NERVOUS SYSTEM
Every organism must somehow become aware of what is going
on around it and accordingly perform actions for its survival. Apart from
actions which provide adjustments to the external environment, there are so
many activities going on inside our body of which we are unaware. All such
actions have to be properly timed and coordinated. Such coordination occurs by
two agencies — the nervous system and the hormonal system.
NEED OF NERVOUS SYSTEM
The nervous system in our body performs the following major
functions:
1. Keeps us informed
about the outside world through the sense organs.
2. Enables us to remember,
to think and to reason
out.
3. Controls and harmonises all
voluntary muscular activities. Ex- running or even, holding this book in
your hand while you are reading it.
4. Regulates involuntary
activities such as breathing or the beating of the heart, without our
thinking about them.
NEURON (NERVE CELL) THE UNIT OF THE NERVOUS SYSTEM
Our nervous system consists of brain, spinal cord, sense
receptors and a whole lot of nerves. The brain and spinal cord are madeup of
neurons or nerve cells. So, let us first learn about these cells, the neurons.
STRUCTURE OF THE NEURON
The three main parts of a neuron are the cell body,
dendrites and axon:
(i) The cell body
(Perikaryon or Cyton) (peri: surrounding, karyon: nucleus)-
* It contains a well-defined nucleus, surrounded by
granular cytoplasm.
* It has all the cell organelles like other cells, only centrosome is absent
because nerve cells have lost the ability to divide.
(ii) Dendrites (dendron: tree/branch)-
These are branched cytoplasmic projections of the cell
body. They conduct nerve impulses to the cyton.
(iii) Axon-
It is a long process from the cell body.
It varies in size from a few millimetres to even more than
one metre in length.
In most neurons, the axon is surrounded by a white
insulating sheath known as myelin (or medullary) sheath which is covered by an outermost
thin sheath called (neurolemma).
The myelin sheath shows gaps throughout the length, which
are called Nodes of Ranvier.
Some axons may have side branches called collaterals.
The end portions of the axons have swollen ends like
“bulbs”, which store certain chemicals called neurotransmitters.
Axon terminals are closely placed near the dendrites of
another one or more neurons but are not connected Such gaps in between are
called synaptic clefts
Transmission of the nerve impulse
In the normal (resting) condition, the outer side of the nerve
fibre carries positive (+) charge. This is called polarised state. This
polarisation is due to more Na+ ions outside the axon membrane.
On stimulation (mechanical, electrical, chemical or heat, etc.), the axon membrane at that spot becomes more permeable to Na+ ions which move inwards and cause loss of polarisation (depolarisation). This is known as the excited region.
The point of depolarisation becomes a stimulus for the next neighbouring area of the membrane which in turn becomes depolarised.
Meanwhile, the previous area becomes repolarised due to active transport of Na+ ions again to the outside. This transport is achieved by what is called “sodium pump” using energy through ATP.
Do not compare conduction of nerve impulse in a nerve fibre with the flow of electricity through an electric wire. In the latter, the electrons actually do move along the wire whereas neither any substance nor any electrons or the ions move along the nerve fibre.
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.
Here, the nerve impulse “jumps” into the next neuron. This
is a chemical process. As the impulse reaches the terminal end of an axon, a
chemical acetylcholine is released. This chemical sets a new impulse in the
dendrites of the adjacent (next) neuron. The chemical is soon broken down by an
enzyme to make the synapse ready for the next transmission.
TYPES OF NEURONS
Sensory neurons convey the impulse from the receptor (sense organ)
to the main nervous system (the brain or spinal cord).
Motor neurons carry the impulse from the main nervous system
to an effector (muscle or gland).
Association (connecting) neurons are located in the brain and spinal cord, which
interconnect the sensory and motor neurons.
NERVES
Nerves are the thread-like white structures which emerge
from the brain and spinal cord and branch out to almost all parts of the body.
A nerve may be compared to an underground electric cable containing numerous
conducting wires, each insulated from the other. The myelin sheath of the axon
acts like an insulation and prevents mixing of impulses in the adjacent axons.
Three kinds of nerves are as follows:
Sensory nerves contain only sensory fibres bringing impulses
from the receptors (sense organs) to the brain or spinal cord.
Example: Optic nerve arising from the eye and ending in the
brain.
Motor nerves contain only motor fibres carrying impulses
from the brain or spinal cord to effector organs (muscles or glands) to bring them
into action.
Example: a nerve arising from the brain and supplying the
muscles of the eyeball for rotating the eye.
Mixed nerves are
those, that contain both sensory and motor fibres.
Example: a spinal nerve.
