The Hoffman reflex (H-reflex) is the electrophysiological analog of the monosynaptic stretch reflex. It is elicited by:

• A Direct electrical stimulation of motor nerve fibers (A-alpha) bypassing the muscle spindle
• B Surface EMG recording of spontaneous motor unit firing during muscle relaxation
• C Repetitive transcranial magnetic stimulation activating the motor cortex to produce descending motor volleys
• D Electrical stimulation of the sensory (Ia afferent) fibers, which synapse onto alpha motor neurons producing a reflex motor response — equivalent to tapping the patellar tendon but bypassing the mechanical receptor ✓

Explanation

The H-reflex is obtained by electrically stimulating the Ia sensory fibers in a peripheral nerve (e.g., tibial nerve in the popliteal fossa) with a submaximal stimulus that preferentially activates large-diameter Ia fibers. The impulse travels antidromically to the spinal cord and synaptically activates alpha motor neurons (monosynaptic reflex arc), producing a muscle response (H-wave) with a longer latency than the direct M-wave. It is the electrophysiological equivalent of the ankle jerk. The H-reflex is absent or reduced in S1 radiculopathy, is exaggerated in upper motor neuron lesions, and is used to measure central conduction time and assess the integrity of the monosynaptic reflex arc.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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