Class 11-science NCERT Solutions Biology Chapter 21 - Neural Control And Coordination

328

329

Neural Control And Coordination Exercise 328

Solution 1(a)

Structure of the brain:

The human brain is protected in the skull.
The brain is covered by three membranes called cranial meninges.
The outer layer of the meninges is called dura mater. It is a tough and fibrous membrane.
The middle layer is called arachnoid. It is thin and delicate.
The inner layer is the pia mater. It is in continuation with the brain tissue. It is highly vascular and richly supplied with blood.
Three primary regions of the brain are

Forebrain
Midbrain
Hindbrain

Forebrain:

The forebrain consists of cerebrum, thalamus and hypothalamus.
Cerebrum:

It forms the major part of the brain.
The cerebrum is divided into halves longitudinally by a deep cleft. Each half is called the cerebral hemisphere.
Both hemispheres are connected by the corpus callosum. It is a tract of nerve fibres.
Cerebral hemispheres are hollow internally.
The walls of the cerebrum have an outer cortex and inner medulla.
The cerebral cortex contains cell bodies of neurons and hence appears greyish. It is called grey matter.
The grey matter is thrown into many grooves and folds called sulci and gyri, respectively.
A higher number of convolutions leads to greater intelligence.
The cerebral cortex contains motor areas, sensory areas and association areas. Association areas are neither sensory nor motor. These areas are responsible for complex functions such as memory, communication and intersensory associations.
The cerebral medulla consists of axons of nerve fibres and appears whitish. It is called white matter.
The inner part of the cerebral hemispheres and a group of associated deep structures such as hippocampus and amygdala form a complex structure called the limbic lobe or limbic system.
Functions:
- The cerebrum is the centre of intelligence, memory, consciousness, will power and voluntary actions.

Thalamus:

It is made of grey matter.
It is situated superior to the midbrain.
Functions:

The thalamus relays motor and sensory impulses to the cerebrum.
It also regulates the manifestation of emotions and recognises heat, cold and pain.

Hypothalamus:

It lies at the base of the thalamus.
It consists of optic chiasma, a point where the fibres of optic nerves cross to opposite sides.
Behind the optic chiasma is the infundibulum. It is a greyish protuberance of the hypothalamus.
The infundibulum holds the pituitary gland.
Functions:

Hypothalamus contains the centres which control body temperature, blood pressure and homeostasis.
It contains the centres to control hunger, thirst, sleep, fatigue, emotions, anger, pleasure and penance.
The neurosecretory cells of the hypothalamus secrete certain hormones or releasing factors which control the activity of the pituitary hormones.
The hypothalamus along with the limbic system is involved in the regulation of sexual behaviour.

Midbrain:
- The midbrain consists of cerebral peduncles and corpora quadrigemina.
- Cerebral Peduncles:
  - Cerebral peduncles are fibrous thick tracts.
  - They connect the cerebrum and the cerebellum.
  - Functions:
- Corpora Quadrigemina:
Hindbrain:
- The hindbrain consists of the cerebellum, pons varolii and medulla oblongata.
- Cerebellum:
- Pons varolii:
- Medulla oblongata:
  - It is located at the base of the skull.
  - It is conical in shape.
  - It continues behind the brain as the spinal cord.
  - Injury to the medulla oblongata results in death.
  - Functions:
    - It acts as a pathway and conducts impulses from the spinal cord to the brain.
    - It controls the activities of the internal organs, heartbeat and breathing.

Solution 1(b)

Structure of the eye:

• The human eye is spherical.

• It is located in the bony socket of the skull.

• The wall of the eyeball is formed of three layers:

Outer fibrous coat
Middle vascular coat
Inner neurosensory coat

Outer fibrous coat:

It is thick and tough.
It protects the eyeball and helps maintain its form.
Its two distinct regions are sclera and cornea.
Sclera:

It is composed of a dense white fibrous connective tissue.
Most of its part remains in the orbit.
Only the 'white of eye' is visible.
The white of eye contains many collagen fibres.
Functions:

It protects and maintains the shape of the eyeball.

