Exercises
Briefly describe the structure of the brain.
Answer
The human brain is a highly complex organ located in the cranial cavity and protected by the skull (cranium) and three meninges (dura mater, arachnoid mater, pia mater). The brain is the master control center of the body and weighs about 1.4 kg in adults.
The brain is divided into three main parts:
1. Forebrain
• Cerebrum: The largest part of the brain, divided into left and right cerebral hemispheres by the longitudinal fissure. Connected by the corpus callosum. The outer layer is the cerebral cortex (gray matter), deeply folded into gyri (ridges) and sulci (grooves). Functions: thinking, voluntary movement, language, sensation, emotion, learning, and memory. Four lobes: frontal, parietal, temporal, occipital.
• Thalamus: Lies below the corpus callosum. Acts as a relay station — all sensory signals (except olfaction) pass through it to reach the cerebral cortex.
• Hypothalamus: Lies below the thalamus. Controls many body functions including hunger, thirst, body temperature, sleep, sex drive. Links nervous system to endocrine system via the pituitary gland. Contains the "master clock" (suprachiasmatic nucleus).
2. Midbrain
• Located between the forebrain and hindbrain; connects them via the cerebral aqueduct.
• Contains four rounded elevations — the corpora quadrigemina (superior and inferior colliculi) — involved in visual and auditory reflexes.
• Contains the cerebral peduncles (nerve fiber bundles connecting brain regions).
3. Hindbrain
• Cerebellum: Second largest part; two hemispheres with a folded cortex. Coordinates voluntary movements, maintains balance, and controls posture.
• Pons: A bridge-like structure relaying signals between cerebrum, cerebellum, and medulla. Also controls breathing.
• Medulla oblongata: Continuous with the spinal cord. Controls vital automatic functions: heart rate, breathing, blood pressure, swallowing, vomiting.
• Brain stem: Collectively, the midbrain + pons + medulla = brain stem; connects cerebrum to spinal cord.
• Limbic system: A ring of structures around the thalamus (including amygdala, hippocampus, cingulate gyrus) involved in emotions, memory, and behavior.
Q2. Compare the following
(a) Central neural system (CNS) and Peripheral neural system (PNS)
(b) Resting potential and action potential
Ans.
(a) CNS vs PNS:
Feature | Central Nervous System (CNS) | Peripheral Nervous System (PNS) |
Components | Brain and spinal cord | All nerves outside the brain and spinal cord (cranial nerves, spinal nerves, ganglia) |
Location | Enclosed in skull and vertebral column | Distributed throughout the body |
Protection | Protected by bone (skull/vertebrae) and meninges | Less protection (peripheral location) |
Function | Integrates and processes information; formulates responses | Carries signals to and from the CNS (afferent and efferent) |
Divisions | Brain (forebrain, midbrain, hindbrain) and spinal cord | Somatic NS (voluntary) and Autonomic NS (involuntary: sympathetic and parasympathetic) |
(b) Resting potential vs Action potential:
Feature | Resting Potential | Action Potential |
Definition | The electrical potential difference across a neuron membrane when NOT conducting an impulse | The rapid, transient change in membrane potential when a nerve impulse is generated |
Value | About –70 mV (inside negative) | Peaks at about +30 to +40 mV (inside briefly becomes positive) |
Ion distribution | Na⁺ outside; K⁺ inside (maintained by Na⁺-K⁺ pump) | Na⁺ rushes in (depolarization); K⁺ rushes out (repolarization) |
State of neuron | Polarized (not stimulated) | Depolarized → repolarized → hyperpolarized |
Duration | Maintained indefinitely as long as no stimulus | Lasts about 1 ms; self-propagating along the axon |
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
Ans.
(a) Polarisation of the membrane of a nerve fibre:
A resting (unstimulated) neuron maintains a resting membrane potential of about –70 mV (inside is negative relative to outside). This is the polarized state, maintained by:
1. Unequal ion distribution: High Na⁺ and Cl⁻ outside; high K⁺ and large organic anions (A⁻) inside.
2. Na⁺-K⁺ pump (ATPase): Actively pumps 3 Na⁺ out and 2 K⁺ in per ATP, maintaining the concentration gradient.
3. K⁺ leak channels: Allow K⁺ to leak out down its concentration gradient, making interior more negative.
4. Na⁺ channels: Mostly closed at rest; Na⁺ outside cannot enter freely.
The net result is that the outside is positive and inside is negative — the membrane is polarized.
