1. Presynaptic inhibition of muscle spindle Ia afferents by afferents from the same and other muscles has been studied in the human lower limb. The experiments have utilized conditioning of test monosynaptic reflexes by vibration of both the test and other muscles. 2. The pattern of inhibition invariably includes autogenetic actions. 3. There are powerful effects from flexor to extensor Ia afferents. Actions from flexor to flexor, and from extensor to extensor, are weaker. Actions from extensors to flexors are very weak. 4. The strength of presynaptic inhibition from one muscle type to another weakens as the muscles considered become more anatomically distant. 5. The inhibition studied both by vibration and by electrical conditioning stimulation of nerves becomes weaker during voluntary isometric contraction of the test muscle. It is strongest at rest and during antagonist contraction. 6. Evidence is provided suggesting that descending control is the primary cause of this modulation of inhibition during contraction. 7. Stimulation of afferents in cutaneous nerves reduces group I presynaptic inhibition of Ia afferents.
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