🧠 Nervous System – Full Breakdown
ICSE Class 10
Biology
June 2026
The nervous system is the body's electrical wiring — it processes information at lightning speed. While the endocrine system works with slow chemical messages (hormones), the nervous system uses fast electrical impulses. ICSE loves this chapter for neuron diagrams, brain structure, reflex arcs, and comparing sympathetic vs parasympathetic systems. Written by someone who scored 93.4% in ICSE 2026.
1) What is the Nervous System?
Nervous System: The master control system of the body that detects stimuli, processes information, and coordinates responses via electrical impulses (nerve impulses).
Three basic functions:
- Sensory input: Receptors detect stimuli (touch, light, sound, temperature, pain, etc.)
- Integration: Brain & spinal cord process the information and decide on a response
- Motor output: Signals sent to effectors (muscles or glands) to produce a response
Divisions of the Nervous System
Nervous System → CNS (Central) + PNS (Peripheral)
| Division | Components | Function |
|---|---|---|
| Central Nervous System (CNS) | Brain + Spinal Cord | Integration & command centre — processes information, initiates responses |
| Peripheral Nervous System (PNS) | Cranial nerves (12 pairs) + Spinal nerves (31 pairs) | Connects CNS to the rest of the body |
| → Somatic NS | Sensory & motor nerves to skeletal muscles | Voluntary movements — conscious control |
| → Autonomic NS | Nerves to smooth muscle, cardiac muscle, glands | Involuntary functions — unconscious control |
| • Sympathetic | "Fight or flight" (adrenaline-like) | Prepares body for stress/emergency |
| • Parasympathetic | "Rest and digest" | Conserves energy, maintains routine functions |
2) The Neuron — Functional Unit
Neuron (Nerve Cell): The structural and functional unit of the nervous system. It is specialised to receive, conduct, and transmit nerve impulses.
Structure of a Neuron
| Part | Description | Function |
|---|---|---|
| Dendrites | Short, branched cytoplasmic extensions from the cell body | Receive impulses from other neurons or receptors → carry them toward the cell body |
| Cell Body (Soma) | Contains nucleus, mitochondria, Nissl granules (RER) | Metabolic centre of the neuron — integrates incoming signals |
| Axon | Single, long fibre that extends from the cell body | Carries impulse away from the cell body toward the next neuron or effector |
| Myelin Sheath | Fatty insulating layer around the axon, made by Schwann cells | Insulates the axon → speeds up impulse conduction (saltatory conduction) |
| Node of Ranvier | Gaps in the myelin sheath between Schwann cells | Allows ions to flow in/out → impulse "jumps" from node to node (faster transmission) |
| Axon Terminals (Synaptic knobs) | Branched endings of the axon | Release neurotransmitters into the synapse to pass the signal to the next cell |
⚠️ Myelinated vs Non-myelinated neurons — board favourite:
Myelinated: Has myelin sheath (Schwann cells) + Nodes of Ranvier → faster impulse conduction (saltatory — impulse jumps from node to node). Found in spinal nerves and most cranial nerves.
Non-myelinated: No myelin sheath → slower impulse conduction. Found in autonomic nerves.
Multiple sclerosis is a disease where the immune system destroys the myelin sheath → impulse conduction slows down → muscle weakness, vision problems, etc.
Myelinated: Has myelin sheath (Schwann cells) + Nodes of Ranvier → faster impulse conduction (saltatory — impulse jumps from node to node). Found in spinal nerves and most cranial nerves.
Non-myelinated: No myelin sheath → slower impulse conduction. Found in autonomic nerves.
Multiple sclerosis is a disease where the immune system destroys the myelin sheath → impulse conduction slows down → muscle weakness, vision problems, etc.
