Jethalal S. Zaveri
The human body is equipped with a versatile assortment of sensory outposts, making up considerably more than the traditional 'five senses'. Without a continued flow of information the brain would be cut off not only from the outside world, but also from an awareness of the body's internal environment. Our eyes, ears, nose, mouth and the entire body surface are endowed with a vast variety of receptors for collecting useful information from outside. The inner senses such as muscle tone enable us to know where the various body parts are, as well as what the current state of the various body system is.
Touch sensations result from stimulation of tactile nerves, and convey information on the size, shape, texture and localization of objects.
Sense of smell functions in attraction of food and warning of danger. Taste nerves (taste buds,) respond to chemical substances dissolved in saliva on the surface of the tongue.
A large portion of the stream of incoming impulses consists of messages from the eyes. Human eyes are capable of not only detecting light, but of distinguishing between different colours, shades of colours and degrees of light and darkness. Analysed and integrated, these messages put together a picture of the outside environment.
The ears are two organs in one. They are the organs of hearing—the perception of sound. They also contain receptors for the sense of equilibrium—the perception of changes in position and movement of the body.
Human senses include;
Sense of Hearing
Sense of Sight
Sense of Smell
Sense of Taste
Sense of Touch
Sense Receptors
The body's sense receptors are the peripheral endings of functional dendrites of sensory neurons. Each receptor is specialized to respond to a particular type of stimuli.
Modality of Sensation and Adaptation
Sensation is the conscious result of the sequence: stimulus→receptor→conducting pathway→sensory area in the brain,
All main types of sensation—sight, sound, etc. are produced by nerve fibres that transmit impulses. Those from the ear are connected by pathways to auditory area; those from the retina terminate in the vision area; and touch fibres to specific touch areas in the brain. Thus we actually perceive all sensations in the brain.
Intensity of a sensation is directly proportional to the strength of the stimulus. Our receptors operate efficiently over a wide range of intensities. For example, we can hear a barely audible whisper and can also distinguish the words blaring out of a loudspeaker.
When a stimulus of constant intensity, continues to act on a receptor, it 'adapts' to the stimulus decreasing the frequency of the response. Thus in a swimming pool, at first the water may feel cold but soon it will feel comfortable
l. SIGHT AND THE EYE
The eye is a special organ of the sense of sight. Man depends on sight more than upon any other sense to supply information about his environment. The mobility of his eyes and head and stereoscopic[1] vision give him a panoramic view of the world i.e. information on depth, distance, dimension and movement Eye's sensitivity to colours and contours enables him to distinguish between different colours and shades of colours. In addition, eyes can be extremely expressive of moods and emotions.
Structure of the Eye
The Eyeball is situated in the orbital cavity. It has a highly complex structure. It is almost spherical in shape and is approximately 2.5 cm. in diameter. The bony walls of the cavity and the fatty cushion help to protect the eye from injury. Further protection is provided by eyelids, eyelashes and eyebrows.
Structurally, the two eyes are the same but some of their activities are coordinated so that they function as a pair.
The anterior one-sixth of the eyeball is covered with a transparent layer called cornea, while the posterior five-sixths is covered by a tough fibrous layer called sclera. The Cornea, the bulge at the front is a window that lets light rays into the eye and bends or refracts them. A flat circular coloured membrane, the iris, lies behind the cornea and gives the eyes their characteristic colours. Between the cornea and the iris is a small compartment containing a clear fluid, the acqueous humour, which nourishes the cornea.
The iris governs the size of the pupil, a small adjustable hole in its centre that regulates the amount of light entering the eye. The crystalline lens, lying behind the pupil, further refracts the light to fucus a sharp image on the retina, the inner layer of the wall ol the eyeball. This thin screen, containing specialized photo-receptor cells, transforms light energy into electrical messages that are transmitted to the brain by the optic nerve which runs from the back of the eye. A large compartment, containing a viscous fluid called the vitreous humour, lies between the lens and the retina and makes up most of the volume of the eyeball. The optic and other nerves as well as arteries that supply the eye-muscles pass through two openings at the back. Six external muscles connect the eyeball to the orbital cavity and provide movement and support.
