Neural Control and Coordination Class 11 Biology NCERT Solutions

Step-by-step solution

1. 1The brain is the centre of all neural control and the master coordinating centre of the body, and it is the largest part of the central nervous system.
2. 2It is protected by three coats called meninges — the outer pia mater, the middle arachnoid and the inner dura mater — and it is filled with cerebrospinal fluid that cushions shocks.
3. 3Structurally the brain is divided into three major regions: the forebrain, the midbrain and the hindbrain.
4. 4The forebrain is the largest region and consists of the cerebrum and the diencephalon. The cerebrum has left and right cerebral hemispheres joined by the corpus callosum, and its outer cerebral cortex carries the areas for sensory perception, voluntary motor control, association, memory and speech.
5. 5The diencephalon contains the thalamus, which relays and integrates sensory signals and acts as the master clock of the body, and the hypothalamus, which maintains homeostasis and links the nervous system with the endocrine system.
6. 6The midbrain is small and lies between the forebrain and hindbrain. It contains the cerebral peduncles and the corpora quadrigemina, and it holds the centres for reflexes such as the pupillary reflex and the visual and auditory reflexes.
7. 7The hindbrain consists of the pons, the cerebellum and the medulla oblongata. The pons carries respiratory and salivation centres, the cerebellum maintains balance, posture and the precision and timing of voluntary movement, and the medulla controls respiration, cardiovascular activity and vomiting and carries nerve pathways between the brain and the spinal cord.

Step-by-step solution

1. 1(a) Definition: the CNS is the brain and the spinal cord, whereas the PNS is the remaining nervous tissue, that is all the nerves and ganglia lying outside the brain and spinal cord.
2. 2(a) Role: the CNS is the command centre in which all sensory information is integrated and from which motor commands originate, whereas the PNS simply carries sensory impulses in and motor impulses out.
3. 3(a) Division: the CNS has no further subdivision, whereas the PNS is divided into the somatic nervous system and the autonomic nervous system.
4. 4(b) Resting potential: this is the electrical potential across the axonal membrane of a neuron at rest, about −60 to −90 mV, with the inside negative relative to the outside. It exists because the membrane is far more permeable to K⁺ than to Na⁺, and the Na⁺/K⁺ pump keeps K⁺ concentrated inside.
5. 5(b) Action potential: this is a rapid, self-propagating reversal of the membrane potential to about +30 to +40 mV, produced when a stimulus brings the membrane to threshold. It involves a rapid influx of Na⁺ followed by an efflux of K⁺.
6. 6(b) Duration and course: the resting potential is a steady state maintained continuously by the Na⁺/K⁺ pump, whereas the action potential is a brief, transient event that travels along the axon and is followed by a refractory period while the ion gradients are restored.

Step-by-step solution

1. 1(a) Polarisation is the resting condition of the axonal membrane. At rest the membrane is comparatively more permeable to K⁺ and almost impermeable to Na⁺, so K⁺ diffuses out of the axon, leaving the axoplasm negatively charged, while negatively charged proteins stay inside. The Na⁺/K⁺ pump maintains a high K⁺ concentration inside and a high Na⁺ concentration outside.
2. 2(a) The result is a potential difference of about −60 to −90 mV across the membrane, with the inside negative relative to the outside; this resting, polarized state is the resting potential.
3. 3(b) Depolarisation begins when a stimulus, or a graded potential from the dendrites, brings the membrane to threshold. Voltage-gated sodium channels then open widely and Na⁺ rushes into the axon down its electrochemical gradient.
4. 4(b) The influx of positively charged Na⁺ makes the inside of the axonal membrane positive for a moment, so the potential overshoots zero and reaches about +30 to +40 mV. This rapid reversal of the resting potential is the depolarisation phase and is the rising phase of the action potential.
5. 5(b) The Na⁺ channels are then inactivated and voltage-gated K⁺ channels open, so K⁺ flows out; the membrane returns to, and briefly goes below, its resting value, which is repolarisation.
6. 6(c) At a chemical synapse the impulse arrives at the axon terminal, which is separated from the next cell by a narrow fluid-filled synaptic cleft, so the membranes of the two cells never touch directly.
7. 7(c) The arrival of the impulse causes vesicles of neurotransmitter in the presynaptic terminal to fuse with the presynaptic membrane and release their contents into the cleft by exocytosis.
8. 8(c) The neurotransmitter diffuses across the cleft and binds to specific receptor molecules on the postsynaptic membrane, which opens channels in it and generates a new electrical signal in the postsynaptic cell.
9. 9(c) The effect is excitatory if the transmitter opens Na⁺ channels, giving a depolarisation that may reach threshold and fire an action potential, and inhibitory if it opens Cl⁻ or K⁺ channels.
10. 10(c) Transmission is one-way because the transmitter is released only from the presynaptic terminal and the receptors are located only on the postsynaptic membrane. The transmitter is then removed by enzymatic degradation, such as acetylcholinesterase, or by reuptake into the terminal.

