Receptors & Human Eye | Biology A-Level - A Level PDF Download

SENSORY SYSTEM

Animals possess some specialised structures to perceive the different type of changes (stimuli)  occurring in their external environment. These structures are known as sense organs. After receiving these stimuli, sensory organs transmit these to the central nervous system through the sensory nerve  fibers.

A sensory organ is only sensitive to a specific kind of stimulus to which it is specialized like temperature, chemicals, touch, light etc. Based on their location in the body sensory organs are of three types :

1. Exteroceptors : - These sense organs receive stimuli from external environment because they remain in contact with the external environment. Example – nose, eyes, tongue, ears and skin.

2. Interoceptors : These sensory organs are associated with internal environment of body and receive the changes taking place in the internal environment. Examples – changes in the composition of blood, concentration of carbon-dioxide, hunger, thirst, asphyxia etc.

3. Proprioceptors : These sensory organs are present in joints, tendons, muscles and connective tissues which perceive the tension and pressure exerted during the activities of equilibrium maintenance and orientations of body.

In human body, five types of exteroceptors are found which are known as sense organs. The main sense organs include skin, eye, nose, ear and tongue.

CUTANEOUS RECEPTORS

Skin envelopes the entire body, and it is considered as tango receptor (fig.) Numerous sensory papillae are found in the dermis of skin to receive the stimuli of touch, pressure, cold, heat, temperature and pain. All these are of simple type of receptors. These sensory structures of skin receive impulses from the nerve endings in skin. These are of following types - 

1. Tactile Receptors : - These receptors are present nacked endings of sensory nerve fibres on the hair follicles in the dermis of skin. These are excited when the hairs come in contact with some object.

Extensively branched (arborized) endings are of sensory nerves are found in the papillae of dermis. These endings are very small sized encapsulated structures called sensory corpuscles. Those which are cylindrical and sensitive to touch are called Meissner's corpuscles. Their number is much more in nipples, lips, glans penis, palm, sole and in fingers. The number of these corpuscles decreases as a person grows older. The corpuscles sensitive to the strong and sustained contacts (pressure) and situated deep in the dermis and called as Pacinian corpuscles. 

2. Pain Receptors (Algesireceptors) : Numerous branched sensory nerve fibres are scattered among the epidermis as well as dermal cells of skin. These possess nacked nerve endings. These nerve endings are sensitive to chemical, electrical, and mechanical stimuli, which cause the sensations of pain in body.

3.  Thermo Receptors : A network of sensory nerve fibres is situated closely to the hair follicles in dermis of skin. These nerve fibres are sensitive to the stimuli related to temperature, These thermoreceptors make a person aware to stimuli of cold, heat etc. Because of this sense perception our hairs get erected during excessive cold. The sensory organs excited by cold and heat are known as frigidoreceptors.

EYE

Eye and ear also called "teleoreceptor", because these receive impulse from far places.

(A)  Eye (Photoreceptor) : -

These are photosensitive organs. Eye ball measures about 2.5 cm in diameter. 

Each eye is an empty ball like round structure, it is called eye ball. Each eye  ball is situated in the notch of frontal  bone of the skull. It is called "Eye orbit". Human eyes are situated in eye orbit lateral to nose. 

Only 1/5th part of whole eye is seen from out side the eye orbit, called as cornea.

Remaining 4/5th part is in the eye orbit, called sclera.

(1) Eye lids or palpebrae : -
There are two muscular eyelids for the protection which having lashes at one side. Both the eyelids are named according to their situation i.e. upper & lower eyelids.

Eyelids are immovable in snakes. Eyelids of fishes are absent.

There is present one more transparent membrane on the cornea. It is called nictitating membrane or third eye lid. It is actively working in rabbit. It is found form of constricted condition at one corner of eye ball, but at the time of need, it may be expanded over entire eye ball.

Nictitating membrane is vestigeal in human. It is also called "Plicasemilunaris"

Eye lashes are found at both the eyelids.

(2)  Glands : -  For the cleaning and for lubrication/moisturising the exposure part of eye.
Following glands are founds in each eye.
    
(a)  Meibomian glands : - These are present at Inner surface of eyelids. They secrete an oily substance, which spread over cornea.   It prevent firction between two eye lid. It also help in frictionless movement of eyelid. 