Physiology of Sight
Light is necessary for seeing. Light is reflected into the eyes by the various objects within the field of vision. The eye uses refraction[2] to focus the light rays it receives from the object to the retina. The four refracting media of the eye are, the cornea, the aqueous humour, the crystalline lens and the vitreous humour. The process of focussing begins when the light passes through the cornea and is refracted. This is referred to as the coarse focus. The aqueous humour which is a clear dilute solution of salts (mainly sodium chloride) is renewed every few hours. It has hardly any effect on the light rays as it is of a similar density to the cornea. Its main function is to nourish the internal structures of the eye that do not possess a blood supply of their own.
The pupil determines the amount of light let in as a result of the contraction and expansion of the muscles of the iris. The crystalline lens lies immediately posterior to the iris- It is a semisolid body with biconvex surface consisting of 2000 thin layers of transparent tissue enclosed in a thin elastic capsule. To keep the object focussed on the retina the lens thins for distant vision and thickens for the near vision. In old age, it becomes denser and less elastic. As a result, old people need glasses for reading etc. In cataract it loses its transparency and blocks the passage of light rays resulting in a loss of vision Cataract is treated by removing the opaque lens and fitting an artificial intra ocular lens or compensating with external glasses.
The vitreous body is a jelly-like material enclosed in a thin membrane and fills the posterior four-fifths of the eyeball. Its function is to nourish and support the retina (which will collapse inward otherwise) and maintain the spherical shape of the eyeball. It does not interfere with light passing through it.
Accommodation
In a camera, the distance between the lens and the film is adjusted until the correct focus is obtained. In the human eye, the distance is fixed. Instead, the curvature (shape) of the lens itself is changed. This adjustment for near and distant vision is called accommodation.
For focussing on a distant object, the lens becomes flatter while to view a nearby object it bulges, becoming, more convex.
The Pathway of Vision
An optic nerve from each eye passes into the cranial cavity, converge briefly, separate again after a partial crisscrossing and continue towards the visual area of the cerebral cortex in the occipital lobes of the cerebrum. The images from the left half of the visual field are transmitted entirely to the right hemisphere while those from the right half to the left one. Since there is continual interchange of information between the two hemispheres, a whole image is perceived rather than two halves.
Rods and Cones
The retina, the screen on which light rays are projected is a network of two types of light sensitive receptors, rods and cones, There are about 125 million rods and 5.5 million cones. The cones operate in bright daylight while the rods are used for seeing in dim light.
Colour Vision
Humans possess the ability to see a full range of colours of the rainbow. White sunlight is a mixture of seven colours called the visible spectrum. The red rays have the longest wave-length while the violet rays are the shortest. The human eye can distinguish not only these basic colours but also more than seventeen thousand intermediate hues.
Colour-blindness occurs when one or more types of cones is absent. In the red-green colour blindness either the red or the green cones are missing. It is a hereditary condition and appears far more frequently in males than in females.
2. HEARING AND THE EAR
Sound
Sound itself is both a physical and psychological phenomenon. Waves of sound caused by a vibrating object are basically minute changes in air pressure. The waves have no significance, no message until they reach the ear. The hearing process is a chain of events in which the waves are conducted through the ear and translated into nerve impulses for interpretation by the brain, The hearer instantly distinguishes the meaning of those signals.
The Three Compartments of the Ear
The ear is divided into three sections:
The external ear
The middle ear, and
The internal ear.
The outer ear consists of the auricle which acts as a sound-gathering funnel and the ear-canal which leads to the eardrum.
The middle ear begins at the tympanic membrane or eardrum. The tympanic cavity is air-filled and the internal air pressure is maintained at the same level as the external atmosphere. It contains the ossicles, three tiny bones named for their distinctive shapes: the malleus (meaning hammer), incus (anvil) and stapes (stirrup). The three ossicles stretch from the eardrum to the oval window, an opening in a very thin bony wall which seperates the middle and the internal ear. The three bones are linked together but not rigidly, so that the vibrations of the eardrum are magnified in force by the jiggling motion of the ossicles. The amplified vibrations of sound pressure are transmitted to the fluid-filled inner ear via the oval window-
The internal ear contains the organ of hearing and is described in two parts:
The bony labyrinth.
