18.6 Exercise Problems

Q1. Briefly describe the structure of the Brain

Answer: Brain
Brain is the main coordinating centre of the body. It is a part of nervous system that controls and monitors every organ of the body. It is well protected by cranial meninges that are made up of 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-forebrain, midbrain and hindbrain.
Forebrain
It is the main thinking part of the brain. It consists of cerebrum, thalamus and hypothalamus.

1. Cerebrum: Cerebrum is the largest part of the brain and constitutes about four-fifths of its weight. Cerebrum is divided into two cerebral hemispheres by a deep longitudinal cerebral fissure. These hemispheres are joined by a tract of nerve fibre known as corpus callosum. The cerebral hemispheres are covered by a layer of cells known as cerebral cortex or grey matter. Cerebrum has sensory regions known as association areas that receive sensory impulses from various receptors as well as from motor regions that control the movement of various muscles. The innermost part of cerebrum gives an opaque white appearance to the layer and is known as the white matter.
2. Thalamus: Thalamus is the main centre for sensory and motor signalling. It is wrapped by cerebrum.
3. Hypothalamus: It lies at the base of thalamus and contains a number of centres that regulate body temperature and the urge for eating and drinking. Some regions of cerebrum, along with 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 pons region of hindbrain. The dorsal surface of midbrain consists of superior and inferior corpora bigemina and four round lobes called corpora quadrigemina. A canal known as cerebral aqueduct passes through the midbrain. Midbrain is concerned with the sense of sight and hearing.

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

1. Pons: Pons is a band of nerve fibre that lies between medulla oblongata and midbrain. It connects the lateral parts of cerebellar hemisphere together.
2. Cerebellum: Cerebellum is a large and well-developed part of hindbrain. Which is located below the posterior sides of cerebral hemispheres and above medulla oblongata. It is responsible for maintaining posture and equilibrium of the body.
3. Medulla Oblongata: Medulla oblongata is the posterior and simplest part of the brain. Which is located beneath the cerebellum. It lowers and extends in the form of spinal cord and leaves the skull through foramen magnum.

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)

\(
\begin{array}{|l|l|}
\hline \text { Central neural system (CNS) } & \text { Peripheral neural system (PNS) } \\
\hline \begin{array}{l}
\text { Consists of the spinal cord and the } \\
\text { brain }
\end{array} & \begin{array}{l}
\text { It consists of the spinal nerves and the } \\
\text { cranial nerves }
\end{array} \\
\hline \begin{array}{l}
\text { The spinal column is protected by the } \\
\text { vertebral column, whereas the brain } \\
\text { is protected by the skull }
\end{array} & \text { No protective structures } \\
\hline \text { No subdivisions } & \begin{array}{l}
\text { It is divided into the autonomic } \\
\text { nervous system and the somatic } \\
\text { nervous system }
\end{array} \\
\hline \begin{array}{l}
\text { Processes information and regulates } \\
\text { the responses to impulses. }
\end{array} & \begin{array}{l}
\text { Nerves of PNS pass impulses to the } \\
\text { CNS and responses from the CNS to } \\
\text { various structures of the body }
\end{array} \\
\hline \text { Group of neurons known as nuclei } & \text { Group of neurons known as ganglia } \\
\hline
\end{array}
\)

(b) Resting potential and action potential

\(
\begin{array}{|l|l|}
\hline \text { Resting potential } & \text { Action potential } \\
\hline \begin{array}{l}
\text { When the neuron is at the resting } \\
\text { phase, it is the potential difference } \\
\text { across membrane }
\end{array} & \begin{array}{l}
\text { When the neuron is triggered, it is } \\
\text { the potential difference across the } \\
\text { membrane }
\end{array} \\
\hline \begin{array}{l}
\text { The exterior side of the neuron is } \\
\text { positively charged, while the interior } \\
\text { side is negatively charged }
\end{array} & \begin{array}{l}
\text { The exterior side of the neuron is } \\
\text { negatively charged, and the interior } \\
\text { side of the neuron is positively } \\
\text { charged }
\end{array} \\
\hline \begin{array}{l}
\text { Permeability of K-ions is observed to be } \\
\text { more by the plasma membrane of } \\
\text { neurons }
\end{array} & \begin{array}{l}
\text { Permeability of Na-ions is observed } \\
\text { to be more by the plasma membrane } \\
\text { of the neurons }
\end{array} \\
\hline \begin{array}{l}
\text { To maintain the resting potential, the } \\
\text { sodium-potassium ATPase pump is } \\
\text { activated, sending Na-ions outside the } \\
\text { neuron }
\end{array} & \begin{array}{l}
\text { It functions in a reverse pattern } \\
\text { wherein the sodium-potassium } \\
\text { ATPase pump sends Na-ions to the } \\
\text { neuron. }
\end{array} \\
\hline
\end{array}
\)