(b) Lacrimal glands : - At outer angle of each eye ball and assoicated with accessory lacrimal gland, which secrete water like substance, which moistures the cornea, eyelids and conjunctiva and cleans it. This water like substance is called "Tear". (Slightly alkaline contains bacteriolytic enzyme Lysozyme) Tear glands activate after four months of birth in human child.
     
(c) Gland of zeis : It is situated in margin of eye lid.
Harderian glands : - These are found inside the lower eye lids. These moisten the nictitating membrane.    
Harderian glands are absent in rabbit & human.    
In place of harderian glands, in mammals, meibomian glands are present. But in some mammals e.g. rats, shrews, whales etc., these harderin glands are found.        
These glands are also found in frog and birds.

(d)  Gland of moll : - These are modified sweat gland found in the eye lashes.

(3)     Muscles of eye balls : -

   
There are present 6 voluntary muscles in the eyeball which help to rotate the eye ball into eye orbit. Out of these 4 are rectus muscles and 2 are oblique muscles. They are also called as extra occular muscles.

Lateral or External Rectus muscle rotates the eyeball towards outside i.e. from nose to ear. Medial or Internal rectus muscle rotates the eyeball toward inside i.e. from ear to nose.

Superior rectus muscle and inferior oblique muscle collectively help the eyeball to rotate upwards.

Inferior rectus muscle and superior oblique muscle collectively help the eyeball to rotate downwards. 

Rectus and oblique muscles collectively rotate the eye ball to all the sides around its axis.

Any defect in one of the these eyeball muscles (e.g. muscle may remain small or extra large than required) causes strabismus or squint eyes. In this defect, eye ball remains inclined to any of the one side. Eye muscles are innervated by occulomotor (III). Pathetic (IV) and Abducens (VI) Nerve.    

INTERNAL STRUCTURE OF EYE BALL : 
    
The wall of remaining eye ball has three layers.

(1) Fibrous tunic : -

It is mesodermal in origin.

It is the outermost covering of eye ball. It is made up of hard and thick fibrous connective tissue. It is also called as sclerotic layer

The layer is divided into 2 parts.

(a) Cornea : -

It is the outer visible part of fibrous tunic. 

The joint between cornea and sclerotic layer is called "Limbus" or "Sclero – corneal junction".

Cornea transplantation is successful because it lacks blood vessels.
        
(b)  Sclera : -
It is made up of white, hard, opaque thick fibrous connective tissue in mammals but in frog, it is made up of cartilage. It is the inner portion of eye ball. It is non-vascularised. This layer is of white colour, so it is also called "White of eye"
Inner layer of eyelids remain streched over anterior part of sclera (limbus) in the form of translucent membrane. It is called conjunctiva. It is made up of epidermis of skin. The thinnest epidermis is extend up to margin of cornea i.e. conjunctiva is the thinnest epidermis in animal body.

(2) Vascular tunic : - - 

It is also mesodermal in origin. 
It is made up of network of blood capillaries. 
It is the middle layer of eyeball Due to the presence of network of blood capillaries it is highly vascularized. Melanin pigment is found in this layer. Due to the presence melanin pigment eye looks like green, blue, brown, black in colour. Eyes of rabbit are red due to red melanin pigments, and in man eyes may be brown, black, blue, green according to the melanin present in it.
    
This layer has three parts : -

(a) Choroid layer : -

• Choroid layer is the part of vascular tunic which lie below the sclerotic layer.
• It contains abundant pigment cells & blood vessels.
• It is dark brown. It darkens the cavity of eyeball to prevent internal reflection of light.
• It nourishes the retina.

(b) Ciliary body : - 

• It is the lower swollen portion below limbus.
• It has ciliary processes which project into eyeball.
• It has ciliary muscles (i) circular (ii) meridional.
• Inner end of meridional is attached to choroid & outer end at the junction of sclera and cornea.

(c) IRIS : -
   
Choroid layer of vascular tunic separates from sclerotic layer (Just after the cornea) inclines towards inner side and forms a coloured screen, it is called Iris. Muscles of Iris are ectodermal in origin. There is present an aperture in the Centre of Iris, it is called Pupil. Light rays enter in the eye ball through pupil.
   