TWO MAJOR DIVISIONS OF THE NERVOUS SYSTEM
Central nervous system (CNS) includes the brain lying in the skull and the
spinal cord contained within the vertebral column.
Peripheral nervous system (PNS) includes the nerves that emerge from and enter
into the brain and spinal cord.
The PNS consists of two subdivisions:
A. Somatic nervous system (SNS) Conveys
information to skeletal (voluntary) muscles.
B. Autonomic nervous system (ANS) includes a pair of chains of ganglia and nerves
which control the involuntary actions of many internal organs (smooth muscles,
heart muscles and
glands).
THE BRAIN
In proportion to the size of the body, the human brain is
the largest among all animals. The brain is a very delicate organ well protected
inside the brain box (cranium) of the skull.
Bulk- The adult brain weighs about 1.35 kg and constitutes about 2% of the total body weight.
It is 80% water and consumes more than 25% (one-fourth) of
the total oxygen taken into the body.
Coverings (Meninges)- The brain is protected by 3 membranous coverings called meninges which continue backwards on the spinal cord.
Dura mater - the
outermost tough fibrous membrane.
Arachnoid - the
thin delicate middle layer giving a web-like cushion.
Pia mater- the
innermost highly vascular membrane, richly supplied with blood.
The space between the covering membranes is filled with a watery fluid-cerebrospinal fluid which acts like a cushion to protect the brain from shocks. The same fluid also fills the central spaces (ventricles) of the brain and the central canal of the spinal cord.
PARTS OF THE BRAIN
The brain has three main parts visible externally,
(1) cerebrum, (2) cerebellum and (3) medulla oblongata.
CEREBRUM (“Brain”)
The cerebrum is the largest portion of the brain.
*It is divided into two (right and left) halves called
cerebral hemispheres. Their outer surface is highly convoluted with ridges and
grooves.
* Each cerebral hemisphere is hollow internally and the
walls have two regions- an outer (cortex) and an inner portion (medulla).
* The outer portion (cortex) of the cerebrum contains cell
bodies of the neurons and, being grayish in colour, is called the gray matter.
* It is the layer of gray matter which is folded to form
the convolutions. The folds are called gyri and the grooves are called sulci
(creases). Such a system increases surface area to accommodate more nerve
cells.
* It is believed that the higher number of convolutions in
the human brain is due to the larger number of brain (nerve) cells and hence greater
intelligence.
* The inner portion of the cerebrum consists of “white
matter” which mainly contains the axons (nerve fibres) of the neurons.
* Corpus callosum (“hard body”) is a sheet of fibres
connecting the two cerebral hemispheres. Its function is to transfer information
from one hemisphere to the other.
The highly developed cortex (gray matter) enables us to think, reason out, invent, plan and memorise. Overall, the cerebrum is the seat of intelligence, consciousness and will-power. It controls all voluntary actions.
What is generally called the subconscious or unconscious mind is also located in the cerebrum. Many past experiences are covered up by more recent impressions which dominate conscious activity.
In dreams or when hypnotised and skilfully
questioned, past experiences may be recalled.
CEREBELLUM (“Little Brain”)
The cerebellum is a much smaller area of the brain located just at the base and
under the large cerebrum. It has no convolutions, but has numcrous furrows.
This also has an outer cortex made of gray matter. Centrally, it has white
matter which, in a median section, appears like a branching tree.
The main function of the cerebellum is to maintain ‘balance’ of the body and coordinate muscular activity. The impulse for performing a muscular act, originates in the cerebrum and not in the cerebellum. for example- if you stand up and walk, the impulse for this activity arises in the cerebrum (conscious part). The act of walking involves coordinated working of many muscles. Proper coordination and timing of their contraction and relaxation is the responsibility of the cerebellum.
MEDULLA OBLONGATA
The medulla oblongata is the lowest portion of the brain
located at the base of the skull. It is roughly triangular and is continued
behind as the spinal cord. Its function is to control the activities of the internal
organs, for example- peristaltic movement of the alimentary canal,
movement of breathing, beating of the heart and many other involuntary actions.
Injury to the medulla generally results in death.
THREE PRIMARY REGIONS OF THE BRAIN
All parts taken together, the brain may be said to consist
of three primary regions-forebrain, midbrain and hindbrain. The various parts
under each region and their principal functions are as follows:
FOREBRAIN
(1) Cerebrum (cerebral hemispheres) (seat of intelligence,
memory, consciousness, will power, voluntary actions).
(2) Diencephalon
(a) Thalamus (relays pain and pressure impulses to
cerebrum).
