber, open at the surface. This chamber contains 15 to 20 spindle-shaped taste cells, each of which ends in a fine projection sticking through the surface pore. These taste cells reproduce themselves so quickly that in a child or young adult each taste bud has a completely new set of taste cells every seven days. Nerve fibres come from the bases of the taste cells and collect to join other nerves that then lead to the taste centre of the brain.
The four sensations of taste – sweet, salt, sour (acid) and bitter – are recognized on different parts of the tongue. The tip of the tongue is mainly responsible for detecting sweet tastes, the edges of the tongue for salt and sour, while the back of the tongue carries the receptors for bitter. A bitter pill is tasted when it is too late, just before it is swallowed. Saliva plays an important role in taste perception because particles of food have to be moist in order to come into close contact with the taste cells.