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) During resting condition, the concentration of \(\mathrm{K}+\) ions is more inside the axoplasm while the concentration of \(\mathrm{Na}+\) ions is more outside the axoplasm. As a result, the potassium ions move faster from inside to outside as compared to sodium ions. Therefore, the membrane becomes positively charged outside and negatively charged inside.

(b) When an electrical stimulus is given to a nerve fibre, an action potential is generated. The membrane becomes permeable to sodium ions than to potassium ions. This results into positive charge inside and negative charge outside the nerve fibre.

(c) Synapse is a small gap that occurs between the last portion of the axon of one neuron and the dendrite of next neuron. An impulse reaches at the end plate of axon, vesicles consisting of chemical substance or neurotransmitter, such as acetylcholine, fuse with the plasma membrane. This chemical moves across the cleft and attaches to chemo-receptors present on the membrane of the dendrite of next neuron. This binding of chemical with chemo-receptors leads to the depolarization of membrane and generates a nerve impulse across nerve fibre.
The chemical, acetylcholine, is inactivated by enzyme acetylcholinestrase. The enzyme is present in the post-synaptic membrane of the dendrite. It hydrolyses acetylcholine and this allows the membrane to repolarize.

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
The neural system provides rapid coordination among the organs of the body. This coordination is in the form of electric impulses and is quick and short-lived. For example, during exercise, our body requires more oxygen and food. Hence, the breathing rate increases automatically and the heart beats faster. This leads to a faster supply of oxygenated blood to the muscles. Moreover, the cellular functions require regulation continuously. These functions are carried out by the hormones. Hence, the neural system along with the endocrine system control and coordinate the physiological processes.

(b) Forebrain
It is the main thinking part of the brain. It consists of cerebrum, thalamus, and hypothalamus.
i. Cerebrum: Cerebrum is the largest part of the brain and constitutes about four-fifth of its weight. Cerebrum is divided into two cerebral hemispheres by a deep longitudinal cerebral fissure. These hemispheres are joined by a tract of nerve fibre known as corpus callosum. The cerebral hemispheres are covered by a layer of cells known as cerebral cortex or grey matter. The innermost part of cerebrum gives an opaque white appearance to the layer and is known as the white matter.
ii. Thalamus: Thalamus is the main centre for sensory and motor signalling. It is wrapped by cerebrum.
iii. Hypothalamus: It lies at the base of thalamus and contains a number of centres that regulate body temperature and the urge for eating and drinking. Some regions of cerebrum, along with hypothalamus, are involved in the regulation of sexual behaviour and expression of emotional reactions such as excitement, pleasure, fear, etc.

(c) Midbrain
It is located between the thalamus region of the forebrain and pons region of hindbrain. The dorsal surface of midbrain consists of superior and inferior corpora bigemina and four round lobes called corpora quadrigemina. A canal known as cerebral aqueduct passes through the midbrain. Midbrain is concerned with the sense of sight and hearing.

(d) Hindbrain
It consists of three regions-pons, cerebellum, and medulla oblongata.
i. Pons is a band of nerve fibre that lies between medulla oblongata and midbrain. It connects the lateral parts of cerebellar hemisphere together.
ii. Cerebellum is a large and well developed part of hindbrain. Which is located below the posterior sides of cerebral hemispheres and above medulla oblongata. It is responsible for maintaining posture and equilibrium of the body.
iii. Medulla oblongata is the posterior and simplest part of the brain. Which is located beneath the cerebellum. It lowers and extends in the form of spinal cord and leaves the skull through foramen magnum.

