SolveItClass 11 · NCERT

NCERT Solutions · Class 11 Biology Neural Control and Coordination

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Exercises 18.1–18.10

  1. Exercise 18.1

    Briefly describe the structure of the Brain

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    The brain is the central information processing organ of the body and acts as the 'command and control system'; it is protected by the skull and, inside it, by three cranial meninges — the outer dura mater, a very thin middle arachnoid and an inner pia mater that touches the brain tissue.
    The brain is divided into three major parts: forebrain, midbrain and hindbrain.
    Forebrain — cerebrum, thalamus and hypothalamus.
    Cerebrum forms the major part of the human brain. A deep cleft divides it longitudinally into left and right cerebral hemispheres, connected by a tract of nerve fibres, the corpus callosum.
    The cell layer covering each hemisphere is the cerebral cortex, thrown into prominent folds. It looks greyish because neuron cell bodies are concentrated in it, and is called the grey matter.
    The cortex contains motor areas, sensory areas and large association areas responsible for intersensory associations, memory and communication.
    The myelinated fibre tracts inside the hemisphere give an opaque white appearance and form the white matter.
    Thalamus, around which the cerebrum wraps, is a major coordinating centre for sensory and motor signalling.
    Hypothalamus lies at the base of the thalamus. It has centres controlling body temperature and the urge for eating and drinking, and groups of neurosecretory cells that secrete hypothalamic hormones.
    The inner parts of the cerebral hemispheres with deep structures such as amygdala and hippocampus form the limbic system, which with the hypothalamus regulates sexual behaviour, emotional reactions and motivation.
    Midbrain
    Located between the thalamus/hypothalamus of the forebrain and the pons of the hindbrain.
    A canal, the cerebral aqueduct, passes through it.
    Its dorsal portion bears four round swellings called corpora quadrigemina.
    Hindbrain — pons, cerebellum and medulla.
    Pons consists of fibre tracts that interconnect different regions of the brain.
    Cerebellum has a very convoluted surface, which provides additional space for many more neurons.
    Medulla oblongata is connected to the spinal cord and contains centres controlling respiration, cardiovascular reflexes and gastric secretions.
    Midbrain, pons and medulla oblongata together make up the brain stem, which forms the connection between the brain and the spinal cord.
    NCERT_Solution_Class11_Biology_Ch18_Q18-1

  2. Exercise 18.2

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

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    (a) Central neural system (CNS) and Peripheral neural system (PNS)
    Parts: CNS includes the brain and the spinal cord; PNS comprises all the nerves of the body associated with the brain and spinal cord.
    Role: CNS is the site of information processing and control; PNS only carries information — its nerve fibres bring impulses to the CNS and take regulatory impulses away from it.
    Fibres: PNS nerve fibres are of two types — afferent fibres, which transmit impulses from tissues/organs to the CNS, and efferent fibres, which transmit regulatory impulses from the CNS to the peripheral tissues/organs.
    Divisions: CNS is divided into brain and spinal cord; PNS is divided into the somatic neural system (impulses from CNS to skeletal muscles) and the autonomic neural system (impulses from CNS to involuntary organs and smooth muscles), the latter further classified into sympathetic and parasympathetic neural systems.
    (b) Resting potential and action potential
    Definition: Resting potential is the electrical potential difference across the plasma membrane of a resting (non-conducting) neuron; action potential is the potential difference across the membrane at a stimulated site, and it is what is termed the nerve impulse.
    State of the membrane: at resting potential the membrane is polarised — outer surface positively charged, inner surface negatively charged; at action potential the polarity is reversed (depolarised) — outer surface negative, inner surface positive.
    Permeability: in the resting state the membrane is comparatively more permeable to \(\displaystyle K^{+}\) and nearly impermeable to \(\displaystyle Na^{+}\); on stimulation the membrane at that site becomes freely permeable to \(\displaystyle Na^{+}\), causing a rapid influx of \(\displaystyle Na^{+}\).
    Ion distribution: resting potential goes with high \(\displaystyle K^{+}\) and negatively charged proteins inside and high \(\displaystyle Na^{+}\) outside, maintained by the sodium-potassium pump ($\displaystyle 3$ \(\displaystyle Na^{+}\) out for $\displaystyle 2$ \(\displaystyle K^{+}\) in); action potential arises when this gradient drives \(\displaystyle Na^{+}\) inwards.
    Duration: resting potential is the steady, maintained condition; action potential is extremely short-lived — a rise in \(\displaystyle K^{+}\) permeability follows within a fraction of a second and restores the resting potential.
    Movement: resting potential stays where it is; the action potential is regenerated at successive points and is thus conducted along the axon.

