Science — CBSE Class 10 board question

(i) Reflex Arc:
A reflex arc is the pathway taken by a nerve impulse during a reflex action — from receptor to effector without involving the brain. It is formed in the spinal cord itself.

Why evolved: Reflex arcs evolved because the brain's thinking process is too slow for urgent situations. Even animals with little or no complex neuron networks need quick responses to danger.

Sequence of events when touching a hot object:

1. Heat receptors in the skin (dendrite tips) detect the stimulus.
2. An electrical impulse travels along the sensory neuron to the spinal cord.
3. The spinal cord (reflex arc) relays the impulse via a relay neuron to the motor neuron.
4. The motor neuron carries the impulse to the muscle (effector).
5. The muscle contracts and the hand is pulled away. Simultaneously, the information also reaches the brain (we feel pain).

(ii) The Peripheral Nervous System (PNS) facilitates communication between the central nervous system and other parts of the body.

Two components:

• Cranial nerves — arising from the brain
• Spinal nerves — arising from the spinal cord

Source: Chapter 6, Sections 6.1, 6.1.1, 6.1.2

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• Part (i) carries ~3 marks: definition (1) + reason for evolution (1) + sequence (1). List the sequence as numbered steps — examiners follow them point by point.
• Part (ii) carries ~2 marks: name the PNS (1) + both components (1). Don't just say "nerves"; specify cranial and spinal nerves.
• The key phrase for reflex arc definition is "pathway of nerve impulse from receptor to effector through spinal cord, bypassing the brain."
• Always mention that the information also reaches the brain — this is a common detail students miss.