NCERT Solutions for Class 11 Biology Chapter 18 Neural Control and Coordination

NCERT
SOLUTIONS
CLASS - 11th

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Class : 11th
Subject : Biology
Chapter : 21
Chapter Name : Neutral Control and Coordination

Q1 Brie y describe the structure of the following:
(a) Brain
(b) Eye
(c) Ear

Answer. (A) Brain: Brain is the main coordinating centre of the body. It is a part of nervous system
that controls and monitors every organ of the body. It is well protected by cranial meninges that
are made up of an outer layer called dura mater, a thin middle layer called arachnoid, and an inner
layer called pia mater.
It is divided into three regions — forebrain, midbrain, and hindbrain.

Forebrain: It is the main thinking part of the brain. It consists of cerebrum, thalamus, and
hypothalamus.

(a) Cerebrum: Cerebrum is the largest part of the brain and constitutes about four- fth of its
weight. Cerebrum is divided into two cerebral hemispheres by a deep longitudinal cerebral ssure.
These hemispheres are joined by a tract of nerve bre known as corpus callosum. The cerebral
hemispheres are covered by a layer of cells known as cerebral cortex or grey matter. Cerebrum has
sensory regions known as association areas that receive sensory impulses from various receptors
as well as from motor regions that control the movement of various muscles. The innermost part
of cerebrum gives an opaque white appearance to the layer and is known as the white matter.

(b) Thalamus: Thalamus is the main centre of coordination for sensory and motor signalling. It is
wrapped by cerebrum.

(c) Hypothalamus: It lies at the base of thalamus and contains a number of centres that regulate
body temperature and the urge for eating and drinking. Some regions of cerebrum, along with
hypothalamus, are involved in the regulation of sexual behaviour and expression of emotional
reactions such as excitement, pleasure, fear, etc.

Midbrain: It is located between the thalamus region of the forebrain and pons region of hindbrain.
The dorsal surface of midbrain consists of superior and inferior corpora bigemina and four round
lobes called corpora quadrigemina. A canal known as cerebral aqueduct passes through the
midbrain. Midbrain is concerned with the sense of sight and hearing.

Hindbrain: It consists of three regions - pons, cerebellum, and medulla oblongata.

(a) Pons is a band of nerve bre that lies between medulla oblongata and midbrain. It connects the
lateral parts of cerebellar hemisphere together.

(b) Cerebellum is a large and well developed part of hindbrain. It is located below the posterior
sides of cerebral hemispheres and above medulla oblongata. It is responsible for maintaining
posture and equilibrium of the body.

(c) Medulla oblongata is the posterior and simplest part of the brain. It is located beneath the
cerebellum. Its lower end extends in the form of spinal cord and leaves the skull through foramen
magnum.

(B) Eye: Eyes are spherical structures that consist of three layers.

(a) The outer layer is composed of sclera and cornea.

(i) Sclera is an opaque tissue that is usually known as white of the eye. It is composed of a dense
connective tissue.
(ii) Cornea is a transparent anterior portion of eye that lacks blood vessels and is nourished by
lymph from the nearby area. It is slightly bulged forward and helps in focusing light rays with the
help of lens.

(b) The middle layer of eye is vascular in nature and contains choroid, ciliary body, and iris.

(i) Choroid lies next to the sclera and contains numerous blood vessels that provide nutrients and
oxygen to the retina and other tissues.
(ii) Ciliary body: The choroid layer is thin over posterior region and gets thickened in the anterior
portion to form ciliary body. It contains blood vessels, ciliary muscles, and ciliary processes.
(iii) Iris: At the junction of sclera and cornea, the ciliary body continues forward to form thin
coloured partition called iris. It is the visible coloured portion of eye. The eye contains a
transparent, biconvex, and elastic structure just behind the iris. It is known as lens. The lens is
held in position by suspensory ligaments attached to the ciliary body. The lens divides the eye ball
into two chambers — an anterior aqueous and posterior vitreous chamber.

(c) The innermost nervous coat of eye contains retina. Retina is the innermost layer. It contains
three layers of cells — inner ganglion cells, middle bipolar cells, and outermost photoreceptor
cells.

The receptor cells present in the retina are of two types — rod cells and cone cells.
(a) Rod cells —The rods contain the rhodopsin pigment (visual purple) that is highly sensitive to
dim light. It is responsible for twilight vision.
(b) Cone cells —The cones contain the iodopsin pigment (visual violet) and are highly sensitive to
high intensity light. They are responsible for daylight and colour visions. The innermost
ganglionic cells give rise to optic nerve bre that forms optic nerve in each eye and is connected

with the brain.

