Every living organism is sensitive and responds to changes in the external and internal environment of the body. The external and internal stimuli are received by varied types of receptors or sense organs. Every sense organ is supplied with the sensory nerve fibres which convey these stimuli to the C.N.S. These sense organs are grouped into three divisions—
On the basis of their position :
(i) External receptors or exteroceptors : Situated on the outer surface of the body.
(ii) Internal receptors or interoceptors : Situated within the digestive system.
(iii) Proprio-receptors : Situated internally but other than digestive system.
On the basis of their function :
(i) Proprioceptors : Present in the muscles, tendons and joints.
(ii) Noiceptors : These are the pain sensory receptors present in deep position.
(iii) Teleceptors : Vision and statoacoustic organs.
On the basis of nature of stimuli :
(i) Chemo-receptors : Olfactory, gustatory, common chemoreceptors.
(ii) Radio-receptors : Photorecep-tors, noiceptors, thermoreceptors.
(iii) Machano-receptors : Presso-receptors or baro-receptors.
Sense organs : Eyes, ears, nose, tongue and skin.
Orbits : These are the deep cavities in which eyes are lodged.
Eye
Eyes are the sense organ of vision or sight.
Accommodation : It is the unique ability of the eye lens to adjust its shape to focus the objects of near and far away distance.
Myopia or Short Sightedness : It is a defect of the eye which can be corrected by lenses. A person suffering with this defect cannot see distant objects but can see nearby objects clearly. This can be corrected by using a concave lens. Student usually suffer with this defect.
Presbyopia or old age sight : It results due to a reduction in elasticity of the lens in old age (above 40 years). It leads to failure of accomodation for viewing near as well as far objects. It is corrected by using convex and concave lenses e.g. by using bifocal lenses.
Hypermetropia or long sightedness : A person suffering with this defect can see only distant objects but not nearby objects. It can be corected by using convex lenses. It is common in old people.
Astigmatism : It develops due to unequal curvature in lens or cornea. It is corrected by using cylinderical lenses.
Cataract : It is due to the opacity of eye lens. It is caused by denaturation of lens proteins due to various reasons. It results in loss of vision and is corrected by the surgical removal of the opaque lens and the use of spectacle with convex lenses. It can be corrected by introducing an artificial lens surgically into the eye called intraocular lens.
Speed of the nerve impulses : The nerve impulses can travel along an axon at different speeds or velocities e.g. in a mammalian nerve it is 100-125 m/s.
Blind spot : No rods or cones are present at the optic nerve in the retina of eye. When an image falls on the retina of the eye a part of the image falling at the optic nerve is not perceived and so this place is called the blind spot.
Night blindness : If the rods of the retina do not contain a brown pigment, called rhodopsin, one is unable to see anything in dim light. This defect is called night blindness.
Colour blindness : If a person is not having cones in retina, he cannot identify different colours and he is said to suffer from colour blindness. Usually a person can not distinguish between red and green colour.
Ears or stato-acoustic organs
Human ears have double function. One is of hearing and other of equilibrium which is sensitive to the sound wave frequencies and change in relation to gravity. These have external ear pinna and middle ear containing tympanic cavity and three ear bones or ear ossicles. These are from outer side to the inner side malleus, incus and stapes. The balancing structure are semicircular canals and utriculus. Hearing is concerned with the sacculus and cochlea.
(a) External ear : The cartilaginous part of external ear is called pinna. At the base is the external auditory meatus, inner side of meatus is having a membrane called tympanic membrane. In the walls of external ear are found the wax-like substance producing glands called ceruminous glands.
(b) Middle ear : Its cavity is called tympanic cavity. Eustachian tube connects this cavity with buccal cavity. This connection is helpful in maintaining equal pressure in and outside the ear. Tympanic cavity is joined with cavity of internal ear by upper fenestra ovalis and lower fenestra rotunda. In between tympanic membrane and fenestra ovalis, three ear ossicles are found namely meatus, incus and stapes.
(c) Internal ear : It is also called membranous labyrinth which is enclosed within bony labyrinth. Endolymph is filled in membranous labyrinth. Crystals of Ca CO3 called otolith are found dispersed in endolymph.
Parts of membranous labyrinth :
1. Semicircular canals—Three in number.
2. Utriculus
3. Sacculus
4. Cochlea
Anterior and posterior semicircular canals are joined at their basal ends. This junction is called crus commune.
Cochlea : Its duct is called cochlea duct which is divided into three parts by two membranes. Three parts are scala vestibuli, scala media and scala tympani and two membranes are Ressner’s membrane and basilar membrane respectively.
Organ of corti is found in the scala media. Cells of organ of corti bear cilia. Tectorial membrane envelops these cilia. Tectorial membrane is highly innervated and perceives slight changes in the movement of cilia.
Cochlea is the main centre of hearing while semicircular canals are related mainly with balancing.
Nose : It is the organ of smell.
Tounge : Organ of taste. A man recognizes four taste by taste buds :
Sweet : tip of the tongue.
Sour : along the lateral edges of tongue.
Salty : upper surface on the front half of tongue.
Bitter : back of the tongue.
Sour : It is evoked by H+ ions produced by the ionization of acids.
Sweet : It is evoked by organic substances such as sugar, dextrin, glycerol etc.
Bitter : It is evoked by quinine, morphine, caffeine, urea and nicotine.
Salty : It is evoked by cations like Na+ of inorganic salts and also by some organic substances.
Afferent nerves : These are nerve fibres which carry impulses to the central nervous system from the various parts of the body.
Efferent nerves : These are nerve fibres which carry impulses from the nerve cell to the various parts of the body.
Motor nerves : Motor nerves influence the nutrition, metabolism and growth of the tissue to which they are distributed.
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