(e) Synapse
Synapse is a junction between the axon terminal of one neuron and the dendrite of next neuron. It is separated by a small gap known synaptic cleft. There are two types of synapses.
i. Electrical synapse
ii. Chemical synapse
In electrical synapses, the pre and post-synaptic neurons lie in close proximity to each other. So, the impulse can move directly from one neuron to another across the synapse. It represents a faster method of impulse transmission.
In chemical synapses, the pre and post-synaptic neurons are not in close proximity. They are separated by a synaptic cleft. The transmission of nerve impulses is carried out by chemicals such as neurotransmitters.

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

Answer: Mechanism of synaptic transmission
Synapse is a junction between two neurons. It is present between the axon terminal of one neuron and the dendrite of next neuron separated by a cleft. There are two ways of synaptic transmission.
i. Chemical transmission: When a nerve impulse reaches the end plate of axon, it releases a neurotransmitter (acetylcholine) across the synaptic cleft. The acetylcholine diffuses across the cleft and binds to the receptors present on the membrane of next neuron. This causes depolarization of membrane and initiates an action potential.
ii. Electrical transmission: an electric current is formed in the neuron. This electric current generates an action potential and leads to transmission of nerve impulse across the nerve fibre. This represents a faster method of nerve conduction than the chemical method of transmission.

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

Answer: Sodium ions play an important role in the generation of action potential. When a nerve fibre is stimulated, the membrane potential decreases. The membrane becomes more permeable in \(\mathrm{Na}+[latex] ions than to [latex]\mathrm{K}+\) ions. As a result, \(\mathrm{Na}+\) diffuses from the outside to the inside of the membrane. This causes the inside of the membrane to become positively charged, while the outer membrane gains a negative charge. This reversal of polarity across the membrane is known as depolarisation. The rapid inflow of \(\mathrm{Na}+\) ions causes the membrane potential to increase, thereby generating an action potential.

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

\(
\begin{array}{|l|l|l|l|}
\hline & \text { Myelinated axons } & & \text { Non-myelinated axons } \\
\hline \text { 1. } & \begin{array}{l}
\text { Transmission of nerve impulse is } \\
\text { faster }
\end{array} & \text { 1. } & \begin{array}{l}
\text { Transmission of nerve } \\
\text { impulse is slower }
\end{array} \\
\hline \text { 2. } & \begin{array}{l}
\text { Myelinated axon has a myelin } \\
\text { sheath. }
\end{array} & \text { 2. } & \text { Myelin sheath is absent } \\
\hline \text { 3. } & \begin{array}{l}
\text { Node of Ranvier is present } \\
\text { between adjacent myelin } \\
\text { sheaths. }
\end{array} & \text { 3. } & \text { Node of Ranvier is absent } \\
\hline \text { 4. } & \begin{array}{l}
\text { Found in the brain, the spinal } \\
\text { cord, the cranial and spinal } \\
\text { nerves }
\end{array} & \text { 4. } & \begin{array}{l}
\text { Found in autonomous and } \\
\text { somatic neural systems }
\end{array} \\
\hline \text { 5. } & \begin{array}{l}
\text { Schwann cells are observed } \\
\text { inside the myelin sheath }
\end{array} & \text { 5. } & \begin{array}{l}
\text { Schwann cells are not } \\
\text { observed inside the myelin } \\
\text { sheath }
\end{array} \\
\hline
\end{array}
\)

(b) Dendrites and axons

\(
\begin{array}{|l|l|l|l|}
\hline & \text { Dendrites } & & \text { Axons } \\
\hline \text { 1. } & \begin{array}{l}
\text { Dendrite is a small projection } \\
\text { arising from the neuron. It } \\
\text { conducts the nerve impulse } \\
\text { toward the cell body. }
\end{array} & \text { 1. } & \begin{array}{l}
\text { Axon is a single, long } \\
\text { projection that conducts the } \\
\text { nerve impulse away from cell } \\
\text { body to the next neuron. }
\end{array} \\
\hline \text { 2. } & \begin{array}{l}
\text { Nissl’s granules are present in } \\
\text { dendrites. }
\end{array} & \text { 2. } & \begin{array}{l}
\text { Nissl’s granules are absent } \\
\text { from axons. }
\end{array} \\
\hline \text { 3. } & \begin{array}{l}
\text { Dendrites are always non- } \\
\text { myelinated. }
\end{array} & \text { 3. } & \begin{array}{l}
\text { Axons can be myelinated or } \\
\text { non-myelinated. }
\end{array} \\
\hline
\end{array}
\)