(b) Hypothalamus (controls the body temperature and
pituitary).
2. MIDBRAIN
A small tubular part (Reflexes involving eyes and ears).
3. HINDBRAIN
(1) Cerebellum (coordinates muscular activity, balance of
the body),
(2) Pons. It is located in the centre of the brain below
the cerebellum. It carries impulses from one hemisphere of the cerebellum to
the other hemisphere and coordinates muscular movements on both the sides of
the body.
(3) Medulla oblongata (controls activities of internal
organs, heartbeat, breathing, etc.).
THE SPINAL CORD
The spinal cord extends from the medulla of the brain down
almost the whole length of the backbone to end at the second lumbar vertebra
and lies within the neural canal of the vertebrae. Figure
Shows the internal structure of the spinal cord in a cross section.
Here the arrangement of white and the gray matter is
reversed from that in the brain. The gray matter containing the cell bodies of
motor (efferent) and association neurons lies on the inner side and the white
matter on the outer side. The white matter contains axons running longitudinally
to and from the brain and even crossing from one side to the other. There is a
small central canal in the centre which runs the entire length and is
continuous with the cavities of the brain. It is also filled with cerebrospinal
fluid which acts as a shock proof cushion and forms a medium for the exchange
of food materials, waste products, and respiratory gases with neurons. Externally,
the spinal cord is covered by the same three membranes dura mater, arachnoid and
pia mater in continuation with those of the brain.
FUNCTIONS OF THE SPINAL CORD
The spinal cord is concerned with the following three
functions:
(i) Reflexes below the neck.
(ii) Conducts sensory impulses from the skin and muscles to
the brain, and
iii) Conducts motor responses from the brain to muscles of
the trunk and limbs.
PERIPHERAL NERVOUS SYSTEM
The peripheral nervous system (PNS) includes the nerves
which carry impulses to and from the central nervous system. It is divided into
somatic nervous system and autonomic nervous system.
SOMATIC NERVOUS SYSTEM
This consists of two sets of nerves:
(1) Cranial nerves and
(2) Spinal nerves
1. Cranial nerves emerge from the brain. There are twelve
pairs of cranial nerves, some of which are-
sensory like the olfactory (for nose) the optic (for eyes) and auditory (for
ears),
motor nerves like the ones going to the eye muscles, and
mixed nerves like those going to and coming from the face
and tongue.
2. Spinal nerves emerge from the spinal cord. There are thirty-one pairs:- 8 pairs in the neck region, 12 pairs in the thorax, 5 pairs in the lumbar region, 5 pairs in the sacral and 1 pair in the coccygeal region.
A typical spinal nerve originates from the spinal cord by means of two roots- a dorsal root and a ventral root. Each dorsal root has an ovoid dorsal ganglion. Every spinal nerve is a mixed nerve having both sensory and motor fibres. At the junction of the two roots, the sensory and the motor fibres separate out-the sensory fibres continue in the dorsal root and the motor fibres into the ventral root. Both the roots enter the gray matter of the spinal cord and end in the corresponding dorsal and ventral projecting “horns” of the gray matter.
AUTONOMIC NERVOUS SYSTEM
The autonomic nervous system (ANS) consists of a pair of chains
of nerves and ganglia on either side of the backbone. This system controls the
involuntary actions of the internal organs. There are two parts of the
autonomic nervous system-
1. Sympathetic and 2. Parasympathetic.
Nerves of sympathetic system arise from the spinal cord between the neck and the waist region. The parasympathetic system is located at two places, one anteriorly in the head and neck and the other posteriorly in sacral region.
These two parts are in general, opposite to each other in action (see Table).
The sympathetic nervous system is stimulated by the hormone adrenaline secreted by the adrenal gland, located on kidneys.
EMOTIONS AND AUTONOMIC NERVOUS SYSTEM
Autonomic nervous system is strongly influenced by emotions
such as grief, anger,
fear, sexual stimulation, etc. High blood pressure, stomach ulcers
and some other troubles may arise due to long continued emotional stress.
REFLEXES (INVOLUNTARY ACTIONS)
There are two types of actions which occur in our body:
Voluntary (performed consciously) and Involuntary (performed unconsciously).
Examples:
1. Voluntary actions
* You wish to watch some programme on TV and you switch it
on and press the remote for a particular channel.
* You pick up an apple and eat it.
* To know the time, you raise your arm and look at the
watch on your wrist.
2. Involuntary actions (occurring unknowingly) (also called
Reflexes)
* Some particle falls into your eye and there is immediate flushing of tears to wash
out the particle (glandular secretion).
