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Difficulty: MediumNervous Coordination and Sense Organs

During a routine medical examination, a physician taps the patellar tendon just below the knee cap, triggering a sudden involuntary forward movement of the leg. Which of the following sequences correctly traces the route of the nerve impulse produced during this knee-jerk reflex?

  1. Patellar receptor → Sensory neuron → Spinal cord → Motor neuron → Quadriceps muscleAnswer
  2. B
    Patellar receptor → Motor neuron → Spinal cord → Sensory neuron → Quadriceps muscle
  3. C
    Quadriceps muscle → Sensory neuron → Spinal cord → Motor neuron → Patellar receptor
  4. D
    Patellar receptor → Spinal cord → Motor neuron → Sensory neuron → Quadriceps muscle

Answer

Patellar receptor → Sensory neuron → Spinal cord → Motor neuron → Quadriceps muscle
The correct response accurately outlines the unidirectional flow of an electrical impulse in a simple spinal reflex arc: mechanical stimulation of the patellar stretch receptor generates an impulse carried by an afferent sensory neuron into the spinal cord, where it synapses directly with an efferent motor neuron that transmits the signal to the quadriceps muscle effector to cause contraction.

Step-by-Step Solution

1
Identify the receptor and stimulus point
Tapping the tendon stimulates stretch receptors in the patellar tendon/quadriceps muscle area.
Receptors are specialized structures that detect physical stimuli and initiate electrical impulses.
2
Trace the afferent nerve pathway to the central nervous system
The impulse travels along an afferent sensory neuron into the spinal cord.
Sensory neurons conduct impulses from peripheral receptors toward the central nervous system.
3
Trace the efferent nerve pathway to the responding organ
The impulse passes directly across a synapse in the spinal cord to an efferent motor neuron, which carries it to the quadriceps effector muscle.
Motor neurons conduct nerve impulses away from the spinal cord to effectors to produce a mechanical response.

Key Concept

Reflex Arc Neural Pathway
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Topics

Nervous Coordination and Sense Organs

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