· They are made up of sensory cells and are connected with sensory receptors that receive the stimuli and transmit the impulses to the brain.
· There are five sense organs in the body. They are eyes, ears, nose, skin and tongue.
Classification of Sense Organs
A. On the basis of location in the body, sense organs are classified into two types.a. Exteroceptors (External receptors)
· These are the receptors found outside the body and receive the stimuli from the external environment.
· They are the organs of touch, smell, taste, sight and hearing.
· These are the receptors found outside the body and receive the stimuli from the external environment.
· They are the organs of touch, smell, taste, sight and hearing.
b. Interoceptors (Internal receptors)
· These are the receptors located inside the body and respond the internal changes like pain, thirst, hunger, temperature and fatigue.
B. On the basis of type of stimulus, they are:
a. Mechanoreceptors: They respond to mechanical stimuli. Example: nerve fibres around hairs.
b. Thermoreceptors: These respond to changes in the temperature. Example: Skin.
c. Chemoreceptors: These respond to changes in the concentration of specific chemical substances. Examples: the cells of taste and smell. They are of two types.
i. Olfactoreceptors: They respond to smell. Example: Sensory epithelium of nasal passages.
ii. Gustatoreceptors: They respond to taste. Example: Taste buds on the surface of the tongue.
On the basis of their functions, sense organs are:
a. Proprioceptors: They are found in muscles, tendons and joints. They respond to the movement of muscles.
b. Nociceptors: These are deep pain receptors. They respond to injury and any damage to stimuli.
c. Teleceptors: They respond to the changes in distant places such as eyes, ears.
EYES OR PHOTORECEPTORS
· Eye is an organ of sight.· It is situated in the orbital cavity and is supplied by II cranial nerve (optic nerve).
· It is almost spherical in shape and is about 2.5 cm in diameter.
· The bony walls of the orbit and the fat (present between the eyeball and orbital cavity) help to protect the eye from injury.
Structure of Human Eye:
· The wall of an eye is made up of three layers of tissues.
a. Outer fibrous layer: Sclera and cornea
b. Middle vascular layer: Choroid, ciliary body, iris, pupil and lens
c. Inner nervous tissue layer: Retina
· The wall of an eye is made up of three layers of tissues.
a. Outer fibrous layer: Sclera and cornea
b. Middle vascular layer: Choroid, ciliary body, iris, pupil and lens
c. Inner nervous tissue layer: Retina
A. Outer fibrous layer: Sclera and Cornea
· The sclera or white of an eye is the outermost layer of the lateral and posterior aspects of the eyeball and is continuous anteriorly with the transparent cornea.
· It maintains the shape of the eye and gives attachment to the extrinsic muscles of the eyeball.
· The light rays pass through the cornea to reach the retina.
· It is convex anteriorly and is involved in refracting light rays to focus them on the retina.
B. Middle Vascular Layer
· It consists of the choroid, ciliary body, iris, pupil and lens.
i. Choroid
· It lines the posterior 5/6th of the inner surface of the sclera.
· It consists of the choroid, ciliary body, iris, pupil and lens.
i. Choroid
· It lines the posterior 5/6th of the inner surface of the sclera.
· It is rich in blood vessels and is deep chocolate brown in colour.
· Light enters through the pupil, stimulates sensory receptors in the retina and is then absorbed by the choroid.
· Light enters through the pupil, stimulates sensory receptors in the retina and is then absorbed by the choroid.
ii. Ciliary body
· The ciliary body is the anterior continuation of the choroid consisting of ciliary muscles and secretory epithelial cells.
· Ciliary body is attached with a suspensory ligament which, at its other end, is attached to the lens capsule.
· Contraction and relaxation of the ciliary muscle alter the shape of the lens i.e. spherical and flattened respectively.
· The epithelial cells secrete aqueous fluid into the anterior and posterior chambers. The ciliary body is supplied by the oculomotor nerve whose stimulation causes contraction.
iii. Iris
· It is a visible, coloured part of the eye and extends anteriorly from the ciliary body and lies behind the cornea and in front of the lens.
· It consists of pigment cells and two layers of smooth muscle fibres (circular and radiating).
· If the pigment is dense, the iris is brown, if little, the iris is blue.
· If there is no pigment, then the eye is white.
· Albinos lack pigments in the eyes.
· In the centre, there is an aperture called the pupil.
iv. Lens
· It is the biconvex, transparent, elastic body lying immediately behind the pupil.
· It is held in position by a suspensory ligament.
· Its thickness is controlled by the ciliary muscle through the suspensory ligament.
· When ciliary muscle contracts, the thickness of the lens is increased to view the nearer objects.