Types of Neurons
| Type | Direction of Impulse | Location | Function |
|---|---|---|---|
| Sensory (Afferent) | Receptors → CNS | From skin, sense organs to spinal cord/brain | Carries information toward CNS (incoming signals) |
| Motor (Efferent) | CNS → Effectors | From spinal cord/brain to muscles/glands | Carries commands away from CNS (outgoing signals) |
| Interneurons (Association) | Within CNS only | Brain & spinal cord (between sensory & motor) | Relay signals, process information, make decisions |
💡 Memory trick:
Sensory = Afferent = Arriving (entering CNS)
Motor = Efferent = Exiting (leaving CNS)
Interneurons = Intermediaries (connecting the two)
Most sensory neurons are unipolar (one process from cell body, splits into two). Most motor neurons are multipolar (many dendrites, one axon). ICSE usually doesn't require this level of detail, but it helps for understanding.
Sensory = Afferent = Arriving (entering CNS)
Motor = Efferent = Exiting (leaving CNS)
Interneurons = Intermediaries (connecting the two)
Most sensory neurons are unipolar (one process from cell body, splits into two). Most motor neurons are multipolar (many dendrites, one axon). ICSE usually doesn't require this level of detail, but it helps for understanding.
3) Synapse — How Neurons Communicate
Synapse: The microscopic gap between the axon terminal of one neuron and the dendrite/cell body of the next neuron (or between a neuron and an effector). It's where the nerve impulse is transmitted chemically.
Structure of a Synapse
- Pre-synaptic knob: Swollen end of the axon terminal — contains synaptic vesicles filled with neurotransmitters
- Synaptic cleft: The tiny gap (~20 nm) between the two neurons
- Post-synaptic membrane: Dendrite or cell body of the next neuron — has receptor proteins for neurotransmitters
How a Nerve Impulse Crosses the Synapse
- Impulse arrives at the pre-synaptic knob → depolarisation opens Ca²⁺ channels
- Ca²⁺ enters → synaptic vesicles move to the membrane and fuse with it
- Neurotransmitters (e.g., acetylcholine, dopamine, serotonin) are released into the synaptic cleft by exocytosis
- Neurotransmitters diffuse across the cleft and bind to receptors on the post-synaptic membrane
- Receptor binding opens ion channels → depolarisation of post-synaptic membrane → new impulse generated in the next neuron
- Neurotransmitters are then broken down by enzymes (e.g., acetylcholinesterase breaks down acetylcholine) or recycled — prevents continuous stimulation
⚠️ Why the synapse is important:
1. Ensures one-way transmission — impulses travel only from pre-synaptic to post-ynaptic (never backwards).
2. Allows integration — multiple signals can be summed (excitatory vs inhibitory).
3. Drugs can affect synapses — caffeine blocks adenosine (keeps you alert), SSRIs block serotonin reuptake (antidepressants), anaesthetics block pain signals at synapses.
1. Ensures one-way transmission — impulses travel only from pre-synaptic to post-ynaptic (never backwards).
2. Allows integration — multiple signals can be summed (excitatory vs inhibitory).
3. Drugs can affect synapses — caffeine blocks adenosine (keeps you alert), SSRIs block serotonin reuptake (antidepressants), anaesthetics block pain signals at synapses.
4) Nerve Impulse — How It Works
A nerve impulse is an electrochemical signal that travels along the neuron's membrane.
Resting potential (polarised): When a neuron is NOT conducting an impulse, the inside of the axon is negative (-70 mV) relative to the outside (positive). This is maintained by the Na⁺/K⁺ pump (pumps 3 Na⁺ out, 2 K⁺ in).
Action potential (depolarisation): When stimulated, Na⁺ channels open → Na⁺ rushes in → inside becomes positive (+40 mV). This is the impulse.
Repolarisation: K⁺ channels open → K⁺ rushes out → inside becomes negative again. Na⁺/K⁺ pump restores resting potential.
All-or-none principle: A neuron either fires fully or not at all — there's no "partial" impulse. The strength of a stimulus is coded by the frequency of impulses, not their size.
📖 ICSE level: You don't need to memorise the voltage values or detailed ion movements. Just know: resting = inside negative, active = inside positive (depolarisation), refractory = returning to resting (repolarisation). The impulse travels along the axon as a wave of depolarisation.
5) Central Nervous System (CNS) — Brain
The brain is the command centre — it weighs ~1.4 kg, contains ~100 billion neurons, and uses ~20% of the body's oxygen despite being only 2% of body weight.