  3. Exercise 18.3

    Explain the following processes:
    (a)
    Polarisation of the membrane of a nerve fibre
    (b)
    Depolarisation of the membrane of a nerve fibre
    (c)
    Transmission of a nerve impulse across a chemical synapse

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    (a) Polarisation of the membrane of a nerve fibre
    Polarisation is the resting condition of the axonal membrane in which its outer surface is positively charged and its inner surface negatively charged.
    The neural membrane carries different types of ion channels that are selectively permeable to different ions.
    In a resting (non-conducting) neuron the membrane is comparatively more permeable to \(\displaystyle K^{+}\), nearly impermeable to \(\displaystyle Na^{+}\), and impermeable to the negatively charged proteins of the axoplasm.
    Consequently the axoplasm holds a high concentration of \(\displaystyle K^{+}\) and negatively charged proteins and a low concentration of \(\displaystyle Na^{+}\), while the fluid outside holds low \(\displaystyle K^{+}\) and high \(\displaystyle Na^{+}\) — a concentration gradient across the membrane.
    These ionic gradients are maintained by the active transport of the sodium-potassium pump, which transports $\displaystyle 3$ \(\displaystyle Na^{+}\) outwards for every $\displaystyle 2$ \(\displaystyle K^{+}\) into the cell.
    The electrical potential difference across this polarised resting membrane is called the resting potential.
    (b) Depolarisation of the membrane of a nerve fibre
    Depolarisation is the reversal of the polarity at a stimulated site — the outer surface becomes negatively charged and the inner surface positively charged.
    When a stimulus is applied at a site (say site A), the membrane there becomes freely permeable to \(\displaystyle Na^{+}\).
    This leads to a rapid influx of \(\displaystyle Na^{+}\), which reverses the polarity at that site.
    The electrical potential difference now across the membrane at site A is the action potential, which is in fact termed a nerve impulse.
    The stimulus-induced rise in \(\displaystyle Na^{+}\) permeability is extremely short-lived; it is quickly followed by a rise in permeability to \(\displaystyle K^{+}\), and within a fraction of a second \(\displaystyle K^{+}\) diffuses outside, restoring the resting potential so the fibre becomes responsive to further stimulation.
    (c) Transmission of a nerve impulse across a chemical synapse
    At a chemical synapse the impulse crosses as a chemical, not as a current, because the pre- and post-synaptic membranes are separated by a fluid-filled space, the synaptic cleft.
    The axon terminals contain vesicles filled with chemicals called neurotransmitters.
    When an impulse (action potential) arrives at the axon terminal, it stimulates the movement of the synaptic vesicles towards the membrane.
    The vesicles fuse with the plasma membrane and release their neurotransmitters into 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 generate a new potential in the post-synaptic neuron.
    The new potential developed may be either excitatory or inhibitory.
    NCERT_Solution_Class11_Biology_Ch18_Q18-3

