BIOLOGY-STDX THE NERVOUS SYSTEM. Nervous co - ordination is brought about by, means of a nervous system.

Transcription

1 BIOLOGY-STDX THE NERVOUS SYSTEM Nervous co - ordination is brought about by, means of a nervous system. SOME BASIC TERMS OF THE NERVOUS SYSTEM 1) STIMULUS - Any change in the external or internal environment of an organism to which the organism responds in the form of an activity. 2) RESPONSE - The activity of the organism due to stimulus. 3) IMPULSE - A wave of irritability (electrical depolarization) that sweeps over a neuron. 4) RECEPTORS - Nerve cells which receive the stimulus and pass it on to the brain or spinal cord. 5) EFFECTORS - Muscles and glands which receive the stimuli from the brain and the spinal cord and respond to it. FUNCTIONS OF THE NERVOUS SYSTEM Regulates involuntary actions. Controls and coordinates voluntary muscular activities. It enables us to think, reason and remember. Keeps us informed about the outside world through sense organs BASIC UNIT OF NERVOUS SYSTEM: The basic structural and functional unit of the nervous system is a special cell called a nerve cell or neuron. STRUCTURE OF A NEURON: The three main parts of of a neuron are the cyton (cell body), dendrites and axon (i) The cell body (Perikaryon or Cyton): It contains a well defined nucleus and nucleolus surrounded by a granular cytoplasm. It contains Nissl's granules, rich in RNA and concerned with protein synthesis are prominent. Typical cell organelles like lysosomes mitochondria and golgi complex are found in the cytoplasm. Only centrosome is absent in the nerve cell because these cells have lost the ability to divide. The cyton controls the metabolism in the axon and dendrites. (ii) Dendrites (singular: dendron) Dendrites are usually highly branched extensions of the cytoplasm of the cyton. 1

2 (iii) Axon: Their fine branching nature allows them to reach the tiniest part of the body from where they conduct nerve impulse to the cyton. Axon is a single, long, thin fibre, highly specialised to conduct nerve impulses away from the neuron. Axons vary in length from a few millimetres to even more than one metre in length. In most neurons, the axon is surrounded by white insulating sheath called covering cells or neurolemma. Axon is enclosed by a white multilayered sheath called myelin sheath. Axons without myelin covering are called non-myelinated axon. Nodes on Ranvier are non-myelinated gaps between the segments of myelin sheath. Some axons may have side branches called collaterals. The distal end of the axon terminals have swollen ends to form bulbs which store chemicals called neurotransmitters. These are responsible for passing the impulse from one neuron to another or from a neuron to a tissue. (Acetylcholine is an important neurotransmitter.) DIAGRAM SHOWING STRUCTURE OF A NEURON SYNAPSE : Synapse is the point of contact between the terminal branches of the axon of a neuron with the dendrites of another neuron separated by a fine gap. 2

3 As the impulse reaches the terminal end of the axon, a neurotransmitter acetylcholine is released and the nerve impulse travels through it to the dendrites of the next neuron to continue ahead. The chemical neurotransmitter is soon broken down by an enzyme to make the synapse ready for the next transmission. Transmission of the nerve impulse : Normally the outer surface of a neuron carries a positive charge. This state is called polarized state. It is due to more Na + ions outside the axon membrane. On being stimulated, the axon membrane at that spot becomes permeable to Na + ions which move inwards. Hence, the interior of the neuron becomes positively charged while the outer surface becomes negatively charged. It causes depolarization. This is known as excited region This point of depolarization becomes a stimulus for the next neighbouring area of the membrane, which in turn becomes depolarized. The previous area now becomes repolarised due to active transport of Na + ions again to the outside.this transport is called the sodium pump using energy through ATP. Tramission of the nerve impulse through a nerve fibre Resting region (Polarised) Excited region (Depolarised) Resting region (Polarised) Recovery region (Repolarised) Excited region (Depolarised) Resting region (Polarised) Resting region Recovery region (Repolarised) Excited region (Depolarised) Resting region (Polarised) 3