Protection of the Brain
- Skull (cranium): Bony protection
- Meninges: Three protective membranes (outer → inner):
- Dura mater: Tough, fibrous outer layer ("tough mother")
- Arachnoid mater: Web-like middle layer (contains CSF in subarachnoid space)
- Pia mater: Thin, delicate inner layer that directly covers the brain ("tender mother")
- Cerebrospinal Fluid (CSF): Clear fluid in the subarachnoid space & ventricles — cushions the brain, removes wastes, provides buoyancy
⚠️ Meningitis: Inflammation of the meninges (usually due to bacterial or viral infection). Symptoms: severe headache, fever, stiff neck, sensitivity to light. Can be life-threatening if not treated. Board loves this as a "what happens if..." question.
Parts of the Brain
| Part | Location | Functions (ICSE Focus) |
|---|---|---|
| Cerebrum | Largest part — front & middle of skull. Divided into 2 cerebral hemispheres (left & right), connected by corpus callosum | Highest centre: Voluntary actions, thinking, memory, intelligence, speech, reasoning, emotions. Sensory areas (vision, hearing, touch, taste, smell). Motor areas control voluntary movement. Right hemisphere controls left body, left hemisphere controls right body. |
| Cerebellum | Below the cerebrum, at the back of the skull (hindbrain) | Coordination & balance: Maintains posture, controls fine motor movements (walking, writing, picking up objects). It does not initiate movement — it fine-tunes it. |
| Medulla Oblongata | Lowest part of brainstem, continues into spinal cord | Vital centre: Controls involuntary actions — heartbeat, breathing rate, blood pressure, swallowing, coughing, sneezing, vomiting. Damage here is usually fatal. |
| Hypothalamus | Below thalamus, above pituitary | Body temperature regulation, hunger, thirst, sleep-wake cycle, emotional responses. Links nervous system to endocrine system (controls pituitary). |
| Pons | Between medulla & midbrain | Relays signals between cerebrum & cerebellum. Also involved in sleep regulation and respiration control. |
💡 Mnemonic for brain parts & functions:
Cerebrum: "Cerebrum = CEO" — thinking, planning, voluntary actions
Cerebellum: "Cerebellum = Coordination" — both start with C, like "C for Control (of balance)"
Medulla: "Medulla = Mandatory" — heartbeat, breathing (can't stop them voluntarily)
Hypothalamus: "Hypo = Below (below thalamus), controls housekeeping" — temp, hunger, thirst
Or: "Cerebrum thinks, Cerebellum coordinates, Medulla keeps you alive."
Cerebrum: "Cerebrum = CEO" — thinking, planning, voluntary actions
Cerebellum: "Cerebellum = Coordination" — both start with C, like "C for Control (of balance)"
Medulla: "Medulla = Mandatory" — heartbeat, breathing (can't stop them voluntarily)
Hypothalamus: "Hypo = Below (below thalamus), controls housekeeping" — temp, hunger, thirst
Or: "Cerebrum thinks, Cerebellum coordinates, Medulla keeps you alive."
6) Central Nervous System (CNS) — Spinal Cord
Spinal Cord: A long, cylindrical bundle of nerve fibres that extends from the medulla oblongata down to L1–L2 in the vertebral column. Protected by the vertebral column and meninges.
Structure:
- Grey matter (inside, H-shaped) — contains cell bodies of neurons, interneurons, synapses. The "processing" part.
- White matter (outside) — contains myelinated axons (nerve fibres) that run up and down, connecting the brain to the body. The "highway" part.
- Central canal: Small tube in the centre filled with CSF
Functions:
- Reflex centre: Many reflex arcs are processed directly in the spinal cord without involving the brain (faster response)
- Conduction pathway:
- Sensory (ascending) tracts: Carry impulses from body → brain (touch, pain, temperature, pressure)
- Motor (descending) tracts: Carry impulses from brain → body (voluntary movements)
Spinal nerves: 31 pairs emerge from the spinal cord. Each spinal nerve has a dorsal root (sensory — incoming) and a ventral root (motor — outgoing).
