Zoology Assignment Help >> Receptors, Functions of Receptors, Structure of Retina

Theory of Receptors and their Functions, Structure of Retina, Zoology Assignment Help

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Receptors

Every animal is sensitive and responds to changes in the external and internal environment of the body. These changes are stimuli. With effect to these stimuli the animals react for performing the life activities easier. The external and internal stimuli are received by varied types of receptors or sense organs. Every sense organ is supplied with the sensory nerve fibres which convey these stimuli to the C.N.S. These sense organs are grouped into three divisions-

On the basis of their position-

(i) External receptors or exteroceptors: Situated on the outer surface of the body.

(ii) Internal receptors or inferoceptors: Situated within the digestive system.

(iii) Proprio-receptors: Situated internally but other than digestive system.

On the basis of their function

(i) Proprioceptors: Present in the muscles, tendons and joints.

(ii) Noiceptors: Pain sensory receptors present in deep position.

(iii) Teleceptors: Vision and stato¬acoustic organs.

On the basis of nature of stimuli¬

(i) Chemo-receptors: Olfactory, gustatory, common chemo¬receptors.

(ii) Radio-receptors: Photo¬receptors, noiceptors, thermo¬receptors.

(iii) Mechano-receptors: Presso¬receptors or bare-receptors.

Extero-receptors: Extero-receptors are of following types-

(1) Cutaneous receptors: Free nerve endings in the dermis-sensory to touch or tango-receptors or algesi¬receptors. Tactile organs are also present. These are corpuscles for cold-forgido¬receptors, for heat-calo-receptors, for touch-pacinian corpuscles.

(2) Chemo-receptors: Sensible to solutions only.

These are-

(a) Organ of taste or gustato¬receptors: Taste buds on tongue and roof of the buccal cavity.

(b) Organ of smell or olfacto¬receptors: Ethmoturbinals, maxillo¬turbinals and naso-turbinals bones are present in the nasal passage. These bones are covered by the olfactory or schneidarian epithelium.

EARS OR STATO-ACOUSTIC ORGANS

Ear has double function of hearing and equilibrium which is sensitive to the sound wave frequencies and changes in relation to gravity. These have external ear pinna and middle ear containing tympanic cavity and three ear bones or ear ossicles. These are from outer side to the inner side malleus, incus and stapes. The balancing structure are semicircular canals and utriculus. Hearing is concerned with the sacculus and cochlea.

(a) External ear: The cartilaginous part of external ear is called pinna. At the base is the external auditory meatus, inner side of meatus is having a membrane called tympanic membrane. In the walls of external ear are found the wax like substance producing glands called ceruminous glands.

(b) Middle ear: Its cavity is called tympanic cavity. Eustachian tube connects this cavity with buccal cavity. This connection is helpful in maintaining equal pressure in and outside the ear. Tympanic cavity is joined with cavity of internal ear by upper fenestra ovalis and lower fenestra rotunda. In between tympanic membrane and fenestra ovalis, three ear ossicles are found namely meatus, incus and stapes.

(c) Internal ear: It is also called membranous labyrinth which is enclosed within bony labyrinth. Endolymph is filled in membranous labyrinth. Crystals of CaCO3 called otolith are found dispersed in endolymph.

Parts of membranous labyrinth:

1. Semicircular canals-Three in number.

2. Utriculus

3. Sacculus

4. Cochlea

Anterior and posterior semicircular canals are joined at their basal ends. This junction is called crus commune.

Cochlea: Its duct is called cochlear duct which is divided into three parts by two membranes. Three parts are scala vestibuli, scala media and scala tympani and two membranes are Reissner's membrane and basilar membrane respectively.

Organ of corti is found in the scala media. Cells of organ of corti bear cilia. Tectorial membrane envelops these cilia. Tectorial membrane is highly innervated and perceives slight changes in the movement of cilia.

Cochlea is the main centre of hearing while semicircular canals are related mainly with balancing.

EYES OR PHOTO-RECEPTORS

These are the light receptors. In man these are spherical and situated in the orbits. Each eye ball is made up of three concentric layers. The outermost layer is opaque, fibroelastic scleroid. The highly vascular pigmented middle layer is the choroid. The inner light sensitive layer is the retina. The photosensitive cells rods and cones and bipolar neurons are embedded in the retina. A biconvex transparent lens is suspended and attached with the suspensory ligament from the ciliary body which divides the eye ball internally into two chambers. The outer to the lens is aqueous chamber containing aqueous humour and inner one is vitreous chamber containing the vitreous humour.

Eyes are sensitive to the wavelength ranging from 380-760 nanometers. Rods are sensitive to the dim light whereas the cones are sensitive to the bright light and are also able to differentiate the colours.

Structure of retina:

1. Pigmented layer - Outer layer.

2. Sensory layer - Inner layer

Inner layer is again divided into-

1. Layer of rods and cones: In the rods rhodopsin is found. They perceive the stimulus of light and dark. It is destroyed in light. Cones are made up of iodopsin and are related with the perception of colour. Basically the cones are sensitive to three types of colour viz. red, green and blue. Yellow spot is the point where only cones are found. It is the point for most optimum image.

2. Bipolar layer: It is made up of bipolar neurons.

3. Ganglionic layer: It is also made up of bipolar neurons. Axons of all neurons join and form optic nerve. The point where optic nerve passes retina is the point with no rods and cones, hence called blind spot.

Eye diseases

1. Strabismus: Due to disfunctioning of any of the eye muscles.

2. Glaucoma: Due to increase in the intra-occular pressure.