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

Concept. The brain is the central information processing organ of the body and acts as the “command and control system”. It controls voluntary movements, balance of the body, functioning of vital involuntary organs (lungs, heart, kidneys), thermoregulation, hunger and thirst, circadian (24-hour) rhythms, activities of several endocrine glands and human behaviour. It is also the site for processing of vision, hearing, speech, memory, intelligence, emotions and thoughts.

Protection. The human brain is well protected by the skull. Inside the skull the brain is covered by three cranial meninges: an outer dura mater, a very thin middle arachnoid, and an inner pia mater which is in contact with the brain tissue.

Parts. The brain is divided into three major parts — forebrain, midbrain and hindbrain.

1. Forebrain — consists of the cerebrum, thalamus and hypothalamus. The cerebrum forms the major part of the brain; a deep cleft divides it into left and right cerebral hemispheres connected by the corpus callosum. The outer cerebral cortex (grey matter, folded) has motor, sensory and association areas; the inner white matter is made of myelinated fibres. The thalamus is wrapped by the cerebrum and coordinates sensory and motor signalling; the hypothalamus at its base controls body temperature, eating and drinking and secretes hypothalamic hormones. The limbic system (inner cerebral parts, amygdala, hippocampus) deals with emotions and sexual behaviour.
2. Midbrain — lies between the thalamus/hypothalamus and the pons; a canal, the cerebral aqueduct, passes through it; its dorsal portion has four round lobes, the corpora quadrigemina.
3. Hindbrain — consists of pons (fibre tracts interconnecting regions of the brain), cerebellum (very convoluted surface with many neurons) and medulla oblongata (connected to the spinal cord; centres for respiration, cardiovascular reflexes and gastric secretions).

Extra depth. The brain stem consists of the midbrain, pons and medulla oblongata and forms the connection between the brain and the spinal cord.

Conclusion: The human brain, protected by the skull and three meninges, has a forebrain (cerebrum, thalamus, hypothalamus), a midbrain (corpora quadrigemina) and a hindbrain (pons, cerebellum, medulla), which together control and coordinate the activities of the body.

Concept. The human neural system has two parts — the CNS and the PNS — and its basic unit, the neuron, transmits impulses as electrical changes called resting and action potentials. Each pair is compared below.

(a) CNS and PNS

(b) Resting potential and action potential

Extra depth. After an action potential, K⁺ diffusing out restores the resting potential, so the neuron can respond again.

Conclusion: The CNS (brain and spinal cord) processes information while the PNS (nerves) carries it to and from the CNS; the resting potential is the polarised state of an inactive neuron, whereas the action potential is the brief reversal of polarity that constitutes the nerve impulse.

Concept. Neurons are excitable cells because the membranes of their axons are in a polarised state. A nerve impulse is a wave of depolarisation and repolarisation along the axon and is passed to the next cell at a synapse.

(a) Polarisation of the membrane

1. Different ion channels, selectively permeable to different ions, are present on the neural membrane.
2. When the neuron is at rest the axonal membrane is comparatively more permeable to K⁺ and nearly impermeable to Na⁺ and to the negatively charged proteins of the axoplasm.
3. Consequently the axoplasm has a high concentration of K⁺ and negatively charged proteins and a low concentration of Na⁺, whereas the fluid outside has a low concentration of K⁺ and a high concentration of Na⁺.
4. The gradients are maintained by the active transport of ions by the sodium–potassium pump, which moves 3 Na⁺ outwards for 2 K⁺ into the cell.
5. As a result the outer surface of the axonal membrane is positively charged and the inner surface negatively charged: the membrane is polarised; the potential difference is the resting potential.

(b) Depolarisation of the membrane

1. A stimulus applied at a site (A) on the polarised membrane makes the membrane at A freely permeable to Na⁺.
2. There is a rapid influx of Na⁺, reversing the polarity: the outer surface becomes negatively charged and the inner side positively charged — depolarisation.
3. The potential difference across the membrane at A is the action potential (nerve impulse).
4. The current flows from A to B on the inner surface and from B to A on the outer surface, so site B also depolarises; the sequence repeats along the axon (conduction of the impulse).
5. The rise in Na⁺ permeability is extremely short-lived; it is quickly followed by a rise in K⁺ permeability, K⁺ diffuses out and restores the resting potential (repolarisation).

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

1. At a chemical synapse the pre- and post-synaptic membranes are separated by a fluid-filled synaptic cleft; the axon terminal contains synaptic vesicles filled with neurotransmitters.
2. When an 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 the neurotransmitters into the synaptic cleft.
3. The neurotransmitters bind to specific receptors on the post-synaptic membrane.
4. This binding opens ion channels allowing the entry of ions, which can generate a new potential in the post-synaptic neuron — excitatory or inhibitory.

Conclusion: The resting axonal membrane is polarised (outside positive) by K⁺ permeability and the Na⁺–K⁺ pump; stimulation causes Na⁺ influx and depolarisation (action potential), K⁺ efflux restores the resting state, and at a chemical synapse neurotransmitters released into the cleft bind receptors and generate a new potential in the next neuron.