* Instantaneous
withdrawal of hand when it accidentally touches a hot iron (muscular movement).
* Shivering
when it is too cold (muscular contractions) or sweating when too hot (glandular secretion).
* Dilation
of the eye pupil to look in the dark and vice versa (muscular movement).
* Pushing
along of the swallowed food through the food canal (muscular movement).
* Non-stop
beating of the heart (muscular movement)
All involuntary actions or reflexes are initiated by some kind of sensory stimulation resulting in either a muscular action or a glandular secretion.
Differences between reflexes and voluntary actions
TYPES OF REFLEXES
Reflexes are of two types :-
(1) natural (inborn) reflexes and (2) conditioned (acquired) reflexes.
*Natural (inborn) reflex is one in which no previous experience or learning is required These reflexes are inborn, i.e. inherited from the parents.
Example: - Blinking,
coughing, sneezing: these are protective.
Salivation, swallowing, peristalsis: provide functional efficiency.
*Conditioned (acquired) reflex is one which develops
during lifetime due to experience or learning. For example, what you experience
simply at the sight or by the smell of a familiar tasty food resulting in
watering (salivation) of your mouth. This means that if you have not eaten that
food earlier, the response would not occur.
Salivation can occur as a natural reflex also. Saliva starts flowing down when you chew or eat food. But in the above example of salivation, the Sight or the smell of food was enough for the response. There, your brain actually remembered the taste of the food and worked in an unconscious way. Such conditioned reflexes are not inborn and are thus also called acquired reflexes.
A conditioned reflex is an involuntary, spontaneous automatic
response brought about due to a previously learned experience.
PAVLOV’S EXPERIMENT ON A DOG
A classic example of developing a conditioned reflex is
that of Pavlov’s experiment on a dog. This famous Russian biologist noted that
under normal conditions, no animal will secrete saliva in response to the
ringing of a bell or to any other sound. But when a bell was repeatedly sounded
simultaneously with the presentation of food during an adequate period of
training, the dog salivated at the sound of the bell even when there was no
food at that time. In other words, a stimulus which previously had no relation with the organism’s
salivary response, now elicits a response in that gland. He ere the condition
(stimulus) is the sound (not the sight of the food) and the reflex is
salivation.
SOME COMMON REFLEXES IN HUMANS
A. Natural (inborn)
Reflexes
Knee-jerk in which the leg is involuntarily extended as a
result of a sharp tap below the knee-cap in a relaxed leg rested on the other leg
while sitting.
Quick closing of the eyelids when an object suddenly approaches the eye or when a strong beam of light is flashed across.
Withdrawal of the hand
or
any part of the body when suddenly pricked.
Peristaltic reflexes: Chyme
(food) is propelled through the small intestine by peristaltic waves. When a portion
of the intestine becomes distended with chyme, the stretching of the intestinal
wall becomes a stimulus and its contraction pushing the food along is the response.
Coughing reflex when
the food swallowed enters the wind pipe.
Sneezing reflex when
any irritant enters the nose.
Most of the daily habits and acts are conditioned reflexes.
Seeing the teacher entering the classroom, you stand up.
You tie
your shoe-laces while talking and not knowing whether you are first
putting the right lace over the left or vice versa.
Playing on a musical
instrument.
Using keys of the
key-board while working on the computer.
Giving a hand signal to your
right automatically, when you are turning your cycle or car to the right.
Applying brakes in
your vehicle (car or bicycle), if someone suddenly comes in front.
Now you can better differentiate between a simple and an acquired reflex as follows:
NERVOUS PATHWAYS IN REFLEXES
A reflex action must be quick to have value. Therefore, the
pathway for receiving and sending information must be short. In a simple
reflex, impulses are not conducted up and down the spinal cord, but leave at
the same level at which they enter.
This may involve two neurons or three neurons. When it involves
two neurons, these are: a dorsal sensory neuron located in the dorsal root and a ventral motor neuron located in the
ventral part of the gray matter of the spinal cord with its fibre in the ventral root.
The impulse from the sense receptor is received through the
sensory (afferent meaning carrying towards) fibres of the dorsal root of the spinal
nerve, and as it reaches the motor neuron in the spinal cord, a response
impulse is immediately
flashed "Like a reflection" through the same spinal nerve
to the effector muscle (or organ) to react.
Sometimes, a third neuron (called intermediate or connecting or association neuron or relay neuron) is included in the reflex arc lying between the sensory fibre and the motor neuron inside the spinal cord as is shown in Fig. 9.7B. After the instant response, the sensation is also carried to the brain via the interconnecting longitudinal nerve fibres (ascending neurons) in the spinal cord.