Cornea:

It is the front transparent part of the outer fibrous coat.
It is non-vascular.
It is covered by a thin, transparent vascular layer of stratified epithelium called conjunctiva.
Conjunctiva is in continuation with the lining of eyelids.
Functions:

The cornea refracts light entering the eye and converges it into the lens.

Middle vascular coat:

It is made of three regions-choroid, ciliary body and iris.
Choroid:
- It is in continuation on the inner side of the sclera.
- Choroid is highly vascular and made of loose fibrous connective tissue.
- It is thin over the posterior two-thirds of the eye ball and becomes thick in front.
- It is bluish in colour.
- It also contains some pigmented cells.
- Functions:
Ciliary body:

The ciliary body is the thick anterior part of the choroid.
It is a less vascular and less pigmented region.
It is composed of ciliary muscles and ciliary processes.
Ciliary muscles:

They are smooth muscles.
There are two types of muscles-circular muscles and meridional muscles.

Ciliary processes:

The inner side of the ciliary body is thrown into many folds called ciliary processes.
Functions:

The ciliary processes secrete aqueous humour.

Iris:

The iris is present at the junction of sclera and cornea.
It is a thin, pigmented and opaque structure.
The pigmented cells of the choroid are responsible for the colour of the iris. The colour may be dark-brown, black, blue or green.
It has an aperture in the centre called the pupil.
The iris has two types of smooth muscles-circular muscles and radial muscles.
Functions:

The iris regulates the size of the eye and hence controls the amount of light entering the eye.
The contraction of circular muscles or sphincters makes the pupil smaller in bright light.
The contraction of radial muscles enlarges the pupil in dim light.

Inner neurosensory coat:

The retina is the innermost, neurosensory, thin layer of the eyeball.
The external surface of the retina is in contact with the choroid and its inner surface is in contact with the vitreous humour.
The external surface consists of four layers:

Pigmented layer:

This layer is made of single layer of cells. The cells contain dark-brown pigment.

Layer of photoreceptor cells:

It contains two types of photoreceptor cells-rods and cones.
Rods:

Rod cells are elongated and rod-shaped.
They contain a purplish-red protein pigment called rhodopsin or visual purple. Rhodopsin contains derivative of vitamin A.
Rods are sensitive to dim light and provide vision in dark called twilight vision or scotopic vision.
Rods do not respond to colours.

Cones:

Cone cells are sensitive to bright light and colours. Hence, they are responsible for photopic vision or daylight vision.
The pigment present in the cone cells is iodopsin.
There are three kinds of cone cells which respond to red, green and blue light.
The other colours are detected by the simultaneous stimulation of more than one kind of cone cells.
When all the three types of cells are stimulated simultaneously, a sensation of white light is produced.
Cone cells are insensitive to dim light, and hence, colour cannot be recognised in the dark.

Layer of bipolar neurons:
Layer of ganglionic cells:

This layer contains the cell bodies of ganglion cells which form the optic nerve.

Blind spot:

The optic nerve leaves the brain and the retinal blood vessels enter the brain at a point where the photoreceptor cells are absent. This is called a blind spot.

Macula lutea:

Lateral to the blind spot is a yellowish pigmented spot called macula lutea or yellow spot.
It lies exactly opposite to the centre of the cornea.
Macula lutea has a central pit called fovea.
Fovea lack blood vessels and rods.
Fovea has only cone cells and it is the region of most distinct vision.

Lens:

Lens lies just behind the iris.
It is transparent, biconvex and elastic.
It is covered by a thin, transparent elastic membrane called lens capsule.
The lens is held in position by ligaments called suspensory ligaments.
The lens and suspensory ligaments divide the cavity of the eye ball into two chambers-aqueous chamber and vitreous chamber.

Aqueous chamber:

Lens lies just behind the iris.
It is present between the cornea and the lens.
It is filled with aqueous humour which is a clear, watery fluid.
Aqueous humour provides nourishment to cornea and the lens.
It supports the lens, keeps it moist and protects it from physical shock.
It provides shape to the eye and inflates the eye ball.
It also removes the metabolic waste from the neighbouring cells.

Vitreous chamber:
- It is a larger chamber behind the lens.
- It is filled with vitreous humour which is a jelly-like, transparent fluid.
- Vitreous fluid also provides shape to the eye.
- It supports the lens and the retina.
- It also protects the retina and its nerve endings.