  4. Exercise 18.4

    Draw labelled diagrams of the following:
    (a)
    Neuron
    (b)
    Brain

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    (a) Neuron — draw the structure printed as Figure $\displaystyle 18.1$.
    Draw the neuron as three connected parts: cell body, dendrites and axon.
    Cell body: a broad, irregular body at one end containing the nucleus and cytoplasm with typical cell organelles; scatter dots inside it for the granular bodies called Nissl's granules.
    Dendrites: several short fibres branching repeatedly out of the cell body; show Nissl's granules in them too, and an arrow pointing towards the cell body, since dendrites transmit impulses towards it.
    Axon: a single long fibre leaving the opposite side of the cell body, drawn much longer than the dendrites, with an arrow pointing away from the cell body.
    Myelin sheath: cover the axon with a row of sausage-shaped sheaths formed by Schwann cells; leave a small gap between two adjacent sheaths and label it node of Ranvier.
    Axon terminals: branch the distal end of the axon and end each branch in a swollen synaptic knob; draw small circles inside the knob for the synaptic vesicles, which contain neurotransmitters.
    Labels needed: cell body, nucleus, Nissl's granules, dendrites, axon, Schwann cell, myelin sheath, node of Ranvier, axon terminal, synaptic knob.
    (b) Brain — draw the sagittal section printed as Figure $\displaystyle 18.4$.
    Draw a side view of a lengthwise (sagittal) section of the human brain and group the parts as forebrain, midbrain and hindbrain.
    Forebrain: the large, deeply folded cerebrum filling the top and front, its outer folded layer marked as cerebral cortex; below it an arched band of fibres, the corpus callosum; the thalamus wrapped by the cerebrum; and the hypothalamus just at the base of the thalamus.
    Midbrain: a short region drawn between the thalamus/hypothalamus and the pons; mark the four round swellings, corpora quadrigemina, on its dorsal side and the cerebral aqueduct passing through it.
    Hindbrain: the pons below the midbrain; the cerebellum behind and below it, drawn with a very convoluted (finely folded) surface; the medulla oblongata below the pons, tapering downward into the spinal cord.
    Mark brain stem as a bracket covering midbrain + pons + medulla oblongata, the connection between brain and spinal cord.
    If shown, label the three covering cranial meninges: dura mater (outer), arachnoid (middle), pia mater (inner, in contact with brain tissue).
    Labels needed: cerebrum, cerebral cortex, corpus callosum, thalamus, hypothalamus, midbrain, corpora quadrigemina, pons, cerebellum, medulla oblongata, spinal cord.
    NCERT_Solution_Class11_Biology_Ch18_Q18-4NCERT_Solution_Class11_Biology_Ch18_Q18-4-2

  5. Exercise 18.5

    Write short notes on the following:
    (a)
    Neural coordination
    (b)
    Forebrain
    (c)
    Midbrain
    (d)
    Hindbrain
    (e)
    Synapse

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    NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.