4 Types Of Neurons: Based on their functions, neurons are of the following types 1) Sensory Neuron: Conveys impulse from the receptor (sense organ) to the central nervous system (the brain or spinal cord). 2) Motor Neuron: Conveys impulse from central nervous system to the effector (muscle or gland). 3) Association (Connecting) Neuron: These cells are typically confined to the CNS. These connect the sensory neuron to the motor neurons. NERVES Nerves are thread like, glistening white structures that emerge from the brain and the spinal cord and branch out to almost all parts of the body. Nerve is a bundle of axons (nerve fibres ) of separate neurons, enclosed in a tubular sheath. There are three kinds of nerves 1) Sensory Nerve : carrying only sensory neurons. E.g. Optic Nerve. 2) Motor Nerve : carrying only motor neurons. E. g. A nerve arising from the brain, supplying the muscles of the eyeball for rotating the eye. 3) Mixed Nerves: carrying both sensory and motor neuron. E.g. Spinal Nerve. SOME IMPORTANT INFORMATION Ganglia (singular : ganglion) are aggregates of the cytons( cell bodies) from which the axons (nerve fibres ) may arise or enter into. Conduction of nerve impulse is a wave of depolarization followed by repolarisation. Functions of the myelin sheath - To increase the speed of nerve impulse conduction. - To insulate the axon. ############### Exercise 1 Q1 Explain the role of : (i) A Neuro transmitter (ii) Sensory Neuron (iii) Motor Neuron (iv) Association Neuron (v) Myelin Sheath Q2. State whether true or false. Correct the incorrect statements. (i) The outer covering of the nerve fibre is called medullary sheath. (ii) The axon can regenerate itself if cut or injured. Q3. Draw a well labelled diagram of a myelinated neuron. 4

5 COMPONENTS OF THE NERVOUS SYSTEM CENTRAL NERVOUS SYSTEM ( CNS) (PNS) PERIPHERAL NERVOUS SYSTEM System BRAIN Somatic Nervous System SPINAL CORD Cranial Nerves Spinal Nerves Autonomic Nervous Parasympathetic Sympathetic 1) Central Nervous System (CNS): It includes the brain and the spinal cord. a) The Brain: An adult brain weighs about 1.35 kg and constitutes 2% of the total body weight. The brain is protected by the cranium of the skull and the three meninges. The meninges, which are continuous with the spinal cord, consist of three layers the outermost tough, fibrous dura mater, the middle web like arachnoid and the innermost,highly vascular pia mater. Inflammation of the meninges causes meningitis. The space between the coverings is filled with cerebro spinal fluid. It serves as a shock absorbing medium and protects the brain against jerks and jolts. It also supplies the neurons in the brain with respiratory gases and nutrients and removes wastes. Pons The brain has the following divisions Medulla oblongata 5

6 (i) Forebrain Cerebrum and Diencephalon (ii) Midbrain (iii) Hindbrain Cerebellum, Pons and Medulla Oblongata (i) FOREBRAIN: It consists of cerebrum and diencephalon. CEREBRUM It is the largest part of the brain and is divided in right and left halves called cerebral hemispheres. The cerebral hemispheres are connected together by a sheet of nerve fibres called corpus callosum. Its function is to transfer information from one hemisphere to another. The walls of the cerebrum has two regions. The outer region is called the cortex and contains the cytons of the neurons,thus it is grayish in colour. It is called the grey matter. The grey matter is highly convoluted with ridges and grooves called gyri and sulci. These convolutions increase the surface area to accommodate more neurons.the number of convolutions are believed to be associated with the degree of intelligence. The inner region of cerebrum has white matter; which is composed of axons of the neurons. Cerebrum is the seat of consciousness, thinking, memory, reasoning, perception and stimulus interpretation. There are also areas concerned with intellect, imagination, foresight, emotions, moods. The cerebrum is the store house of past experiences, which may be recalled during dreams or hypnotization. Diencephalon It lies concealed by the cerebrum. Diencephalon is distinguishable in two parts, thalamus and hypothalamus. Thalamus is concerned with relaying pain and pressure impulses to the cerebrum. Hypothalamus controls the pituitary gland, the body temperature, water balance, blood pressure. The inferior (lower) surface of the diencephalon bears the optic chaisma and pituitary gland. (ii) MIDBRAIN: It is a small tubular part connecting the forebrain to the hindbrain. It has the reflexes involving the eyes and ears. (iii) HINDBRAIN: It consists of cerebellum, pons and medulla oblongata. CEREBELLUM It is a much smaller area of the brain located just at the base of the large cerebrum. It is also divide into two hemispheres. It has no convolutions, but numerous furrows. It also has the outer cortex made up of grey matter and inner white matter. The white matter, in median section, appears like a branching tree. Its main function is to maintain body balance and co ordinate muscular activity. PONS It is located in the centre of the brain below the cerebellum. 6