Step-by-step solution

1. 1(a) A neuron is the structural and functional unit of the nervous system. Label, in the order the impulse travels: dendrites, cell body (cyton or soma) containing the nucleus and Nissl granules, axon hillock, axon, myelin sheath, nodes of Ranvier, and axon terminals or synaptic knobs.
2. 2(a) The direction of the impulse is from the dendrites and cell body down the axon to the axon terminals.
3. 3(a) The basic plan of a multipolar motor neuron, showing the receptive end, the integrating region and the output end:
4. 4
   $$\underset{\text{receive}}{\underbrace{\text{dendrites}}}⟶\underset{\text{integrate}}{\underbrace{\text{cell body }(\text{nucleus})}}⟶\underset{\text{conduct}}{\underbrace{\underset{\text{saltatory}}{\underbrace{\text{myelin sheath}}}\mid \underset{\text{gap}}{\underbrace{\text{node of Ranvier}}}\text{...}\text{axon}}}⟶\underset{\text{secrete transmitter}}{\underbrace{\text{axon terminals}}}$$
5. 5(b) For the brain, the outer covering consists of three meninges — dura mater, arachnoid and pia mater — and inside them the forebrain, midbrain and hindbrain are shown, with the cerebral hemispheres at the top, the thalamus and hypothalamus in the diencephalon, and the pons, cerebellum and medulla oblongata in the hindbrain, continuing into the spinal cord.
6. 6(b) The cerebellum must be drawn at the posterior region beneath the cerebrum, since that is where it lies.

Step-by-step solution

1. 1(a) Neural coordination is the coordination of the activities of the body by the nervous system. Receptors detect a stimulus, sensory neurons carry the information as an impulse to the central nervous system, the CNS integrates it and decides a response, and motor neurons carry the impulse to an effector such as a muscle or gland. Its two components are the conduction of impulses and the reflex action.
2. 2(b) The forebrain is the largest and most anterior region of the brain and consists of the cerebrum and the diencephalon. The cerebrum has two hemispheres joined by the corpus callosum, and its cerebral cortex contains separate areas for sensation, voluntary motor control, association, memory and speech, so it is the seat of intelligence, reasoning and conscious behaviour.
3. 3(b) The diencephalon contains the thalamus, which relays and integrates sensory inputs and acts as the master clock, and the hypothalamus, which maintains homeostasis of body temperature, hunger and thirst, regulates the pituitary gland and sex drive, and houses centres for reward, anger and fear.
4. 4(c) The midbrain is a small region lying between the forebrain and the hindbrain. It contains the cerebral peduncles and the corpora quadrigemina, and it carries the reflex centres for the pupil and for visual and auditory reflexes, so it functions as a relay and reflex centre rather than as a centre of thought.
5. 5(d) The hindbrain comprises the pons, the cerebellum and the medulla oblongata. The pons contains centres for respiration and salivation, the cerebellum maintains body balance, posture and the precision and timing of voluntary movements, and the medulla oblongata contains vital centres for respiration, cardiovascular activity and vomiting and relays tracts between the brain and spinal cord.
6. 6(e) A synapse is the junction between two neurons, or between a neuron and an effector cell. The two membranes are separated by a narrow gap called the synaptic cleft, and transmission across it is by a chemical neurotransmitter released from the presynaptic terminal, which makes the synapse one-way.

Step-by-step solution

1. 1The mechanism operates at the junction between the axon terminal of one neuron and the dendrite or cell body of the next, across the synaptic cleft.
2. 2When the action potential reaches the presynaptic terminal, the depolarisation opens voltage-gated Ca²⁺ channels and Ca²⁺ rushes into the terminal.
3. 3The raised Ca²⁺ level causes the synaptic vesicles to move to and fuse with the presynaptic membrane.
4. 4The neurotransmitter, such as acetylcholine, is then released into the cleft by exocytosis — this is the step that makes the synapse chemical.
5. 5The neurotransmitter diffuses across the cleft in a very short time and binds to specific receptors on the postsynaptic membrane.
6. 6Receptor binding opens channels in the postsynaptic membrane. If the transmitter is excitatory, Na⁺ channels open and the postsynaptic membrane depolarises, producing an excitatory postsynaptic potential; if the summed potential reaches threshold, a fresh action potential is fired in the next neuron.
7. 7If the transmitter is inhibitory, Cl⁻ or K⁺ channels open instead and the postsynaptic membrane hyperpolarises, so no impulse is generated.
8. 8The neurotransmitter is then removed so that the synapse can transmit again — by enzymatic breakdown in the cleft, as with acetylcholinesterase breaking down acetylcholine, or by reuptake into the presynaptic terminal.
9. 9Because release happens only on one side and the receptors are present only on the other, transmission across a chemical synapse is unidirectional; the brief delay across the cleft is the synaptic delay.