Concept. The neuron is the structural and functional unit of the neural system, and the brain is its central information processing organ. The labelled diagrams below follow the NCERT figures; every important label is explained under them.

(a) Neuron

1. Cell body (soma): contains cytoplasm with typical cell organelles, the nucleus and granular Nissl’s granules.
2. Dendrites: short fibres which branch repeatedly and project out of the cell body; they also contain Nissl’s granules and transmit impulses towards the cell body.
3. Axon: a long fibre whose distal end is branched; each branch terminates as a bulb-like synaptic knob (axon terminal) with synaptic vesicles containing neurotransmitters; it transmits impulses away from the cell body to a synapse or neuromuscular junction.
4. Myelin sheath and Schwann cells: in myelinated fibres the Schwann cells form the myelin sheath around the axon; the gaps between adjacent sheaths are the nodes of Ranvier.

(b) Brain (sagittal section)

• Forebrain: cerebrum (cerebral hemispheres joined by the corpus callosum), thalamus, hypothalamus.
• Midbrain: between forebrain and pons; cerebral aqueduct and corpora quadrigemina.
• Hindbrain: pons, cerebellum and medulla oblongata, continuous with the spinal cord.

Conclusion: A neuron consists of a cell body with Nissl’s granules, dendrites and a (myelinated) axon ending in synaptic knobs; the brain is made of the forebrain (cerebrum, thalamus, hypothalamus), midbrain and hindbrain (pons, cerebellum, medulla).

(a) Neural coordination. Coordination is the process through which two or more organs interact and complement the functions of one another to maintain homeostasis (for example muscles, lungs, heart, blood vessels and kidneys during exercise). In our body the neural system and the endocrine system jointly coordinate and integrate all the activities of organs. The neural system gives point-to-point connections for quick coordination: sensory (afferent) neurons carry information from receptors to the CNS, which integrates it and sends regulatory impulses along efferent neurons to effectors. Neural coordination involves generation and conduction of the nerve impulse (resting potential, action potential) and its transmission across synapses.

(b) Forebrain. Consists of the cerebrum, thalamus and hypothalamus. The cerebrum has two hemispheres connected by the corpus callosum; the cortex (grey matter) contains motor, sensory and association areas (memory, communication, intersensory association) and the inner white matter has myelinated tracts. The thalamus is the major coordinating centre for sensory and motor signalling; the hypothalamus controls body temperature, eating and drinking and secretes hypothalamic hormones; the limbic system (with amygdala and hippocampus) regulates sexual behaviour, emotions and motivation.

(c) 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 consists of four round swellings called corpora quadrigemina. The midbrain receives and integrates visual, tactile and auditory inputs.

(d) Hindbrain. Comprises the pons (fibre tracts interconnecting different regions of the brain), the cerebellum (highly convoluted surface giving space for many more neurons; integrates information from the semicircular canals and auditory system, controlling balance and coordination) and the medulla oblongata (connected to the spinal cord; centres controlling respiration, cardiovascular reflexes and gastric secretions). Midbrain, pons and medulla form the brain stem.

(e) Synapse. A synapse is the junction through which a nerve impulse passes 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, the synaptic cleft. Electrical synapses have the membranes in very close proximity so that current flows directly; they are faster but rare. Chemical synapses have a fluid-filled cleft and use neurotransmitters, released from synaptic vesicles of the axon terminal, which bind to receptors on the post-synaptic membrane and generate an excitatory or inhibitory potential.

Conclusion: Neural coordination integrates organ functions through neurons and synapses; the forebrain, midbrain and hindbrain each control specific functions, and the synapse transmits impulses between neurons electrically or chemically.

Concept. A nerve impulse is transmitted from one neuron to another through junctions called synapses. Most synapses in the human body are chemical synapses, in which the membranes of the pre-synaptic and post-synaptic neurons are separated by a fluid-filled synaptic cleft and chemicals called neurotransmitters carry the signal across.

1. Arrival of the action potential. The impulse (action potential) travels along the axon of the pre-synaptic neuron and reaches the axon terminal.
2. Movement of vesicles. The axon terminal contains synaptic vesicles filled with neurotransmitters. The arrival of the impulse stimulates the vesicles to move towards the pre-synaptic membrane.
3. Release of neurotransmitters. The vesicles fuse with the plasma membrane and release their neurotransmitter into the synaptic cleft.
4. Binding to receptors. The released neurotransmitter molecules bind to their specific receptors on the post-synaptic membrane.
5. Generation of a new potential. Binding opens ion channels allowing the entry of ions, which can generate a new potential in the post-synaptic neuron, which may be either excitatory (impulse passes on) or inhibitory (impulse is stopped).

Extra depth. Transmission across a chemical synapse is one-way (from pre- to post-synaptic neuron) because only the axon terminal has vesicles and only the post-synaptic membrane has receptors. The neurotransmitter is then removed or broken down so that the synapse can transmit the next impulse (general knowledge); at the neuromuscular junction the neurotransmitter is acetylcholine.