7 It carries impulses from from one hemisphere of the cerebellum to the other hemisphere. It co ordinates muscular movement on both hemispheres of the cerebellum. MEDULLA OBLONGATA It is the lowest part of the brain located at the base of the brain. It is roughly triangular in shape and continues below with the spinal cord. It has white matter on the outside and grey matter on the inside. Its function is to control the involuntary activities such as breathing, heartbeat, peristalsis, swallowing, vomiting etc. POINTS TO REMEMBER 1. All parts of the brain consist of outer grey matter and inner white matter except the medulla oblongata, which has outer white matter and inner grey matter. 2. An alcoholic person when drunk generally walks clumsily. The cerebellum, due to the effect of the alcohol, is unable to coordinate muscular movements properly. 3. Injury to the medulla oblongata causes death, as the medulla oblongata controls involuntary activities like heartbeat and breathing. Exercise 2 Q1 Name the following (i) The protective coverings located around the human brain and spinal cord. (ii) A neurotransmitter stored at the terminal end of the axon. (iii) Inflammation of the meninges. Q2. Mention the location and function of : (i) Corpus callosum (ii) Pons Q3. Give Reasons : (i) Injury to the medulla results in death. (ii) An alcoholic person walks unsteadily when drunk. ****************************************************** SPINAL CORD Location: Spinal cord is located in the spinal canal of vertebral column. It is a continuation of the medulla oblongata and ends at about the second lumbar vertebrae. Protection: Spinal cord is well protected by the bony canal, meninges and the cerebrospinal fluid. Structure: Spinal cord consists of a series of 31 segments. Each segment gives rise to a pair of spinal nerves. Each pair of spinal nerves is connected to a segment of the spinal cord by 2 points of attachment called roots. 7

8 i) Posterior or dorsal root: It contains sensory fibres only and conducts nerve impulses from the periphery to the spinal cord. Each dorsal root also has a swelling called the dorsal root ganglion. It contains the cell bodies of the sensory neurons. ii) Ventral root: It contains motor neuron axons only and conducts impulses from the spinal cord to the periphery. In cross-section, the arrangement of white and the gray matter in the spinal cord is reversed from that in the brain. Spinal cord consists of an inner H shaped gray matter containing the cell bodies of motor and association neuron and the white matter on the outer side. In the cross bar of the H of the gray matter is a small space called the central canal. The central canal runs the entire length of the spinal cord and contains the cerebrospinal fluid. Function of cerebrospinal fluid: It acts as shock proof cushion and forms a medium for the exchange of food materials, waste products and respiratory gases with neurons. Functions of the spinal cord: It conveys sensory nerve impulses from the periphery to the brain and to conduct motor impulses from the brain to the periphery (skin muscle). It serves as a reflex centre. REFLEX ARC AND REFLEX ACTION Reflex arc is the shortest route that can be taken by an impulse from a receptor to an effector. Reflex action is an automatic, quick and involuntary action in the body brought about by a stimulus. Components of the Reflex arc: Receptor: The nerve cell dendrites which respond to stimulus and convert it into an impulse in a sensory neuron. 8

9 Sensory neuron: The nerve impulse from the receptor passes through an axon terminal branch of the sensory neuron in the spinal cord. Centre: It is a region in the spinal cord where incoming sensory impulse generates an outgoing motor impulse. The centre usually contains one or more association neuron between the sensory neuron and the motor neuron. Motor neuron: It carries impulse generated by the association neuron in the centre to the effector organ (muscle/ gland). Effector: It is an organ (muscle/ gland) that responds to motor nerve impulse. Types of reflexes: Natural reflex and Conditioned reflex. Natural reflex Conditioned reflex These reflexes are inborn and inherited. Response to the stimulus requires no previous experience, learning or judgement. Conditioned reflexes are acquired reflexes which develop during lifetime due to experience or learning. Eg : Swallowing, blushing, blinking. Eg : Spontaneous application of brakes to avoid accident, Knitting while watching television. PERIPHERAL NERVOUS SYSTEM A. SOMATIC NERVOUS SYSTEM-made up of the cranial nerves and the spinal nerves. Cranial nerves - Emerge from the brain. - There are 12 pairs of cranial nerves. Out of which 10 arise from brain stem. - Some cranial nerves contain only sensory fibres and are called sensory nerves. The others contain both sensory and motor fibres and are called mixed nerves. Spinal nerves - Emerge from the spinal cord. - There are 31 pairs of spinal nerves. - All spinal nerves are mixed nerves. They are named and numbered according to the region and level of the spinal cord from which they emerge.. 9

