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

Q1. Briefly describe the structure of the brain.

Answer:

The brain is the main coordinating centre of the body. It is a part of the nervous system that controls and monitors every organ of the body.

It is well protected by cranial meninges, an outer layer called dura mater, a thin middle layer called arachnoid, and an inner layer called pia mater.

It is divided into three regions —

1. Forebrain
2. midbrain
3. hindbrain

Forebrain:

It is the main thinking part of the brain. It consists of the cerebrum, thalamus, and hypothalamus.

1. Cerebrum: The cerebrum is the largest part of the brain and constitutes about four-fifths of its weight. It is divided into two cerebral hemispheres by a deep longitudinal cerebral fissure. These hemispheres are joined by a tract of nerve fibre known as the corpus callosum.

2. Thalamus: Thalamus is the main centre of coordination for sensory and motor signalling. It is wrapped by the cerebrum.

3. Hypothalamus: Hypothalamus lies at the base of the thalamus and contains several centres that regulate body temperature and the urge to eat and drink. Some regions of the cerebrum, along with the hypothalamus, are involved in the regulation of sexual behaviour and expression of emotional reactions such as excitement, pleasure, fear, etc.

Midbrain:
It is located between the thalamus region of the forebrain and the pons region of the hindbrain. The dorsal surface of the midbrain consists of superior and inferior corpora bigemina and four round lobes called corpora quadrigemina.

Hindbrain:
It consists of three regions - pons, cerebellum, and medulla oblongata.

• Pons is a band of nerve fibre that lies between the medulla oblongata and the midbrain. It connects the lateral parts of the cerebellar hemisphere.
• The cerebellum is a large and well-developed part of the hindbrain. It is located below the posterior sides of cerebral hemispheres and above the medulla oblongata. It is responsible for maintaining posture and equilibrium of the body.
• The medulla oblongata is the posterior and simplest part of the brain. It is located beneath the cerebellum. Its lower end extends in the form of the spinal cord and leaves the skull through the foramen magnum.

Q2. Compare the following:

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

Answer: The major difference between the Central neural system (CNS) and the Peripheral neural system (PNS) is discussed below in the table:

Q2. Compare the following:

(b) Resting potential and action potential

Answer:

Q3. Explain the following processes:

(b) Depolarisation of the membrane of a nerve fibre

Answer:

When the nerve fibre is stimulated, it is said to be in a 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 depolarization 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 makes 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 it experiences an action potential.

Q3. Explain the following processes:

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

Answer:

A synapse is created by the pre-synaptic and post-synaptic neuron membranes and can be or not be divided by a gap known as the synaptic cleft. The pre-synaptic and post-synaptic neurons are always divided by this cleft in a chemical synapse. Upon arrival of an impulse at the axon terminal, calcium ions found in the synaptic cleft enter the synaptic knobs of the pre-synaptic neuron. This initiates the transport of synaptic vesicles towards the plasma membrane, where they fuse and release the neurotransmitter acetylcholine into the synaptic cleft. The emptied vesicles go back to the cytoplasm of the pre-synaptic neuron to be replenished. The released acetylcholine attaches to protein receptors on the plasma membrane of the post-synaptic neuron, allowing sodium ions to flow in and potassium ions to exit. This interaction creates an action potential in the post-synaptic membrane so that the impulse can be passed on to the subsequent neuron.

Q4. Draw labelled diagrams of the following:

(a) Neuron

Answer:

Neuron-

(b) Brain

Answer:

Brain-

Q5. Write short notes on the following:

(a) Neural coordination

Answer:

Coordination is a basic feature of living things, which allows various organs to communicate and supplement one another's functions. In humans and other higher animals, coordination occurs through neural coordination and chemical coordination. Neural coordination is performed by specialized cells referred to as neurons, which constitute a sophisticated network of point-to-point connections that function through nerve impulses.