(C) Ear: Ear is the sense organ for hearing and equilibrium. It consists of three portions ear, middle
ear, and internal ear.

1. External ear:
It consists of pinna, external auditory meatus, and a tympanic membrane.
(a) Pinna is a sensitive structure that collects and directs the vibrations into the ear to produce
sound.
(b) External auditory meatus is a tubular passage supported by cartilage in external ear.
(c) Tympanic membrane is a thin membrane that lies close to the auditory canal. It separates the
middle ear from external ear.

2. Middle ear:
It is an air- lled tympanic cavity that is connected with pharynx through eustachian tube.
Eustachian tube helps to equalize air pressure in both sides of tympanic membrane. The middle
ear contains a exible chain of three middle bones called ear ossicles. The three ear ossicles are
malleus, incus, and stapes that are attached to each other.

3. Internal ear:
It is also known as labyrinth. Labyrinth is divided into bony labyrinth and a membranous
labyrinth.
Bony labyrinth is lled with perilymph while membranous labyrinth is lled with endolymph.
Membranous labyrinth is divided into 2 parts.
(a) Vestibular apparatus
Vestibular apparatus is a central sac-like part that is divided into utriculus and sacculus. A special
group of sensory cells called macula are present in sacculus and utriculus. Vestibular apparatus
also contains three semi-circular canals. The lower end of each semi-circular canal contains a
projecting ridge called crista ampularis. Each ampulla has a group of sensory cells called crista.
Crista and macula are responsible for maintaining the balance of body and posture.
(b) Cochlea:
Cochlea is a long and coiled outgrowth of sacculus. It is the main hearing organ. Cochlea consists
of three membranes. The organ of corti, a hearing organ, is located on the basilar membrane that
has hair cells.

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Q2 Compare the following:
(a) Central neural system (CNS) and Peripheral neural system (PNS)
(b) Resting potential and action potential
(c) Choroid and retina

Answer. (a) Central neural system (CNS) and Peripheral neural system (PNS)

(b) Resting potential and action potential

(c) Choroid and retina

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Q3 Explain the following processes:
(a) Polarisation of the membrane of a nerve bre
(b) Depolarisation of the membrane of a nerve bre
(c) Conduction of a nerve impulse along a nerve bre
(d) Transmission of a nerve impulse across a chemical synapse

Answer. (a) Polarisation of the membrane of a nerve bre During resting condition, the
concentration of K+ ions is more inside the axoplasm while the concentration of Na+ ions is more
outside the axoplasm. As a result, the potassium ions move faster from inside to outside as
compared to sodium ions. Therefore, the membrane becomes positively charged outside and
negatively charged inside. This is known as polarization of membrane or polarized nerve.

(b) Depolarisation of the membrane of a nerve bre When an electrical stimulus is given to a nerve
bre, an action potential is generated. The
membrane becomes permeable to sodium ions than to potassium ions. This results into positive
charge inside and negative charge outside the nerve bre. Hence, the membrane is said to be
depolarized.

(c) Conduction of a nerve impulse along a nerve bre
There are two types of nerve bres — myelinated and non-myelinated. In myelinated nerve bre,
the action potential is conducted from node to node in jumping manner. This is because the
myelinated nerve bre is coated with myelin sheath. The myelin sheath is impermeable to ions. As
a result, the
ionic exchange and depolarisation of nerve bre is not possible along the whole length of nerve
bre. It takes place only at some point, known as nodes of Ranvier, whereas in non-myelinated
nerve bre, the ionic exchange and depolarization of nerve bre takes place along the whole
length of the nerve bre. Because of this ionic exchange, the depolarized area becomes repolarised
and the next polarized area becomes depolarized.
(d) Transmission of a nerve impulse across a chemical synapse
Synapse is a small gap that occurs between the last portion of the axon of one neuron and the
dendrite of next neuron. When an impulse reaches at the end plate of axon, vesicles consisting of
chemical substance or neurotransmitter, such as acetylcholine, fuse with the plasma membrane.
This chemical moves across the cleft and attaches to chemo-receptors present on the membrane
of the dendrite of next neuron. This binding of chemical with chemo-receptors leads to the
depolarization of membrane and generates a nerve impulse across nerve bre. The chemical,
acetylcholine, is inactivated by enzyme acetylcholinestrase. The enzyme is present in the post
synaptic membrane of the dendrite.
It hydrolyses acetylcholine and this allows the membrane to repolarise.