COMPLEX REFLEX ACTION
Complex reflex action involves neurons at different levels
of the spinal cord. While walking in the dark on a road, if you happen to step
on a coiled piece of rope, you suddenly jump aside with the fear of a suspected
snake. Virtually, all The skeletal muscles are involved in this reflex.
The term “reflex” comes from the Latin word “reflexus” meaning reflected or directed back. Can you make out wherefrom the impulses are reflected back in a reflex?
The above path can be expressed schematically as follows:
The various terms/components ‘in it’ ‘are explained below:
Reflex arc- is the shortest route that can be taken by an impulse from a receptor to an effector.
Receptor- The specialised epithelial cells in contact with the terminal endings of the nerve cells which respond to stimulus and convert it into an impulse in a sensory neuron.
Sensory neuron- The neuron in the spinal cord that receives nerve impulses through its axon/terminal endings which are in contact with a receptor cell.
CNS
(Central nervous system - Spinal cord/ brain)- It is a region in the
spinal cord/brain where’ incoming sensory impulse generates an outgoing motor
impulse. In the centre, there may be an association neuron between sensory and motor
neuron.
Motor
neuron- It
carries impulse generated by the association neuron in the CNS to the effector organ
(muscle or gland).
Effector- It is an organ (muscle or
gland) that responds to motor nerve impulse.
SENSE ORGANS — EYE AND EAR
THE SENSE ORGANS
The sense organs enable us to be aware of the conditions of the environment.
The major sense organs in our body are the eyes, ears,
tongue, nose and skin which are sensitive to light, sound, taste, smell and
touch respectively. In addition, there are also the senses of balance, body movements,
hunger and thirst, pain, etc. The actual sensation is perceived by the sensory
cells located in these organs-such cells are categorised as receptors.
THE EYES
Orbits : The two eyes are located in deep sockets or orbits
on the front side of the head. Each eye is in the form of a ball and can be
rotated with the help of six muscles.
Eyelids : The upper and the lower movable eyelids protect
the outer (front) surface of the eyes and can shut out light. Each eyelid
carries outwardly curved eyelashes which prevent falling of larger particles
into the eye.
Eyebrows are
virtually not a part of the eye, yet these are also protective; they prevent
the rain drops or the trickling perspiration from getting into the eyes.
Tear glands (lacrimal glands) are located at the upper sideward portion of the orbit.
Six to twelve ducts of the gland pour the secretion over
the front surface.
* The movements of the eyelids (blinking) spread the liquid which mainly serves as a lubricant.
* The tears also keep the front surface of the eye clean by washing away dust particles.
* They also have an antiseptic property due to the enzyme lysozyme which kills the germs.
* The tear ducts drain off the liquid into a sac lying at the inner angle of the eye.
A nasolacrimal duct conducts the secretion into the nasal
cavity. All of us have sometimes experienced that medicines dropped in the eyes
come into the nose and even into the throat. This happens through the
above-mentioned duct. Due to irritation or in certain emotional states, the
tear glands pour out a lot of liquid which “waters the eyes” or overflows as
“tears”. You may shed tears both in grief and in extreme joy.
The conjunctiva is a thin membrane covering the entire front part of the eye (Fig). It is continuous with the inner lining of the eyelids. Over the cornea, it is reduced to a single layer of transparent epithelium. You must have often heard of a very common eye disease “conjunctivitis” in which this outermost layer turns red due to a viral infection.
The bony orbit socket,
eyebrows, tear glands and conjunctiva serve for eye protection in their own
ways.
STRUCTURE OF THE EYEBALL
The wall of the eyeball is composed of three concentric layers :
(1) outer sclerotic, (2) middle choroid, and (3) inner retina.
(1) The sclerotic layer (or sclera) is made of tough fibrous tissue and is white in colour. The white portion on the front of the eye is the sclerotic layer, itself visible through the conjunctiva. 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.
Sometimes, the cornea of certain patient turns opaque (white) and non-functional. In such cases, the
defective cornea can be replaced by a healthy cornea from a
donated eye.
(2) The choroid layer is richly supplied with blood vessels for providing nourishment to the eye. It contains a dark black pigment (melanin) which prevents light rays from reflecting and scattering inside the eye. In the front of the eye, the choroid expands to form the ciliary body (containing circular muscles). The smooth muscles in the ciliary body alter the shape of the lens.
