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Difficulty: MediumNervous Coordination and Sense Organs

When an individual accidentally steps on a sharp pin and instantly withdraws their foot, through which path does the nerve impulse travel to produce this involuntary reflex response?

  1. Pain receptor → sensory neuron → relay neuron in spinal cord → motor neuron → leg muscle effectorAnswer
  2. B
    Pain receptor → motor neuron → relay neuron in spinal cord → sensory neuron → leg muscle effector
  3. C
    Leg muscle effector → motor neuron → relay neuron in spinal cord �� sensory neuron → pain receptor
  4. D
    Pain receptor → relay neuron in brain → motor neuron → sensory neuron → leg muscle effector

Answer

From pain receptors, along a sensory neuron to a relay neuron in the spinal cord, then via a motor neuron to the leg muscle effector.
The correct response accurately details the unidirectional flow of an electrical nerve impulse in a reflex arc: from the sensory receptor (pain receptor in skin), along the afferent (sensory) neuron, through the synaptic interneuron (relay neuron) within the spinal cord, and out along the efferent (motor) neuron to the effector (leg muscle).

Step-by-Step Solution

1
Identify the primary stimulus and receptor
The sharp pin stimulates cutaneous pain receptors in the foot.
Sensory receptors detect environmental stimuli and convert them into action potentials.
2
Trace afferent signal conduction to the central nervous system
Impulses travel along the afferent (sensory) neuron into the grey matter of the spinal cord.
Sensory neurons conduct impulses from peripheral receptors toward the central nervous system.
3
Trace synaptic relay and efferent output to effector
The impulse passes across synapses via an interneuron (relay neuron) to an efferent (motor) neuron, which stimulates contraction in the leg muscle.
Spinal interneurons integrate the response without brain delay, and motor neurons carry signals out to the effector muscle.

Key Concept

Spinal Reflex Arc Pathway
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Topics

Nervous Coordination and Sense Organs

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