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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.
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Their fine branching nature allows them to reach the tiniest part of the body from where they
conduct nerve impulse to the cyton.
(iii) Axon:
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.
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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)
Resting region (Polarised)
Excited region
(Depolarised)
Recovery region
(Repolarised)
Resting region
Excited region
(Depolarised)
Recovery region
(Repolarised)
Resting region (Polarised)
Excited region
(Depolarised)
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Resting region (Polarised)
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.
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Exercise 1
Q1 Explain the role of : (i) A Neuro transmitter
(ii) Sensory Neuron
(iii) Motor Neuron
(v) Myelin Sheath
(iv) Association Neuron
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.
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COMPONENTS OF THE NERVOUS SYSTEM
CENTRAL NERVOUS SYSTEM ( CNS)
(PNS)
PERIPHERAL NERVOUS SYSTEM
Somatic Nervous System
System BRAIN
Autonomic Nervous
SPINAL CORD
Cranial Nerves Spinal Nerves
Sympathetic
Parasympathetic
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
Medulla oblongata
The
brain has
the
following divisions –
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(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.
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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.
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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.
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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.
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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
Spinal nerves
-
Emerge from the brain.
-
Emerge from the spinal cord.
-
There are 12 pairs of cranial nerves.
Out of which 10 arise from brain
stem.
-
There are 31 pairs of spinal nerves.
-
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.
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.
.
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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.
Dilates all blood vessels except
coronary vessels which constrict.
3.
Lungs
Dilates bronchi and
bronchioles
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.
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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
Natural Reflexes
ii.
Knitting without
looking
Typing
iii.
Eating.
Natural reflexes.
i.
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.
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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.
THE SENSE ORGANS
THE EYE
ACCESSORY STRUCTURES OF 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.
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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.
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 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
Blind 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.
 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
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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 steps1. 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.
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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
DISTANT 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.
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
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Light adaptation
Dark adaptation
The pupils constrict to prevent entry of
light into the eye.
The pupils dilate to allow more light to
enter the eye.
The visual purple is bleached to reduce
their sensitivity of the rods.
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.
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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.
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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
 Inherited condition.
 Cannot discriminate between certain colours such as the red and the green.
 Affects the males more than the females.
8. Squint
 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.
QUESTIONS:
Q.1
Differentiate between
a)
b)
c)
d)
e)
f)
Q.2
aqueous humour and vitreous humor.
Yellow spot and blind spot.
Cones and rods.
Myopia and hypermetropia.
Light adaptation and dark adaptation.
Night blindness and colour blindness.
Answer the following questions:
a.
b.
c.
d.
What is a Lacrimal Gland?
Name an old age eye defect. What happens in it?
What is meant by power of accommodation of the eye?
Mention the characteristics of the image that falls on the retina of the eye.
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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.
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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.
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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.
Functions of the ear:
A] Hearing:
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.
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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”?
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