Solution 1(c)

Structure of ear:

Structure of Ear

Middle Ear

Structure of Cochlea

The human ear consists of three parts:
- External ear
- Middle ear
- Internal ear
- External ear:
- Middle ear:
- Inner ear:
  - It consists of bony labyrinth and membranous labyrinth.
  - The bony labyrinth is filled with a fluid called perilymph.
  - The membranous labyrinth floats in the perilymph.
  - The membranous labyrinth is filled with the endolymph.
  - There are three regions of the membranous labyrinth; they are vestibule, semicircular canals and cochlea.
  - Vestibule (Otolith organ):
  - Semicircular canals:
  - Cochlea:
    - It arises from the saccule.
    - Cochlea is a bony coiled structure.
    - The cochlear cavity is divided into three chambers by two membranes-Reisnner's membrane and basilar membrane.
    - The upper chamber is scala vestibule, the middle chamber is scala media and the lower chamber is scala tympani.
    - Scala tympani and scala vestibule are filled with the perilymph, while scala media is filled with the endolymph.
    - The basilar membrane bears the organ of Corti.
    - Organ of Corti is connected with the nerve fibres of the auditory nerve which connects to the brain.
    - Organ of Corti:
      - It is the organ of hearing.
      - It contains hair cells which are auditory receptor cells.
      - The apical ends of the hair cells have processes called stereo cilia.
      - The basal parts of the hair cells have synaptic contacts with the afferent nerve fibres.
      - Above the rows of hair cells is a smooth gelatinous layer called the tectorial membrane.

Solution 2(a)

Central Neural System (CNS)	Peripheral Neural System (PNS)
1. CNS is made of the brain and the spinal cord.	1. PNS consists of cranial nerves and spinal nerves.
2. Brain is protected by the skull and the spinal cord by the vertebral column.	2. There are no protective structures for PNS.
3. CNS does not have any subdivisions.	3. PNS is divided into somatic nervous system and autonomic nervous system.
4. CNS processes the information and controls the response to impulses.	4. Nerves of PNS carry impulses to the CNS and responses from the CNS to different organs of the body.
5. Group of neurons in CNS is called nuclei.	5. Group of neurons in PNS is called ganglia.

Solution 2(b)

Resting Potential	Action Potential
1. It is the potential difference (difference in electric charge) across the membrane when the neuron is at the resting phase.	1. It is the potential difference (difference in the electric charge) across the membrane when the neuron is stimulated or excited.
2. The outer side of the neuron is positively charged and the interior of the neuron is negatively charged.	2. The outer side of the neuron is negatively charged and the interior of the neuron is positively charged.
3. The plasma membrane of the neuron is more permeable to K⁺ ions.	3. The plasma membrane of the neuron is more permeable to Na⁺ ions.
4. The sodium-potassium-ATPase pump is active and sends Na⁺ ions outside the neuron to maintain the resting potential.	4. The sodium-potassium-ATPase pump functions in a reverse manner and sends Na⁺ ions into the neuron.

Solution 2(c)

Choroid	Retina
1. It is the middle coat of the eye ball.	1. It is the inner coat of the eye ball.
2. It does not have photoreceptor cells.	2. It has two kinds of photoreceptor cells-rods and cones.
3. It is the vascular layer of the eyeball.	3. It is the neurosensory layer of the eyeball.
4. It nourishes the retina and prevents reflection of light in the eye.	4. It imparts vision.

Solution 3(a)

Polarisation of the membrane of a nerve fibre:

When the nerve fibre is at the resting phase, it is said to be in the polarised state.
In a polarised state, the membrane of the nerve fibre experiences resting potential.
The following steps take place during the process of polarisation of the membrane of a nerve fibre:
- When a depolarised region of a nerve fibre starts becoming polarised initially, there are more K⁺ ions outside the nerve fibre and the axon membrane contains large amount of Na⁺ ions.
- As the region of the membrane starts attaining the polarised state, the membrane becomes more permeable to K⁺ ions and impermeable to Na⁺ ions and negatively charged proteins.
- 3 Na⁺ ions are sent outside the axon and 2 K⁺ ions are sent into the axon by a sodium-potassium pump by active transport.
- The inner side of the membrane becomes electronegative (negatively charged) and the outer side becomes electropositive (positively charged) because of the movement of sodium and potassium ions. This makes the nerve fibre polarised.

