Nervous System

• Neuron — dendrite, cell body, axon, nerve ending; electrical impulse.
• Synapse — chemical neurotransmitters cross gap between neurons.
• Central nervous system — brain and spinal cord.
• Reflex arc — receptor → sensory neuron → spinal cord → motor neuron → effector.
• Brain parts — cerebrum (thinking), cerebellum (balance), medulla (involuntary).

1. Neuron structure — impulse travels dendrite → axon → synapse.
2. Reflex action — fast, involuntary; bypasses brain initially (hand withdrawal from flame).
3. Reflex arc components — receptor, sensory nerve, relay neuron, motor nerve, effector (muscle/gland).
4. Cerebrum — voluntary actions, memory, reasoning; largest part.
5. Cerebellum — posture, balance, precision of movement.
6. Medulla oblongata — breathing, heartbeat, swallowing (involuntary).
7. Spinal cord — reflex centre; link between brain and body.
8. Protection — skull (brain), vertebral column (spinal cord).
9. Peripheral nerves — cranial and spinal nerves carry impulses.
10. NCERT activity — knee-jerk or hand withdrawal illustrates reflex.

Reflex arc when touching hot object

Step 1 — Thermoreceptor in skin detects heat (stimulus).
Step 2 — Sensory neuron carries impulse to spinal cord.
Step 3 — Relay neuron in spinal cord connects to motor neuron.
Step 4 — Motor neuron to arm muscle → contraction → hand withdraws.
Step 5 — Brain informed afterward — conscious pain felt.
Conclusion: reflex arc enables rapid protective response.

• Saying brain initiates all reflexes (spinal cord mediates many).
• Misconception: electrical signal crosses synapse directly (uses chemicals).
• Confusing cerebellum with cerebrum functions.
• Forgetting effector can be gland (salivation reflex).
• Drawing impulse from axon to dendrite within same neuron (dendrite receives).

• Draw and label parts of a neuron.
• What is a reflex arc?
• Which brain part controls balance?
• Difference between voluntary and involuntary actions?

Open the Practice tab for graded questions on Nervous System.