Question

Observe the given figure and identify the labelled parts P, Q, R and S.

Hint:

Reflex arc pathway: Receptor → Sensory neuron → Spinal cord → Motor neuron → Effector.

Answer 1

Identification of Labelled Parts P, Q, R and S:

The given diagram represents a reflex arc, which shows the pathway of nerve impulses during a reflex action (such as withdrawing the hand from a hot object).

Label P:
\[ \boxed{\text{Receptor (in the skin)}} \] This part detects the stimulus (heat from the hot vessel). The pain receptors in the skin sense the heat and initiate the nerve impulse.

Label Q:
\[ \boxed{\text{Sensory neuron (afferent neuron)}} \] This neuron carries the nerve impulse from the receptor (skin) to the spinal cord.

Label R:
\[ \boxed{\text{Relay neuron (interneuron) in spinal cord}} \] This neuron is present inside the spinal cord and transfers the impulse from the sensory neuron to the motor neuron.

Label S:
\[ \boxed{\text{Motor neuron (efferent neuron)}} \] This neuron carries the impulse from the spinal cord to the effector (muscles of the hand), causing the hand to withdraw.

Conclusion:
The diagram shows a reflex arc where the receptor (P) detects the stimulus, the sensory neuron (Q) carries the message to the spinal cord, the relay neuron (R) processes it, and the motor neuron (S) sends the response to the muscles.