Solution 3(b)

Depolarisation of the membrane of a nerve fibre:

When the nerve fibre is stimulated, it is said to be in the depolarised state.
In a depolarised state, the membrane of the nerve fibre experiences an action potential.
The following steps take place during the process of depolarisation of the membrane of a nerve fibre:

In a polarised state, the axon has more concentration of K⁺ ions and outside the axon, the concentration of Na⁺ ions is more.
When the nerve fibre gets excited by the stimulus, the permeability of the membrane for Na⁺ ions and K⁺ ions is reversed.
The membrane becomes highly permeable for Na⁺ ions.
There is a rapid influx of Na⁺ ions into the axon.
This make the inner side of the membrane positively charged and the outside of the membrane becomes negatively charged.
This results in depolarisation of the membrane of the nerve fibre and its experiences an action potential.

Solution 3(c)

Conduction of a nerve impulse along a nerve fibre:

A nerve impulse is conducted across the length of a nerve fibre in an organised manner.
On the nerve fibre during the conduction of an impulse, a region is always depolarised and a region next to it will be polarised. To send the impulse forward, the depolarised region repolarises and the polarised region depolarises. This is repeated across the length of the nerve fibre which helps in the conduction of impulse.
It occurs in following steps:
- At a depolarised region, consider site A, there will be positive charge on the inner surface of the membrane and negative charge on the outer surface of the membrane.
- The region next to it which is polarised, consider site B, there will B positive charge on the outer surface of the membrane and negative charge on the inner surface of the membrane.
- Hence, at site A, the current will flow on the inner surface of the membrane from A to B, and at site B, the current will flow on the outer surface from B to A. This will complete the circuit of the current flow.
- This will help site B to depolarise, so that the impulse is conducted to site B.
- As soon as the impulse is conducted to site B, site A will get repolarised.
- When site B will be in the depolarised state, the region next to it, consider site C, will be polarised.

Solution 3(d)

Transmission of a nerve impulse across a chemical synapse:

A synapse is formed by the membranes of the pre-synaptic neuron and the post-synaptic neuron.
A synapse may or may not be separated by a gap which is called the synaptic cleft.
At a chemical synapse, the pre-synaptic and post-synaptic neurons are separated by the synaptic cleft.
When an impulse arrives at the axon terminal, the calcium ions present in the synaptic cleft enter the synaptic knobs present at the axon terminals of the pre-synaptic neuron.
The synaptic vesicles in the synaptic knobs of the pre-synaptic neuron move towards the plasma membrane and fuse with it.
The vesicles release the neurotransmitter acetylcholine in the synaptic cleft. (Empty synaptic vesicles return to the cytoplasm of the pre-synaptic neuron where they are refilled.)
The molecules of the acetylcholine bind to the protein receptors present on the plasma membrane of the post-synaptic neurons.
This binding opens the channels and sodium ions enter the post-synaptic neuron, while potassium ions leave the post-synaptic membrane.
This generates an action potential in the membrane of the post-synaptic neuron, and hence, the impulse is transmitted to the post-synaptic neuron.

Solution 4(a)

Neuron:

Solution 4(b)

Brain:

Solution 4(c)

Eye:

Solution 4(d)

Ear:

Neural Control And Coordination Exercise 329

Solution 5(a)

Neural coordination:

Coordination is a characteristic feature of living organisms.
Coordination is the process through which two or more organs interact and complement the functions of one another.
Coordination is achieved by two ways in humans and other higher order animals—neural coordination and chemical coordination.
Neural coordination is carried out by highly specialised cells called neurons.
The neural system is a network of point-to-point connections between the neurons and the organs and it operates through nerve impulses.
Neural coordination is always between the stimulus and the response—receptors and effectors.
All body functions are carried out and controlled by neural coordination.
The stimulus is received from organs such as the skin and a response is generated which is sent to the muscles or glands.
The previous stimulus is always stored in memory by the neural system.
Neural coordination helps in controlling and harmonising voluntary actions such as running, walking, writing and talking.
It helps us to remember, analyse, think and reason because the brain, a part of the neural system, is the site of intelligence.
All vital functions such as breathing, working of the heart and digestion are controlled by neural coordination.
It helps maintain homeostasis by coordinating between various metabolic activities of the body.