(b) Depolarisation of the membrane of a nerve fibre:
When a nerve fiber receives a stimulus that reaches the threshold level, voltage-gated Na⁺ channels open and Na⁺ ions rush into the cell along their electrochemical gradient. This rapid influx of positive ions causes the inside of the membrane to become less negative, then neutral, then positive (+30 to +40 mV). This reversal of polarity is called depolarization.
• The point of depolarization travels along the nerve fiber as a wave — this is the nerve impulse (action potential).
• After depolarization, Na⁺ channels close and K⁺ channels open, allowing K⁺ to flow out, restoring the resting potential (repolarization).
• A brief refractory period follows when the membrane cannot be re-stimulated.
(c) Transmission of a nerve impulse across a chemical synapse:
At a chemical synapse, the nerve impulse is transmitted from one neuron (presynaptic) to another (postsynaptic) via chemical neurotransmitters:
1. The action potential arrives at the synaptic knob (axon terminal) of the presynaptic neuron.
2. Depolarization of the presynaptic membrane causes voltage-gated Ca²⁺ channels to open; Ca²⁺ ions rush in.
3. Rising Ca²⁺ triggers synaptic vesicles (containing neurotransmitters) to fuse with the presynaptic membrane and release neurotransmitter (e.g., acetylcholine, dopamine) into the synaptic cleft by exocytosis.
4. Neurotransmitter molecules diffuse across the synaptic cleft and bind to specific receptors on the postsynaptic membrane.
5. Binding opens ion channels on the postsynaptic membrane — if excitatory neurotransmitter: Na⁺ enters → depolarization → action potential in postsynaptic neuron. If inhibitory (GABA): Cl⁻ enters or K⁺ exits → hyperpolarization → inhibition.
6. Neurotransmitter is then degraded (e.g., acetylcholinesterase breaks down acetylcholine) or reabsorbed (reuptake) by the presynaptic neuron to terminate the signal.
Q4. Draw labelled diagrams of the following
(a) Neuron (b) Brain
Ans.
(a) Neuron (diagram description):
A neuron (nerve cell) has three main parts:
• Cell body (soma/cyton): Contains the nucleus, mitochondria, Nissl's granules (RER and ribosomes for protein synthesis), and cytoplasm.
• Dendrites: Short, branched extensions of the cell body that receive impulses from other neurons. Multiple dendrites per neuron.
• Axon: Single, long extension that conducts impulses away from the cell body. Key features: axon hillock (where axon originates), myelin sheath (phospholipid insulation produced by Schwann cells), nodes of Ranvier (gaps in myelin sheath), axon terminals (synaptic knobs at the end with synaptic vesicles).
Labels on diagram: dendrites, cell body (soma), nucleus, Nissl's granules, axon hillock, axon, myelin sheath (produced by Schwann cell), node of Ranvier, axon terminal (synaptic knob), synaptic vesicles.
(b) Brain (diagram description):
A sagittal section showing: Cerebrum (large folded structure on top, divided into lobes — frontal, parietal, occipital, temporal), corpus callosum (connecting the two hemispheres), thalamus (central relay), hypothalamus (below thalamus), pituitary gland (hanging from hypothalamus), midbrain (corpora quadrigemina — four bumps), pons (bridge), cerebellum (folded structure at back), medulla oblongata (connecting to spinal cord), spinal cord, brain stem (midbrain + pons + medulla), meninges (three-layered covering).
Q5. Write short notes on the following
(a) Neural coordination (b) Forebrain (c) Midbrain (d) Hindbrain (e) Synapse
Ans.
(a) Neural coordination:
Neural coordination is the mechanism by which the nervous system regulates and integrates the activities of different organs and organ systems so they work together in a harmonious and coordinated manner. It is mediated through electrical impulses conducted by neurons. Neural coordination is fast and short-lived. It involves the central nervous system (CNS) receiving sensory input, processing it, and sending appropriate motor commands to effectors (muscles/glands). Neural coordination also works in conjunction with the endocrine system for overall body homeostasis.
(b) Forebrain:
The forebrain is the anterior and largest part of the brain, consisting of:
• Cerebrum: Highest center of conscious thought, perception, voluntary movement, memory, speech, and sensory integration. Divided into two hemispheres with four lobes each.
• Thalamus: Relay station for all sensory signals (except smell) traveling to the cerebral cortex.
• Hypothalamus: Controls homeostatic functions (temperature, hunger, thirst, circadian rhythms), regulates the pituitary gland, and controls the ANS. The limbic system is also associated with the forebrain.
(c) Midbrain:
Located between the forebrain and hindbrain. Contains:
• Corpora quadrigemina: Four rounded elevations. Superior colliculi — visual reflexes (orienting eyes/head toward stimuli). Inferior colliculi — auditory reflexes (orienting toward sounds).