· When the eyes are accommodated for distant vision, the ciliary muscle is relaxed and choroid due to its elastic property recoils and pushes the suspensory ligament peripherally, so the lens is flattened and the object is correctly focused on the retina.
· When ciliary muscle contracts, the thickness of the lens is increased to view the nearer objects.
· When the eyes are accommodated for distant vision, the ciliary muscle is relaxed and choroid due to its elastic property recoils and pushes the suspensory ligament peripherally, so the lens is flattened and the object is correctly focused on the retina.
C. Inner nervous layer: Retina
· This is the innermost delicate structure adapted for the stimulation of light rays.
· The light-sensitive layer of the retina consists of rods and cones.
· Near the centre of the posterior part of the retina is called the Macula lutea or yellow spot.
· A depression at the centre of the macula lutea is a fovea centralis that consists of only cones.
· The rods contain photosensitive pigment ‘Rhodopsin’ and cones ‘Iodopsin’ that convert light rays into nerve impulses.
· Rods are sensitive to dim light and cones to bright light.
· It is considered that there are three different types of cones, each of which contains a different light-sensitive pigment.
· A depression at the centre of the macula lutea is a fovea centralis that consists of only cones.
· The rods contain photosensitive pigment ‘Rhodopsin’ and cones ‘Iodopsin’ that convert light rays into nerve impulses.
· Rods are sensitive to dim light and cones to bright light.
· It is considered that there are three different types of cones, each of which contains a different light-sensitive pigment.
· Cones that contain erythrolabe are most sensitive to red light.
· Cones that contain chlorolabe are most sensitive to green light.
· Cones that contain cyanolabe are most sensitive to blue light.
· Combinations of these three colours of light produce all the colours that humans can see.
· The area of the retina from where the optic nerve leaves the eye is called a blind spot or optic disc.
· Blindspot is devoid of rods and cones so no image is formed here.
Physiology of Sight (Vision)
· Light, which travels at the speed of 3108 m/s, is reflected into the eyes by objects within the field of vision.
· White light is the combination of VIBGYOR. Among them, violet has the shortest and red has the longest wavelength.
· Light, which travels at the speed of 3108 m/s, is reflected into the eyes by objects within the field of vision.
· White light is the combination of VIBGYOR. Among them, violet has the shortest and red has the longest wavelength.
The Electromagnetic Spectrum
· It is broad but only a small part of it is visible to human eyes called the spectrum of visible light. A specific colour is perceived when only one wavelength is reflected by the object and others are absorbed.
· For example: an object appears red when only a red wavelength is reflected.
· An object appears white when all wavelengths are reflected and black when all are absorbed.
· An object appears white when all wavelengths are reflected and black when all are absorbed.
· UV light is not normally visible because it is absorbed by a yellow pigment in a lens.
· UV light is visible after cataract extraction and it has been suggested that long-term exposure may damage the retina.
· To have a clear vision, light reflected from the object within the visual field is correctly focused onto the retina of each eye.
· The processes involved in producing a clear image are the refraction of the light rays, change in the size of the pupil and accommodation.
· To have a clear vision, light reflected from the object within the visual field is correctly focused onto the retina of each eye.
· The processes involved in producing a clear image are the refraction of the light rays, change in the size of the pupil and accommodation.
Refraction of Light Rays
· When the rays of light pass from a medium of one density to a medium of a different density, then they are bent.
· For example, in the eye, the biconvex lens bends and focuses the light rays in the retina. Before reaching the retina, light rays pass through the conjunctiva, cornea, aqueous fluid, lens and vitreous body.
· They all are denser than air and, with the exception of the lens; they have constant refractory power, similar to that of water.
· They all are denser than air and, with the exception of the lens; they have constant refractory power, similar to that of water.
· It is the only structure in the eye that changes its refractory power which is achieved by changing its thickness.
· The amount of refraction is less when the light comes from distant objects and more when it comes from nearby objects.
· When the ciliary muscle contracts, it releases its pull on the suspensory ligament and lens bulges, increasing its convexity; so there is an increase in refractory power that focuses light rays from nearer objects in the retina.
· To focus the light rays from the distant object to the retina ciliary muscle relaxes, it slips backwards and pulls the suspensory ligament, making the lens thinner.
· When the ciliary muscle contracts, it releases its pull on the suspensory ligament and lens bulges, increasing its convexity; so there is an increase in refractory power that focuses light rays from nearer objects in the retina.
· To focus the light rays from the distant object to the retina ciliary muscle relaxes, it slips backwards and pulls the suspensory ligament, making the lens thinner.
· Refractory power decreases, focal length increases, the convexity of a lens decreases in order to view the distant objects.