Solution 5(b)

Forebrain:

The forebrain consists of cerebrum, thalamus and hypothalamus.
- Cerebrum:
- Thalamus:
- Hypothalamus:
  - It lies at the base of the thalamus.
  - It consists of the optic chiasma, a point where the fibres of optic nerves cross to opposite sides.
  - Behind the optic chiasma is the infundibulum. It is a greyish protuberance of the hypothalamus.
  - The infundibulum holds the pituitary gland.
  - Functions:
    - The hypothalamus contains the centres which control body temperature, blood pressure and homeostasis.
    - It contains the centres to control hunger, thirst, sleep, fatigue, emotions, anger, pleasure and penance.
    - The neurosecretory cells of the hypothalamus secrete certain hormone or releasing factors which control the activity of the pituitary hormones.
    - The hypothalamus along with the limbic system is involved in the regulation of sexual behaviour.

Solution 5(c)

Midbrain:

The midbrain consists of cerebral peduncles and corpora quadrigemina.
- Cerebral Peduncles:
- Corpora Quadrigemina:
  - On the dorsal portion of the brain, there are two pairs of solid lobes present. These lobes are called corpora quadrigemina.
  - One pair is called superior colliculi and the other pair is called inferior colliculi.
  - Functions:

Solution 5(d)

Hindbrain:

The hindbrain consists the cerebellum, pons varolii and medulla oblongata.
- Cerebellum:
- Pons varolii:
- Medulla oblongata:
  - It is located at the base of the skull.
  - It is conical in shape.
  - It continues behind the brain as the spinal cord.
  - Injury to the medulla oblongata results in death.
  - Functions:
    - It acts as a pathway and conducts impulses from the spinal cord to the brain.
    - It controls the activities of the internal organs, heartbeat and breathing.

Solution 5(e)

Retina:

Retina is the innermost, neurosensory, thin layer of the eyeball.
The external surface of the retina is in contact with the choroid and its inner surface is in contact with the vitreous humour.
The retina is the site of image formation.
The external surface consists of four layers:

Pigmented layer:

This layer is made of single layer of cells. The cells contain dark-brown pigment.

Layer of photoreceptor cells:

It contains two types of photoreceptor cells-rods and cones.
Rods:

Rod cells are elongated and rod-shaped.
They contain a purplish-red protein pigment called rhodopsin or visual purple. Rhodopsin contains a derivative of vitamin A.
Rods are sensitive to dim light and provide vision in dark called twilight vision or scotopic vision.
Rods do not respond to colours.

Cones:

Cone cells are sensitive to bright light and colours. Hence, they are responsible for photopic vision or daylight vision.
The pigment present in the cone cells is iodopsin.
There are three kinds of cone cells which respond to red, green and blue light.
The other colours are detected by the simultaneous stimulation of more than one kind of cone cells.
When all the three types of cells are stimulated simultaneously, a sensation of white light is produced.
Cone cells are insensitive to dim light, and hence, colour cannot be recognised in the dark.

Layer of bipolar neurons
Layer of ganglionic cells:

This layer contains the cell bodies of ganglion cells which form the optic nerve.

Blind spot:

The optic nerve leaves the brain and the retinal blood vessels enter the brain at a point where the photoreceptor cells are absent. It is called the blind spot.

Macula lutea:
- Lateral to the blind spot is a yellowish pigmented spot called macula lutea or yellow spot.
- It lies exactly opposite to the centre of the cornea.
- Macula lutea has a central pit called fovea.
- Fovea lack blood vessels and rods.
- Fovea has only cone cells, and it is the region of most distinct vision.e cells, and it is the region of most distinct vision.

