Q1. [5] medium thorough-understanding
[long_answer] You accidentally prick your finger with a pin. Trace the complete path of the nervous impulse — from the moment the pain receptor in your finger detects the stimulus to the moment the muscles in your arm withdraw the hand. Name all the structures the impulse passes through in order, and explain how the signal crosses the gap between two consecutive neurons.
Generated by claude-sonnet-4-6 · 2026-06-26 01:00 · grounding rag
Model Answer
Path of the Nervous Impulse (Reflex Arc):
- Pain receptor (dendritic tip of sensory neuron) in the finger detects the pin-prick stimulus.
- A chemical reaction generates an electrical impulse that travels along the dendrite → cell body → axon of the sensory (afferent) neuron toward the spinal cord.
- The impulse reaches the spinal cord, where a reflex arc is formed. The sensory neuron connects to a relay neuron in the spinal cord.
- The relay neuron passes the impulse to the motor (efferent) neuron, which carries it out to the muscle cells of the arm.
- The muscles contract, withdrawing the hand. (The signal also travels to the brain, but the withdrawal happens before conscious perception.)
How the signal crosses the gap (synapse):
At the end of each axon, the electrical impulse triggers the release of chemicals (neurotransmitters). These chemicals cross the synapse (gap between two neurons) and generate a similar electrical impulse in the dendrite of the next neuron.
Source: Chapter 6, Sections 6.1 & 6.1.1
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Explanation
- Examiners expect the sequence: receptor → sensory neuron → spinal cord (reflex arc) → motor neuron → muscle. Missing any link costs marks.
- The term reflex arc is essential; mention that it is formed in the spinal cord.
- For the synapse, you must state both parts: electrical impulse causes chemical release, chemicals cross the gap and restart the electrical impulse — one part alone won't fetch full marks.
- "Impulse passes through dendrite → cell body → axon" must be stated explicitly as it is a direct textbook line.
- At 5 marks, examiners typically award 1 mark per correctly named step/structure and 1 mark for the synapse explanation.