Pupil
· In bright light, pupils are constricted and in dim light, they are dilated to enter less and more light into eyes respectively.
· The iris consists of one layer of circular and another layer of radiating smooth muscle fibre. Contraction of the circular muscles constricts the pupil and contraction of radiating fibres dilates it.
· The size of the pupil is controlled by the autonomic nervous system.
· Sympathetic stimulation dilates the pupil and parasympathetic stimulation causes constrict them.
· Near-less focal length- high will be the refractory power.
Accommodation
· The adjustment of the eye to enable it to focus on objects at various distances is called accommodation.
Close vision
· In order to focus on near objects i.e. within about 6 metres, the accommodation is made by following adjustments- constriction of the pupil, convergence and changing the power of the lens.
Constriction of pupil
· It reduces the width of the beam of light entering the eye so that light passes through the central curved part of the lens only.
Convergence
· Light rays from the nearby objects enter the two eyes at different angles and for clear vision, they must stimulate the corresponding area of the two retinae.
· To obtain a clear image, extrinsic muscles rotate the eyes so that they converge on the object to be viewed.
Changing the power of the Lens
· The lens is thicker for near vision and is thinnest when focusing on objects at more than 6 metres distance.
· This change in the thickness of the lens is made to focus the light rays on the retina.
· Objects more than 6 metres away from the eye are focused on the retina without adjustment of lens or convergence of the eyes.
Monocular and Binocular vision
· When both the eyes can be focused simultaneously on a common object, as in human eyes, it is called binocular vision.
· When both the eyes can be focused simultaneously on a common object, as in human eyes, it is called binocular vision.
· The images from the two eyes are fused in the cerebrum so that only one image is perceived. Binocular vision provides better judgements over the distance, depth and height.
· When each eye focuses its own object and both the eyes can’t focus on one object, it is called monocular vision.
· When each eye focuses its own object and both the eyes can’t focus on one object, it is called monocular vision.
Extrinsic muscles of the eye, their actions and cranial nerve supply
| Name | Action | Cranial nerve supply |
|---|---|---|
| Medial rectus | rotates eyeball inwards | Oculomotor nerve |
| Lateral rectus | rotates eyeball outwards | Abducent nerve |
| Superior rectus | rotates eyeball upwards | Oculomotor nerve |
| Inferior rectus | rotates eyeball downwards | Oculomotor nerve |
| Superior oblique | rotates eyeball downwards and outwards | Trochlear nerve |
| Inferior oblique | rotates eyeball upwards and outwards | Oculomotor nerve |
Working of the Eye
· The human eye is sensitive to light wavelengths ranging from 380-760 nanometres.
· It consists of two functional parts: the focusing and the receptor part.
i. Focusing part
· It consists of the conjunctiva, cornea, aqueous humour, lens and vitreous humour. These parts are transparent and act as lenses.
· They refract the light rays to focus them on the retina.
· Maximum refraction is caused by the cornea, which places the image approximately on the retina.
· The lens maintains the fine adjustments and brings the image into sharp focus.
ii. Receptor part
· It comprises the retina.
· The image formed on the retina is inverted and smaller.
· It converts the energy of the specific wavelengths of light into action potentials (sensory impulses) of nerve fibres.
· The nerve impulses are carried by the optic nerve to the visual areas of the cerebral hemispheres where the real sensation of the sight occurs and one sees the object upright.
Accommodation
· The adjustment of the eye to enable it to focus on objects at various distances is called accommodation. Human eyes have a remarkable power of accommodation by changing the convexity of the lens.
· The changes in the convexity of the lens i.e. the increased convexity of the lens during accommodation can be determined by the Phacoscope of Helmholtz.
· The size of the pupil can be increased or decreased by the action of iris muscles. In bright light, the pupil is constricted and in dim light, it is dilated.
· Due to the action of the ciliary muscles and suspensory ligament, the focal length of the lens can be changed.
· Then the objects can be focused in different intensities of light from varying distances.
Mechanism of Accommodation
· When the eyes are accommodated for distant vision, the ciliary muscle is relaxed and choroid due to its elastic property, recoils and pushes the suspensory ligament peripherally, so the anterior surface of the lens is flattened and the object is correctly focused on the retina.
· When the eyes are accommodated for near vision, the contraction of ciliary muscles draws the choroid inward, causing the relaxation of the suspensory ligaments. As a result, there is the relaxation of the lens capsule.
· Due to the relaxation of the capsule, the lens assumes a more convex form.
· The anterior surface bulges more than the posterior because it is prevented by the vitreous humour.
Biochemistry of vision
· We will discuss here mainly the photochemistry of rhodopsin but we can apply almost exactly the same principles to the cone pigments.