10 Kinds of Nerves: 1. Afferent nerves: The sensory nerves of the PNS form the afferent system which conveys information from the periphery of the body to the CNS. 2. Efferent nerves: The motor nerves of the PNS form the efferent system which conveys information from the CNS to the muscles and glands. 3. Mixed nerves: These nerves carry both sensory and motor fibres. B AUTONOMIC NERVOUS SYSTEM The autonomic nervous system includes nerves conveying impulses to glands, involuntary (smooth) muscles and heart muscles. It consists of 22 pairs of chains of ganglia located close to or embedded in the organs they control. The autonomic nervous system is made up of the sympathetic nervous system and the parasympathetic nervous system. Both of them exert an opposite (antagonistic) effect on organs. The sympathetic nervous system is stimulated by the hormone adrenaline. Effects of the two parts of the autonomic nervous system Sr. no Organs Sympathetic nervous system Parasympathetic nervous system 1. Heart Accelerates heart beat Retards heart beat. 2. Blood vessels Constricts all blood vessels except coronary vessels which dilate. 3. Lungs Dilates bronchi and bronchioles Dilates all blood vessels except coronary vessels which constrict. Constricts bronchi and bronchioles. 4. Intestines Peristalsis decreased. Peristalsis increased. 5. Urinary bladder Sphincter contraction, muscle relaxed. Sphincter relaxation, muscle contraction. 6. Pupil of eye Dilation Constriction. 7. Salivary glands Inhibits secretion of saliva. Stimulates secretion of saliva. 10

11 8. Lacrimal glands Stimulates secretion Inhibits secretion. 9. Erector muscles of skin Stimulates contraction. Relaxes. 10. Body (as a whole) Prepares body for action. Prepares body for relaxation. QUESTION AND ANSWERS: A) Rearrange the following in correct sequence pertaining to what is given in bracket: i) Effector, Sensory neuron, Receptor, Motor neuron, Stimulus, Central nervous system Response. (Reflex arc) Stimulus, Receptor, Sensory neuron, Central nervous system, Motor neuron, Effector, Response. B) Pick up the odd man out and write the category to which the others belong: i) Blinking, Knitting without looking, Smiling, Blushing, Crying. ii) Sneezing, Coughing, Typing, Blinking. iii) Coughing, Sneezing, Eating,Blinking. Sr. No Odd one Category i. Knitting without Natural Reflexes looking ii. Typing Natural reflexes. iii. Eating. Natural reflexes. C) Explain Reflex action. Reflex action is an automatic, quick and involuntary action in the body brought about by a stimulus. D) State whether the following statement is true or false, If false, correct the statement by changing the first or last word only: Dilation of the pupil is brought about by the sympathetic nervous system. True. E) Classify the following actions as natural reflex or conditioned reflex: i) Playing a guitar: Conditioned reflex. ii) Removing your hand suddenly when pricked by a thorn: Natural reflex. iii) Applying sudden brakes when a dog crosses the path: Conditioned reflex. iv) Blinking of eyelids on exposure to light: Natural reflex. 11

12 v) Tying ones shoe lace: Conditioned reflex. F) Differentiate between Cerebrum and Spinal cord ( Arrangement of cyton and axon of neurons) Cerebrum Spinal cord Cytons outside and axons inside. Axon outside and cyton inside. G) During a street fight between two individuals, mention the effects on the following organs by the autonomous nervous system, in the table given below: Organ Sympathetic system Parasympathetic system 1. Heart. Accelerates heart beat Retards heart beat. 2. Pupil of the eye. Dilation Constriction. 3. Salivary gland. Inhibits secretion of saliva. Stimulates secretion of saliva. ACCESSORY STRUCTURES OF THE EYE THE SENSE ORGANS THE EYE Orbits the two eyes are located in deep sockets or orbits on the front side of the head. Eyebrows the coarse, lateral hair protect the eyes from perspiration and the direct rays of the sun. 12

