Classification of sensory organs
The sense organs or the receptors are structures specialized to perceive conditions outside and to some extent inside the body.
They provide information to the central nervous system upon which action may be taken.
Sense organs occur commonly in all organisms from invertebrates to vertebrates.
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They can be classified under the following categories with respect to their sensitivity to various stimuli.
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1. Mechanoreceptors:
These receptors are responsive to tactile stimuli, to sound, to other vibrations or to pressure changes. Pacinian corpuscles are pressure receptors found in deeper layer of the skin.
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Touch receptors are located in the dermis or in sub-epidermal connective tissue. Tactile receptors are found all over the skin. In mammals, the part of the ear concerned with the sense of hearing is known as cochlea, which possesses approximately 24,000 hair cells. These hair cells are the actual receptors of hearing. Sound producing organs in insects are known as tymbal organs.
2. Thermoreceptors:
Thereceptors whichrespond to temperature changes are termed thermoreceptors. In warm-blooded animals, these are located in the hypothalamus, while in cold-blooded animals; the temperature is regulated through behaviour responses. In arthropods, thermoreceptors are found on the antennae or legs.
3. Chemoreceptors:
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Chemoreceptors are concerned with chemical reception. In vertebrates and invertebrates, two types of cells are concerned with chemical reception, (i) bipolar nerve cell and (ii) columnar epithelial cells.
Olfactoreceptors are found in the skin of fish and contain mucous membranes made of basal cells. These acts as organs of smelling in many tetrapods, there is a pair of vomeronasal organs (organ of Jacobson) which also act as smelling organs.
Gustato-receptors (organ of taste) consist of taste buds for testing substances. Taste buds are confined to the tongue, oral cavity and pharynx in higher vertebrates while it is distributed in mouth, pharynx, bronchial cavity and outer surface of head in fishes.
4. Photoreceptors:
The main photoreceptor organ in animals is the eye.
Eyes in Mammals (Man):
The human eye is a hollow sphere composed of the following structure:
Sclera:
This is the external covering of the eye; very tough, containing collagen fibres protects and maintains shape of eyeball.
Cornea:
It is the transparent part of sclera and acts as the main structure refracting light towards the retina.
Conjunctiva:
A transparent layer of cells protecting the cornea and continuous with the epithelium of eyelids
Eyelid:
It protects the cornea from mechanical and chemical damage.
Choroid:
Rich in blood vessels and covered with black pigment cells to prevent reflection of light within the eye.
Lens:
This is a transparent, elastic biconvex structure. It provides fine adjustment for focusing light on to the retina and separates the aqueous and vitreous humours.
Aqueous humour:
A clear solution of salts secreted by ciliary body of the eye.
Iris:
A circular, muscular diaphragm containing the pigment which gives the eye its colour. It controls the amount of light entering the eye.
Pupil:
A hole in the iris, all light enters the eye through it.
Vitreous humour:
A clear, semi-solid substance supporting the eyeball.
Retina:
It contains photoreceptor cells, rods and cones, cell bodies and neurons supplying the optic nerve.