• Cerebral peduncles: Fiber tracts connecting the cerebrum with lower brain regions.
• Connects via cerebral aqueduct (carries CSF).
• Part of the reticular activating system (RAS) involved in consciousness and alertness.
(d) Hindbrain:
The posterior part of the brain, consisting of:
• Cerebellum: Coordinates voluntary movement, balance, and posture. Acts as "autopilot" for learned motor skills (cycling, typing). Has folded cortex (folia).
• Pons: Bridges cerebellum and medulla with higher brain centers. Contains pneumotaxic and apneustic centers for breathing regulation.
• Medulla oblongata: Controls vital autonomic functions — cardiac center (heart rate), vasomotor center (blood pressure), respiratory center, centers for swallowing, vomiting, and coughing.
(e) Synapse
A synapse is the specialized junction between two neurons (or between a neuron and an effector cell) across which nerve impulses are transmitted. It consists of:
• Presynaptic terminal: Axon terminal of the sending neuron containing synaptic vesicles with neurotransmitter.
• Synaptic cleft: Narrow gap (~20–40 nm) between the two neurons.
• Postsynaptic membrane: Membrane of receiving neuron (or effector) bearing receptor proteins for neurotransmitter.
Synapses can be excitatory (trigger action potential) or inhibitory (prevent action potential). Chemical synapses use neurotransmitters (e.g., acetylcholine, dopamine, GABA); electrical synapses use gap junctions and are faster but rarer.
Give a brief account of mechanism of synaptic transmission.
Answer
Mechanism of synaptic transmission at a chemical synapse:
1. Arrival of action potential: The nerve impulse (action potential) travels along the axon and reaches the presynaptic terminal (synaptic knob).
2. Ca²⁺ influx: Depolarization of the presynaptic membrane opens voltage-gated calcium (Ca²⁺) channels. Ca²⁺ flows into the synaptic knob from the extracellular fluid.
3. Neurotransmitter release: The rise in intracellular Ca²⁺ causes synaptic vesicles to move toward and fuse with the presynaptic membrane (exocytosis), releasing neurotransmitter molecules into the synaptic cleft.
4. Diffusion across cleft: Neurotransmitter molecules diffuse across the narrow synaptic cleft (~20 nm).
5. Receptor binding: Neurotransmitter binds to specific receptor proteins on the postsynaptic membrane, causing conformational changes and opening of ion channels.
Postsynaptic potential:
Answer
— Excitatory NT (e.g., acetylcholine, glutamate): Na⁺ enters
⇒ depolarization (EPSP — Excitatory Postsynaptic Potential)
⇒ action potential if threshold is reached.
— Inhibitory NT (e.g., GABA, glycine): Cl⁻ enters or K⁺ exits
⇒ hyperpolarization (IPSP — Inhibitory Postsynaptic Potential)
⇒ inhibition.
Termination: Neurotransmitter is removed by:
Answer
— Enzymatic degradation: E.g., acetylcholinesterase breaks down acetylcholine into acetate + choline.
— Reuptake: Transported back into the presynaptic terminal for reuse.
— Diffusion: Some diffuse away.
Explain the role of Na⁺ in the generation of action potential.
Answer
Sodium ions (Na⁺) play a central role in generating the action potential:
At rest (Resting potential)
• The resting membrane potential is about –70 mV (inside negative).
• Voltage-gated Na⁺ channels are closed. High [Na⁺] outside and low [Na⁺] inside establishes a steep electrochemical gradient for Na⁺ to enter.
During stimulation (Generation of action potential)
1. When a threshold stimulus (~–55 mV) is applied, voltage-gated Na⁺ channels in the axon membrane rapidly open.
2. Na⁺ ions rush into the cell along both their concentration gradient (high outside → low inside) and electrical gradient (outside positive → inside negative attracting positive ions).
3. This rapid influx of Na⁺ rapidly depolarizes the membrane — inside becomes positive (+30 to +40 mV). This is the rising phase (depolarization) of the action potential.
4. The Na⁺ channels quickly inactivate (close automatically after ~0.5 ms), stopping further Na⁺ influx.
5. K⁺ channels open → K⁺ flows out → repolarization (falling phase).
6. Briefly, the membrane becomes hyperpolarized (more negative than rest), then the Na⁺-K⁺ pump restores the original ion distribution.
Without Na⁺ influx, depolarization cannot occur, and therefore, no action potential can be generated. Na⁺ is thus the key ion responsible for the depolarizing phase of the action potential.
Q8. Differentiate between
(a) Myelinated and non-myelinated axons
(b) Dendrites and axons
(c) Thalamus and Hypothalamus
(d) Cerebrum and Cerebellum
Ans.