(c) Thalamus and Hypothalamus

\(
\begin{array}{|c|c|}
\hline \text { Thalamus } & \text { Hypothalamus } \\
\hline \text { Thalamus is the part of the } & \text { Hypothalamus is the part of the } \\
\text { forebrain that receives nerve } & \text { forebrain that controls involuntary } \\
\text { impulses of pain, temperature, } & \text { functions such as hunger, thirst, } \\
\text { touch, etc., and conducts them } & \text { sweating, sleep, fatigue, sexual } \\
\text { to the cerebral hemisphere. } & \text { desire, temperature regulation, etc. } \\
\hline
\end{array}
\)

(d) Cerebrum and Cerebellum

\(
\begin{array}{|c|c|}
\hline \text { Cerebrum } & \text { Cerebellum } \\
\hline \text { It is the part of the forebrain that } \\
\text { controls voluntary functions. It is the } \\
\text { place where intelligence, will power, } & \text { It is the part of the hindbrain } \\
\text { memory, etc., reside. } & \text { functions and controls the } \\
\text { equilibrium. } \\
\hline
\end{array}
\)

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) Forebrain is largest and the most developed part of the human brain.
(b) Hypothalamus acts as a master clock of the human body.

Q10. Distinguish between:
(a) afferent neurons and efferent neurons
(b) impulse conduction in a myelinated nerve fibre and unmyelinated nerve fibre
(c) cranial nerves and spinal nerves.

Answer: (a) Afferent neurons and efferent neurons
\(
\begin{array}{|c|c|}
\hline \text { Afferent neurons } & \text { Efferent neurons } \\
\hline \begin{array}{l}
\text { Afferent neuron conducts } \\
\text { nerve impulses toward the } \\
\text { brain or the spinal cord. }
\end{array} & \begin{array}{l}
\text { Efferent neuron conducts nerve impulses } \\
\text { from the brain or spinal cord to the } \\
\text { effector organs such as muscles or } \\
\text { glands. }
\end{array} \\
\hline
\end{array}
\)

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

\(
\begin{array}{|l|l|l|l|}
\hline & \begin{array}{l}
\text { Impulse conduction in a } \\
\text { myelinated nerve fibre }
\end{array} & & \begin{array}{l}
\text { Impulse conduction in an } \\
\text { unmyelinated nerve fibre }
\end{array} \\
\hline \text { 1. } & \begin{array}{l}
\text { In a myelinated nerve } \\
\text { fibre, the action potential } \\
\text { is conducted from one } \\
\text { node to another. }
\end{array} & \text { 1. } & \begin{array}{l}
\text { In an unmyelinated nerve fibre, the } \\
\text { action potential is not conducted } \\
\text { from node to node. It is carried along } \\
\text { the whole length of the nerve fibre. }
\end{array} \\
\hline \text { 2. } & \begin{array}{l}
\text { The conduction of } \\
\text { impulses is faster. }
\end{array} & \text { 2. } & \text { The conduction of impulses is slower. } \\
\hline
\end{array}
\)

c) cranial nerves and spinal nerves.

\(
\begin{array}{|l|l|l|l|}
\hline & \text { Cranial nerves } & & \text { Spinal nerves } \\
\hline \text { 1. } & \begin{array}{l}
\text { Cranial nerves arise from the } \\
\text { brain. }
\end{array} & \text { 1. } & \begin{array}{l}
\text { Spinal nerves arise from the } \\
\text { spinal cord. }
\end{array} \\
\hline \text { 2. } & \begin{array}{l}
\text { There are 12 pairs of cranial } \\
\text { nerves. }
\end{array} & \text { 2. } & \begin{array}{l}
\text { There are } 31 \text { pairs of spinal } \\
\text { nerves. }
\end{array} \\
\hline
\end{array}
\)

Exemplar Section

VERY SHORT ANSWER TYPE QUESTIONS

Q1. Rearrange the following in the correct order of involvement in electrical impulse movement Synaptic knob, dendrites, cell body, Axon terminal, Axon

Answer: Dendrites—Cell body—Axon—Axon terminal—Synaptic knob.

Q2. Comment upon the role of ear in maintaining the balance of the body and posture.

Answer: The crista and macula are the specific receptors of the vestibular apparatus responsible for maintenance of balance of the body and posture.