Conclusion: At a chemical synapse the action potential makes synaptic vesicles release neurotransmitters into the cleft; these bind to receptors on the post-synaptic membrane, open ion channels and generate an excitatory or inhibitory potential in the next neuron.

Concept. The ions Na⁺ and K⁺ are unequally distributed across the neural membrane: at rest the axoplasm has a low concentration of Na⁺ and a high concentration of K⁺, while the fluid outside has a high concentration of Na⁺ and a low concentration of K⁺. The sodium–potassium pump maintains these gradients by pumping 3 Na⁺ out for every 2 K⁺ taken in. At rest the membrane is nearly impermeable to Na⁺, so the outer surface is positive and the inner surface negative (resting potential).

1. Stimulus. When a stimulus is applied at a site (A) on the polarised membrane, the membrane at that site suddenly becomes freely permeable to Na⁺ (voltage-gated Na⁺ channels open).
2. Na⁺ influx. Because the outside concentration of Na⁺ is high and the inside negative, Na⁺ rapidly rushes into the axon.
3. Reversal of polarity. The inflow of positive ions reverses the polarity: the outer surface becomes negatively charged and the inner side positively charged — depolarisation.
4. Action potential. The potential difference across the membrane at A, reversed by Na⁺ influx, is the action potential, i.e. the nerve impulse.
5. Propagation. At the neighbouring site B, which is still polarised, a current flows (inside: A to B; outside: B to A); this depolarises B by the same Na⁺ influx. In this way the impulse is conducted along the axon.
6. End of Na⁺ role. The rise in permeability to Na⁺ is extremely short-lived; it is quickly followed by a rise in K⁺ permeability, and K⁺ diffusing out restores the resting potential.

Conclusion: Na⁺ generates the action potential by rushing into the axon when the stimulated membrane becomes freely permeable to it, reversing the polarity (depolarisation); the same Na⁺ influx at successive sites conducts the impulse along the axon.

Concept. Each pair compares two structures of the neural system: the two kinds of nerve fibres, the two processes of a neuron, two parts of the forebrain, and two parts of the brain with different functions.

(a) Myelinated and non-myelinated axons

(b) Dendrites and axons

(c) Thalamus and hypothalamus

(d) Cerebrum and cerebellum

Conclusion: Myelinated axons have a myelin sheath with nodes of Ranvier and conduct faster; dendrites carry impulses towards and axons away from the cell body; the thalamus coordinates sensory–motor signals while the hypothalamus controls temperature, hunger and thirst and secretes hormones; the cerebrum is the seat of intelligence while the cerebellum controls balance and coordination.

Concept. The brain is the “command and control system” of the body. Its different parts are specialised for different tasks, and the question asks for the part that is the most highly developed in humans and the part that regulates the body’s 24-hour rhythms.

(a) The most developed part of the human brain is the CEREBRUM (the cerebral cortex of the forebrain).

• The cerebrum forms the major part of the human brain, divided into left and right cerebral hemispheres by a deep cleft and connected by the corpus callosum.
• The cerebral cortex is thrown into prominent folds, which increase its surface area enormously and accommodate a vast number of neuron cell bodies (grey matter).
• It contains motor areas, sensory areas and large association areas responsible for complex functions like intersensory associations, memory and communication; it is the seat of intelligence, speech, thoughts and voluntary actions — the functions that make humans different from other animals.

(b) The part acting as the master clock is the HYPOTHALAMUS (specifically a group of neurons in it called the suprachiasmatic nucleus — general knowledge).

• The brain controls circadian (24-hour) rhythms of the body; the centre for these rhythms lies in the hypothalamus, which acts as the “biological clock”.
• The hypothalamus lies at the base of the thalamus and also contains centres controlling body temperature and the urge for eating and drinking, and neurosecretory cells that secrete hypothalamic hormones; through the pituitary it links the neural and endocrine systems.
• Light information from the eyes reaches this clock and synchronises sleep–wake and hormone cycles with day and night (general knowledge).

Conclusion: The cerebrum, with its much folded cortex and association areas, is the most developed part of the human brain, and the hypothalamus, which controls the circadian rhythms, acts as the master clock of the central neural system.

Concept. The peripheral neural system consists of nerves connected to the CNS; its fibres are of two functional types (afferent and efferent), and the nerves are classified by their origin as cranial or spinal. Impulse conduction differs according to whether the fibre has a myelin sheath.

(a) Afferent neurons and efferent neurons

(b) Impulse conduction in myelinated and unmyelinated fibres

(c) Cranial nerves and spinal nerves (the printed question marks this part as (f); the textbook of this edition does not detail them, so the standard facts are given)

Extra depth. Saltatory conduction is why myelinated fibres in spinal and cranial nerves can carry impulses so rapidly; the diseases that damage myelin slow down nerve conduction.

Conclusion: Afferent neurons carry impulses to the CNS and efferent neurons from it; impulses jump from node to node (saltatory) in myelinated fibres but travel continuously in unmyelinated fibres; and cranial nerves (12 pairs) arise from the brain while spinal nerves (31 pairs) arise from the spinal cord.