⚠️ Spinal cord injury: Damage to the spinal cord can cause paralysis below the level of injury. If the injury is in the cervical (neck) region → quadriplegia (all 4 limbs paralysed). If in the thoracic/lumbar region → paraplegia (lower body paralysed). The spinal cord cannot regenerate (unlike some peripheral nerves).
7) Peripheral Nervous System (PNS)
| Division | Components | Function |
|---|---|---|
| Cranial Nerves | 12 pairs originating from the brain | Supply head, neck, and some organs. Examples: Optic (vision), Olfactory (smell), Auditory (hearing), Vagus (heart, lungs, digestion) |
| Spinal Nerves | 31 pairs originating from the spinal cord | Supply trunk and limbs. Each nerve carries both sensory & motor fibres. |
| Somatic NS | Voluntary — sensory & motor nerves to skeletal muscles | Conscious movements: walking, talking, writing. Single neuron from CNS to muscle. |
| Autonomic NS | Involuntary — to smooth muscle, cardiac muscle, glands | Unconscious regulation: heartbeat, digestion, breathing, sweating. Two-neuron chain from CNS to effector. |
Autonomic Nervous System — Sympathetic vs Parasympathetic
| Feature | Sympathetic ("Fight or Flight") | Parasympathetic ("Rest & Digest") |
|---|---|---|
| Origin | Thoracic & lumbar spinal cord (T1–L2) | Cranial nerves + sacral spinal cord |
| Neurotransmitter | Noradrenaline (norepinephrine) | Acetylcholine |
| Overall effect | Prepares body for stress/emergency | Conserves energy, maintains routine |
| Heart rate | Increases ↑ | Decreases ↓ |
| Breathing | Bronchodilation — wider airways ↑ | Bronchoconstriction — normal ↓ |
| Pupils | Dilate (widen) ↑ | Constrict (narrow) ↓ |
| Digestion | ↓ Decreased (blood diverted to muscles) | ↑ Increased (stimulates peristalsis, enzyme secretion) |
| Blood glucose | Increases ↑ (liver releases glucose) | Stable/normal |
| Saliva production | ↓ Reduced (dry mouth) | ↑ Normal |
| Urination | ↓ Inhibited | ↑ Promoted |
💡 Easy way to remember Sympathetic vs Parasympathetic:
Sympathetic: "S" for Stress, Speed, Stimulate — prepares you to run from a tiger. ↑ HR, ↑ breathing, ↓ digestion.
Parasympathetic: "P" for Peace, Process, Preserve — "rest and digest". ↓ HR, ↓ breathing, ↑ digestion.
Think: Sympathetic = accelerator pedal. Parasympathetic = brake pedal.
Sympathetic: "S" for Stress, Speed, Stimulate — prepares you to run from a tiger. ↑ HR, ↑ breathing, ↓ digestion.
Parasympathetic: "P" for Peace, Process, Preserve — "rest and digest". ↓ HR, ↓ breathing, ↑ digestion.
Think: Sympathetic = accelerator pedal. Parasympathetic = brake pedal.
8) Reflex Arc
Reflex Arc: The neural pathway that mediates a reflex action — an automatic, involuntary, rapid response to a stimulus that does not require conscious thought.
Importance of reflexes: They protect the body from harm by producing a rapid response without waiting for the brain. For example, pulling your hand away from a hot stove happens before you consciously feel pain.
The Reflex Pathway (5 Components)
- Receptor: Detects the stimulus (e.g., thermoreceptors in skin detect heat)
- Sensory neuron: Carries impulse from receptor → spinal cord
- Interneuron (Relay neuron): In the spinal cord's grey matter — connects sensory neuron to motor neuron; may also send the signal up to the brain (so you feel pain after the reflex)
- Motor neuron: Carries impulse from spinal cord → effector
- Effector: Muscle (contracts) or gland (secretes) — produces the response
⚠️ Example — Withdrawal reflex (hand on hot object):
Stimulus (heat) → Receptor in skin → Sensory neuron → Spinal cord → Interneuron → Motor neuron → Arm muscle contracts → Hand pulled away. The signal ALSO goes to the brain, but by the time you feel pain, the reflex has already happened. This is why you pull your hand back from a flame BEFORE you consciously register the pain.