Q3. Which cells of the retina enable us to see coloured objects around us?

Answer: Cone cells of the retina enable us to see the coloured objects around us.

Q4. During resting potential, the axonal membrane is polarised, indicate the movement of +ve and -ve ions leading to polarisation diagrammatically.

Answer: Neurons are excitable cells because their membranes are in a polarised state. Different types of ion channels are present on the neural membrane. These ion channels are selectively permeable to different ions. When a neuron is not conducting any impulse, i.e., resting, the axonal membrane is comparatively more permeable to potassium ions \(\left(\mathrm{K}^{+}\right)\)and nearly impermeable to sodium ions \(\left(\mathrm{Na}^{+}\right)\). Similarly, the membrane is impermeable to negatively charged proteins present in the axoplasm. Consequently, the axoplasm inside the axon contains high concentration of \(\mathrm{K}^{+}\)and negatively charged proteins and low concentration of \(\mathrm{Na}^{+}\). In contrast, the fluid outside the axon contains a “low concentration of \(\mathrm{K}^{+}\), a high concentration of \(\mathrm{Na}^{+}\)and thus form a concentration gradient. These ionic gradients across the resting membrane are maintained by the active transport of ions by the sodium-potassium pump which transports \(3 \mathrm{Na}^{+}\)outwards for \(2 \mathrm{~K}^{+}\)into the cell. As a result, the outer surface of the axonal membrane possesses a positive charge while its inner surface becomes negatively charged and therefore is polarised. The electrical potential difference across the resting plasma membrane is called as the resting potential.

Q5. Name the structures involved in the protection of the brain.

Answer: The human brain is well protected by the skull. Inside the skull, the brain is covered by cranial meninges consisting of an outer layer called dura mater, a very thin middle layer called arachnoid and an inner layer (which is in contact with the brain tissue) called pia mater. Piamater is a vascular membrane which is richly supplied with blood capillaries. Space between the duramater and arachnoid is called subdural space. Space between the arachnoid and pia mater is called subarachnoid space. Subarachnoid space is filled with the cerebrospinal fluid (CSF) which acts as a cushion for CNS from shocks.

Q6. Our reaction like aggressive behaviour, use of abusive words, restlessness etc. are regulated by brain, name the parts involved.

Answer:  Limbic system and hypothalamus

Q7. What do grey and white matter in the brain represent?

Answer: The layer of cells which covers the cerebral hemisphere is called cerebral cortex and is thrown into prominent folds. The cerebral cortex is referred to as the grey matter due to its greyish appearance. The neuron cell bodies are concentrated here giving the colour. Fibres of the tracts are covered with the myelin sheath, which constitute the inner part of cerebral hemisphere. They give an opaque white appearance to the layer and, hence, is called the white matter.

Q8. Where is the hunger centre located in human brain? 

Answer: Hypothalamus

Q9. Complete the statement by choosing appropriate match among the following:
\(
\begin{array}{|l|l|}
\hline \text { a. Resting potential } & \text { i. Chemicals involved in the transmission of impulses at synapses. } \\
\hline \text { b. Nerve impulse } & \text { ii. Gap between the pre synaptic and post synaptic neurons } \\
\hline \text { c. Synaptic cleft } & \text { iii. Electrical potential difference across the resting neural membrane } \\
\hline \text { d. Neurotransmitters } & \text { iv. An electrical wave like response of a neuron to a stimulation } \\
\hline
\end{array}
\)

Answer: 

\(
\begin{array}{|l|l|}
\hline \text { a. Resting potential } & \text { iii. Electrical potential difference across the resting neural membrane } \\
\hline \text { b. Nerve impulse } & \text { iv. An electrical wave like response of a neuron to a stimulation. } \\
\hline \text { c. Synaptic cleft } & \text { ii. Gap between the pre synaptic and post synaptic neurons } \\
\hline \text { d. Neurotransmitters } & \text { i. Chemicals involved in the transmission of impulses at synapses } \\
\hline
\end{array}
\)

SHORT ANSWER TYPE QUESTIONS

Q1. The major parts of the human neural system is depicted below. Fill in the empty boxes with appropriate words.

Answer: 

Q2. What is the difference between electrical transmission and chemical transmission?