Step-by-step solution

1. 1At rest the axonal membrane is polarised, with the inside negative at about −70 mV, and it is comparatively impermeable to Na⁺ while being highly permeable to K⁺.
2. 2A stimulus, or the summed graded potentials from the dendrites, brings the membrane to its threshold potential, which is the level of depolarisation that must be reached to open the voltage-gated channels.
3. 3At threshold the voltage-gated Na⁺ channels open rapidly, and because the electrochemical gradient for Na⁺ is strongly inward, Na⁺ rushes into the axon.
4. 4This massive inward movement of positive charge makes the inside of the membrane positive, so the potential rises from about −70 mV to about +30 to +40 mV. This is the depolarisation, or rising, phase of the action potential.
5. 5Na⁺ is therefore the ion that generates the rising phase: it supplies the charge that reverses the polarity of the membrane. This can be proved experimentally — blocking the Na⁺ channels with tetrodotoxin, or removing Na⁺ from the extracellular fluid, prevents the action potential altogether.
6. 6The Na⁺ channels then inactivate almost immediately, so the influx stops on its own, while voltage-gated K⁺ channels open and K⁺ flows out, producing repolarisation and briefly hyperpolarising the membrane.
7. 7The Na⁺/K⁺ pump is not what creates the action potential but what restores the gradients afterwards, by exporting 3 Na⁺ for every 2 K⁺ imported, using ATP.

Step-by-step solution

1. 1(a) A myelinated axon is wrapped in a fatty insulating sheath made by Schwann cells in the PNS and oligodendrocytes in the CNS, and the sheath is interrupted at intervals by the nodes of Ranvier; a non-myelinated axon has no such sheath.
2. 2(a) Because the myelin insulates the axoplasm, current spreads rapidly from node to node, giving saltatory conduction; a non-myelinated axon conducts continuously and therefore much more slowly.
3. 3(a) Myelinated fibres are usually of larger diameter and conduct impulses faster, up to about 120 m/s, and need less energy per impulse; non-myelinated fibres are of smaller diameter, conduct at only about 0.5 to 2 m/s, and consume more energy.
4. 4(b) Dendrites are short, highly branched and tapering, are numerous, and their receptive surface receives stimuli and carries the resulting signal toward the cell body.
5. 5(b) An axon is a single, long, cylindrical fibre of uniform diameter that arises from the cell body at the axon hillock, may be myelinated, and carries impulses away from the cell body to the axon terminals.
6. 6(c) The thalamus lies in the forebrain and is chiefly concerned with relaying and integrating sensory information and with the sleep–wake rhythm and other body rhythms; it is described as the master clock of the body.
7. 7(c) The hypothalamus lies below the thalamus and maintains homeostasis — body temperature, hunger, thirst and water balance — and also regulates the pituitary and gonadal activity, the sex drive, and expresses anger, fear and reward behaviour.
8. 8(d) The cerebrum is the largest part of the brain, has two hemispheres joined by the corpus callosum, has gyri and sulci, and governs voluntary action, sensation, memory, intelligence, reasoning and speech.
9. 9(d) The cerebellum is a much smaller part lying at the posterior end beneath the cerebrum, has folia and a vermis with a narrow cortex, and its function is control of balance, posture and the coordination, precision and timing of voluntary movements.

Step-by-step solution

1. 1(a) The cerebrum is the most developed part of the human brain, and this is what distinguishes human brain size from that of other mammals.
2. 2(a) The two cerebral hemispheres are separated by a deep groove except where they are joined by the corpus callosum, and the cerebral cortex is the seat of intelligence, memory, reasoning and speech.
3. 3(b) The pineal gland, which lies on the dorsal side of the forebrain, acts as the master clock, or a biological clock.
4. 4(b) It secretes melatonin, and because the secretion of this hormone is linked to the length of the day, it regulates the diurnal rhythm of the body, such as the sleep–wake cycle and the timing of reproductive maturity.

Step-by-step solution

1. 1A note on the numbering: three comparisons are listed, so they run (a), (b) and (c). Some printings of this Reprint 2026-27 set label the third one (f), which is a typo in the book — the answer below uses (c) so it matches the three items.
2. 2(a) An afferent or sensory neuron carries impulses from a receptor to the central nervous system, and its cell body lies in the dorsal root ganglion of the spinal cord; an efferent or motor neuron carries impulses from the central nervous system to an effector such as a muscle or gland, and its cell body lies in the ventral horn of the spinal cord.
3. 3(a) In a typical afferent neuron the dendrites are long and the axon short, while in a typical efferent neuron the dendrites are short and the axon long, so the direction of the long fibre matches the direction of the impulse.
4. 4(b) In a myelinated fibre the impulse effectively jumps from one node of Ranvier to the next, which is saltatory conduction, so it travels fast at a lower cost in energy; in an unmyelinated fibre the impulse travels continuously along the whole axonal membrane, so it is much slower and consumes more energy.
5. 5(c) Cranial nerves arise directly from the brain, are numbered I to XII, and are named after the brain region they supply; spinal nerves arise from the spinal cord, are 31 pairs, and are not named.
6. 6(c) Each spinal nerve is formed by the union of a dorsal root carrying sensory fibres and a ventral root carrying motor fibres, so all of them are mixed; the cranial nerves carry both sensory and motor fibres, and the optic nerve is purely sensory and the oculomotor partly motor.
7. 7(c) Cranial nerves supply the head, neck and shoulder region, while spinal nerves supply the rest of the body and the limbs.