Solution 5(f)

Ear Ossicles:

Three small bones called ear ossicles are present in the tympanic cavity of the middle ear.
They are malleus, incus and stapes.
The handle of the malleus is attached to the central part of the tympanic membrane which is called umbo.
The other end of the malleus is attached to the incus by ligaments.
The incus on its other end is attached to the stapes by ligaments.
The other end of the stapes covers an opening called fenestra ovalis or oval window of cochlea.
The ear ossicles transfer vibration from the external ear to the inner ear.

Solution 5(g)

Cochlea:

It is the part of the inner ear. It arises from the saccule.
The cochlea is a bony coiled structure.
The cochlear cavity is divided into three chambers by the two membranes-Reisnner's membrane and basilar membrane.
The upper chamber is called scala vestibule, the middle chamber is scala media and the lower chamber is scala tympani.
Scala tympani and scala vestibule are filled with the perilymph, while scala media is filled with the endolymph.
The basilar membrane bears the organ of Corti.
The organ of Corti is connected with the nerve fibres of the auditory nerve which connects to the brain.
The organ of Corti is the organ of hearing.

Solution 5(h)

Organ of Corti:

It is located on the basilar membrane of the inner ear.
It is the organ of hearing.
It contains hair cells which are auditory receptor cells.
These cells are present in rows on the internal side of the organ.
The apical ends of the hair cells have processes called stereo cilia.
The basal parts of the hair cells have synaptic contacts with the afferent nerve fibres.
Above the rows of hair cells is a smooth gelatinous layer called the tectorial membrane.

Solution 5(i)

Synapse:

A synapse is formed by the membranes of the pre-synaptic neuron and the post-synaptic neuron.
A synapse may or may not be separated by a gap which is called the synaptic cleft.
There are two kinds of synapses-electrical synapse and chemical synapse.
Mechanism of electrical synapse:

In this case, the pre-synaptic and post-synaptic membranes are in proximity.
Impulse in the form of electric current directly flows from the pre-synaptic neuron to the post-synaptic neuron.
Transmission is faster than the chemical synapse.

Mechanism of chemical synapse:

Here, the pre-synaptic and post-synaptic neurons are separated by the synaptic cleft.
When an impulse arrives at the axon terminal, the calcium ions present in the synaptic cleft enter the synaptic knobs present at the axon terminals of the pre-synaptic neuron.
The synaptic vesicles in the synaptic knobs of the pre-synaptic neuron move towards the plasma membrane and fuse with it.
The vesicles release the neurotransmitter acetylcholine in the synaptic cleft. (Empty synaptic vesicles return to the cytoplasm of the pre-synaptic neuron where they are refilled.)
The molecules of acetylcholine bind to the protein receptors present on the plasma membrane of the post-synaptic neurons.
This binding opens the channels, and sodium ions enter the post-synaptic neuron, while potassium ions leave the post-synaptic membrane.
This generates an action potential in the membrane of the post-synaptic neuron, and hence, the impulse is transmitted to the post-synaptic neuron.

Solution 6(a)

Mechanism of synaptic transmission:

A synapse is formed by the membranes of the pre-synaptic neuron and the post-synaptic neuron.
A synapse may or may not be separated by a gap called the synaptic cleft.
There are two kinds of synapses—electrical synapse and chemical synapse.
Mechanism of synaptic transmission at the electrical synapse:
- In this case, the pre-synaptic and post-synaptic membranes are in proximity.
- Impulse in the form of electric current directly flows from the pre-synaptic neuron to the post-synaptic neuron.
- Transmission is faster than the chemical synapse.
Mechanism of synaptic transmission at the chemical synapse:

Here, the pre-synaptic and post-synaptic neurons are separated by the synaptic cleft.
When an impulse arrives at the axon terminal, the calcium ions present in the synaptic cleft enter the synaptic knobs present at the axon terminals of the pre-synaptic neuron.
The synaptic vesicles present in the synaptic knobs present in the pre-synaptic neuron move towards the plasma membrane and fuse with it.
The vesicles release the neurotransmitter acetylcholine in the synaptic cleft. (Empty synaptic vesicles return to the cytoplasm of the pre-synaptic neuron where they are refilled.)
The molecules of the acetylcholine bind to the protein receptors present on the plasma membrane of the post-synaptic neurons.
This binding opens the channels, and sodium ions enter the post-synaptic neuron, while potassium ions leave the post-synaptic membrane.
This generates an action potential in the membrane of the post-synaptic neuron, and hence, the impulse is transmitted to the post-synaptic neuron.