Answer:

\(
\begin{array}{|l|l|}
\hline \text { Electrical transmission } & \text { Chemical transmission } \\
\hline 1 . \text { Mediated by electrical synapses. } & \text { 1. } \text { Mediated through neuro transmitters. } \\
\hline \text { 2. } \begin{array}{l}
\text { The membranes of pre- and post- } \\
\text { synaptic neurons are in very close } \\
\text { proximity. }
\end{array} & \text { 2. } \begin{array}{l}
\text { The membranes of pre- and post- synaptic } \\
\text { neurons are separated by synaptic cleft. }
\end{array} \\
\hline \begin{array}{l}
\text { 3. } \text { Electrical current can flow directly } \\
\text { from one neuron into the other across } \\
\text { the synapses. }
\end{array} & \text { 3. } \begin{array}{l}
\text { Neurotransmitters are involved in the } \\
\text { transmission of impulses at the synapses. }
\end{array} \\
\hline 4. \text { This transmission is faster. } & 4 . \text { This transmission is slower. } \\
\hline 5. \text { These are rare in our system. } & 5. \text { These are common in our system. } \\
\hline
\end{array}
\)

Q3. Neural system and computers share certain common features. Comment in five lines. (Hint: CPU, input-output devices).

Answer: Neural system and computers share certain common features. The neural system has brain as command and control centre similar to the computer that has CPU (Central processing unit). Sensory organs are input devices of neural system like the mouse and keyboard of the computer. Responses of the body are the output of the neural system.like the data analysis and typed material of the computer. Nerves are comparable to the wires of the computers.

Q4. If someone receives a blow on the back of neck, what would be the effect on the person’s CNS? 

Answer: If someone receives a blow on the back of neck, it may result in the dislocation of the cervical vertebrae that may lead to the injury of the spinal cord passes through neural canal. Injury of spinal cord may lead to paralysis.

LONG ANSWER TYPE QUESTIONS

Q1. Explain the process of the transport and release of a neurotransmitter with the help of a labelled diagram showing a complete neuron, axon terminal and synapse.

Answer: A nerve impulse is transmitted from one neuron to another through junctions called synapses. A synapse is formed by the membranes of a presynaptic neuron and a postsynaptic neuron, which may or may not be separated by a gap called synaptic cleft. At a chemical synapse, the membranes of the pre- and post-synaptic neurons are separated by a fluid-filled space called synaptic cleft. Chemicals called neurotransmitters are involved in the transmission of impulses at these synapses. The axon terminals contain vesicles filled with these neurotransmitters. When an impulse (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 their neurotransmitters in the synaptic cleft. The released neurotransmitters bind to their specific receptors, present on the postsynaptic membrane. This binding opens ion channels allowing the entry of ions which can generate a new potential in the postsynaptic neuron. The new potential developed may be either excitatory or inhibitory.

Q2. Name the parts of human forebrain indicating their respective functions.

Answer: Forebrain:
The forebrain consists of cerebrum, thalamus and hypothalamus. Cerebrum forms_ the major part of the human brain. A deep cleft divides the cerebrum longitudinally into two halves, which are termed as the left and right cerebral hemispheres. The hemispheres are connected by a tract of nerve fibres called corpus callosum. The layer of cells which covers the cerebral hemisphere is called cerebral cortex and is thrown into prominent folds. The cerebral cortex is referred to as the grey matter due to its greyish appearance. The neuron cell bodies are concentrated here giving the colour. The cerebral cortex contains motor areas, sensory areas and large regions that are neither clearly sensory nor motor in function. These regions called as the association areas are responsible for complex functions like intersensory associations, memory and communication. Fibres of the tracts are covered with the myelin sheath, which constitute the inner part of cerebral hemisphere. They give an opaque white appearance to the layer and, hence, is called the white matter. The cerebrum wraps around a structure called thalamus, which is a major coordinating centre for sensory and motor signaling. Another very important part of the brain called hypothalamus lies at the base of the thalamus. The hypothalamus contains a number of centres which control body temperature, urge for eating and drinking. It also contains several groups of neurosecretory cells, which secrete hormones called hypothalamic hormones. The inner parts of cerebral hemispheres and a group of associated deep structures like amygdala, hippocampus, etc., form a complex structure called the limbic lobe or limbic system. Along with the hypothalamus, it is involved in the regulation of sexual behaviour, expression of emotional reactions (e.g., excitement, pleasure, rage and fear), and motivation.