Nervous System

NCERT Solution

Question 1: Briefly describe the structure of the following:
(a) Brain (b) Eye (c) Ear

Answer:

(a) Structure of Brain

The human brain is well protected by the skull. Inside the skull, the brain is covered by cranial meninges consisting of an outer layer called dura mater, a very thin middle layer called arachnoid and an inner layer (which is in contact with the brain tissue) called pia mater. The brain can be divided into three major parts: forebrain, midbrain and hindbrain.


Forebrain

The forebrain consists of cerebrum, thalamus and hypothalamus. Cerebrum forms the major part of the human brain. A deep cleft divides the cerebrum longitudinally into two halves, which are termed as the left and right cerebral hemispheres. The hemispheres are connected by a tract of nerve fibres called corpus callosum.

The cerebrum wraps around a structure called thalamus, which is a major coordinating centre for sensory and motor signaling. Another very important part of the brain called hypothalamus lies at the base of the thalamus.

The inner parts of cerebral hemispheres and a group of associated deep structures like amygdala, hippocampus, etc., form a complex structure called the limbic lobe or limbic system.

Midbrain

The midbrain is located between the thalamus/hypothalamus of the forebrain and pons of the hindbrain. A canal called the cerebral aqueduct passess through the midbrain. The dorsal portion of the midbrain consists mainly of four round swellings (lobes) called corpora quadrigemina. Midbrain and hindbrain form the brain stem.

Hindbrain

The hindbrain comprises pons, cerebellum and medulla (also called the medulla oblongata). Pons consists of fibre tracts that interconnect different regions of the brain. Cerebellum has very convoluted surface in order to provide the additional space for many more neurons. The medulla of the brain is connected to the spinal cord.

(b) Structure of Eye

The adult human eye ball is nearly a spherical structure. The wall of the eye ball is composed of three layers.

The external layer is composed of a dense connective tissue and is called the sclera. The anterior portion of this layer is called the cornea.

The middle layer, choroid, contains many blood vessels and looks bluish in colour. The choroid layer is thin over the posterior two-thirds of the eye ball, but it becomes thick in the anterior part to form the ciliary body.

The ciliary body itself continues forward to form a pigmented and opaque structure called the iris which is the visible coloured portion of the eye.

The eye ball contains a transparent crystalline lens which is held in place by ligaments attached to the ciliary body. In front of the lens, the aperture surrounded by the iris is called the pupil. The diameter of the pupil is regulated by the muscle fibres of iris.

The inner layer is the retina and it contains three layers of cells – from inside to outside – ganglion cells, bipolar cells and photoreceptor cells.

There are two types of photoreceptor cells, namely, rods and cones. These cells contain the light-sensitive proteins called the photopigments. The daylight (photopic) vision and colour vision are functions of cones and the twilight (scotopic) vision is the function of the rods. The rods contain a purplish-red protein called the rhodopsin or visual purple, which contains a derivative of Vitamin A.

The optic nerves leave the eye and the retinal blood vessels enter it at a point medial to and slightly above the posterior pole of the eye ball. Photoreceptor cells are not present in that region and hence it is called the blind spot.

The space between the cornea and the lens is called the aqueous chamber and contains a thin watery fluid called aqueous humor. The pace between the lens and the retina is called the vitreous chamber and is filled with a transparent gel called vitreous humor.

(c) Structure of Ear

Anatomically, the ear can be divided into three major sections called the outer ear, the middle ear and the inner ear.

Outer Ear: The outer ear consists of the pinna and external auditory meatus (canal). The external auditory meatus leads inwards and extends up to the tympanic membrane (the ear drum). The tympanic membrane is composed of connective tissues covered with skin outside and with mucus membrane inside.

Middle Ear: The middle ear contains three ossicles called malleus, incus and stapes which are attached to one another in a chain-like fashion.

A Eustachian tube connects the middle ear cavity with the pharynx. The Eustachian tube helps in equalising the pressures on either sides of the ear drum.

Inner Ear: The fluid-filled inner ear called labyrinth consists of two parts, the bony and the membranous labyrinths. The bony labyrinth is a series of channels. Inside these channels lies the membranous labyrinth, which is surrounded by a fluid called perilymph. The membranous labyrinth is filled with a fluid called endolymph. The coiled portion of the labyrinth is called cochlea.


Question 2: Compare the following:

(a) Central nervous system (CNS) and Peripheral nervous system (PNS)

Answer: Central Nervous System and Peripheral Nervous System: The CNS includes the brain and the spinal cord and is the site of information processing and control. The PNS comprises of all the nerves of the body associated with the CNS (brain and spinal cord).