2 types of muscles are found with Iris. 

(a) Radial dilatory muscles : -

   
These are outer Involuntary muscles, these are expanded in the iris breadth wise. Iris becomes thick and narrow if these muscles contract and diameter of pupil is increased at that time. It happens in dim light, it is called Mydriasis.

(b) Circular sphincter muscles : -
   
These are scattered in inner part of iris. Due to the contraction (In bright day light or high flashes of light) of these muscles in high light, Iris expands breadth wise and diameter of pupil is decreased. It is called miosis.
Iris controls the intensity of light by increasing or decreasing the diameter of pupil i.e. Iris acts as diaphragm of a camera. Except muscle of iris & ciliary body all vascular tunic of eye ball is mesodermal in origin. The parasympathetic fibres constrict & sympathetic fibre dilate the pupil. Colour of eye like blue, gray, brown is layer of pigmented cells.

3. Neurosensory tunic : - ,

This layer is ectodermal in origin. 

It is the inner most layer of eye ball and has 3 parts : - 

(1) Pars ciliaris : - This part is attached with ciliary bodies. There are present spine like projections at the surface of ciliary body, these are called "Orra serrata:

(2)  Pars iridica : - This part lies just after the iris. It has a layer of pigmented cells.
Pars iridica and pars ciliaris are made up of simple cuboidal epithelium.

(3) Pars optica : - It is also called Retina It is the part just below the choroid layer.
Structure of retina is complicated. It has following layers.
    
(a) Pigmented layer : - It is the outer most layer. In the cells of this layer, pigment is found called melanin (receptor cells.)

(b) Sensory layer : - This layer is made up of specialized sensory cells. Rods and cones are found in this layer. Receptor cells are also known as photoreceptors/visual cells.

Rods are long, thin, cylindrical structures/cells. These are numerous in number. (1110 – 1125 Lacs)

Rods are differentiates the light and dark. They are more sensitive than cones.

A purple coloured pigment is found in rods called Rhodopsin/Visual purple.

Cones are thick and small cells which differentiate among different colours in full light (65 Lacs) Iodopsin/Visual violet is present in cones.

Only rods are found in the retina of owl, because it is nocturnal animal, unlike hen which has only cones in its retina.

Cones are absent in the retina of most of the nocturnal mammals like shrews, bats etc. squirrel has only cones in its retina.

A neurons layer is present just below the rods and cones. The synapse layer which, developes between photo sensory cells and bipolar neurons are called as outer plexiform layer. 

Each bipolar neuron has a dendron and one axon. 

 
Axons are jointed together by specific nerve cells, called Amacrine cells. Such neurons do not have nerve fibres.

In between bipolar neurons, supporting cells are found which, are called as Muller's cells.

(c) Ganglionic layer : - This layer is made up of multiple nerve. The  dendrites of multipolar nerve form synapses with axons of bipolar neurons. This layer of synapse is called as inner plexiform layer.
Axons of all nerve cells combine to form optic nerve. This optic nerve penetrates the retina and goes to brain.
At the point place, at which retina is pierced by optic nerve, cones and rods are absent. So no image will be percieve at that place. This point place is known as "Blind spot".
Just above the blind spot at the optical axis of eye ball, there is a place, where only cones are present. Yellow pigments is found (xanthophyll) in these cones. So this place is known as yellow spot or macula lutea or Area centralis.   
A groove or notch is found in area centralis, called fovea centralis. Fovea centralis contain only cone cells. 

Fovea centralis is most sensitive part of eye. Cones are some what obliquely placed at this place. An enlarged image of object is formed here. 

Lens : - It is ectodermal origin.

A transparent, biconvex lens is present just after iris. In frog, lens is spherical in eyeball. 

Lens is connected by ciliary body with the help of "Suspensory ligaments" also called as zonula of zinn" or zonules. These ligaments are flexible and this can slide the lens and can change it's focal length. Lens  and suspensory ligament divides the cavity of eyeball into two chambers.

(a) Aqueous chamber : -
The part of eye ball which lies between cornea and lens is filled with an akaline liquid, it is called aqueous humor. It is a type of transparent tissue fluid. It's divided aqueous chamber into two parts.   

