Quick Summary
The nervous system provides rapid communication and coordination. Neurons transmit electrical impulses; synapses pass them between cells via neurotransmitters. The brain and spinal cord form the central nervous system (CNS). Reflex arcs allow fast automatic responses. The structure and functions of brain regions are among the most-tested biology topics in NEET.
Organisation of the Nervous System
Neuron — Quick Recap
- Cell body (soma): Nucleus, Nissl granules (RER)
- Dendrites: Receive signals
- Axon: Transmit signals; covered by myelin sheath; nodes of Ranvier for saltatory conduction
- Myelinated fibres: Faster conduction (saltatory) than unmyelinated
Nerve Impulse — Action Potential
Resting potential: −70 mV (inside negative; maintained by Na⁺/K⁺ pump — 3Na⁺ out, 2K⁺ in)
Action potential (impulse): 1. Stimulus → Depolarisation: Na⁺ channels open → Na⁺ rushes IN → inside becomes + (+30 mV) 2. Repolarisation: K⁺ channels open → K⁺ rushes OUT → inside returns to negative 3. Hyperpolarisation → Refractory period: Brief period when neuron cannot fire again 4. Na⁺/K⁺ pump restores resting potential
All-or-none law: Impulse is either full-strength or nothing — no partial action potentials.
Synapse
Components:
- Pre-synaptic terminal (knob)
- Synaptic cleft (~20 nm gap)
- Post-synaptic membrane (receptors)
Mechanism: 1. Impulse reaches pre-synaptic knob → Ca²⁺ enters 2. Synaptic vesicles release neurotransmitter (acetylcholine, dopamine, serotonin, GABA) 3. Neurotransmitter binds receptors on post-synaptic membrane 4. New impulse generated in post-synaptic neuron (or inhibited if inhibitory) 5. Neurotransmitter broken down (acetylcholinesterase breaks down ACh)
Reflex Arc
Path: Receptor → Afferent (sensory) nerve → Spinal cord → Efferent (motor) nerve → Effector
- Spinal reflex: Does not involve brain (e.g., knee-jerk, withdrawal from pain)
- Advantage: Very fast (milliseconds) — protective
Human Brain
Forebrain
| Structure | Function |
|---|---|
| Cerebrum | Largest part; 2 hemispheres; voluntary actions, intelligence, memory, speech, learning |
| Thalamus | Relay station for sensory signals |
| Hypothalamus | Homeostasis (hunger, thirst, temperature, sleep); controls pituitary gland |
Cerebrum lobes:
- Frontal: voluntary movement, speech (Broca's area)
- Parietal: sensory integration, touch
- Temporal: hearing, smell, memory
- Occipital: vision
Midbrain
- Visual and auditory reflexes
- Connects forebrain and hindbrain
Hindbrain
| Structure | Function |
|---|---|
| Pons | Relay; breathing rhythm |
| Medulla oblongata | Vital centres: breathing, heartbeat, vomiting, swallowing |
| Cerebellum | Balance and coordination of voluntary movements |
Spinal Cord
- Extends from medulla to lumbar region (filum terminale below)
- White matter (outside): myelinated axons; ascending (sensory) and descending (motor) tracts
- Grey matter (inside, H-shaped): cell bodies; dorsal horn (sensory), ventral horn (motor)
- Central canal: Contains CSF (cerebrospinal fluid)
Sense Organs (Brief)
| Organ | Receptor | Sensation |
|---|---|---|
| Eye | Rods (dim light/B&W), Cones (colour) | Vision |
| Ear | Hair cells in organ of Corti | Hearing; semicircular canals (balance) |
| Nose | Olfactory epithelium | Smell |
| Tongue | Taste buds | Taste |
| Skin | Meissner's corpuscles (touch), Pacinian (pressure) | Touch, pressure, pain |
Important Questions
One Mark
- What is the resting potential of a neuron?
- Name the neurotransmitter at a neuromuscular junction.
- Which part of the brain controls body temperature?
- What is saltatory conduction?
Short Answer (3 marks)
- Explain the generation of an action potential.
- Describe the reflex arc with a diagram. Give one example.
- What are the functions of cerebellum and medulla oblongata?
Long Answer (5 marks)
- Explain the mechanism of synaptic transmission.
- Describe the structure of the human brain with the functions of each major part.
Common Mistakes
- During depolarisation: Na⁺ rushes IN (not out). During repolarisation: K⁺ rushes OUT.
- Cerebellum controls coordination and balance, NOT voluntary movement (that is cerebrum).
- Hypothalamus controls pituitary (endocrine connection) AND homeostasis — students often limit it to one function.
- Reflex arc: nerve centre is in the SPINAL CORD (not brain) for spinal reflexes.
Board Exam Tips
- Brain structure (forebrain, midbrain, hindbrain) with functions is a standard 5-mark question.
- "Explain action potential" is a reliable 5-mark question — include resting potential and all stages.
- Synapse mechanism and neurotransmitters appear as 3-mark questions.
- NEET: Brain parts functions, action potential, synapse, and reflex arc contribute 4–5 questions per paper — one of the highest-yield chapters.