EXERCISES

Q1. Briefly describe the structure of the human brain.
Answer: The human brain is the control centre of the body, protected by the skull and three meninges namely dura mater, arachnoid, and pia mater. It is divided into three main parts:

1. Forebrain: Composed of the cerebrum, thalamus, and hypothalamus. The cerebrum, with two hemispheres joined by the corpus callosum, controls voluntary actions, memory, intelligence, and senses. The hypothalamus regulates body temperature, hunger, and emotions.
2. Midbrain: Located between the forebrain and hindbrain, it contains corpora quadrigemina, which control visual and auditory reflexes.
3. Hindbrain: Includes the pons, cerebellum, and medulla oblongata. The cerebellum maintains balance and posture, while the medulla controls involuntary actions like heartbeat and breathing.

Together, the midbrain, pons, and medulla form the brainstem, which connects the brain to the spinal cord.

Q2. Compare the following:
(a) Central neural system (CNS) and Peripheral neural system (PNS)
(b) Resting potential and action potential
Answer:

(a) Central neural system (CNS) and Peripheral neural system (PNS)
The CNS includes the brain and spinal cord and acts as the main centre for information processing and control. The PNS consists of all the nerves connecting the CNS to the rest of the body and is divided into somatic and autonomic systems.

(b) Resting potential and action potential
Answer: The resting potential is the electrical difference across the neuron membrane when it is not conducting impulses. The outer surface is positive due to more Na⁺ ions outside. The action potential occurs when a stimulus causes Na⁺ ions to rush in, reversing the polarity of the membrane and generating an electrical impulse.

Q3. 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
Answer: 

(a) Polarisation of the membrane of a nerve fibre
In the resting state, a neuron’s membrane is polarised because it is more permeable to K⁺ and almost impermeable to Na⁺. The sodium–potassium pump actively sends 3 Na⁺ out and 2 K⁺ in, making the outside positive and inside negative, creating the resting potential.

(b) Depolarisation of the membrane of a nerve fibre
Answer: When a stimulus is applied, the membrane becomes freely permeable to Na⁺, which rapidly enters the cell. This reverses the charge—the inside becomes positive and the outside negative—creating the action potential.

(c) Transmission of a nerve impulse across a chemical synapse
Answer: When an impulse reaches the axon terminal, synaptic vesicles release neurotransmitters into the synaptic cleft. These neurotransmitters bind to receptors on the post-synaptic membrane, opening ion channels and generating a new potential in the next neuron.

Q4. Draw labelled diagrams of the following:(a) Neuron (b) Brain
Answer:
(a) Neuron

(b) Brain

Q5. Write short notes on the following:
(a) Neural coordination (b) Forebrain (c) Midbrain
(d) Hindbrain (e) Synapse
Answer:

(a) Neural coordination: Neural coordination refers to how the nervous system ensures quick and precise control of body activities. It connects different organs through neurons and works with the endocrine system to maintain balance and coordination in the body.

(b) Forebrain: The forebrain consists of the cerebrum, thalamus, and hypothalamus. It controls sensory perception, memory, intelligence, emotions, and voluntary movements. The hypothalamus also regulates temperature, hunger, and thirst.

(c) Midbrain: The midbrain connects the forebrain and hindbrain. It contains the corpora quadrigemina and the cerebral aqueduct, helping in visual and auditory processing.

(d) Hindbrain: The hindbrain includes the pons, cerebellum, and medulla. The cerebellum maintains balance and coordination, while the medulla controls involuntary actions like heartbeat and breathing.

(e) Synapse: A synapse is a junction between two neurons where nerve impulses are transmitted through neurotransmitters. It can be electrical (direct impulse flow) or chemical (involving neurotransmitters across a gap).

Q6. Give a brief account of the mechanism of synaptic transmission.
Answer: When a nerve impulse reaches the axon terminal, it causes synaptic vesicles to release neurotransmitters into the synaptic cleft. These chemicals bind to receptors on the post-synaptic membrane, opening ion channels and creating a new potential in the next neuron. This may excite or inhibit the following neuron depending on the neurotransmitter.

Q7. Explain the role of Na⁺ in the generation of action potential.
Answer: Sodium ions play a key role in creating the action potential. In the resting state, the neuron membrane blocks Na⁺ entry. When a stimulus occurs, Na⁺ channels open and Na⁺ rapidly enters the cell, reversing the charge of the membrane. This depolarisation forms the action potential, which is later followed by K⁺ movement to restore the resting state.

Q8. Differentiate between:
(a) Myelinated and non-myelinated axons
(b) Dendrites and axons
(c) Thalamus and Hypothalamus
(d) Cerebrum and Cerebellum
Answer:

(a) Myelinated and non-myelinated axons: Myelinated axons have a myelin sheath with nodes of Ranvier and conduct impulses quickly through saltatory conduction. Non-myelinated axons lack the sheath and nodes, causing slower conduction.

(b) Dendrites and axons: Dendrites are short, branched extensions that carry impulses toward the cell body and contain Nissl’s granules. Axons are long fibres that carry impulses away from the cell body and end in synaptic knobs containing neurotransmitters.

(c) Thalamus and hypothalamus: The thalamus acts as a relay centre for sensory and motor signals, while the hypothalamus controls body temperature, hunger, thirst, and hormone secretion.

(d) Cerebrum and cerebellum: The cerebrum, part of the forebrain, controls higher mental functions like thinking and memory. The cerebellum, in the hindbrain, coordinates balance, posture, and muscular movements.

Q9. 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?
Answer:

(a) Which part of the human brain is the most developed?
Answer: The cerebrum is the most developed part of the human brain, responsible for intelligence, reasoning, and memory.

(b) Which part of our central neural system acts as a master clock?
Answer: The hypothalamus acts as the master clock by regulating the body’s circadian rhythms and maintaining internal balance.

Q10. 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.
Answer:

(a) Afferent neurons and efferent neurons
Answer: Afferent neurons carry impulses from body parts to the CNS (sensory input), while efferent neurons carry impulses from the CNS to muscles or glands (motor output).

(b) Impulse conduction in myelinated and unmyelinated nerve fibres
Answer: In myelinated fibres, impulses jump from one node of Ranvier to another, making transmission faster. In unmyelinated fibres, impulses travel continuously along the membrane, so conduction is slower.

(c) Cranial nerves and spinal nerves
Answer: Cranial nerves arise from the brain and are 12 pairs, mainly serving the head and neck. Spinal nerves arise from the spinal cord and are 31 pairs, serving the trunk and limbs.