13 Eyelids also a protective movable structure. They shade the eye during sleep, protect the eyes from excessive light and foreign particles, and spread lubricating secretions over the eyeballs. Tear gland/ Lacrimal glands they manufacture tears. Each gland is about the size and shape of almonds and is located at the upper, outer end of the eyeball beneath the eyelid. The Lacrimal secretion (tears) is a watery solution containing salts and lysosome. It cleans and lubricates the eyeball and kills the germs if any. The tear ducts drain off the liquid into a sac lying at the inner angle of the eye Nasolacrimal duct conducts secretion into the nasal cavity. Conjunctiva - a thin membrane covering the entire front part of the eye. It is a epithelium lining the inside of eyelids. Functions of Tears: Wash and lubricate the eye surface. Destroy germs. Communicate emotions. The Internal structure of the Eye: The Eyeball is composed of three layers outer sclera, middle choroid and inner retina. The Sclera A tough, white non elastic fibrous coat that surrounds the eyeball. It bulges out and becomes transparent in the front region where it covers the coloured part of the eye; this part is called the Cornea. The anterior portion is covered by the cornea and its posterior surface is pierced by the optic nerve. Choroid Inner to the sclera, deeply pigmented layer of tissue, richly supplied with blood vessels. 13

14 Ciliary body is the thickest portion arising from a point just behind the junction of the sclera and the cornea. The smooth muscles in the ciliary body alter the shape of the eye lens. Iris is an opaque, coloured disc of tissue continuous with the choroid. It is suspended between the cornea and the eye lens and is attached at its outer margin to the ciliary process. It consists of circular and radial smooth muscle fibres. In the centre is a circular window called the pupil through which light enters the eye. Retina It is the innermost, light sensitive layer of the eyeball covering the choroid and ending at the edge of the ciliary body. The retina contains two kinds of light sensitive neurons the rods and the cones. Rods contain the pigment rhodopsin (visual purple) and are concentrated more towards the periphery of the retina. They are sensitive to dim light but do not respond to colour. Cones contain the pigment iodopsin and are most densely concentrated in the yellow spot.they are sensitive to bright light and are responsible for colour vision. Optic nerve fibres convey the information received from the rods and the cones (through the ganglion neuron) to the brain in the form of nerve impulses. Yellow spot Region of best vision. Lies on the retina, almost at the centre, in line with the horizontal axis of the eyeball. This spot has the high concentration of cones and hence the region of brightest and sharpest vision. Blind spot Region of no vision. This is the point, below the yellow spot, where the nerve fibres from all the sensitive cells of the retina converge and bundle together to leave the eyeball in the form of optic nerve. No sensory cells here and therefore this is the point of no vision. Eye lens is a transparent biconvex crystalline body located just behind the pupil. It contains transparent lens fibres. The eye lens is held in position by suspensory ligament which attaches it to a ciliary body. The eye lens refracts light and converge it on the retina. Change in its focal length by the combined effort of the suspensory ligament and ciliary muscles 14

15 help focus light rays such that a clear vision of near and distant objects is obtained. Two chambers of the Eye Aqueous Humor clear watery fluid found in front chamber between the eye lens and the cornea. It keeps the eye lens moist and protects from mechanical shocks. It also refracts light. Vitreous Humor transparent jelly like thick fluid found in the larger cavity of the eyeball behind the eye lens. It refracts light rays and maintains the shape of the eyeball. It protects the retina and its nerve endings. Image Formation This involves four steps- 1. Entry of light rays: reflected light rays from the object enter the eyes through conjunctiva to cornea to aqueous humor to eye lens to vitreous humor. 2. Focusing of image: the curvature of the cornea converges the light rays to some extent and the eye lens converges them further to form an image on the retina. The image on the retina is real and inverted. 3. Nerve impulse produced in retina transmitted to brain: the light energy of the image produces chemical changes which generate nerve impulses that travel through the optic nerve and reach the occipital lobe of the cerebrum where they give the sensation of sight. 4. Brain interprets: translates the information received and here an inverted image is perceived as an erect image. 15

