Tapping the patellar tendon with a reflex hammer produces a brief contraction of the knee extensors. What is the cause of the muscle contraction?
**Core Concept**
The patellar tendon tap elicits a stretch reflex, a type of monosynaptic reflex arc that involves the afferent input from muscle spindle receptors, processing in the spinal cord, and efferent output to the muscle. This reflex helps maintain muscle tone and posture.
**Why the Correct Answer is Right**
When the patellar tendon is tapped, it stretches the quadriceps muscle, activating the muscle spindle receptors. These receptors send a signal through the afferent neurons to the spinal cord, where it is processed in the reflex arc. The processed signal is then transmitted through the efferent neurons to the quadriceps muscle, causing a brief contraction. This contraction is a result of the activation of the alpha motor neurons, which innervate the muscle fibers.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the patellar tendon tap does not directly stimulate the alpha motor neurons; instead, it activates the muscle spindle receptors.
**Option B:** This option is incorrect because the stretch reflex is a monosynaptic reflex, not a polysynaptic reflex, which involves multiple synapses in the reflex arc.
**Option C:** This option is incorrect because the patellar tendon tap does not stimulate the Golgi tendon organs, which are involved in the proprioceptive feedback but not in the stretch reflex.
**Clinical Pearl / High-Yield Fact**
The stretch reflex can be used clinically to assess the integrity of the reflex arc, particularly in the context of neurological examination. An absent or diminished reflex may indicate a lesion in the afferent or efferent pathway, such as a peripheral nerve injury or spinal cord damage.
**Correct Answer:** C. Activation of muscle spindle receptors.