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Organs of Sense

FEBRUARY 24, 2016

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A sense is an organism’s capacity to receive information for perception. Usually, the organism’s nervous system has a specific sensory system or organ, dedicated to each sense. Humans have multiple senses -- sight, hearing, taste, smell, and touch are the five traditionally recognized senses. Other animals also have receptors to sense the world around them, with varying degrees of capability. Humans possess a weaker sense of smell compared to many other mammals while some animals may lack one or more sense. Some animals can sense the world in a way that humans cannot, as they can sense electrical and magnetic fields, and detect water pressure and currents.

A sense is a group of sensory cells which responds to a specific physical phenomenon, and that corresponds to a particular brain region, where the signals are received and interpreted.


A. Sense of Touch, Temperature, Pressure and Pain

The sense of touch is due to the touch corpuscles present in the upper layer i.e. epidermis of the human skin. The sense of balance or vestibular sense allows an organism to sense body movement, direction, and acceleration, and to attain equilibrium and balance. The organ responsible for this is present in both the inner ears. The vestibular nerve conducts information from sensory receptors which sense motion of fluid in three semicircular canals caused by rotation of the head.
The sense of heat and cold is maintained by the skin and includes internal skin passages. There are specialized receptors for cold and for heat. These are quite different from the homeostatic thermoceptors in the brain (hypothalamus), which provide feedback on internal body temperature.

B. Sense of Taste

The sense of taste is due to the taste buds in the papillae of the tongue. The tip of the tongue is the most sensitive to sweet taste, and the rear is sensitive to bitter taste. The middle part of the tongue is not sensitive at all i.e. can be used to place a bitter pill while ingesting it.

Humans receive tastes through sensory organs called taste buds on the upper surface of the tongue. There are five basic tastes: sweet, bitter, sour, salty and umami. Other tastes such as calcium and free fatty acids may also be basic tastes but have yet to gain wide acceptance.

C. Sense of Sight

Humans can perceive light rays in the range of 3800-7600 Angstroms. The rays having a wavelength less than 3800 Angstrom are called ultra-violet rays while those with a wavelength of more than 7600 Angstrom are termed infra-red rays. Neither of these rays is detectable by the human eye. 

To see an object, its image must be formed on the retina. The convexity of the lens of the eye increases when the object is nearer and decreases when the object is far away to get the image on the retina in each case. The property of the lens by which it can adjust its convexity to form distinct image of the object at various distances is called accommodation. When the ciliary muscles do no work properly to bring about accommodation of the lens, the eye becomes defective.


The human retina contains two types of cells i.e.

1. Cone Cells - Meant for daylight vision, also responsible for colour vision
2. Rod Cells: Meant for nighttime or low intensity vision.

Since humans have many more cone cells as compared to rod cells, they are much better at seeing the objects in bright intensity as compared to others. Humans see with both the eyes together and both the eyes form separate images of an object. These two separate images are then fused in the brain to create an impression of depth. (Otherwise, the human retina is a 2-D structure and cannot give a perception of depth). This difference in the images formed in the two eyes is referred to as binocular disparity. You can easily observe this phenomenon by focusing on a pencil placed vertically in the line of your nose. First, see it with the left eye closes and later, see it with the right eye closed. In each of the cases, you will find a different view of the pencil. When you see it with both the eyes, these two images are fused by the brain to give you a perception of depth.


1. Myopia (Short-sightedness)

When a person can see the nearer objects clearly but not those lying far away, he is said to be suffering from myopia. In this case, the convexity of his lens cannot be sufficiently decreased to form the image of the distant object on the retina. It is remedied by the use of concave glasses.

2. Hypermetropia (Long-sightedness)

It refers to a person’s ability to see the distant objects clearly but not the near ones. It is remedied by the use of convex glasses.

3. Astigmatism

A person suffering from it can see the vertical lines clearly but not the horizontal ones, or vice versa.  It is remedied by the use of cylindrical glasses.

4. Colour Blindness

If a person cannot distinguish between different colours (especially between red and green), he is said to be suffering from this defect. It is a sex-linked disorder i.e. occurs due to recessive genes on the 23rd pair of chromosomes.

D. Sense of Smell

Owes to the presence of smell-perceiving cells present in the lining of nasal chambers. Unlike taste, there are hundreds of olfactory receptors, each binding to a particular molecular feature. Odour molecules have many features and, thus, excite specific receptors strongly. This combination of excitatory signals from different receptors makes up the smell of a particular molecule. In the brain, olfaction is processed by the olfactory system.


E. Sense of Hearing

Is due to the vibration of the ear membrane in the inner ear. Made possible by the incidence of sound waves on it.

Since sound waves are basically vibrations propagating through a medium like air, their detection is the sense of hearing. It is a mechanical sense as these vibrations are mechanically conducted from the eardrum through tiny bones to hair-like fibres in the inner ear, which detect their mechanical motion within 20 -20,000 hertz.


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