(b) Resting potential and action potential

Answer: Resting Potential and Action Potential: The relatively static membrane potential of membrane is called resting potential. At this stage, there are more sodium ions outside the neuron and more potassium ions inside the neuron. When the membrane potential of an axon rapidly rises and falls, this stage is called action potential. At this stage, there are more potassium ions outside the neuron and more sodium ions inside the neuron.

(c) Choroid and retina

Answer: Choroid and Retina: The middle layer of eyeball is called choroid, while the inner layer is called retina. Choroid contains many blood vessels, while retina contains photoreceptors.

Question 3: Explain the following processes:

(a) Polarisation of the membrane of a nerve fibre

Answer: Polarisation of the membrane of a nerve fibre: The fluid inside the membrane contains high concentration of K+ and negatively charged proteins and low concentration of Na+. In contrast, the fluid outside the axon contains a low concentration of K+, a high concentration of Na+ and thus forms a concentration gradient.

These ionic gradients across the resting membrane are maintained by the active transport of ions by the sodium-potassium pump which transports 3 Na+ outwards for 2 K+ into the cell. As a result, the outer surface of the axonal membrane possesses a positive charge while its inner surface becomes negatively charged and therefore is polarised.

(b) Depolarisation of the membrane of a nerve fibre

Answer: Depolarisation of the membrane of a nerve fibre: When a stimulus is applied at a site on the polarised membrane, the membrane at the site A becomes freely permeable to Na+. This leads to a rapid influx of Na+ followed by the reversal of the polarity at that site, i.e., the outer surface of the membrane becomes negatively charged and the inner side becomes positively charged. The polarity of the membrane at the site is thus reversed and hence depolarised.

(c) Conduction of a nerve impulse along a nerve fibre

Answer: Conduction of a nerve impulse along a nerve fibre: When a stimulus is applied at a site on the polarised membrane, the membrane at the site becomes freely permeable to Na+. This leads to a rapid influx of Na+ followed by the reversal of the polarity at that site, i.e., the outer surface of the membrane becomes negatively charged and the inner side becomes positively charged.

At sites immediately ahead, the axon membrane has a positive charge on the outer surface and a negative charge on its inner surface. As a result, a current flows on the inner surface from site A to site B.

On the outer surface current flows from site B to site A to complete the circuit of current flow. Hence, the polarity at the site is reversed, and an action potential is generated at site B. Thus, the impulse (action potential) generated at site A arrives at site B.

The sequence is repeated along the length of the axon and consequently the impulse is conducted.

The rise in the stimulus-induced permeability to Na+ is extremely shortlived. It is quickly followed by a rise in permeability to K+. Within a fraction of a second, K+ diffuses outside the membrane and restores the resting potential of the membrane at the site of excitation and the fibre becomes once more responsive to further stimulation.

(d) Transmission of a nerve impulse across a chemical synapse

Answer: Transmission of a nerve impulse across chemical synapse: At a chemical synapse, the membranes of the pre- and post-synaptic neurons are separated by a fluid-filled space called synaptic cleft. Chemicals called neurotransmitters are involved in the transmission of impulses at these synapses. The axon terminals contain vesicles filled with these neurotransmitters. When an impulse (action potential) arrives at the axon terminal, it stimulates the movement of the synaptic vesicles towards the membrane where they fuse with the plasma membrane and release their neurotransmitters in the synaptic cleft. The released neurotransmitters bind to their specific receptors, present on the post-synaptic membrane. This binding opens ion channels allowing the entry of ions which can generate a new potential in the post-synaptic neuron. The new potential developed may be either excitatory or inhibitory.

Question 4: Give a brief account of:

(a) Mechanism of vision

Answer: Mechanism of Vision

(b) Mechanism of hearing

Answer: Mechanism of Hearing: The outer part of the ear collects sound. That sound pressure is amplified through the middle portion of the ear and, in land animals, passed from the medium of air into a liquid medium. The change from air to liquid occurs because air surrounds the head and is contained in the ear canal and middle ear, but not in the inner ear. The inner ear is hollow, embedded in the temporal bone, the densest bone of the body. The hollow channels of the inner ear are filled with liquid, and contain a sensory epithelium that is studded with hair cells. The microscopic "hairs" of these cells are structural protein filaments that project out into the fluid. The hair cells are mechanoreceptors that release a chemical neurotransmitter when stimulated. Sound waves moving through fluid push the filaments; if the filaments bend over enough it causes the hair cells to fire. In this way sound waves are transformed into nerve impulses.