The iris (Latin iris : rainbow) is also an extension of the choroid, partially covering the lens and leaving a circular opening in the centre, the pupil. The blue, brown or black colour of the eye refers to the colour of the iris. [“PUPIL” name has been derived from the Latin word “pupa” meaning “doll”, which in this context refers to the tiny image of oneself seen reflected in another’s eye]. The iris contains radial muscles to widen and circular muscles. to constrict the pupil. This adjustment of the size of the pupil regulates the amount of light entering the eye. You can easily observe this by throwing a torch light into the eyes while looking in a mirror. In dim or dark light, the pupil is dilated, while in bright light, it is constricted. The pattern and arrangement of the iris muscles is unique for every individual and therefore these are also a source of an individual’s identification.
(3) The retina or the innermost layer is sensitive to light. It contains two types of sense cells called rods and cones.
The rod cells (inner ends rod-like) are sensitive to dim light but do not respond to colour. They contain the pigment rhodopsin or visual purple. The rod cells are distributed almost throughout the retina.
The cones (inner ends conical) are sensitive to bright light and are responsible for colour vision. They contain the pigment iodopsin. The cone cells are mostly confined to the yellow spot.
YELLOW SPOT - The area of best vision
The distribution of rods and cones is not uniform. A
particular spot called the macula lutea or simply yellow spot or fovea
centralis lies at the back of the eye almost at the centre on the horizontal
axis of the eyeball. This spot contains the maximum number.
of sensory cells and particularly the cones. As a result,
this is the region of brightest vision and also of the colour vision. The rest
of the retina has fewer cones and more rods.
BLIND SPOT - The area of no vision
Lateral to the yellow spot on the nasal side is the blind
spot. There are no sensory cells here and, therefore, this is the point of no
vision. This is the point at which the nerve fibres from all the sensory cells
of the retina converge and bundle together to leave the eyeball in the form of
the optic nerve.
LENS
The lens is a transparent biconvex crystalline body located
just behind the pupil. It contains transparent lens fibres (long thin cells
which have lost their nuclei). The lens is collectively held in position by
fibres called the suspensory ligament, which attaches it to the ciliary body.
The ciliary body lies at the junction of the choroid and the iris, and is
itself a part of the choroid. The ciliary body contains muscles which on
contraction and relaxation, change the shape of the lens (being somewhat
elastic) for viewing objects at different distances.
TWO CHAMBERS OF THE EYE - AQUEOUS AND VITREOUS CHAMBERS
The lens, together with its suspending structures, divides the inner cavity of the eyeball into two chambers: aqueous chamber in front of the lens and vitreous chamber behind the lens.
(1) Aqueous chamber is the front chamber between the lens and the cornea. It is filled with a clear watery liquid called aqueous.
* The aqueous humour serves in two ways :
(i) Keeps the lens moist and protects it from physical
shock,
(ii) It refracts light.
(2) Vitreous chamber is the larger cavity of the eyeball behind the lens. It is filled with a transparent jelly-like thicker fluid called vitreous humour.
* The vitreous humour serves two functions :
(i) It helps in keeping the shape of the eyeball,
(ii) It protects the retina and its nerve endings.
The four major steps in seeing an object are as
follows:
(1) Entry of light rays : Light rays from the object enter the eyes through the transparent structures (conjunctiva, cornea, aqueous humour, lens, vitreous humour).
(2) Focusing of image : First, the curvature of the cornea converges the light rays to some extent and the lens converges them further to form an image on the retina. The image on the retina is inverted and real.
(3) Nerve impulse produced in retina transmitted to brain : The light energy of the image produces chemical changes in the sensitive cells (rods and cones). These changes generate nerve impulses which travel through the optic nerve and reach the visual area of the cerebrum, where they give the sensation of sight.
(4) Brain interprets : Our brain interprets the image in many ways, e.g., it “sees” the objects upright even if the image formed in the eye is inverted.
Accommodation (viewing objects in sharp focus). To see an object clearly, its image should be in sharp focus in each eye. The process of focusing the eye to see objects at different distances is called accommodation.
This is mainly brought about by a change in the curvature of the elastic lens making it thinner or fatter.
* For distant vision, the lens is more flattened or
thinner.
* For near vision (nearer than 6 metres), the lens becomes
more convex or rounded. These changes * in the shape of the lens is brought about
by the ciliary muscles.
* In the normal condition (ciliary muscles relaxed), the
lens remains stretched by the suspensory ligaments and it is less convex,
suited for viewing distant objects.
When we look at nearby objects, the ciliary muscles (which are circular) contract and tend to pull the ciliary body slightly forward. This releases the tension on the suspensory ligament making it loose and the lens, on account of its elasticity, becomes thicker and more rounded or convex.