The membranous labyrinth.
The membranous labyrinth has the same shape as the bony labyrinth and fits into it like a tube within a tube. Both are filled with fluid. Both consists of three parts: a vestibule, a cochlea and three semicircular canals. The receptors for hearing are found within a complex structure which lies within the cochlear duct. More than 20,000 stiff hair-like fibres run across the floor (Basilar membrane) of the duct. Their lengths increase progressively from about 0.04 m.m. to 0 5 m.m. They can vibrate like the reeds of a harmonica; shorter ones at high frequency and longer ones at low frequency. The transmitted sound waves set different portions or the basilar fibres vibrating, stimulating different cells of the receptors which relay their impulses along the auditory nerve to the brain.
The Physiology of Hearing
Sound waves enter the external auditory canal and strike the eardrum setting it vibrating. The vibrations are transmitted to the three tiny ossicles—the hammer, the anvil and the stirrup—in sequence. The foot plate of the stirrup presses against the covering membrane of the oval window, setting up pressure waves in the fluid which winds through the spiral coils of the snail. The pressure produces a wave that travels along the floor i.e., the basilar membrane. This looks something like a long xylophone which gets wider as it stretches out along the coil of snail. At the end, it is 12 times as wide as at the base near the oval window. Each sound sets up sympathetic (resonant) vibrations in a particular place. These, in turn, stimulate the hair cells generating a receptor potential, which activates neurons of the cochlear nerve. The impulses transmitted to the brain carry data on the place of the basilar membrane to enable the cerebral cortex to determine the frequency of the sound.
Nervous Pathway for Hearing
Like the messages of the other sense organs, the sounds detected by the ears are not meaningful, until they are analysed and interpreted in the brain. The main nervous pathway for hearing go upward to the cerebral cortex in the upper part of the temporal lobe. Each ear sends impulses to both sides of the brain and even a total destruction of the hearing centre in one hemisphere would not interfere with hearing.
Sense of Equilibrium
Ears are two organs in one. Besides being the organs of hearing, they also contain receptors for the sense of equilibrium. Two small membranous sacs in the labyrinth of the inner ear are the organs of static equilibrium, and the three semicircular canals are those of dynamic balance. A movement of the head in any direction will set the fluid in at least one of the canals in motion stimulating the hair cells and initiating impulses that are relayed to the brain. To maintain the body's equilibrium corrective reflexes are initiated in the cerebellum.
SENSES Of TASTE AND SMELL - -
TONGUE AND NOSE
Taste and smell are both chemical senses, that is they are triggered by the chemical content of substances in the environment. Both senses are very much interconnected.
The Taste Buds
The tongue is a most versatile organ. Its dexterity permits it to function in speech, chewing swallowing and sucking besides tasting. Taste buds, the specialized gustatory receptors are situated on the upper surface of the tongue, soft palate and epiglottis. A taste bud consists of a small bundle of cells which have hair-like dendrites protruding through tiny pores into the mouth cavity. Taste impulses from different parts of the tongue etc. are transmitted to the cerebral cortex by the nerve fibres at the other end of the taste cells.
Physiology of Taste
In order to be tasted the chemicals of the food must be dissolved in the fluid medium of the saliva. There are five primary tastes: sweet, sour, salt, bitter and pungent (that of chilli). The large variety of other tastes are either a combination of two or more of these or are associated with the sense of smell. By far the greatest influence on the sense of taste is the sense of smell. What is generally referred to as the taste, is strictly the flavour which is combination of its taste and smell. Food tends to lose much of its taste when the nasal passages are blocked as with a head-cold.
Taste receptors are particularly sensitive to bitter taste. This is an important protective mechanism because many deadly toxins are bitter and are automatically rejected.
A loss of taste is called hypogeusia; in dysgeusia, things taste wrong or even offensive. Research indicates that trace metal zinc is involved in the proper functioning of the taste buds.
The Nose and Olfactory Receptors
The human nose can detest lower concentration of a volatile substance than are detectable by a gas chromatograph. The sensation of smell requires an actual contact of odour-producing substance with the receptors.
