Neural Control and Coordination — NCERT Solutions

Given: The human brain is the central organ of the neural system enclosed within the skull.

Structure of the Human Brain:

The human brain is protected by the bony cranium (skull) and is covered by three meningeal layers: dura mater (outermost), arachnoid (middle), and pia mater (innermost). The brain is divided into three major regions:

I. Forebrain:
It consists of:

• Cerebrum: The largest part of the brain. It is longitudinally divided into two cerebral hemispheres connected by the corpus callosum. The outer layer is the cerebral cortex (grey matter), which is highly folded into gyri (ridges) and sulci (grooves), increasing surface area. The inner region contains white matter. The cerebrum is divided into four lobes: frontal, parietal, temporal, and occipital. It controls voluntary movements, memory, intelligence, and sensory perception.
• Thalamus: Acts as a relay centre for sensory and motor signals to and from the cerebral cortex.
• Hypothalamus: Controls body temperature, hunger, thirst, sleep, and regulates the pituitary gland. It forms the floor of the diencephalon.
• Limbic System: Formed by inner parts of cerebral hemispheres and associated deep structures. It is concerned with olfaction, autonomic responses, sexual behaviour, emotional reactions, and motivation.

II. Midbrain:

• Located between the forebrain and hindbrain.
• The dorsal portion has four rounded lobes called corpora quadrigemina (two superior and two inferior colliculi).
• It receives and integrates visual, tactile, and auditory inputs.
• The midbrain and hindbrain together form the brain stem.

III. Hindbrain:
It consists of:

• Pons: Contains fibre tracts that interconnect different regions of the brain. It also helps in regulating respiration.
• Cerebellum: Has a highly convoluted surface. It integrates information from the semicircular canals of the ear and the auditory system. It coordinates voluntary movements, maintains posture and balance.
• Medulla Oblongata: Connects the brain to the spinal cord. It contains vital centres that control respiration, cardiovascular reflexes, and gastric secretions.

Conclusion: The brain is a highly complex organ that integrates and coordinates all body functions through its three major divisions.

(a) Central Neural System (CNS) vs. Peripheral Neural System (PNS):

(b) Resting Potential vs. Action Potential:

(a) Polarisation of the Membrane of a Nerve Fibre:

Concept: The resting nerve membrane is said to be in a polarised state.

Explanation:

• In a resting (non-conducting) neuron, the neural membrane is selectively permeable to $\text{K}^+$ ions and nearly impermeable to $\text{Na}^+$ ions.
• The $\text{Na}^+$/$\text{K}^+$ pump actively transports 3 Na$^+$ ions out and 2 K$^+$ ions in per cycle, maintaining a concentration gradient.
• As a result:
• $\text{Na}^+$ concentration is high outside the membrane.
• $\text{K}^+$ concentration is high inside the membrane.
• Large negatively charged proteins are trapped inside.
• This creates a charge difference: the outer surface is positively charged and the inner surface is negatively charged.
• The resting membrane potential is approximately $-70$ mV (inside relative to outside).
• This state of charge separation across the membrane is called polarisation.

(b) Depolarisation of the Membrane of a Nerve Fibre:

Concept: When a threshold stimulus is applied, the polarised membrane undergoes depolarisation.

Explanation:

• When a stimulus of sufficient intensity (threshold stimulus) is applied to a polarised nerve fibre, the membrane permeability to $\text{Na}^+$ ions increases drastically.
• Voltage-gated $\text{Na}^+$ channels open and $\text{Na}^+$ ions rapidly rush inside the membrane (down the concentration gradient).
• This causes the inner surface to become positively charged and the outer surface to become negatively charged at the stimulated point.
• The membrane potential reverses from $-70$ mV to approximately $+30$ mV.
• This reversal of polarity is called depolarisation.
• After depolarisation, $\text{Na}^+$ channels close and $\text{K}^+$ channels open; $\text{K}^+$ ions rush out, restoring the original polarity — this is called repolarisation.
• The wave of depolarisation followed by repolarisation travels along the axon as a nerve impulse (action potential).

(c) Transmission of a Nerve Impulse Across a Chemical Synapse:

Concept: At a chemical synapse, neurotransmitters act as chemical messengers to transmit impulses from one neuron to the next.

Steps:

1. Arrival of impulse: The nerve impulse (action potential) travels along the axon and reaches the axon terminal (pre-synaptic terminal).
2. Depolarisation of pre-synaptic membrane: The action potential causes depolarisation of the pre-synaptic membrane.
3. Influx of Ca$^{2+}$ ions: Depolarisation opens voltage-gated $\text{Ca}^{2+}$ channels; $\text{Ca}^{2+}$ ions enter the pre-synaptic terminal.
4. Fusion of synaptic vesicles: The influx of $\text{Ca}^{2+}$ causes synaptic vesicles (containing neurotransmitters) to fuse with the pre-synaptic membrane.
5. Release of neurotransmitters: Neurotransmitters (e.g., acetylcholine) are released by exocytosis into the synaptic cleft.
6. Binding to receptors: Neurotransmitter molecules diffuse across the synaptic cleft and bind to specific receptor proteins on the post-synaptic membrane.
7. Generation of new impulse: Binding of neurotransmitters opens ion channels in the post-synaptic membrane, causing depolarisation and generation of a new action potential in the post-synaptic neuron.
8. Inactivation: The neurotransmitter is subsequently inactivated by specific enzymes (e.g., acetylcholinesterase breaks down acetylcholine) to stop continuous stimulation.

Conclusion: Thus, the nerve impulse is transmitted from one neuron to the next via chemical neurotransmitters across the synaptic cleft.