LIGHT AND DARK ADAPTATION
When you pass from a brightly lighted area to a dark room (such as a cinema hall), you experience difficulty in seeing objects for a short while. Slowly, your vision is improved. This improvement is called dark adaptation. This change is due to-
(a) regeneration of the visual purple (or rhodopsin), the
pigment of the rods, which was earlier broken down due to bright light, and
(b) dilation of the pupil permitting more light to enter
the eyes.
When a person with dark adapted eyes moves to a brightly lighted area, as in coming out of a cinema hall after the noon show, he experiences a dazzling effect for a short period. After a few seconds, he comes back to normal viewing through light adaptation. The adaptation is due to reverse of the previous changes, i.e.,
(a) the visual purple of the rods is bleached, reducing
their sensitivity, and
(b) the pupil constricts (gets narrower), to reduce the
amount of light entering the eyes.
The partial closure of the eyelids in dazzling light also
serves the same purpose.
Colour Vision
Colour vision is possible only through cones of the retina which are stimulated only in bright light. You cannot make out the red, violet or purple flowers in a garden on a moonlit night, because then only the rods function and not the cones.
COMMON DEFECTS OF THE EYE
1. Near or short-sightedness (Myopia) is a condition in which the near objects can be seen clearly while the distant objects appear blurred. In it, the image of distant objects is formed in front of the retina.
Reasons for myopia : The two possible reasons are
(i) the eye ball is lengthened from front to back OR (ii) the lens is too curved (even both reasons may occur together).
Correction of myopia : This defect can be corrected by suitable concave (diverging) lens which causes the light rays to diverge before they strike the lens of the eye. Most of your classmates using spectacles may be suffering from myopia (power of glasses used is mentioned in minus.
2. Hyperopia or long-sightedness
Far or long-sightedness (Hyperopia, old term- Hypermetropia)
is a condition in which there is a difficulty in seeing near objects. In it,
the image of near object falls behind the retina and a convex (converging) lens
is required to correct it (power of the glasses used is mentioned in plus “+”).
This defect results on account of either shortening of the
eyeball from front to back or the lens is too flat.
3. Astigmatism is a defect in which some parts of the object are seen in focus while others are blurred. It arises due to the uneven curvature of the cornea. This is corrected by cylindrical lenses.
4. Presbyopia is a condition affecting older people who cannot see near objects clearly. Their lens loses flexibility resulting in a kind of far- sightedness. This again is corrected by a convex lens.
5. Cataract is a condition in which the lens turns opaque and the vision is cut down even to total blindness. It can be corrected by surgically removing the lens, and by using spectacles with highly convex lenses, compensating for the missing lens, or in a newer technique, a small plastic lens is implanted behind or in front of the iris.
6. Night-blindness: is a condition in which a person feels difficulty in seeing in dim light as during the night. This is due to non-formation of the pigment visual purple of the rods. Only rods function in dim light and in the absence of the pigment, they cannot function. This is usually due to the deficiency of vitamin A which is required for the synthesis of the pigment.
7. Colour blindness : Some people by birth cannot discriminate between certain colours such as the red and green. This is due to a genetic defect. The males mostly suffer from this defect, whereas it rarely occurs in females.
8. Squint : In this defect, the two eyes somewhat converge leading to what is called “cross eye”. An opposite condition appears when they diverge and is called the “wide eye.” Both conditions may cause double vision or diplopia. Surgery and suitable exercise can correct these defects. Stereoscopic (binocular) vision.
All monkeys/apes and particularly humans can perceive depth or the relative distance of the objects. This is due to simultaneous focusing of an object in both eyes, and their images by a kind of “overlapping” in the brain giving the three dimensional effect.
After-images — the basis of motion pictures. If one looks at a bright object for a moment and then closes the eyes, the sensation of light persists for a short period. In the same way, if one looks at a brightly coloured object and then looks at a dark surface, an image of the object in the same colour will persist. This is known as persistence image or the after-image. It lasts for about one-tenth of a second. This is the principle on which the technique of motion pictures is based.
THE EAR - SENSES OF HEARING AND BALANCE
The human ear is concerned with two functions, hearing and body balance. It has three main divisions- (i) outer ear, (ii) middle ear and (iii) inner ear.
(i) The outer ear consists of the projecting part pinna (also called “auricle”) and the passage auditory canal leading to the ear drum (or tympanum).
(ii) The middle ear contains three tiny bones — malleus, incus and stapes or hammer, anvil and stirrup in popular terms and an Eustachian tube which connects the cavity of the middle ear with the throat. The three bones are collectively called the ear ossicles. The handle of the hammer bone is attached to the inner surface of the ear drum. Its opposite end is connected to the anvil which, in turn, is joined to the stirrup. The flat part of the stirrup fits on the so-called oval window, a membrane-covered opening leading to the inner ear. A second opening, the round window, also covered by a thin membrane, connects the middle and the inner ear.