16 ACCOMMODATION OF THE EYE Ability of the eye lens to change its focal length to view near and far objects is called accommodation. NEAR VISION The ciliary muscle contracts and pulls the ciliary process forward, toward the eye lens. The suspensory ligament slackens as the tension on it is reduced by the contraction of the ciliary muscles. The eye lens becomes more convex (bulges) and the focal length decreases. DISTANT VISION The ciliary muscles relax and move back. The suspensory ligament becomes taut or stretched. The eye lens is pulled thin. It becomes less convex and its focal length is increased. Far vision Near vision LIGHT AND DARK ADAPTATION : The constriction and dilation of the pupil of the eye to adjust to changes in light intensities ability is called adaptation. As a person is exposed to light on leaving a dark room, he is temporarily blinded. The eyes gradually adapt to the light. This is called light adaptation. When entering a dark room from a brightly lit one, he is initially blinded. Gradually the objects are viewed. This is called dark adaptation 16

17 Light adaptation The pupils constrict to prevent entry of light into the eye. The visual purple is bleached to reduce their sensitivity of the rods. Dark adaptation The pupils dilate to allow more light to enter the eye. The pigment of the rods visual purple is regenerated.. ADAPTATION OF THE EYE A constricted pupil- light adaptation A dilated pupil-dark adaptation COMMON DEFECTS OF THE EYE: 1. Myopia / Near sightedness / Short sightedness A condition where near vision is clear while distant vision is blurred. The image of distant objects is formed in front of the retina. Two possible reasons1) the eyeball is lengthened from front to back 2) the eye lens is too curved or too thick Correction can be done by suitable concave lens which causes the light rays to diverge before they strike the lens of the eye and thus bring the image on the retina. The power of glasses used is mentioned in minus. 17

18 Myopia and its correction 2. Hypermetropia / Long sightedness / Far sightedness / Hyperopia A condition where distant vision is clear while near vision is blurred. The image of distant objects is formed behind the retina. Two possible reasons 1) shortening of the eyeball from front to back 2) the eye lens is too flat or thin. Correction can be done by suitable convex lens which intensify ray refraction. The power of glasses used is mentioned in plus. 3. Astigmatism A disorder of cornea or eye lens when curvature of cornea or eye lens is different. Cylindrical lens or contact lenses help in correction. 4. Presbyopia Same as Hypermetropia. Generally after the age of 40. Due to loss of elasticity of the ciliary muscles. Correction can be done by using bifocal lens. 18

19 5. Cataract A loss of transparency/ Opaqueness of the eye lens. Age related disorder that can be surgically corrected by replacing the eye lens. 6. Night blindness Difficulty in seeing at night or in dim light due to defect in metabolism of the visual purple of the rods. Due to deficiency of vitamin A. 7. Colour blindness 8. Squint QUESTIONS: Inherited condition. Cannot discriminate between certain colours such as the red and the green. Affects the males more than the females. A defect where the two eyes may converge (cross eye)or diverge( wide eye). Both conditions may cause double vision or diplopia. Surgery and suitable excercise can correct these defects. Q.1 Differentiate between a) aqueous humour and vitreous humor. b) Yellow spot and blind spot. c) Cones and rods. d) Myopia and hypermetropia. e) Light adaptation and dark adaptation. f) Night blindness and colour blindness. Q.2 Answer the following questions: a. What is a Lacrimal Gland? b. Name an old age eye defect. What happens in it? c. What is meant by power of accommodation of the eye? d. Mention the characteristics of the image that falls on the retina of the eye. 19