    (a) Neural coordination
    Coordination is the process through which two or more organs interact and complement the functions of one another, and the neural system carries it out by providing an organised network of point-to-point connections for quick coordination.
    The neural system is composed of highly specialised cells, neurons, which detect, receive and transmit different kinds of stimuli.
    Example: during physical exercise the energy demand rises, so oxygen supply, rate of respiration, heart beat and blood flow all increase together; when exercise stops, the nerves, lungs, heart and kidney gradually return to normal.
    In the body the neural system and the endocrine system jointly coordinate and integrate the activities of all organs so that they function in a synchronised fashion, the endocrine system doing it chemically through hormones.
    (b) Forebrain
    The forebrain consists of cerebrum, thalamus and hypothalamus.
    Cerebrum is the major part of the human brain; a deep cleft divides it into two cerebral hemispheres joined by the corpus callosum. Its folded outer layer, the cerebral cortex (grey matter), holds motor areas, sensory areas and association areas for intersensory associations, memory and communication; the myelinated tracts within form the white matter.
    Thalamus, around which the cerebrum wraps, is a major coordinating centre for sensory and motor signalling.
    Hypothalamus, at the base of the thalamus, contains centres controlling body temperature and the urge for eating and drinking, and neurosecretory cells that secrete hypothalamic hormones.
    The inner parts of the hemispheres with structures such as amygdala and hippocampus form the limbic system, which with the hypothalamus regulates sexual behaviour, emotional reactions and motivation.
    (c) Midbrain
    The midbrain lies between the thalamus/hypothalamus of the forebrain and the pons of the hindbrain.
    A canal called the cerebral aqueduct passes through it.
    Its dorsal portion consists mainly of four round swellings (lobes) called corpora quadrigemina.
    With the pons and medulla it forms part of the brain stem; the chapter's summary notes that the midbrain receives and integrates visual, tactile and auditory inputs.
    (d) Hindbrain
    The hindbrain comprises pons, cerebellum and medulla (medulla oblongata).
    Pons consists of fibre tracts that interconnect different regions of the brain.
    Cerebellum has a very convoluted surface that provides additional space for many more neurons; it integrates information received from the semicircular canals of the ear and the auditory system.
    Medulla is connected to the spinal cord and contains centres which control respiration, cardiovascular reflexes and gastric secretions.
    (e) Synapse
    A synapse is the junction through which a nerve impulse is transmitted from one neuron to another.
    It is formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron, which may or may not be separated by a gap called the synaptic cleft.
    There are two types — electrical synapses and chemical synapses.
    At electrical synapses the two membranes lie in very close proximity and electrical current flows directly from one neuron into the other; transmission here resembles impulse conduction along a single axon, is always faster than at a chemical synapse, and such synapses are rare in our system.
    At chemical synapses the two membranes are separated by the fluid-filled synaptic cleft, and chemicals called neurotransmitters, stored in synaptic vesicles of the axon terminal, carry the impulse across.

  6. Exercise 18.6

    Give a brief account of Mechanism of synaptic transmission.

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    A nerve impulse is transmitted from one neuron to another at a synapse, formed by the membranes of a pre-synaptic and a post-synaptic neuron, which may or may not be separated by a synaptic cleft.
    At an electrical synapse
    The membranes of the pre- and post-synaptic neurons lie in very close proximity.
    Electrical current flows directly from one neuron into the other, so transmission is very similar to impulse conduction along a single axon.
    It is always faster than transmission across a chemical synapse, but electrical synapses are rare in our system.
    At a chemical synapse
    The two membranes are separated by a fluid-filled space, the synaptic cleft, so the impulse is carried across by chemicals called neurotransmitters.
    The axon terminals contain vesicles filled with neurotransmitters.
    When an impulse (action potential) arrives at the axon terminal, it stimulates the movement of these synaptic vesicles towards the membrane.
    The vesicles fuse with the plasma membrane and release their neurotransmitters into the synaptic cleft.
    The released neurotransmitters bind to specific receptors present on the post-synaptic membrane.
    This binding opens ion channels, allowing the entry of ions which generate a new potential in the post-synaptic neuron.
    The new potential developed may be either excitatory or inhibitory.
    NCERT_Solution_Class11_Biology_Ch18_Q18-6

  7. Exercise 18.7

    Explain the role of Na+\displaystyle Na^{+} in the generation of action potential.

    The book prints no answer for this

    NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.