Stimulus (heat) → Receptor in skin → Sensory neuron → Spinal cord → Interneuron → Motor neuron → Arm muscle contracts → Hand pulled away. The signal ALSO goes to the brain, but by the time you feel pain, the reflex has already happened. This is why you pull your hand back from a flame BEFORE you consciously register the pain.
Types of Reflexes
| Type | Example | Pathway | Purpose |
|---|---|---|---|
| Simple (Monosynaptic) | Knee-jerk reflex (patellar) | Receptor → Sensory neuron → Motor neuron → Effector. Only 2 neurons (one synapse in spinal cord). | Tests spinal cord function (used by doctors with a reflex hammer) |
| Complex (Polysynaptic) | Withdrawal from heat, blinking, sneezing, coughing | Involves interneurons (multiple synapses). Can be modified by brain signals. | Protection from harm |
💡 Reflex vs Voluntary action — board favourite:
Reflex: Involuntary, rapid, doesn't require brain (processed in spinal cord), protective, stereotyped (same response every time).
Voluntary: Conscious, slower, requires brain (cerebrum), learned/planned, can be modified.
Example: Touching a hot stove = reflex. Deciding to lift your hand = voluntary.
Reflex: Involuntary, rapid, doesn't require brain (processed in spinal cord), protective, stereotyped (same response every time).
Voluntary: Conscious, slower, requires brain (cerebrum), learned/planned, can be modified.
Example: Touching a hot stove = reflex. Deciding to lift your hand = voluntary.
9) Common Disorders of the Nervous System
| Disorder | Cause | Symptoms |
|---|---|---|
| Stroke (Cerebrovascular Accident) | Blockage or rupture of blood vessel in brain → brain cells die from lack of oxygen | Sudden weakness/paralysis on one side, speech difficulty, confusion. Medical emergency (FAST: Face, Arm, Speech, Time) |
| Paralysis | Damage to spinal cord or brain (e.g., stroke, injury) | Loss of muscle function — quadriplegia (4 limbs), paraplegia (lower body) |
| Meningitis | Inflammation of meninges (bacterial/viral) | Severe headache, fever, stiff neck, photophobia (light sensitivity), vomiting |
| Parkinson's Disease | Degeneration of dopamine-producing neurons in the brain | Tremors (shaking), muscle rigidity, slow movement (bradykinesia), shuffling gait |
| Alzheimer's Disease | Progressive brain degeneration — loss of neurons, plaques & tangles | Memory loss (especially recent), confusion, personality changes, difficulty with language |
| Epilepsy | Abnormal electrical activity in the brain | Recurrent seizures (convulsions) — can range from brief staring spells to full-body convulsions |
| Multiple Sclerosis | Autoimmune — body attacks myelin sheath | Muscle weakness, numbness, vision problems, fatigue, coordination problems |
10) Common ICSE Board Questions
1-Mark Definitions:
- Define: Reflex arc, Synapse, Neuron, Myelin sheath, Meninges
- What is the difference between CNS and PNS?
- Name the three layers of the meninges.
- Which part of the brain controls: (a) balance (b) heartbeat (c) voluntary actions?
- What is the function of the cerebellum?
- What is a nerve impulse? What is the "all-or-none" principle?
- Name the gap between two neurons.
- List 2 differences between sympathetic and parasympathetic nervous systems.
3-Mark Reasoning Questions:
- Draw a labelled diagram of a neuron and write the function of each part.
- Explain how a nerve impulse crosses a synapse. Why is the synapse important for one-way transmission?
- Differentiate between the cerebrum, cerebellum, and medulla oblongata (location + function).
- Describe the reflex arc with the example of withdrawing a hand from a hot object.
- Differentiate between the sympathetic and parasympathetic nervous systems (any 4 points).
- "The spinal cord serves as both a reflex centre and a conduction pathway." Explain.
- What is the difference between a reflex action and a voluntary action?