(iii) The inner ear or membranous labyrinth has two main parts — the cochlea and the semicircular canals. The cochlea is spiral-shaped and looks like a snail shell. It has two and a half turns. Its inner winding cavity is divided into three parallel canals separated by membranes. The median (cochlear) canal (2) is filled with a fluid called endolymph and the other two (1 & 3) with perilymph. The middle canal contains areas possessing sensory cells, spiral organ called organ of Corti for hearing. The nerve fibres arising from these cells join the auditory nerve. The sensory cells lie on the basilar membrane.
The other part of the inner ear is a set of three semi-circular canals which are arranged at right angles to each other in three different planes so that one is horizontal and the other two are vertical. One end of each canal is widened to form an ampulla which contains sensory cells for dynamic balance while the body is in motion and nerve fibres from them join the auditory nerve.
The short stem joining the bases of semicircular canals to the cochlea shows two parts - a utriculus and a sacculus. These parts also contain sensory cells for static balance when the body is stationary as in standing.
FUNCTIONS OF THE EAR
The internal ear is involved in two sensory functions- hearing
and body balance.
HEARING- The pinna collects the sound waves and conducts them through the external auditory canal. They finally strike on the ear drum which is set into vibration.
The eustachian tube equalises the air pressure on either
side of the ear drum allowing it to vibrate freely.
The vibrating ear drum sets the three ossicles also into
vibration.
The vibration of the last ossicle (stirrup) is magnified
due to lever-like action of the first two ossicles.
The vibrating stirrup transmits the vibration to the membrane
of the oval window which in turn sets the fluid contained in the cochlear canals
also into vibration.
The vibrating movements of the fluid stimulate the hair-like
processes of the sensory cells of the cochlea (in spiral organ) and the
impulses are transmitted to the brain via the auditory nerve.
The different areas of the cochlear canal are suited to
sounds of different pitch. Most of the sounds we hear are combinations of
vibrations at many different rates of speed, i.e., of different pitches. We cannot
pick up vibrations of all rates of speed (frequencies). Our sensory endings can
receive only those from 20 to 20,000 Hertz, but the most keenly heard sounds
are those at frequencies between 1000 and 4000 Hz. The dogs can perceive sounds
of even higher frequencies.
BALANCING- As the head is turned in different directions, the fluid inside the semicircular canals is also shaken. The moving fluid in the canals pushes against sensory hair cells sending the nerve impulse through the nerve fibres attached to them, to the brain via the auditory nerve. The sensory cells in the semicircular canals are concerned with dynamic equilibrium i.e., while the body is in motion. Similar sensory patches are also located in the utriculus and sacculus which register the static (positional) balance with respect to gravity.
THE SENSE OF TASTE (GUSTATION)
The sense of taste is located in the taste buds of the
tongue. A taste bud is an ovoid group of sensory and supporting cells. The
sensory cells end in hair-like processes and have nerve fibres extending from
their bases. The taste hairs project into the outer taste pore located on the
surface of the epithelium. Substances in solution enter these pores-
Sensitivity on different areas of the tongue for the four
elementary (otherwise in hundreds)
Tastes - Sweet, Salt, Bitter and Sour and stimulate the sensory
hairs. The taste buds are located mainly on the upper surface of the tongue and
some are also present on the other areas in the mouth and the throat.
THE SENSE OF SMELL
The sense of smell is located in the delicate epithelial
layers of the nasal chamber. The sense cells for smell have hair-like
projections which respond to particles dissolved in the mucous secretion of the
nose. The impulse from these cells is transmitted to the brain by the olfactory
nerve.
FLAVOUR is a combination of taste and smell while eating or sipping. If you hold your nose, you will find that grated apple and grated onion taste alike - slightly sweet. Cold with blocked nose has the same effect and makes food taste/smell/ flavourless.
SOME BASIC TERMS IN NERVOUS ACTIVITY
Stimulus : an
agent or a sudden change of the external or internal environment that results
in a change in an organism or any of its body parts.
Response : the
change in an organism resulting due to stimulus.
Impulse : a wave of irritability (electrical disturbance
that sweeps over the nerve cell)
Receptors : the
specialised epithelial cells which, on receiving the stimulus, set up waves of
impulses towards the central nervous system.
Effectors : muscles
or glands which, on receiving the impulse from the brain or spinal cord,
contract or secrete substances.