· The light rays focused on the retina generate potential (impulses) in the rod and cones.
· Light splits the photosynthetic pigment rhodopsin (visual purple) into retinene and opsin (protein) resulting in changes in the three-dimensional structure of the opsin.
· The changed structure of the opsin leads to the activation of a regulatory protein called transducin. The activated transducin activates an enzyme called phosphodiesterase.
· Activated phosphodiesterase converts cGMP (cyclic guanosine monophosphate, which binds to Na+ channels) to 5’-GMP (which does not bind to Na+ channels).
· The influx of sodium ions into the outer segment of the rod cell occurs mainly because of cGMP present in the cytoplasm of the cell.
· The sodium influx maintains a slight depolarization up to -40 mV.
· The potential is constant and it is also called the dark current.
· Because of the conversion of the cGMP to 5’-GMP, the concentration of cGMP is reduced in the rod cell. It is due to the light which leads to the rapid decrease in cGMP levels in the photoreceptor cells which results in the closure of the Na channels.
· The sudden closure of the sodium channels prevents the entry of sodium ions. As a result, the photoreceptor cells become hyperpolarized.
· The hyperpolarization of the cell membrane produces a signal that generates action potentials (impulses) in the ganglion cells.
· The axons of all retinal ganglion cells in one eye form optic nerve.
· The action potentials (impulses) generated in the retina are transmitted by the optic nerve to the visual cortex in the occipital lobe of the cerebral hemisphere of the brain where the neural impulses are analyzed and the image formed on the retina is recognized.
Common Defects of the Eye
· Eye defect is the inability of the lens in the eye to change its shape according to the distance of objects to be viewed.
· The common defects of the eye are as follows:
a. Myopia or Short-sightedness
· In this case, the image of a distant object is formed in front of the retina so a person suffering from this defect can’t see the distant objects clearly but can see nearby objects clearly. Hence, it is also called short-sightedness.
· It is caused due to elongated eyeball and thick eye lens (short focal length).
· Diverging lenses of suitable power are used to remove this defect.
· They diverge the parallel rays and bring them to focus on the retina.
b. Hypermetropia or Long-sightedness
· In this case, the image of the nearby object is formed behind the retina so a person suffering from this defect can’t see the nearby objects clearly but can see the distant objects clearly. Hence, it is also called long-sightedness.
· It is caused due to smaller eyeballs and thin eye lenses (long focal length).
· Converging lenses of suitable power are used to remove this defect.
· They converge the parallel rays and bring them to focus at the retina.
c. Astigmatism
· It is the condition in which the cornea or a lens is curved unequally in different planes so that the light rays in one plane are focused at different points from those in another plane, so blurred vision appears.
· It is corrected by wearing a cylindrical lens.
d. Cataract
· Lens becomes opaque i.e. it loses its transparency so interferes with the transmission of light rays to the retina, causing blindness.
· Surgical removal of opaque lens and replacement of artificial lens or use of spectacles with convex lens and laser treatment of opacity is the remedy of this defect.
e. Glaucoma
· It is caused by increased intraocular pressure (Normal pressure ranges from 10–20 mm of Hg).
· It is due to blockage of the canal of Schlemm and aqueous veins through which eye humour pass out or by over secretion of aqueous humour.
· It exerts pressure on the posterior cavity and greatly reduces the blood supply to the retina, dilated pupil.
· Thus, lack of nutrients ultimately damages the nerve cells of the retina leading to blurred vision and blindness.
Treatment is done by reducing intraocular pressure which is done:
i. Putting drops in the eye at regular intervals so as to improve the outflow of aqueous humour from the eye.
ii. By the intake of diuretics that reduce the aqueous humour production.
iii. Fistulerizing or surgical operation to form accessory channels (new canals) to drain out aqueous humour, if the pressure is due to blockage of the canal of Schlemm.
f. Nyctalopia (Night blindness)
· It is the inability to see in the night due to deficiency of vitamin ‘A’ in the diet.
· It can be corrected by the intake of vitamin ‘A’ or vitamin ‘A’ enriched yellow fruits, green vegetables, milk and milk products.
· Deficiency of vitamin ‘A’ leads to decreased synthesis of rhodopsin for rod cells to function well.
g. Colour blindness
· It is caused due to absence or decrease in particular types of cones in the retina.
· It can be corrected by intake of medicines.
h. Presbyopia (Old age long-sightedness)
· It is caused due to loss of elasticity of the lens after the age of 40.
· There is a failure of accommodation to focus near objects.
· It can be corrected by using reading glasses or bifocal lenses, trifocals or contact lenses.