- Explain the structure and function of the synapse.
- What is the role of the myelin sheath in impulse conduction? What happens if it is damaged?
- Explain the protection of the brain (skull, meninges, CSF). What is meningitis?
5-Mark Questions / Diagrams:
- Draw a neat labelled diagram of a neuron. Label: dendrites, cell body, nucleus, axon, myelin sheath, Node of Ranvier, Schwann cell, axon terminals.
- Draw a labelled diagram showing the reflex arc pathway. Label all 5 components and show the direction of impulse with arrows.
- Draw a diagram of the brain showing the cerebrum, cerebellum, medulla oblongata, pons, and hypothalamus. Write the function of each.
- Make a table comparing the sympathetic and parasympathetic nervous systems under at least 6 headings (origin, neurotransmitter, HR, breathing, pupils, digestion).
- Describe the structure of a synapse and explain how a nerve impulse is transmitted across it with the help of neurotransmitters.
- Explain the different types of neurons (sensory, motor, interneurons) with their functions and direction of impulse conduction.
🧠 From my exam (93.4% scorer): The nervous system chapter has three guaranteed diagrams — the neuron, the reflex arc, and the brain. Practice all three.
Neuron diagram tip: Show the myelin sheath as segmented tubes around the axon, with small bare patches (Nodes of Ranvier) in between. Draw at least 3 Schwann cells.
Reflex arc tip: Label the 5 components in a clear flow: Stimulus → Receptor → Sensory neuron → (Spinal cord) Interneuron → Motor neuron → Effector → Response. Use arrows to show the direction.
Brain diagram tip: Show the cerebrum as the largest part (wrinkled surface), cerebellum below it (smaller, folded), and medulla at the base (stem-like). The cerebrum should be clearly distinguished from the cerebellum.
Neuron diagram tip: Show the myelin sheath as segmented tubes around the axon, with small bare patches (Nodes of Ranvier) in between. Draw at least 3 Schwann cells.
Reflex arc tip: Label the 5 components in a clear flow: Stimulus → Receptor → Sensory neuron → (Spinal cord) Interneuron → Motor neuron → Effector → Response. Use arrows to show the direction.
Brain diagram tip: Show the cerebrum as the largest part (wrinkled surface), cerebellum below it (smaller, folded), and medulla at the base (stem-like). The cerebrum should be clearly distinguished from the cerebellum.
11) Quick Reference Tables (Print This)
Brain Parts & Functions
| Part | Function (One-Liner) |
|---|---|
| Cerebrum | Voluntary actions, thinking, memory, speech, senses |
| Cerebellum | Balance, coordination of movement, posture |
| Medulla Oblongata | Involuntary vital functions — heartbeat, breathing, BP |
| Hypothalamus | Temperature, hunger, thirst, links NS to endocrine |
| Pons | Relay between cerebrum & cerebellum, sleep |
Sympathetic vs Parasympathetic — Quick Glance
| Feature | Sympathetic | Parasympathetic |
|---|---|---|
| Motto | Fight or Flight | Rest and Digest |
| Heart Rate | ↑ Increases | ↓ Decreases |
| Pupils | ↑ Dilate | ↓ Constrict |
| Digestion | ↓ Inhibited | ↑ Stimulated |
| Airways | ↑ Dilate (wider) | ↓ Constrict (normal) |
| Neurotransmitter | Noradrenaline | Acetylcholine |
Neuron Parts — One Look
| Part | Function |
|---|---|
| Dendrites | Receive impulse → toward cell body |
| Axon | Carry impulse away from cell body |
| Myelin Sheath | Insulation → speeds up conduction |
| Node of Ranvier | Gap in myelin — saltatory conduction |
| Synaptic Knobs | Release neurotransmitters into synapse |
⚠️ Final advice: The nervous system chapter connects directly to the endocrine system — the two are the body's communication systems. The nervous system is fast but short-lived (electrical impulses). The endocrine system is slow but long-lasting (hormones). ICSE sometimes asks a comparison. Remember: Nerves = electrical wires sending instant messages. Hormones = email that takes time but reaches everywhere.