Question 5: Answer briefly:

(a) How do you perceive the colour of an object?

Answer: Cones are responsible for color vision. They require brighter light to function than rods require. There are three types of cones, maximally sensitive to long-wavelength, medium-wavelength, and short-wavelength light (often referred to as red, green, and blue, respectively, though the sensitivity peaks are not actually at these colors). The color seen is the combined effect of stimuli to, and responses from, these three types of cone cells.

(b) Which part of our body helps us in maintaining the body balance?

Answer: The Inner ear has three semi-circular canals forming cochlea. Cochlea is responsible for maintaining the body balance.

(c) How does the eye regulate the amount of light that falls on the retina.

Answer: The pupil in the eye functions like an aperture. This dilates in case of low light and constricts in case of intense light thereby regulating the amount of light falling on the retina.


Question 6: Differentiate between:

(a) Myelinated and non-myelinated axons

Answer: Myelinated and non-myelinated axons: The myelinated nerve fibres are enveloped with Schwann cells, which form a myelin sheath around the axon. Myelinated nerve fibres are found in spinal and cranial nerves. Unmyelinated nerve fibre is enclosed by a Schwann cell that does not form a myelin sheath around the axon, and is commonly found in autonomous and the somatic nervous systems.

(b) Dendrites and axons

Answer: Dendrites: Short fibres which branch repeatedly and project out of the cell body also contain Nissl’s granules and are called dendrites. These fibres transmit impulses towards the cell body.

Axon: The axon is a long fibre, the distal end of which is branched. Each branch terminates as a bulb-like structure called synaptic knob which possess synaptic vesicles containing chemicals called neurotransmitters. The axons transmit nerve impulses away from the cell body to a synapse or to a neuro-muscular junction.

(c) Rods and cones

Answer: Rods and Cones: There are two types of photoreceptor cells, namely, rods and cones. These cells contain the light-sensitive proteins called the photopigments. The daylight (photopic) vision and colour vision are functions of cones and the twilight (scotopic) vision is the function of the rods. The rods contain a purplish-red protein called the rhodopsin or visual purple, which contains a derivative of Vitamin A.

(d) Thalamus and Hypothalamus

Answer: Thalamus and Hypothalamus: The cerebrum wraps around a structure called thalamus, which is a major coordinating centre for sensory and motor signaling. Another very important part of the brain called hypothalamus lies at the base of the thalamus. The hypothalamus contains a number of centres which control body temperature, urge for eating and drinking. It also contains several groups of neurosecretory cells, which secrete hormones called hypothalamic hormones.

(e) Cerebrum and Cerebellum

Answer: Cerebrum and Cerebellum: The cerebrum is located in the forebrain while cerebellum is located in the hind brain.

Question 7: The region of the vertebrate eye, where the optic nerve passes out of the retina, is called the

Answer: (c) Blind Spot

Question 8: Distinguish between:

(a) afferent neurons and efferent neurons

Answer: The afferent nerve fibres transmit impulses from tissues/organs to the CNS and the efferent fibres transmit regulatory impulses from the CNS to the concerned peripheral tissues/organs.

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

Answer: The conduction velocity v of myelinated neurons varies roughly linearly with axon diameter whereas the speed of unmyelinated neurons varies roughly as the square root of diameter. Myelin has two important advantages: fast conduction speed and energy efficiency. Also, since the ionic currents are confined to the nodes of Ranvier, there is far fewer ions "leak" across the membrane, saving metabolic energy. This saving is a significant selective advantage, since the human nervous system uses approximately 20% of the body's metabolic energy.

(c) aqueous humor and vitreous humor

Answer: The space between the cornea and the lens is called the aqueous chamber and contains a thin watery fluid called aqueous humor. The pace between the lens and the retina is called the vitreous chamber and is filled with a transparent gel called vitreous humor.

(d) blind spot and yellow spot

Answer: The optic nerves leave the eye and the retinal blood vessels enter it at a point medial to and slightly above the posterior pole of the eye ball. Photoreceptor cells are not present in that region and hence it is called the blind spot. At the posterior pole of the eye lateral to the blind spot, here is a yellowish pigmented spot called macula lutea with a central pit called the fovea. The fovea is a thinned-out portion of the retina where only the cones are densely packed. It is the point where the visual acuity (resolution) is the greatest.

(e) cranial nerves and spinal nerves.

Answer: Cranial nerves are nerves that emerge directly from the brain stem in contrast to spinal nerves which emerge from segments of the spinal cord. Peripheral nerves are separated to achieve segmental innervation, cranial nerves are divided to serve one or a few specific functions in wider anatomical territories.



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