    The inward rush of \(\displaystyle Na^{+}\) is what actually creates the action potential — its entry reverses the polarity of the membrane at the stimulated site.
    Before stimulation: the resting axonal membrane is nearly impermeable to \(\displaystyle Na^{+}\), and the sodium-potassium pump transports $\displaystyle 3$ \(\displaystyle Na^{+}\) outwards for every $\displaystyle 2$ \(\displaystyle K^{+}\) inwards. So the axoplasm is kept low in \(\displaystyle Na^{+}\) while the fluid outside is high in \(\displaystyle Na^{+}\) — a steep concentration gradient is stored across the polarised membrane (outer surface positive, inner negative).
    On stimulation: when a stimulus is applied at a site (say site A), the membrane there becomes freely permeable to \(\displaystyle Na^{+}\).
    Rapid influx: \(\displaystyle Na^{+}\) pours in down that gradient, so the outer surface at site A becomes negatively charged and the inner surface positively charged — the site is depolarised.
    Action potential: the electrical potential difference now across the membrane at site A is the action potential, which is in fact termed the nerve impulse.
    Conduction: the site immediately ahead (site B) is still polarised, so current flows from A to B on the inner surface and from B to A on the outer surface, reversing the polarity at B and generating an action potential there too. Repeated along the length of the axon, this conducts the impulse.
    Short-lived: the stimulus-induced permeability to \(\displaystyle Na^{+}\) is extremely short-lived; it is quickly followed by a rise in permeability to \(\displaystyle K^{+}\), and \(\displaystyle K^{+}\) diffusing outward restores the resting potential so the fibre can respond again.
    NCERT_Solution_Class11_Biology_Ch18_Q18-7

  8. Exercise 18.8

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

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    NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.

    (a) Myelinated and non-myelinated axons
    Sheath: myelinated axons are enveloped by Schwann cells which form a myelin sheath around the axon; a non-myelinated axon is enclosed by a Schwann cell that does not form a myelin sheath.
    Nodes: the gaps between two adjacent myelin sheaths are the nodes of Ranvier, present only in myelinated fibres; non-myelinated fibres have no such nodes.
    Where found: myelinated nerve fibres are found in spinal and cranial nerves; non-myelinated fibres are commonly found in the autonomous (autonomic) and somatic neural systems.
    (b) Dendrites and axons
    Shape and number: dendrites are short fibres which branch repeatedly and project out of the cell body; the axon is a single long fibre whose distal end alone is branched.
    Nissl's granules: dendrites contain Nissl's granules, as the cell body does; the chapter records these granules only in the cell body and the dendrites, not in the axon.
    Direction of impulse: dendrites transmit impulses towards the cell body; axons transmit nerve impulses away from the cell body to a synapse or to a neuro-muscular junction.
    Ending: each branch of the axon terminates as a bulb-like synaptic knob containing synaptic vesicles filled with neurotransmitters; dendrites have no such knobs.
    Sheath: an axon may be myelinated or non-myelinated, with Schwann cells around it; dendrites are not described as being sheathed.
    (c) Thalamus and Hypothalamus
    Position: the cerebrum wraps around the thalamus; the hypothalamus lies at the base of the thalamus. Both belong to the forebrain.
    Function of thalamus: it is a major coordinating centre for sensory and motor signalling.
    Function of hypothalamus: it contains centres which control body temperature and the urge for eating and drinking.
    Secretion: the hypothalamus contains several groups of neurosecretory cells which secrete hypothalamic hormones; no such secretory role is given to the thalamus.
    Limbic system: along with the limbic system the hypothalamus is involved in the regulation of sexual behaviour, expression of emotional reactions and motivation.
    (d) Cerebrum and Cerebellum
    Part of the brain: the cerebrum belongs to the forebrain and forms the major part of the human brain; the cerebellum belongs to the hindbrain.
    Structure: a deep cleft divides the cerebrum longitudinally into two cerebral hemispheres joined by a tract of nerve fibres, the corpus callosum; the cerebellum instead is described by its very convoluted surface, which provides additional space for many more neurons.
    Layers: the cerebrum has an outer folded cerebral cortex (grey matter) of neuron cell bodies over an inner white matter of myelinated tracts.
    Function of cerebrum: its cortex holds motor areas, sensory areas and association areas responsible for complex functions such as intersensory associations, memory and communication.
    Function of cerebellum: it integrates information received from the semicircular canals of the ear and the auditory system.