Solution 6(b)

Mechanism of vision:

The light rays pass through the pupil, lens, aqueous humour, vitreous humour and falls on the retina.
The light induces the dissociation of the photo-pigment rhodopsin into opsin and retinal.
The dissociation of opsin from the retinal brings changes in the structure of opsin.
This generates an action potential in the rods and cones of the retina.
The action potential is further transmitted to the ganglion cells through bipolar neurons.
It is finally transmitted to the visual cortex of the brain via the optic nerve.
The impulses are analysed at the visual cortex and the responses are sent back to form the image on the retina.

Solution 6(c)

Mechanism of hearing:

Sound waves are collected by the pinna of the external ear.
The waves pass through the external auditory meatus to the ear drum.
The ear drum begins to vibrate.
The vibrations through the air drum are passed on to the malleus, incus and stapes of the middle ear. Here, the frequency of vibrations increases.
Through the oval window, vibrations are further passed to the cochlea of the inner ear.
The vibrations set in the endolymph of the cochlea induce vibrations in the basilar membrane.
Vibrations of the basilar membrane cause sensory hair of the organ of Corti to vibrate.
The receptor hair cells press themselves against the tectorial membrane which convert the sound energy into the action potential or nerve impulse.
The nerve impulse is transmitted to the auditory cortex of the brain.
The impulses are analysed at the auditory cortex and the sound is recognised.

Solution 7(a)

Cone cells present in the retina of the eye are responsible for colour vision.
There are three kinds of cone cells which respond to red, green and blue light.
Different cone cells get stimulated at different wavelengths of light.
The other colours are detected by the simultaneous stimulation of more than one kind of cone cells.
When all the three types of cells are stimulated simultaneously, a sensation of white light is produced.

Solution 7(b)

Crista ampullaris present in the three semicircular canals, the macula utriculi present in the utricle and the macula sacculi present in the saccule of the inner ear help us in maintaining body balance.

Solution 7(c)

Light enters the eye through the pupil, an aperture present in the centre of the iris.
The iris has two types of muscles-circular smooth muscles and radial smooth muscles-which regulate the amount of light which falls on the retina.
The smooth circular muscles contract in bright light which makes the pupil smaller in size; hence, lesser amount of light falls on the retina.
In dim light, the pupil is widened by the contraction of radial smooth muscles so that sufficient amount of light falls on the retina.

Solution 8(a)

In a stimulated nerve fibre, sodium channels of the neurilemma get activated and open. Sodium ions diffuse from the outside to the intracellular fluid because of the electrochemical gradient. The potassium ions move out, and the membrane becomes negatively charged from outside and positively charged from inside. This sudden change in the membrane potential is called the action potential, and the membrane is said to be depolarised.

Solution 8(b)

Mechanism of generation of light-induced impulse in the retina:

The photosensitive compounds (photo pigments) in the human eye are composed of opsin and retinal. Light induces dissociation of retinal and opsin which changes the structure of opsin. It generates an action potential in the bipolar neurons. These impulses/action potential are transmitted by the optic nerves to the visual cortex of the brain where the neural impulses are analysed and the erect image is recognised.

Solution 8(c)

The perilymph of the internal ear receives the vibrations through the membrane covering the fenestra ovalis. From the perilymph, the vibrations are transferred to the scala vestibuli of the cochlea and then to the scala media through Reissner's membrane and stimulate the sensory hair of the organ of Corti. The impulses thus received by the hair cells are carried to the brain through the auditory nerve where the sensation of hearing is felt.

Solution 9(a)

Myelinated Axon	Non-myelinated Axon
1. Myelin sheath is present.	1. Myelin sheath is absent.
2. Nodes of Ranvier are present.	2. Nodes of Ranvier are absent.
It is found in the grey matter of the brain, spinal cord and autonomous nervous system.	3. It is found in the white matter of the brain, spinal cord and autonomous nervous system.
4. The conduction of nerve impulse is node to node.	4. The conduction of nerve impulse is smooth.
5. The speed of conduction of impulse is 50 times faster than in non-myelinated axon.	5. The speed of conduction of impulse is slow.