(i) Anterior chamber : - 
This chamber lies between cornea and iris, it is also called as Venous chamber. Veins carry CO₂, metabolic wastes outside from here.

(ii)  Posterior Chamber : -
This chamber lies between iris and lens, it is also called as arterial chamber. Arteries supply O₂ and nutrients here. 

(b) Vitreous chamber : - Cavity of eye ball which lies between lens and retina is called vitreous chamber. A jelly like liquid (transparent and thick like albumin) is filled in this chamber, This is called vitreous humor. 

In this liquid 99% water, some salts, a mucoprotein called vitrin and a mucopolysaccharide-Hyaluronic acid are present. Gelatinous nature of vitreous humor is depend upon fibrillar protein & hyaluronic acid. It is form during embryonic stage. In this chamber Hyalocytes cells are found.    

Aquous humor and vitreous humor both the liquids are secreted by the glands of cilliary body. Canal of Schlemm is present between limbus and ciliary body . Aqeous humor leak out by canal of schlemm into blood capillaries and again reach upto their veins. 

Both these liquids maintain proper pressure inside the cavity of eye ball. These check the eye ball from collapsing.

If this canal of schlemm is blocked by any reason and fluids do not return back to veins liquid is increased in the chambers of  eye.

When amount of this humor is increased in the eye chambers then pressure is increased inside the eye ball . Thus retina pressure is increased. This is known as glaucoma.

A thin Hyaloid canal or Cloquet's or Cloquet's canal is also found in vitreous humor from blind spot to central point of lens. It provide nourishment to the developing lens which gradually atrophied.  

WORKING OF EYES

Light ryas emitted by any object enter the eye. A small, real and inverted image of object is formed at retinal Sensory cells of retina are sensitized, and optic nerve carries this impulse to brain . At this time animal able to see the object. 

Cornea, aqueous humor biconcave lens and vitreous humor completely refract the light rays coming from object. As a result of this an inverted image is formed at retina. Just like diaphragm of a camera. is Iris of eye, decreases or increases the diamter of pupil acoording to amount of light. Iris expands to decrease the pupil in high intensity of light so a small amount of light touches the retina.  When light is dim, iris itself constricts to increase the diameter of pupil. 

ACCOMODATION OR FOCUSSING - 

The ability to change the focal length of lens by changing the curvature of lens, is called accommodation power. 

Only mammals and birds have this accommodation power in their eyes. 

This ability is very less in frog and it depends on the sliding of lens forward and backward to some extent. 
 
In normal condition muscle fibres of ciliary body remain relaxed and lens is stretched by its suspensory ligaments, and due to this, lens is flat. A flat lens has more focal length. As a result of this eye can see long distant objects easily.

To see near by objects, sphincter muscles of ciliary body contract and ciliary body becomes broad, suspensory ligaments becomes loose and relaxed. As a result of this relaxation lens becomes biconvex, and now this focal length is reduced. Now animal is able to see near by object easily. 

TYPES OF VISION :

(a) Monocular vision or panoramic vision : - Most of the vertebrates have their eyes situated on the lateral  sides of head and due to this animal is capable to see the large area of both the sides. It is called monocular  vision.  
e.g. rabbit, frog, horse (Most of the herbivorous animals have this type of vision)    

(b)  Binocular vision : - Most of the carnivores mammals have eyes in front of their heads and side by side, so as to focus on one object by both the eyes. It is called binocular vision e.g. Man, monkeys and apes. 

(c)  Stereoscopic vision : - It is three dimensional vision found in human. 

(d)  Telescopic vision : - This is found in birds.  

Largest eyeballs are found in horse. 

Sharpest vision is found in eagle.

Shortest sight is found in monkeys.

Chemical explanation of vision - 

Cones and rods of eye are stimulated by light rays. It is a chemical event.

A shiny visual purple pigment is found in rods of retina called Rhodopsin. It is formed by a protien opsin and a coloured pigment Retinal, just like haemoglobin of blood. 

Opsin is also called scotopsin.

In bright light. rhodopsin is decomposed into opsin protein and retinal pigments. This chemical change is sight impulse. This sight impulse is carried by optic nerve to the brain, and animal is able to see. 

In dark, rods synthesize rhodopsin again with the help of opsin, retinal and enzyme. 

