A sense organ is often referred to as a receptor organ.
External stimuli affect the sensory structures which make up the general cutaneous surface of the body, the exteroceptive area, and the tissues of the body wall or the proprioceptive area. These somatic area receptors are known under the general term of exteroceptors.
Internal stimuli which originate in various visceral organs such as the intestinal tract or heart affect the visceral sense organs or interoceptors.
A receptor structure is not necessarily an organ; in many unicellular animals it is a specialized structure within the organism.
Receptors are named on the basis of the stimulus which affects them, permitting the organism to be sensitive to changes in its environment.
Sensory Organ is a part of the body that receives stimuli and transmits them as sensations to the brain.
There are five types of sensory organs:
Eye:
The eye is the organ of vision.
It has a complex structure consisting of a transparent lens that focuses light on the retina. The retina is covered with two basic types of light-sensitive cells - rods and cones. The cone cells are sensitive to color and are located in the part of the retina called the fovea, where the light is focused by the lens. The rod cells are not sensitive to color, but have greater sensitivity to light than the cone cells. These cells are located around the fovea and are responsible for peripheral vision and night vision.
The eye is connected to the brain through the optic nerve. The point of this connection is called the "blind spot" because it is insensitive to light. Experiments have shown that the back of the brain maps the visual input from the eyes.
Color blindness or "Daltonism" is a common abnormality in human vision that makes it impossible to differentiate colors accurately. One type of color blindness results in the inability to distinguish red from green. This can be a real handicap for certain types of occupations. To a colorblind person, a person with normal color vision would appear to have extrasensory perception. However, we want to reserve the term "extrasensory perception" for perception that is beyond the range of the normal.
Ear:
The ear is the organ of hearing.
The outer ear protrudes away from the head and is shaped like a cup to direct sounds toward the tympanic membrane, which transmits vibrations to the inner ear through a series of small bones.
The inner ear, or cochlea, is a spiral-shaped chamber covered internally by nerve fibers that react to the vibrations and transmit impulses to the brain via the auditory nerve. The brain combines the input of our two ears to determine the direction and distance of sounds.
The human ear can perceive frequencies from 16 cycles per second, which is a very deep bass, to 28,000 cycles per second, which is a very high pitch. In addition, the human ear can detect pitch changes as small as 3 hundredths of one percent of the original frequency in some frequency ranges. Some people have "perfect pitch", which is the ability to map a tone precisely on the musical scale. Bats and dolphins can detect frequencies higher than 100,000 cycles per second.
Tongue:
Taste buds are the sensory receptor organs for taste.
The receptors for taste, called taste buds, are situated chiefly in the tongue, but they are also located in the roof of the mouth and near the pharynx. They are able to detect four basic tastes: salty, sweet, bitter, and sour.
The tongue also can detect a sensation called "umami" from taste receptors sensitive to amino acids.
Generally, the taste buds close to the tip of the tongue are sensitive to sweet tastes, whereas those in the back of the tongue are sensitive to bitter tastes. The taste buds on top and on the side of the tongue are sensitive to salty and sour tastes.
At the base of each taste bud there is a nerve that sends the sensations to the brain.
The sense of taste functions in coordination with the sense of smell. The number of taste buds varies substantially from individual to individual, but greater numbers increase sensitivity.
Women, in general, have a greater number of taste buds than men. As in the case of color blindness, some people are insensitive to some tastes.
Smell:
The organ of smell is the olfactory epithelium. This pseudostratified layer is composed of olfactory receptor cells, supporting cells, and basal cells.
The nose is the organ responsible for the sense of smell. The cavity of the nose is lined with mucous membranes that have smell receptors connected to the olfactory nerve.
The smells themselves consist of vapors of various substances. The smell receptors interact with the molecules of these vapors and transmit the sensations to the brain.
The nose also has a structure called the vomeronasal organ whose function has not been determined, but which is suspected of being sensitive to pheromones that influence the reproductive cycle.
The smell receptors are sensitive to seven types of sensations that can be characterized as camphor, musk, flower, mint, ether, acrid, or putrid.
The sense of smell is sometimes temporarily lost when a person has a cold.
Dogs have a sense of smell that is many times more sensitive than man's.
Touch:
Touch is sense the skin with help of sensory organs.
The sense of touch is distributed throughout the body. Nerve endings in the skin and other parts of the body transmit sensations to the brain.
Some parts of the body have a larger number of nerve endings and, therefore, are more sensitive.
Four kinds of touch sensations can be identified: cold, heat, contact, and pain.
Hairs on the skin magnify the sensitivity and act as an early warning system for the body.
The fingertips and the sexual organs have the greatest concentration of nerve endings. The sexual organs have "erogenous zones" that when stimulated start a series of endocrine reactions and motor responses resulting in orgasm.
Contributed by: harinag49 @ yahoo.co.in
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