Coupon Accepted Successfully!


Sensory Reception and Processing

All animals respond to various environmental stimuli. This is necessary for animals to survive. The stimuli from the environment are received by special cells called sense receptors. The sensory nerves carry the stimuli to the central nervous system, from where, the motor nerves carry messages to the effector organs resulting in a response. The major senses of the human body are those of smell, tough, taste, vision and hearing. The receptors for these senses are located in specialised organs called sense organs. The major sense organs on the body are nose (sense of smell), skin (sense of touch), tongue (sense of taste), eyes (sense of vision) and ears (sense of hearing).

Sense of Smell (Nasal Chambers)
The organs of smell are called olfactory sense organs. These are located in the epithelial tissue of the nasal chambers of the human being. A group of specialized cells called the olfactory cells are kept moist by the mucus secreted by the olfactory epithelium. Substances entering the nasal chamber adhere and dissolve in the mucous secretion. The olfactory cells pick up the particular odour and pass it on to the olfactory nerve which, in turn, transmits the impulse to the brain.

Sense of Taste (Taste Buds) 

Diagrams of the Section of the Tongue Showing the Location and Structure of Taste Bud
The organs of taste are called taste buds. These are located on small papillae on the tongue. A taste bud is a somewhat spherical structure consisting of two kinds of cells
  1. sensory cells and
  2. supporting cells.
The sensory cells are elongated and spindle shaped. The outer free end of each sensory cell bears a hair like flagellum. The lower end is innervated by fine nerve endings of 7th, 9th and 10th cranial nerves.

The supporting cells are like the sensory cells, but they are larger in size and have no flagella.

When food comes in contact with the taste buds the sensory cells are stimulated. The impulse thus produced is carried to the brain through the sensory nerves mentioned above.

Sense of Touch (Skin)
The organs of touch are touch corpuscles which lie in the skin below the epidermis. Each touch corpuscle is a group of flattened tactile cells which are innervated below by the fibres of a sensory nerve. These touch corpuscles are sensitive to contact, chemicals and also variations in temperature and humidity.

 A Touch Corpuscle

Sense of Vision (Eye)
Each eye is a hollow spherical body called the eye ball which is situated in a bony socket called the orbit. The eye ball can move with the help of six eye muscles, namely, superior, inferior, external and internal retina and superior and inferior oblique muscles.

The exposed part of the eye ball is protected by upper and lower movable eye-lids. These serve to protect the eye from dust. Each eye-lid bears a few eye-lashes on its margin. Present below the outer angle of the upper eye-lid are the lachrymal glands. They secrete a watery fluid called tears. Tears keep the surface of the eye moist and protect it form external dust and bacterial infection. The wall of the eyeball is composed of three layers or coats. These are sclerotic coat, choroid coat and the retina.





The human eye: (A) Median vertical section; (B) A small part of the retina enlarged to show the nerves, rods, cones, and pigmented cells; (C) The lens serves to form an image (reduced and inverted) on the retina. (D) Changes in the shape of the lens to focus the near and distant objects (accommodation)
  1. The Sclerotic Coat
    It is the outermost tough and opaque layer of the eyeball. It is composed of fibrous connective tissue. In the exposed part of the eye the sclerotic coat is transparent to form the cornea. The cornea is covered by a thin, delicate, transparent epidermal membrane called conjunctiva. It is in continuation with the epidermal lining of the eye-lids. It is highly vascular.
  2. Choroid Coat
    The middle layer of the eyeball inner to the sclerotic coat, is the choroid coat. It is very vascular and is formed of loose connective tissue. In the exposed part of the eyeball behind the cornea, the choroid coat bends inwards to form a flat circular partition called iris. The iris divides the cavity of the eyeball into two chambers. The outer smaller chamber is the aqueous chamber filled with the aqueous humor. The inner larger chamber is the vitreous chamber filled with a gelatinous vitreous humor. The iris is perforated by a central opening called the pupil. The iris contains two sets of muscles: circular and radial muscles. By contraction of the circular muscles, the diameter of the pupil is decreased and by contraction of the radial muscles it is increased. At the level of the outer margin of the iris the choroid coat is thickened to form the ciliary body. It is highly vascular, muscular and glandular. It contains two sets of muscles: the circular ciliary muscles and the radial ciliary muscles. The ciliary body is produced into folds which are arranged radially round the outer edge of the iris forming ciliary processes.
  3. The Retina
    It is the innermost layer, inner to the choroids coat. The outer side of the retina is pigmented and is fused with choroid. The inner part of the retina is semi-transparent and is the most sensitive layer of the eye ball. The sensory layer of the retina consists of two types of cells, the rods and cones. The rods are sensitive to dim light and contain a brown pigment called rhodopsin which is formed from vitamin A. The rods enable the individual to see shades of grey and white in dim light as well as in the dark. If the food is deficient in Vitamin A, it is not possible to see anything in dim light. This defect is called night blindness. This defect is due to insufficiency in rhodopsin in the rods. The cone cells are sensitive to bright light and enable us to see the corresponding colour. Those who have no cones in the retina cannot see any colour and they suffer from what is known as colour blindness. The cone cells are most numerous at the back of the eye opposite the pupil. This is a somewhat depressed part of retina and is known as yellow spot or fovea. Vision is the sharpest here. The sensory fibres from the retina bundle together and emerge from the back of the eyeball as the optic nerve. At the exit of the optic nerve, there are no rods and cones. A part of the image falling at this place is not perceived; hence it is called the blind spot.

    They eye functions like a photographic camera. When an object lies in front of the eye, the incidental light rays emerging from it pass through the lens and fall on the retinal cells. These light rays generate impulses which are carried by the optic nerve to the brain. The impulses are analysed in the visual area of the brain. It is only then an object is seen. If the optic nerve is cut, one cannot see anything, as the impulses do not reach the brain. When one looks at an object, the image is formed on the retina. This image is inverted as it is in the camera. However, once the impulses are analysed in the brain one gets the upright impression of the object. The image is focused by the lens on the yellow spot. In a camera the image is focused by moving the lens forward or backward. In the eye the lens does not change in position for focusing. It does the focusing by a process known as accommodation. This process involves the changing of the curvature of the lens. For focusing distant objects the lens becomes more flattened.  For focusing nearby objects the lens becomes more convex. The amount of light entering the eyes is regulated by increase or decrease in the size of the pupil by the action of the iris muscles. Thus the objects can be clearly focused from varying distance and in different intensities of light.

Test Your Skills Now!
Take a Quiz now
Reviewer Name