Receptors In Man – Organ of hearing (ear) Hearing and balancing:The ears contain receptor cells that are sensitive to sound vibrations between frequencies of 20 Hz to 25000 Hz (hertz). The exact range varies with age of a person.
The human ear has the following divisions.
- Outer ear
- Middle ear
- Inner ear
Outer ear
It consists of an outer flap of cartilage covered by the skin known as the ear pinna and a tube that leads inwards called the external auditory meatus. It contains sebaceous glands that produce secretions.
Middle ear
The external auditory meatus ends at the drum which is a membranous structure. From here onwards (inwards) is the middle ear which is an air filled cavity and communicates with the vassal cavity (cavity of nose) by a narrow tube called Eustachian tube. Also present in the middle ear are three bones known as incus, the second is malleus and the third stapes that fits into the window-like opening in the bone of skull that separates the middle ear from inner ear.
Inner ear
The inner ear consists of the following parts. There are 3-semicircular canals having swellings at one of their ends called the ampullae. The other parts are the utricles, succulus and a coiled cochlea. The semicircular canals open into these parts.
The auditory nerve from brain supplies branches to the cochlea making its inner cells sensitive to sound vibrations. The three semicircular canal, utricles, succulus and cochlea are collective called the membranous labyrinth.
The space around the membranous labyrinth contains a fluid, the perilymph and inside it, is another fluid the endolymph containing certain calcareous particles.
(Study the model of ear)
Fig. 15.7 Human ear
Hearing
The sound vibrations in the air are directed into the external auditory meatus. These strike the ear drum and make it vibrate. The eardrum transfers the vibrations to the three ossicles and through these to the perilymph and the endolymph which are set into motion. The particles in the endolymph strike the sensitive cells of cochlea which is activated sending impulses that travel through the auditory nerve to brain and are interpreted as sound.
Balancing
The 3-semicircular canals, utricles and saccules are the structures that are concerned with maintaining balance and posture. The former responding to changes in the directions and movement and the latter to changes in the posture. Any defect in these causes dizziness and loss of balance. It is common experience that if a person goes round and round on a swing, he loses his balance and is unable to stand upright for some time until the balance is restored.
One of the causes of deafness is puncture in the ear drum due to an injury or infection, thus impairing the drum vibrations.
Other senses: Touch, Taste and Smell
Touch
There are encapsulated nerve endings in the skin (dermis) which respond to slight pressure on skin. The skin responds to heat and cold also. Experiments have shown that the spots in the skin that respond to cold do not respond to heat and vice versa. It is also known that any stimulus that effects the free and exposed nerve endings produce sensation of pain. Some sensory receptors are placed deeper in the tissues and are not stimulated unless the tissues above are firmly compressed.
Taste
It is generally believed that the entire tongue is the organ of taste but it is not so. The tiny organs of taste are located on the surface of tongue. These are called the taste buds. They are spherical in shape and microscopic in size. A taste bud consists of a group of cells that are supplied with nerve endings from the nerve that penetrates the inner part of the mouth cavity.
There are four kinds of taste buds, each kind specified to seek a specific taste i. e. sweet, salty, sour, and bitter. Those for salty taste are situated along with those for bitter taste on the side of tongue and for sweet taste at the tip of tongue.
Fig. 15.8 (a) taste maps (b) the structure of a taste bud.
Smell
The organ of smell lies in the nasal cavity above mouth. It is also called the olfactory organ and its sensitive cells are supplies by the branches of the olfactory nerve from brain. The incoming air currents carry the smell particles of substances into the nasal cavity which arouse the sensitive cells in the organ of smell. The olfactory nerve carries the sensation to the brain, which determines the nature of a substance emitting the smell.
Receptors in man
Receptors are the sensory organs which receives a stimulus. Following are the main receptor organs in man.
- Organ of sight (eyes)
- Organ of hearing (eyes)
- Organ of smell (nose)
- Organs of taste (taste buds on tongue)
- Organ of touch (skin)
Sight
Eyes are the most marvelous of the sense organs as they make us aware of various objects around us, nearby and far away. When we see a tree we do not have to touch it or climb its branches in order to know what it is like. With normal vision we can reach out as far as out eyes can see and are thus become aware of events and circumstances around us. The eyes are organs of sight enabling to appreciate shape, color and movements of objects and persons around us.