This is the reason that we can not see any thing, when we move to dark place from an enlighted place (for some time only) In the same way we are unable to see in light if we are coming from dark place because it will take time to synthesize or decompose the rhodopsin  it is called adaptation of eye.

For resynthesis of rhodopsin, animal blinks its eyelids. 

Retinin is formed by vitamin A so deficiency of vitamin A caused night blindness. 

Cones able us to differentiate among colours and bright light. Cones have a pigment called Iodopsin in place of rhodopsin of rods. It is decomposed into photopsin and retinal.  
There are three types of cones in retina : -

(a)  Erythrolab    -     Red cones (erythropsin – Sensitive to red)

(b)  Chlorolab    -     Green cones (Chloropsin – Sensitive to green)

(c)  Cynolab    -       Blue cones. (Cynopsin – Sensitive to blue)

We are able to acknowledge different colours due to these three types of cones their combination. 

In moon light we cannot see colours because only rods are functioning . Due to low light level cones are not functioning.      

Red, green and blue are the primary colours.
   
Dimlight vision   -    Scotopic vision
   
Bright light vision   -    Photopic vison             

The eyes of some animals shine at night, because in the eyes of these animals, there is a pigment just outside the retina in the choroid layer of eyeball, which reflects the light rays coming from retina. This layer is called Tapetum. Due to this layer, these animals are capable to see in dark also. 

Kangaroo, hoofed mammals, elephants, whales etc. are having a silver shining layer of fibrous connective tissue called Tapetum fibrosum.  

In Elasmobranch fishes a reflecting colour pigment called Guanine is present is tapetum layer so it is called tapetum lucidum. 

Hunters and carnivore mammals like dogs, cats, tiger etc. have a layer in their retina called tapetum cellulosum. 

In the eyes of birds a comb like structure of blood vessel is present which, is called pecten.

Emmetropia : - Normal vision of eyes is called emmetropia. 

Some important defects of eyes : - 

1.  Hypermatropia (far sightedness) : - 

In this defect of eye, person is able to see object placed at far distance but is unable to clear see obejcts close to him or her. 

This defect is due to small size of eyeball or flatness of lens. In this defect image is formed behind the retina. To cure this defect person should wear convex lenses in spectacles.  

Sometimes in old age this defect may occur due to reduction in the flexibility of lens or ciliary body, then it is known as presbyopia. 

2.  Myopia or  Nearsightedness or short sightedness : - 

In this defect of eye, person is able to see objects near/close to him or her but is unable to see objects placed at far distance. 

This is due to enlargement of eyeball or increased convexity of lens. 

In this defects image is formed before the retina because light rays coming from distant objects converge before retina. 

To overcome this defect person should wear concave lenses in spectacles. 

3.  Astigmatism : - In this defect curvature of cornea is changed as a result of that light rays do not focus on macula lutea but somewhere else,  causing incomplete blurred vison. This defect may be cured by cylinderical lenses.   

4. Night blindness : - This is due to deficiency of vit A. In this disorder synthesis of Rhodopsin is decreased. 

5. Xerophthalmia : - it is due to keratinisation of cunjunctiva and cornea, and conjuctiva becomes thick. It is also due to deficeincy of vit A. 

6. Trachoma : - In this defect of eye, a watery fluid oozes out from eyes in excess amount so eyes become red due to irritation. It is caused by a mircrobe Chlamydia trachomatis.  

7. Strabismus : - It is due to loosening or contraction of the any of 6 voluntary muscles which give the proper position to the eye ball in its orbit. Thus eye ball inclines towards one side of orbit. It is strabismus or squint eyes. Particular muscle may be cured by operation and this defect is cured.

8. Cataract : - In this defect, lens becomes more solid, brown or more flat. It occurs in old age mostly. The lens becomes opaque, and reduces its power of accommodation. At this stage person can not see. A new lens is administered in place of defective lens by operation.  

9. Glaucoma : - If the canal of schlemm is blocked in eyeball, aqueous humour can not return to veins again as a result pressure is increased in eye chambers and retina is damaged and person becomes totally blind. 

10. Photophobia : - In this defect proper image is not formed in bright light. 

11. Colour blindness : - It is genetic disorfer of X-chromosome. It is due to recessive gene. Colourblind persons can not different in red and green colour.