(a) Labelled Diagram of a Neuron:

Note: Draw the diagram as described below in the exam.

Description of Neuron diagram:

• Draw a large, roughly star-shaped cell body (soma/cyton) in the centre.
• Show the nucleus (large, round) inside the cell body.
• Show Nissl's granules (dark dots) in the cytoplasm.
• Draw several short, branched projections from the cell body — label them Dendrites.
• Draw one long projection from the cell body — label it Axon.
• At the base of the axon, show the Axon hillock.
• Show the axon covered by a sheath — label it Myelin sheath (Schwann cells).
• Show gaps between myelin segments — label them Nodes of Ranvier.
• Show the outer covering of the myelin sheath — label it Neurilemma.
• At the end of the axon, show branched endings — label them Axon terminals / Synaptic knobs.
• Label the entire structure: Myelinated Neuron.

Key labels: Cell body (Cyton), Nucleus, Nissl's granules, Dendrites, Axon hillock, Axon, Myelin sheath, Neurilemma, Node of Ranvier, Axon terminal (Synaptic knob).

(b) Labelled Diagram of the Human Brain:

Note: Draw a sagittal (longitudinal) section of the human brain as described below.

Description of Brain diagram:

• Draw the large, dome-shaped Cerebrum at the top, showing gyri and sulci (folds and grooves).
• Show the Corpus callosum connecting the two hemispheres (as a thick band).
• Below and behind the cerebrum, draw the Cerebellum with its characteristic folded appearance.
• In the centre, show the Thalamus and below it the Hypothalamus.
• Show the Midbrain connecting forebrain and hindbrain, with Corpora quadrigemina on the dorsal side.
• Show Pons as a bulge on the ventral side of the brainstem.
• Show Medulla oblongata at the base, continuous with the Spinal cord.
• Label the Pituitary gland hanging from the hypothalamus.

Key labels: Cerebrum, Corpus callosum, Thalamus, Hypothalamus, Pituitary gland, Midbrain, Corpora quadrigemina, Pons, Cerebellum, Medulla oblongata, Spinal cord.

(a) Neural Coordination:

• Neural coordination is the process by which the neural system (nervous system) coordinates and integrates the functions of all organs and organ systems of the body.
• It involves the rapid transmission of electrical signals (nerve impulses) from one part of the body to another.
• The functional unit of the neural system is the neuron.
• Neural coordination helps in:
• Receiving stimuli from the environment (sensory input)
• Processing and integrating information (CNS)
• Sending appropriate responses to effector organs (motor output)
• It also regulates metabolic and homeostatic activities of the body.
• Neural coordination is fast but short-lived, unlike hormonal coordination.

(b) Forebrain:

• The forebrain is the anterior-most and most developed part of the human brain.
• It consists of:
• Cerebrum: Largest part; divided into two hemispheres by a longitudinal fissure; connected by corpus callosum. Controls voluntary movements, memory, intelligence, speech, and sensory perception. The outer cortex is grey matter; inner region is white matter.
• Thalamus: Relay centre for sensory and motor signals; located below the cerebrum.
• Hypothalamus: Controls body temperature, hunger, thirst, sleep, and regulates the pituitary gland (master endocrine gland).
• Limbic System: Formed by inner parts of cerebral hemispheres; concerned with olfaction, autonomic responses, sexual behaviour, emotional reactions, and motivation.

(c) Midbrain:

• The midbrain is located between the forebrain (diencephalon) and the hindbrain.
• The dorsal portion consists of four rounded lobes called corpora quadrigemina (two superior and two inferior colliculi).
• The superior colliculi are involved in visual reflexes; inferior colliculi are involved in auditory reflexes.
• The midbrain receives and integrates visual, tactile, and auditory inputs.
• It contains the cerebral aqueduct (Aqueduct of Sylvius) which connects the third and fourth ventricles.
• Along with the hindbrain, it forms the brain stem.
• It also contains the reticular activating system which is involved in maintaining consciousness and alertness.

(d) Hindbrain:

• The hindbrain is the posterior part of the brain and consists of three parts:

1. Pons: A bulge on the ventral surface of the brainstem. Contains fibre tracts that interconnect different regions of the brain. Also helps in regulating respiration (pneumotaxic centre).
2. Cerebellum: Second largest part of the brain. Has a highly folded surface (folia). Integrates information from the semicircular canals of the ear and the auditory system. Coordinates voluntary movements, maintains posture, balance, and muscle tone.
3. Medulla Oblongata: The most posterior part; continuous with the spinal cord. Contains vital reflex centres that control respiration (respiratory rhythm centre), cardiovascular reflexes (cardiac centre), and gastric secretions.

• Together, pons and medulla form the brain stem along with the midbrain.

(e) Synapse:

• A synapse is the junction between two neurons where nerve impulses are transmitted from one neuron to the next.
• It is formed by the membranes of a pre-synaptic neuron (the neuron sending the signal) and a post-synaptic neuron (the neuron receiving the signal).
• Types of synapses:

1. Electrical synapse: Pre- and post-synaptic membranes are in very close proximity; impulse transmission is direct and faster.
2. Chemical synapse: Pre- and post-synaptic membranes are separated by a fluid-filled space called the synaptic cleft (~20 nm wide). Transmission occurs via chemical messengers called neurotransmitters (e.g., acetylcholine, dopamine).

• Structure: The pre-synaptic terminal contains synaptic vesicles filled with neurotransmitters. The post-synaptic membrane has specific receptor proteins.
• Synapses ensure unidirectional transmission of nerve impulses.