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Q4 Draw labelled diagrams of the following:
(a) Neuron
(b) Brain
(c) Eye
(d) Ear

Answer. (a) Neuron

(b) Brain

(c) Eye

(d) Ear

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Q5 Write short notes on the following:
(a) Neural coordination
(b) Forebrain
(c) Midbrain
(d) Hindbrain
(e) Retina
(f) Ear ossicles
(g) Cochlea
(h) Organ of Corti
(i) Synapse

Answer. (a) Neural coordination
The neural system provides rapid coordination among the organs of the body. This coordination is

in the form of electric impulses and is quick and short lived. All the physiological processes in the
body are closed linked and dependent upon each other. For example, during exercise, our body
requires more oxygen and food. Hence, the breathing rate increases automatically and the heart
beats faster. This leads to a faster supply of oxygenated blood to the muscles. Moreover, the
cellular functions require regulation continuously. These functions are carried out by the
hormones.
Hence, the neural system along with the endocrine system control and coordinate the
physiological processes.

(b) Forebrain : It is the main thinking part of the brain. It consists of cerebrum, thalamus, and
hypothalamus.

(i) Cerebrum: Cerebrum is the largest part of the brain and constitutes about four- fth of its
weight. Cerebrum is divided into two cerebral hemispheres by a deep longitudinal cerebral ssure.
These hemispheres are joined by a tract of nerve bres known as corpus callosum. The cerebral
hemispheres are covered by a layer of cells known as cerebral cortex or grey matter. Cerebrum has
sensory regions known as association areas that receive sensory impulses from various receptors
as well as from motor regions that control the movement of various muscles. The innermost part
of cerebrum gives an opaque white appearance to the layer and is known as the white matter.
(ii) Thalamus: Thalamus is the main centre of coordination for sensory and motor signalling. It is
wrapped by cerebrum.
(iii) Hypothalamus: It lies at the base of thalamus and contains a number of centres that regulate
body temperature and the urge for eating and drinking. Some regions of cerebrum, along with
hypothalamus, are involved in the regulation of sexual behaviour and expression of emotional
reactions such as
excitement, pleasure, fear, etc.

(c) Midbrain : It is located between the thalamus region of the forebrain and pons region of
hindbrain. The dorsal surface of midbrain consists of superior and inferior corpora bigemina and
four round lobes called corpora quadrigemina. A canal known as cerebral aqueduct passes through
the midbrain. Midbrain is concerned with the sense of sight and hearing.

(d) Hindbrain : It consists of three regions — pons, cerebellum, and medulla oblongata.

(i) Pons is a band of nerve bres that lies between medulla oblongata and midbrain. It connects
the lateral parts of cerebellar hemisphere together.
(ii) Cerebellum is a large and well developed part of hindbrain. It is located below the posterior
sides of cerebral hemispheres and above the medulla oblongata. It is responsible for maintaining
posture and equilibrium of the body.
(iii) Medulla oblongata is the posterior and simplest part of the brain. It is located beneath the
cerebellum. Its lower end extends in the form of spinal cord and leaves the skull through foramen
magnum.

(e) Retina : Retina is the innermost layer. It contains three layers of cells — inner ganglion cells,
middle bipolar cells, and outermost photoreceptor cells. The receptor cells present in the retina

are of two types rod cells and cone cells.

(i) Rod cells —The rods contain rhodopsin pigment (visual purple), which is highly sensitive to dim
light. It is responsible for twilight vision.
(ii) Cone cells —The cones contain iodopsin pigment (visual violet) and are highly sensitive to high
intensity light. They are responsible for daylight and colour visions. The innermost ganglionic
cells give rise to optic nerve bre that forms optic nerve in each eye and is connected with the
brain. In this region, the photoreceptor cells are absent. Hence, it is known as the blind spot. At
the posterior part, lateral to blind spot, there is a pigmented spot called macula lutea. This spot
has a shallow depression at its middle known as fovea. Fovea has only cone cells. They are devoid
of rod cells. Hence, it is the place of most distinct vision.