This system guarantees a one-to-one correspondence between stimulus and response, involving receptors to sense stimuli and effectors to produce corresponding responses. Neural coordination regulates all the processes of the body, getting stimuli from organs such as the skin and relaying responses to muscles or glands. The neural system also remembers past stimuli in memory, aiding in learning and recollection. It is responsible for voluntary movements such as walking, running, writing, and speaking, as well as cognitive activities such as thinking, reasoning, and analysis. In addition, critical processes of breathing, regulation of heartbeat, and digestion are governed by neural coordination. Through the coordination of different metabolic processes, it ensures homeostasis, keeping the body running smoothly.

Q5. Write short notes on the following:

(b) Forebrain

Answer:

The forebrain is made up of three principal components: the Cerebrum, Thalamus, and Hypothalamus.

The Thalamus is a grey matter structure that lies above the midbrain. It functions as a relay centre, passing motor and sensory impulses to the cerebrum. It has an important role in emotional control as well as assisting in the recognition of sensations such as heat, cold, and pain.

Q5. Write short notes on the following:

(c) Midbrain

Answer:

The midbrain has two large structures: the cerebral peduncles and the corpora quadrigemina. The cerebral peduncles are stout, fibrous tracts that act as links between the cerebrum and cerebellum, with important functions in transferring sensory and motor impulses between the forebrain and hindbrain. There are two pairs of solid lobes on the dorsal side of the brain called the corpora quadrigemina, which are again split into the superior colliculi and inferior colliculi. The corpora quadrigemina controls the visual reflexes, i.e., manages head and eye movement upon stimuli. It also controls auditory reflexes, enabling the head to move towards a sound source for enhanced detection and localization.

Q5 . Write short notes on the following:

(d) Hindbrain

Answer:

The hindbrain comprises three major structures: the cerebellum, pons Varolii, and medulla oblongata. The cerebellum lies at the back of the brain and contains an outer cerebellar cortex that is composed of grey matter and an inner cerebellar medulla that is composed of white matter. White matter comprises fibre tracts linking the cerebellum to the medulla oblongata and cerebrum. The cerebellum is responsible for coordinating muscular activity and body balance.

While the urge to carry out a muscular activity comes from the cerebrum, the cerebellum coordinates voluntary movements to make them smooth. The pons Varolii, situated above the medulla oblongata, consists of a thick bundle of white nerve fibres. It operates by integrating the two lobes of the cerebellum and houses the pneumatic centre that controls breathing. The conical medulla oblongata, which is at the bottom of the skull, runs down into the spinal cord. It acts as a channel through which impulses are carried between the brain and spinal cord. Moreover, it regulates important functions of internal organs like heartbeat and breathing, thereby making it a crucial component of survival. Any damage to the medulla oblongata can prove to be fatal.

Q6. Give a brief account of Mechanism of synaptic transmission.

Answer:

The 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.

The mechanism of synaptic transmission at the 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 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.

Q7. Explain the Role of Na+ in the generation of action potential.

Answer:

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 the outside and positively charged from the inside. This sudden change in the membrane potential is called the action potential, and the membrane is said to be depolarized.

Q8. Differentiate between:

(a) Myelinated and non-myelinated axons

Answer: The major difference is discussed below:

Q8. Differentiate between:
(b) Dendrites and axons

Answer:

Q8. Differentiate between:

(c) Thalamus and Hypothalamus

Answer: The major difference is discussed below:

Q8. Differentiate between:

(d) Cerebrum and Cerebellum

Answer: The major difference is discussed below:

Q9. Answer the following:

(a) Which part of the human brain is the most developed?

Answer:

The cerebrum is the most developed.

Q10. Distinguish between:

(a) afferent neurons and efferent neurons

Answer: The major difference is discussed below:

Q10. Distinguish between:

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

Answer: The major difference is discussed below:

Q10. Distinguish between:

(c) cranial nerves and spinal nerves.

Answer: The major difference is discussed below: