Tapping the patellar tendon stretches the quadriceps, and the leg extends. The afferent limb of this reflex consists of which fibers, and how many synapses lie in its spinal pathway?

• A Ib afferents from the Golgi tendon organ, disynaptic
• B Group II afferents from secondary spindle endings, polysynaptic
• C Ia afferents from primary spindle endings, monosynaptic ✓
• D Gamma efferents to intrafusal fibers, disynaptic

Explanation

The deep tendon (stretch) reflex uses Ia annulospiral afferents arising from the primary endings wrapped around the central region of nuclear bag and chain intrafusal fibers. These large, rapidly conducting fibers enter the dorsal root and synapse directly on the homonymous alpha motor neuron, making it the classic monosynaptic reflex. Ib afferents mediate the inverse stretch reflex from Golgi tendon organs, group II fibers signal static position, and gamma efferents are motor, not sensory, so none fits.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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