(f) Ear ossicles : The middle ear contains a exible chain of three middle bones called ear ossicles.
The three ear ossicles are as follows.
(i) Malleus
(ii) Incus
(iii) Stapes
The malleus is attached to tympanic membrane on one side and to incus on the other side. The
incus is connected with stapes. Stapes, in turn, are attached with an oval membrane, fenestra
ovalis, of internal ear. The ear ossicles act as a lever that transmits sound waves from external ear
to internal ear.

(g) Cochlea : Cochlea is a long, coiled outgrowth of sacculus. It is the main hearing organ. The
cochlea forms three chambers.
(i) Upper — scala vestibule
(ii) Middle — scala media
(iii) Lower — scale tympani
The oor of the scala media is basilar membrane while its roof is Reissner's membrane. Reissner's
membrane gives out a projection called tectorial membrane. The organ of corti, a hearing organ, is
located on the basilar membrane. Organ of corti contains receptor hair cells. The upper scala
vestibule and lower scala tympani contain perilymph.

(h) Organ of corti : Organ of corti is the hearing organ. It is located on the basilar membrane that
contains hair cells. Hair cells act as auditory receptors. They are present on the internal side of
organ of corti.
(i) synapse : Synapse is a junction between the axon terminal of one neuron and the dendrite of
next neuron. It is separated by a small gap known as synaptic cleft.
There are two types of synapses.
(a) Electrical synapse
(b) Chemical synapse
In electrical synapses, the pre and post synaptic neurons lie in close proximity to each other.

Hence, the impulse can move directly from one neuron to another across the synapse. This
represents a faster method of impulse transmission.
In chemical synapses, the pre and post synaptic neurons are not in close proximity. They are
separated by a synaptic cleft. The transmission of nerve impulses is carried out by chemicals such
as neurotransmitters.

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Q6 Give a brief account of:
(a) Mechanism of synaptic transmission
(b) Mechanism of vision
(c) Mechanism of hearing

Answer. (a) Mechanism of synaptic transmission
Synapse is a junction between two neurons. It is present between the axon terminal of one neuron
and the dendrite of next neuron separated by a cleft.
There are two ways of synaptic transmission.
(1) Chemical transmission
(2) Electrical transmission
1. Chemical transmission — When a nerve impulse reaches the end plate of axon, it releases a
neurotransmitter (acetylcholine) across the synaptic cleft. This chemical is synthesized in cell
body of the neuron and is transported to the axon terminal. The acetylcholine diffuses across the
cleft and binds to the receptors present on the membrane of next neuron. This causes
depolarization of membrane and initiates an action potential.
2. Electrical transmission — In this type of transmission, an electric current is formed in the
neuron. This electric current generates an action potential and leads to transmission of nerve
impulse across the nerve bre. This represents a faster method of nerve conduction than the
chemical method of transmission.
(b) Mechanism of vision
Retina is the innermost layer of eye. It contains three layers of cells — inner ganglion cells, middle
bipolar cells, and outermost photoreceptor cells. A photoreceptor cell is composed of a protein
called opsin and an aldehyde of vitamin A called retinal. When light rays are focused on the
retinathrough cornea, it leads to the dissociation of retinal from opsin protein. This changes the
structure of opsin. As the structure of opsin changes, the permeability of membrane changes,

generating a potential difference in the cells. This generates an action potential in the ganglionic
cells and is transmitted to the visual cortex of the brain via optic nerves. In the cortex region of
brain, the impulses are analysed and image is formed on the retina.
(c) Mechanism of hearing
The pinna of the external region collects the sound waves and directs it towards ear drum or
external auditory canal. These waves strike the tympanic membrane and vibrations are created.
Then, these vibrations are transmitted to the oval window, fenestra ovalis, through three ear
ossicles, named as malleus, incus, and stapes. These ear ossicles act as lever and transmit the
sound waves to internal ear. These vibrations from fenestra ovalis are transmitted into cochlear
uid. This generates sound waves in the lymph. The formation of waves generates a ripple in the
basilar membrane. This movement bends the sensory hair cells present on the organ of corti
against tectorial membrane. As a result of this, sound waves are converted into nerve impulses.
These impulses are then carried to auditory cortex of brain via auditory nerves. In cerebral cortex
of brain, the impulses are analysed and sound is recognized.

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Q7 Answer brie y: (a) How do you perceive the colour of an object? (b) Which part of our body
helps us in maintaining the body balance? (c) How does the eye regulate the amount of light that
falls on the retina.