(a) Myelinated vs Non-myelinated axons:
Myelinated axons | Non-myelinated axons |
Covered by myelin sheath (produced by Schwann cells — fatty insulation) | Lack myelin sheath (only neurilemma/Schwann cell membrane) |
Conduction is saltatory (impulse jumps from node to node) | Conduction is continuous along the entire axon length |
Faster conduction velocity (up to 120 m/s) | Slower conduction velocity (0.5–2 m/s) |
Nodes of Ranvier present (gaps in myelin) | Nodes of Ranvier absent |
Found in somatic motor and sensory nerves | Found in autonomic nervous system, C-fibers (pain) |
(b) Dendrites vs Axons:
Dendrites | Axons |
Short, branched extensions of cell body | Single, long extension of cell body |
Multiple per neuron | Only one per neuron |
Receive impulses from other neurons | Conduct impulses away from the cell body |
No myelin sheath | May or may not have myelin sheath |
Increase surface area for receiving signals | Transmit signals to next neuron or effector |
(c) Thalamus vs Hypothalamus:
Thalamus | Hypothalamus |
Located above the hypothalamus, in the center of the forebrain | Located below the thalamus, forms floor of the third ventricle |
Acts as relay center for all sensory signals (except smell) going to cerebral cortex | Controls autonomic functions: hunger, thirst, body temperature, sleep |
Involved in consciousness and alertness | Links nervous system to endocrine system via pituitary gland |
No direct hormonal connection | Secretes releasing/inhibiting hormones for pituitary; contains the body's "master clock" (SCN) |
(d) Cerebrum vs Cerebellum:
Cerebrum | Cerebellum |
Largest part of the brain; part of forebrain | Second largest; part of hindbrain |
Highly folded into gyri and sulci | Folded into folia (leaf-like folds) |
Controls conscious thought, voluntary movements, intelligence, memory, language | Coordinates voluntary movement, balance, posture; fine-tunes motor activity |
Two large hemispheres connected by corpus callosum | Two hemispheres connected by vermis |
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?
Ans.
(a) The cerebrum is the most developed part of the human brain. It is the largest and most complex part, responsible for higher cognitive functions including intelligence, memory, language, decision-making, sensory perception, and voluntary motor control. The human cerebrum is uniquely large and highly convoluted (folded) compared to other mammals, providing the large surface area needed for these advanced functions. The cerebral cortex (outer gray matter layer) contains approximately 14–16 billion neurons.
(b) The suprachiasmatic nucleus (SCN) of the hypothalamus acts as the master clock (circadian pacemaker) of the central nervous system. It receives light input from the retina via the retinohypothalamic tract and synchronizes all biological rhythms (sleep-wake cycle, hormone secretion patterns, body temperature fluctuations) to a ~24-hour cycle.
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
Ans.
(a) Afferent vs Efferent neurons:
Afferent neurons (sensory) | Efferent neurons (motor) |
Carry impulses FROM sense organs/receptors TO the CNS | Carry impulses FROM the CNS TO muscles/glands (effectors) |
Cell body in dorsal root ganglion (spinal nerves) or cranial ganglia | Cell body in ventral horn of spinal cord or motor cortex |
Unipolar or pseudounipolar structure | Multipolar structure (many dendrites) |
Examples: pain fibers, touch fibers, auditory nerve fibers | Examples: motor neurons innervating skeletal muscle (somatic), autonomic fibers to smooth muscle/glands |
(b) Impulse conduction in myelinated vs unmyelinated nerve fibre:
Myelinated Fibre | Unmyelinated Fibre |
Saltatory conduction: impulse "jumps" from one node of Ranvier to the next (salto = jump) | Continuous conduction: impulse travels along the entire axon membrane step by step |
Faster (up to 120 m/s) and more energy-efficient | Slower (0.5–2 m/s) and less energy-efficient |
Ion exchange only at nodes, so less Na⁺-K⁺ pump activity needed | Ion exchange along entire membrane, higher ATP cost |
(f) Cranial nerves vs Spinal nerves
Cranial Nerves | Spinal Nerves |
12 pairs (I–XII) in humans | 31 pairs in humans |
Arise from the brain (mainly brain stem) | Arise from the spinal cord |
Pass through foramina (openings) in the skull | Pass through intervertebral foramina |
May be purely sensory (e.g., olfactory I, optic II, vestibulocochlear VIII), purely motor (e.g., oculomotor III), or mixed | All are mixed (contain both sensory/afferent and motor/efferent fibers) |
Named (e.g., vagus, trigeminal, facial) | Named by region (cervical, thoracic, lumbar, sacral, coccygeal) |
Important Extra Questions
What is reflex action and reflex arc? Explain with an example.