The hair-like dendrites of about 100 million smell receptors occupy an area about the size of a postage stamp in the uppermost portion of the lining of the nasal cavity. The olfactory tract leads into the olfactory area in the cortex of the temporal lobe where the impulses are interpreted.
Physiology of Smell
In order to stimulate these receptors, a substance must be volatile so that it can be inhaled into the nostril. It must at least be slightly water soluble so that it can dissolve in the mucus coating of the membrane.
The multitude of distinct odours that can be recognised, represent various combination of seven primary classes of odour viz. camphoraceous, musky, floral, pepperminty, etherlike, pungent and putrid.
The human sense of smell is almost rudimentary in comparison to that of other animals. Again, it serves more important roles for them than for humans. They secrete odorous pheromones as media of communication, 'no trespassing' sign and sex attractants. However, smell serves a number of functions in human life. The aroma of appetizing food starts a flow of saliva and tones up the digestive organs. It can warn of danger such as toxins lurking in spoiled food. A particular perfume or smell can unlock a whole scene of distant past from the filing cabinet of the brain. Olfactory receptors adapt very rapidly and if the same odour persists, one ceases to notice it.
4. SENSE OF TOUCH ETC.
The sense of touch is the most basic means by which a person makes contact with the world around him.
The ability to feel shapes and textures provides the brain with more precise information than the senses of sight and hearing. By holding an object (with closed eyes) the fingers can tell its size, general contours as well as whether it is rough or smooth, hard or soft, wet or dry. Unlike the other four senses, touch responds to more than one type of energy stimulus, temperature and pressure. Moreover, the sense-organs for touch are distributed all over the body. Thus the sense of touch is more than a single sense.
Sense of touch, pressure, heat, cold and pain are provided by a variety of specialized sense-receptors, which are not uniformly distributed over the surface of the body. While pain receptors are the most numerous, heat-receptors are the most sparse. While touch receptors are in the skin or in the tissues immediately beneath the skin, pressure results from the deformation of deeper tissues. While some types of tactile receptors adapt slowly, others adapt within a fraction of a second. But for the rapid adaptation of many touch receptors, we would be unable to wear clothes.
Touch signals pass through the relay station of the thalamus and reach the appropriate area of the cerebral cortex in the parietal lobe.
Besides providing information about the temperature, texture and weight of an object, the sense of touch performs other important functions in our life. Shaking hands, hand holding, caressing and kissing are some examples of tremendous emotional impact.
Pain
Sensation of pain can be extremely useful. It is the body's warning signal for urgent corrective action to prevent danger to the tissues. Pain receptors are stimulated by intense pressure, heat or cold, if tissue damage is-produced. There is little or no adaptation of these receptors. Pain impulses are transmitted to the thalamus and relayed to the cerebral cortex for recognition of the kind of pain and localization. Psychic reactions to pain, such as anguish, anxiety, are modified by one's personality, emotional state, ethnic and cultural background.
Temperature
The thermal receptors adapt quickly so that a hot bath or a cold swimming pool soon becomes bearable. They are non-uniformly distributed over the body. The lips and mucous membranes of the mouth and rectum are highly temperature-sensitive but other mucous membranes-of the body are insensitive to temperature.
5. VISCERAL RECEPTORS
Sense receptors described above bring in information about the external environment. But the brain needs a continued flow of information about what is happening inside the body as well and the internal organs are also supplied with receptors which are called visceral receptors. They are concerned with respiration, heart-beat, size of the blood vessels and such other vital functions. Usually the conditions are attended to automatically. Mild problems such as hunger or a feeling of fullness in the bowels or bladder, are readily remedied. Visceral pain, on the other hand, may call for a medical assistance.
6. THE KINESTHETIC SENSE
The term Kinesthesia is used to denote the conscious recognition of the orientation of the different parts of the body. This 'position sense' is provided by specialized proprioceptors[3] located in the joints, capsules, ligaments, muscles and tendons. Impulses are transmitted to the cerebellum and result in reflex adjustment of the muscles. All kinesthetic transmissions are performed at a high speed.