Answer. (a) Photoreceptors are cells that are sensitive to light. They are of two types — rods and
cones. These are present in the retina. Cones help in distinguishing colours. There are three types
of cone cells — those responding to green light, those responding to blue light, and those
responding to red light. These cells are stimulated by different lights, from different sources. The
combinations of the signals generated help us see the different colours.
(b) Vestibular apparatus is located in the internal ear, above the cochlea and helps in maintaining
body balance. Crista and macula are the sensory spots of the vestibular apparatus controlling
dynamic equilibrium.
(c) Pupil is the small aperture in the iris that regulates the amount of light entering the eye.

Cornea, aqueous humour, lens, and vitreous humour act together and refract light rays, focussing
them onto the photoreceptor cells of the retina.

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Q8 Explain the following:
(a) Role of Na in the generation of action potential.
+

(b) Mechanism of generation of light-induced impulse in the retina.
(c) Mechanism through which a sound produces a nerve impulse in the inner ear.

Answer. (a) Sodium ions play an important role in the generation of action potential. When a
nerve bre is stimulated, the membrane potential decreases. The membrane becomes more
permeable to Na ions than to K ions. As a result, Na+ diffuses from the outside to the inside of
+ +

the membrane. This causes the inside of the membrane to become positively-charged, while the
outer membrane gains a negatively charge. This reversal of polarity across the membrane is known
as depolarisation. The rapid in ow of Na ions causes the membrane potential to increase,
+

thereby generating an action potential.

(b) Retina is the innermost layer of the eye. It contains three layers of cells — inner ganglion cells,
middle bipolar cells, and outermost photoreceptor cells. Photoreceptor cells are composed of a
protein called opsin and an aldehyde of vitamin A calledretinal. When light rays are focused on

the retina through the cornea, retinal gets dissociated from opsin. As a result, the structure of
opsin gets changed. This in turn causes the permeability of the membrane to change, thereby
generating a potential difference in the cells. Consequently, an action potential is generated in the
ganglion cells and is transmitted to the visual cortex of the brain via the optic nerves. In the cortex
region of the brain, the impulses are analysed and the image is formed on the retina.
(c) The pinna of the external ear collects the sound waves and directs them to the tympanic
membrane (ear drum) via the external auditory canal. The ear drum then vibrates the sound waves
and conducts them to the internal ear through the ear ossicles. The ear ossicles increase the
intensity of the sound waves. These vibrating sound waves are conducted through the oval window
to the uid in the cochlea. Consequently, a movement is created in the lymph. This movement
produces vibrations in the basilar membrane, which in turn stimulate the auditory hair cells.
These cells generate a nerve impulse, conducting it to the auditory cortex of the brain via afferent
bres. The auditory cortex region interprets the nerve impulse and sound is recognised.

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Q9 Differentiate between:
(a) Myelinated and non-myelinated axons
(b) Dendrites and axons
(c) Rods and cones
(d) Thalamus and Hypothalamus
(e) Cerebrum and Cerebellum

Answer. (a) Myelinated and non-myelinated axons

(b) Dendrites and axons

(c) Rods and cones

(d) Thalamus and Hypothalamus

(e) Cerebrum and Cerebellum

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Q10 Answer the following:

(a) Which part of the ear determines the pitch of a sound?
(b) Which part of the human brain is the most developed?
(c) Which part of our central neural system acts as a master clock?

Answer. (a) Cochlea determines the pitch of a sound.
(b) Forebrain is largest and the most developed part of the human brain.
(c) Hypothalamus acts as a master clock in the human body.

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Q11 The region of the vertebrate eye, where the optic nerve passes out of the retina, is called the
(a) fovea
(b) iris
(c) blind spot
(d) optic chaisma

Answer. (c) Blind spot
Blind spot is the part where the optic nerve passes out of the retina. Photoreceptors are absent
from this region.

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Q12 Distinguish between:
(a) afferent neurons and efferent neurons
(b) impulse conduction in a myelinated nerve bre and unmyelinated nerve bre
(c) aqueous humor and vitreous humor
(d) blind spot and yellow spot
(e) cranial nerves and spinal nerves.

Answer. (a) afferent neurons and efferent neurons

(b) impulse conduction in a myelinated nerve bre and unmyelinated nerve bre

(c) aqueous humor and vitreous humor

(d) blind spot and yellow spot

(e) cranial nerves and spinal nerves.

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