Answer
Reflex action is a rapid, automatic, involuntary response to a specific stimulus, mediated by the spinal cord (or lower brain centers) without conscious involvement of the brain. Reflexes are protective and allow quick responses to potentially harmful stimuli.
A reflex arc is the neural pathway that mediates a reflex action. It consists of five components:
1. Receptor: Detects stimulus (e.g., skin receptor for pain/heat).
2. Afferent (sensory) neuron: Carries impulse from receptor to spinal cord.
3. Nerve center (interneuron): In the gray matter of spinal cord (integration center).
4. Efferent (motor) neuron: Carries impulse from spinal cord to effector.
5. Effector: Muscle or gland that carries out the response.
Example — Knee-jerk reflex (patellar reflex): Tapping the patellar tendon stretches the quadriceps muscle
⇒ sensory neuron carries impulse to spinal cord
⇒ monosynaptic connection with motor neuron
⇒ motor neuron stimulates quadriceps to contract
⇒ leg kicks forward. This happens in ~50 ms.
What are the differences between the sympathetic and parasympathetic nervous systems?
Answer
Feature | Sympathetic NS | Parasympathetic NS |
Function | "Fight or flight" — activates body for stress response | "Rest and digest" — maintains routine body functions |
Heart rate | Increases | Decreases |
Digestion | Decreases (inhibits) | Increases (stimulates) |
Pupils | Dilates (mydriasis) | Constricts (miosis) |
Neurotransmitter at target organ | Noradrenaline (norepinephrine) | Acetylcholine |
Origin | Thoracic and lumbar segments of spinal cord (thoraco-lumbar outflow) | Cranial nerves and sacral segments (cranio-sacral outflow) |
What is the blood-brain barrier?
Answer
The blood-brain barrier (BBB) is a highly selective semipermeable barrier between the blood circulation and the brain's extracellular fluid (interstitial fluid). It is formed by:
• Tight junctions between capillary endothelial cells
• Astrocyte foot processes surrounding the capillaries
• Pericytes
Functions
• Protects the brain from harmful pathogens, toxins, and fluctuating ions in the blood.
• Maintains a stable ionic environment essential for neural function.
• Allows passage of essential nutrients (glucose, amino acids, O₂) and lipid-soluble molecules (e.g., some drugs, alcohol, anesthetics).
• Blocks passage of large molecules, most drugs, and water-soluble substances.
Some regions of the brain lack a BBB (circumventricular organs) — these sense blood-borne hormones and signals for homeostasis.
Explain the structure and function of the ear in hearing.
Answer
The ear has three parts involved in hearing:
Outer ear: Pinna collects sound waves
⇒ external auditory canal
⇒ tympanic membrane (eardrum) vibrates.
Middle ear: Three ossicles — malleus (attached to eardrum)
⇒ incus
⇒ stapes (attached to oval window of inner ear). Ossicles amplify vibrations (~20× increase in pressure). Eustachian tube connects to pharynx to equalize pressure.
Inner ear (cochlea): Fluid-filled snail-shaped structure. Vibrations enter via oval window
⇒ fluid waves in scala vestibuli
⇒ through helicotrema
⇒ scala tympani
⇒ exit via round window. Basilar membrane (in scala media) vibrates, bending hair cells of the Organ of Corti. Hair cell stereocilia bending
⇒ opens ion channels
⇒ receptor potential
⇒ action potential in cochlear nerve (part of VIII cranial nerve)
⇒ auditory cortex in temporal lobe
⇒ sound perception.
Different frequencies of sound cause maximum vibration at different places along the basilar membrane (tonotopy), allowing pitch discrimination.
What is the role of the limbic system in behavior?
Answer
The limbic system is a group of interconnected brain structures encircling the thalamus, including the amygdala, hippocampus, cingulate gyrus, hypothalamus, olfactory bulb, and septal nuclei. It is sometimes called the "emotional brain."
Functions
1. Emotions: Amygdala processes fear, aggression, pleasure, and emotional memories.
2. Memory consolidation: Hippocampus is essential for converting short-term memories to long-term memories (declarative/episodic memory). Damage to hippocampus causes anterograde amnesia.
3. Motivation and reward: Nucleus accumbens and ventral tegmental area (dopaminergic pathways) mediate pleasure and reward (addiction involves limbic system).
4. Olfaction: Olfactory bulb directly connects to limbic system (explaining strong emotional/memory associations with smells).
5. Autonomic regulation: Through hypothalamus — mediates emotional effects on heart rate, digestion, hormone release.