Structure of the eye
In order to know how the eyes make us to see things we have to know how an eye is constructed. It is a relatively small organ, spherical in shape except in the front portion (the transparent region) where it bulges out a little. In the average person the eye is less than one inch (about 2.4 cm) in diameter each eye is located in a bony socket called orbit. The orbit is formed partly by the bones that surround brain and partly by the bones of upper face.
The sockets also contain muscles that move the eye ball, the nerves and blood vessels that supply the eye balls. In front of each eye are eye-lids, an upper and a lower, which acting together act as a curtain to shut out light. As lids separate or when the eyes open the front portion of the eye ball is exposed and light can enter. Image is formed by the light rays entering the eye through the anterior transparent portion.
An eye has the following parts.
- Cornea 2. Iris 3. Lense
4. Sclerotic layer 5. Choroid layer 6. Retina
7. Blind spot 8. Aqueous chamber and aqueous humour
9. Vitreous chamber and vitreous humour
A camera which is a human invention is based on the same principle as the human eye.
Fig. 15.5 structure of eye
The sclerotic of eye ball is the out-most layer, white in color and is thick and firm. It is transparent in front and bulges out slightly to form the cornea. Below sclerotic is the second layer of the eye ball called choroid, which is pigmented and vascular. Behind cornea the choroids bends inwards to form the iris having a circular opening called pupil through which the size of pupil can be increased or decreased. Situated behind pupil is a biconvex lens, which undergoes changes in convexity during the process of focusing. It is kept in place by suspensory ligaments and muscles.
Receptors In Man – The third and the inner most layer of eye is the retina. It is the sensitive layer of the eye. It is supplied by branches of optic nerve which enters the eye ball from brain.
The sensory cells of retina are of two types:
- Rods 2. Cones
These cells help in the formation of image on retina. The cones enable to differentiate color while the rods are sensitive to white light. Above the point of entry of optic nerve is a blind spot.
(Animals like cat can see in almost dark as it has additional rods but cannot differentiate colors because cones are absent from its retina.)
The iris divides the cavity or eye into an anterior aqueous chamber containing a fluid the aqueous humour and a large posterior chamber, the vitreous chamber; this contains transparent jelly like vitreous humour. The pupil (opening in the iris), the lens behind the iris, the aqueous humour and the vitreous humour are all involved in the focusing of a clear and sharp image on retina.
(Study the model of human eye and a fresh eye of a sheep / goat.)
There are six muscles present in the orbit attached to the outer side of sclerotic layer that cause the movements of the eye in different directions. Also present in the sockets are the tear glands, the secretion of which washes the eyes.
Image Formation
During focusing, the light rays coming form object pass through cornea and lens. The light rays reflected from a given point at a distance striking the cornea are almost parallel to each other. These rays are focused by the lens on the retina on a special spot called fovea centralis (yellow spot) fro producing sharp image.
The lens plays its role by changing its convexity to produce enough refraction in addition to that produced by cornea to bring to image into focus and from a clear image on retina.
Farsighted and nearsighted vision
Farsightedness
In far or long sightedness the image is formed behind the retina. The cornea and the lens cannot bend the light rays enough to focus on the retina. Artificial lenses are used that bend the incoming light rays before they reach cornea thus aiding the cornea and lenses to focus the image on retina.
Nearsightedness
In the near or short sightedness (myopia), the lens and cornea focus the image of a distant object at a point in front of the retina. This problem is corrected by a concave lens which is thinner in the center than at the edges.
Colorblindness
As already mentioned there are two kinds of light receptor cells in the retina, the rods and the cones. These are the cones that enable us to distinguish colors. There are three kinds of cone cells adapted to receive three primary colors i.e. blue, green and red. To receive other colors the eye requires coordinated activation of the three kinds of its cone cells. In a colorblind person one type of cone cells are lacking. Such a person is unable to distinguish between green and red colors and is called colorblind. Colorblindness is a genetic (hereditary) defect (Fig. 15.6), discussed in detail in chapter 17.
Fig 15.6 color blindness chart
(There are certain other defects of human eye such as night blindness in which a person is unable to see after sun-set; and cataract which is the defect of aqueous humour.)