Solution 9(b)

Dendrites	Axons
1. They are short processes.	1. Axons are long processes.
2. Dendrites carry impulses towards the cell body of the neuron.	2. Axons carry impulses away from the cell body of the neuron.
3. Dendrites are always branched.	3. Axons may or may not be branched.
4. Nissl's granules are present in the neuroplasm.	4. Nissl's granules are absent in the neuroplasm.

Solution 9(c)

Rods	Cones
1. They are sensitive to dim light.	1. They are sensitive to bright light.
2. Rods contain the pigment rhodopsin.	2. Cones contain the pigment iodopsin.
3. They have no role to play in colour vision.	3. They are responsible for colour vision.
4. Only one kind of rods exists.	4. There are three kinds of cones-red, blue and green.

Solution 9(d)

Thalamus	Hypothalamus
1. It is made of only grey matter.	1. It is made of white and grey matter.
2. The thalamus does not secrete any hormone.	2. The hypothalamus secretes certain hormones which control the activity of the pituitary gland.
3. It is situated superior to the midbrain.	3. It is situated at the base of the thalamus.
4. It contains the centres of sensations such as heat, cold and pain.	4. It contains the centres which control body temperature, blood pressure and homeostasis.

Solution 9(e)

Cerebrum	Cerebellum
1. It is the largest part of the brain.	1. It is the second largest part of the brain.
2. It is part of the forebrain.	2. It is part of the hindbrain.
3. The cerebrum is divided into two cerebral hemispheres.	3. The cerebellum is divided into three lobes-central vermis and the two lateral cerebellar hemispheres.
4. It is the site of memory and intelligence.	4. It is the site of body equilibrium and posture.

Solution 10

(a) The cochlea determines the pitch of a sound.

(b) The cerebrum is the most developed.

Solution 11

The blind spot on the retina does not contain any photosensory cells. The optic nerve leaves the brain and the retinal blood vessels enter the brain at the blind spot.

Solution 12(a)

Afferent Neurons	Efferent Neurons
1. They conduct sensory impulses from the receptors to the central nervous system.	1. They conduct motor impulses from the central nervous system to the effector organs such as muscles.
2. They are present in the sense organs.	2. They are present in the brain and the spinal cord.
3. They are sensory neurons.	3. They are motor neurons.

Solution 12(b)

Impulse Conduction in a Myelinated Nerve Fibre	Impulse Conduction in Non-myelinated Nerve Fibre
1. Impulse travels from node to node.	1. Impulse travels along the length of the entire nerve fibre.
2. The speed of conduction is 50 times faster than the non-myelintaed nerve fibre.	2. The speed of conduction is slower.
3. Energy expenditure during impulse transmission is less.	3. Energy expenditure during impulse transmission is more.

Solution 12(c)

Aqueous Humour	Vitreous Humour
1. It is present in the aqueous chamber between the lens and the cornea.	1. It is present in the vitreous chamber between the lens and the retina.
2. It is secreted by the ciliary processes.	2. It is secreted by the retina of the eye.
3. It is a jelly-like fluid.	3. It is a watery fluid.
4. It supports the lens.	4. It supports the lens and retina.

Solution 12(d)

Blind Spot	Yellow Spot
1. It is the point on the retina at which the optic nerve leaves the eye and retinal blood vessels enter the eye.	1. It is the point on the retina which lies exactly opposite the centre of the cornea.
2. It is void of photoreceptor cells.	2. It contains two kinds of photoreceptor cells-rods and cones.
3. It is insensitive to light.	3. It is very much sensitive to light.
4. It has no function with respect to vision.	4. It is the area of most distinct vision.

Solution 12(e)

Cranial Nerves	Spinal Nerves
1. There are 12 pairs of cranial nerves.	1. There are 31 pairs of spinal nerves.
2. They arise from the brain and extend to the other parts of the body.	2. They arise from the spinal cord and extend to other parts of the body.
3. They may be sensory, motor or mixed.	3. They are mixed nerves.