In a classical monosynaptic knee-jerk (patellar tendon) reflex arc, what is the correct anatomical pathway traversed by the nerve impulse?

Answer and explanation

Correct answer: D. Spindle receptor -> Afferent neuron -> Spinal cord -> Efferent neuron -> Quadriceps

  1. Step 1: Tapping the patellar tendon stretches the quadriceps muscle, activating stretch receptors called muscle spindles.
  2. Step 2: The sensory impulse travels along an afferent sensory neuron through the dorsal root ganglion into the dorsal horn of the spinal cord.
  3. Step 3: In this monosynaptic reflex, the sensory neuron synapses directly with an alpha-motor neuron in the ventral horn without an intervening interneuron.
  4. Step 4: The motor neuron transmits an action potential via the ventral root to the quadriceps muscle, causing it to contract and kick the leg forward.
Answer
Muscle spindle receptor -> Afferent sensory neuron -> Dorsal root of spinal cord -> Motor neuron in ventral horn -> Effector quadriceps muscle
Why not the others: 'Golgi tendon organ -> Efferent motor neuron...' reverses the pathway direction and involves the auditory center; 'Skin nociceptor...' describes a polysynaptic withdrawal reflex to painful stimuli, not the tendon stretch reflex; 'Motor endplate -> Interneuron...' reverses normal reflex polarity.

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