  9. Exercise 18.9

    Answer the following:
    (a)
    Which part of the human brain is the most developed?
    (b)
    Which part of our central neural system acts as a master clock?

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    NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.

    (a) The cerebrum is the most developed part of the human brain - the chapter states that the cerebrum forms the major part of the human brain, divided by a deep cleft into the left and right cerebral hemispheres, which are connected by the corpus callosum.
    Its outer layer, the cerebral cortex or grey matter, carries motor areas, sensory areas and very large association areas responsible for intersensory associations, memory and communication - so the cerebrum is the most developed both in sheer bulk and in the complexity of what it does. All of this is from the chapter.
    (b) The hypothalamus acts as the master clock - more precisely the suprachiasmatic nucleus (SCN), a small cluster of neurons within it. This naming is from outside the book.
    What the chapter does give: it says the brain controls the circadian ($\displaystyle 24$-hour) rhythms of our body, and it places the hypothalamus at the base of the thalamus in the forebrain, holding centres that control body temperature and the urge for eating and drinking, plus neurosecretory cells that secrete hypothalamic hormones.
    Which structure actually keeps that $\displaystyle 24$-hour time is not stated anywhere in the chapter - it names the function without naming the clock, so part (b) cannot be settled from the chapter alone.
    From outside the book: the SCN lies just above the optic chiasma and receives light information directly from the retina, which resets it each day. It then drives the daily rhythms of sleep and wakefulness, body temperature, urine formation and hormone release - including melatonin from the pineal gland. Because every other body rhythm is set to its beat, it is called the master clock.

  10. Exercise 18.10

    Distinguish between:
    (a)
    afferent neurons and efferent neurons
    (b)
    impulse conduction in a myelinated nerve fibre and unmyelinated nerve fibre (f) cranial nerves and spinal nerves.

    The book prints no answer for this

    NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.

    (a) Afferent neurons and efferent neurons
    Direction: afferent nerve fibres transmit impulses from tissues/organs to the CNS; efferent fibres transmit regulatory impulses from the CNS to the concerned peripheral tissues/organs.
    Kind of information carried: afferent fibres bring in the information the CNS is to process; efferent fibres carry out the CNS's regulatory instructions.
    Where they belong: both are nerve fibres of the peripheral neural system (PNS), which comprises all the nerves of the body associated with the brain and spinal cord.
    Example of an efferent path: the somatic neural system relays impulses from the CNS to skeletal muscles, and the autonomic neural system transmits impulses from the CNS to involuntary organs and smooth muscles.
    (b) Impulse conduction in a myelinated and an unmyelinated nerve fibre
    What the chapter gives is the structural difference only: a myelinated fibre is enveloped by Schwann cells that form a myelin sheath, with nodes of Ranvier in the gaps between two adjacent sheaths, while an unmyelinated fibre is enclosed by a Schwann cell that does not form a myelin sheath and so has no nodes.
    Myelinated fibres are found in spinal and cranial nerves; unmyelinated fibres are common in the autonomous (autonomic) and somatic neural systems.
    In both, the impulse is conducted the same way as far as this chapter describes it — a stimulus makes the membrane freely permeable to \(\displaystyle Na^{+}\), the site is depolarised, current flows to the next site, and the sequence is repeated along the axon.
    Not in this chapter: the current edition does not describe any difference in how or how fast the impulse travels in the two fibres (the jumping, node-to-node conduction usually quoted for myelinated fibres). Do not attribute it to this chapter.
    (f) Cranial nerves and spinal nerves
    Not answerable from this chapter. Cranial and spinal nerves are mentioned only once, to say that myelinated nerve fibres are found in spinal and cranial nerves.
    What can be said from the chapter: both are nerves of the peripheral neural system, which comprises all the nerves associated with the CNS, and both may carry afferent and efferent fibres.
    The chapter gives no account of where each arises (brain vs spinal cord), how many pairs there are, or what regions they supply, so those points must come from elsewhere.