20 Human Ear Introduction: The human ear, like that of other mammals, contains sense organs that serve two quite different functions: that of hearing and that of body balance. Anatomically the ear has three distinguishable parts: the outer, middle, and inner ear. Structure of the human ear: 1] Outer ear: The outer ear consists of the visible portion called the pinna, which projects from the side of the head, the short external auditory canal- the inner end of which is closed by the tympanic membrane, commonly called the eardrum. a) Pinna: consists of a thin cartilage covered by skin. The main purpose of this cartilage is to collect sound waves. It acts as a funnel collecting the sound to be directed into the ear canal. b) The Ear Canal: also known as the auditory canal, is only about 2 centimeters in length. It is a passage between the pinna and the eardrum and helps to protect the ear from infection. Lining the ear canal are small hairs that help to filter dirt. With the help of glands, which produce earwax, the inner ear is well protected. c) The Eardrum : or the tympanic membrane, is the key to hearing. It is considered either a part of the outer ear and middle ear. The drum vibrates when it is hit by sound waves and turns the sound energy into mechanical energy. Once these sound waves have been changed into mechanical energy they are transferred to the next bones of the middle ear. 20

21 2] Middle ear: The cavity of the middle ear is a narrow, hollow, air-filled space. It consists of Eustachian tube and ear ossicles. a) The Eustachian tube: it is about 45 millimeters (1.75 inches) long, leads downward from the middle ear to the throat. The Eustachian tube maintains equal air pressure on both sides of the tympanic membrane. b) Ear ossicles: Crossing the middle-ear cavity are three tiny bones that link the tympanic membrane with the oval window and inner ear. From the outside inward they are the malleus (hammer), the incus (anvil), and the stapes (stirrup). The stirrup fits on the oval window- a membrane present between the mid and inner ear. A second opening- the round window also covered by a thin membrane connects the middle and the inner ear. 3] Inner ear: The inner ear is also called the membranous labyrinth,it has three parts semicircular canal, vestibule and cochlea. 21

22 a) Semicircular canals: they are filled with fluid called endolymph. They are attached to the cochlea and help in maintaining dynamic balance while the body is in motion.one end of each canal is widened to form an ampulla which contains sensory cells and nerve fibres which then join the auditory nerve. b)the Cochlea :The cochlea itself is the auditory portion of the inner ear. The name comes from the Latin or Greek words for snail or spiral shell. Internally cochlea is divided lengthwise into 3 canals. Upper and lower canal is filled with perilymph and middle canal or median canal is filled with endolymph. The stapes transmits the sound vibrations to the oval window which is on the outside of the cochlea. The median canal of cochlea contains the sensory cells of hearing called the organ of corti. The oval window vibrates the fluid inside the cochlea, known as perilymph. In turn this causes movement of the hairs within the organ of Corti. The thousands of hair cells within the organ of Corti vibrate and send electronic signals. The frequency of the sounds being received effects which location of hair cells are moved. The higher the frequency, the closer to the entrance of the cochlea the hairs will move, due to stiffness in the basilar membrane. Very strong frequencies can actually cause the hair cells to die. This is the common cause of hearing loss. b) Vestibule: is a short stem joining the bases of semicircular canals to the cochlea. Vestibule contains the utriculus and sacculus, these parts contain sensory cells for static balance when the body is stationary. A] Hearing: Functions of the ear: 1. The pinna collects the sound waves and transmits them through the auditory canal to the ear drum. 2. The vibrating ear drum sets three ossicles also into vibrations which are amplified. 3. The stapes relays the sound waves to the oval window membrane which causes vibrations in the fluid (perilymph) contained in the cochlear canal. 4. Sensory cells in the organ of corti are stimulated by the vibrations travelling in the fluids and send nerve impulses via the auditory nerve to the brain. B] Balancing: The sensory cells in the semicircular canals are concerned with dynamic equilibrium while the body is in motion. Similarly sensory cells in utriculus and sacculus register the static balance with respect to gravity. 22

23 Questions: 1. Give exact location and function of following: a) Organ of corti. b) Sacculus. c) Semi circular cannals. d) Eustachian tube 2. With reference to function of ear answer the question that follows. a) Give the technical term for the structure found in the inner ear. b) Name three small bones present in middle ear. What is biological term for them collectively. c) Name the part of the ear associated with 1) static balance, 2)hearing and 3) balance. 3. Name the nerve which transmits messages from ear to brain. 4. We feel pain in the ears at high altitudes, in airplanes why? 5. Which structure: a) Converts sound waves into mechanical vibrations? b) Converts vibrations into nerve impulses? c) Responds to change in position? d) Transmits impulses to the brain? e) Equalizes the atmospheric pressure and pressure in the ear? f) Has a sensory organ called Organ of Corti? *********************************************** 23