H-REFLEXES

• first described and named after Hoffmann
• similar to tendon reflexes except elicited electrically
• electrophysiological test to quantify muscle spasticity
• tibial nerve stimulated behind knee – response recorded from triceps surae
• low intensity stimulation – reflex potential recorded at latency of 30msec – monosynaptic reflex – from I A spindle afferents to motor neurons
• increased intensity – M response – latency of 5msec – simulation of motor neurons with higher threshold than IA afferents – from knee to calf directly
• still intensity increase – plateau appears – maximum amplitude – H-reflexes abolished
• also recorded from FCR by median nerve stimulation and quadriceps femoris by femoral nerve stimulation
• tibial H-reflexes – used to measure stretch reflex excitability
• H/M ratio – maximum H-reflex amplitude to maximum M response amplitude – higher is spastic patients with hyper active tendon jerks
• - vibration inhibits H-reflexes – due to presynaptic inhibition – spastic patients have less vibratory inhibition than others
• = H-reflex recovery curve – in spastic patients inhibition by conditioning reflex is of short duration than others
• H- reflex used to identify different mode of action of different spasmolytic medications- to identify optimal muscle relaxant for a given patient with spasticity

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