Sensory Receptor - Eyes (Organ of Sight) | Biology | NEET

SENSORY RECEPTORS

The principle function of the special sensory receptors is to detect environmental stimuli and transduce their energy into electrical impulses . These are then conveyed along sensory neurons to the central nervous system , where they are integrated and processed and a response is produced . On the basis of stimuli receptors are categorised as :

( i ) Mechanoreceptors : Respond to mechanical forces such as touch , pressure , vibration , etc.

( ii ) Thermoreceptors : Respond to temperature changes Include free nerve endings and Krause's end bulbs .

( iii ) Chemoreceptors : Respond to chemicals. Include olfactory cells in the nasal mucosa and taste buds .

( iv ) Nociceptors : Respond to painful stimuli which may be damaging to tissues . Include free nerve endings .

( v ) Photoreceptors : Respond to light energy. Include cones and rods in the retina.

The most complex sensory receptors , are called sensory organs . They consist of numerous sense cells , sensory neurons and associated accessory structures , e.g. , eye and ear have a level of complexity of sense organs .

Eyes ( Organs of Sight )

The organs of sight are a pair of eyes which are situated in deep protective bony cavities, called the orbits or eye sockets of the skull . Eyes consist of tissues present in three layers :

( i ) Outer fibrous coat : Sclera , an opaque outermost covering , maintains the shape of the eyeball . Cornea is a thin transparent , front part of sclera which lacks blood vessels but is rich in nerve endings . Cornea allows the light to pass into the eye . Conjunctiva protects the cornea and also secretes oils and mucus that moisten and lubricate the eye .

( ii ) Middle vascular coat : Choroid is present beneath the sclera and contains numerous blood vessels and nourishes the retina . It becomes thick in the anterior part to form the ciliary body which holds the lens in position . Iris forms a pigmented circle attached tot he ciliary body in front of the lens . The movement of iris controls the size of pupil .

( iii ) Inner nervous coat : The inner layer is the retina and it contains different layers of cells , from inside to outside such as layers of ganglion cells bipolar cells, photoreceptor cells and pigment cell . The photoreceptors or visual cells are of two types : rod cells and cone cells . Rods contain a photosensitive pigment called rhodopsin In retina of human eyes , cones are of three types which possess characteristic photopigments ( porpyrosin , iodopsin and cyanopsin ) that respond to red , green and blue lights respectively . The daylight ( photopic ) vision and colour vision are functions of cones and the twilight ( scotopic ) vision is the function of the rods .

The space between the cornea and the lens is called the aqueous chamber which contains a thin watery fluid called aqueous humour . It helps to maintain the shape of the front part of the eye and provides nutrients to the lens and cornea . The space between the lens and retina is called the vitreous chamber which is filled with a transparent gel called the vitreous humour 

Optic nerve contains the fibres of the sensory neurons and leaves the eye ball from the back side . The point of departure of optic nerve through the retina does not have any rods or cones and thus produces a blind spot or optic disc . At the posterior pole of the eye lateral to the blind spot , there is a yellowish pigmented spot called macula lutea with a central pit called the fovea centralis . The fovea centralis is a thinned - out portion of the retina where only the cones are densely packed . It is the point where the visual acuity ( resolution ) is the greatest .

Mechanism of vision

The rays of light coming from an object fall on the retina after passing through cornea , aqueous humour , pupil , lens and vitreous humour . The stimulus perceived by rods and cones results in the dissociation of photosensitive pigment ( e.g. , rhodopsin ) into opsin ( a protein ) and retinal ( an aldehyde of vitamin A ) . This dissociation results in changes in structure of the opsin. The stimulated photoreceptors initiate the nerve impulses , which are transmitted by the optic nerves to the visual cortex area of the brain where the neural impulses are analysed and the image formed on the retina is recognised

MANISH MEVADA

M.Sc, M.Phil, B.Ed