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University Laboratory of Physiology, Oxford.

1. Electromyography (surface and intramuscular) has been used to study the reflex responses of the human flexor digitorum profundus (FDP) to angular rotation of the distal interphalangeal joint of the 4th finger. This has been done with the hand in three separate positions which, owing to the arrangement of the various tendons, allow the movement to be transmitted to (a) both the flexor and extensor muscles, (b) FDP alone (extensors disengaged) and (c) neither flexor nor extensor muscles (all muscles disengaged, but cutaneous and joint receptors still potentially activated). The stimuli were applied while the subject was voluntarily contracting FDP to produce a constant level of EMG activity; this remained possible when the muscle was disengaged from the joint. 2. With all muscles connected, FDP behaved similarly to the analogous long flexor of the thumb. 'Stretch' elicited a prolonged complex response starting with a short-latency component corresponding to the tendon jerk. Unloading of the contracting muscle caused a pronounced reduction of its on-going EMG activity. The latency of this latter effect was approximately 20 ms greater than that of the initial stretch-evoked response, thereby demonstrating that it was not due to a disfacilitation via the short-latency pathway (on reduction of the tonic spindle afferent firing from FDP as it shortened). 3. With all muscles disengaged, movement of the joint in either direction evoked simply a weak, variable excitatory response, with a latency somewhat greater than that of the normal unloading response. This was attributed to the activation of cutaneous and/or joint receptors. The effectiveness of the disengagement of the flexor was demonstrated by the abolition of its normal stretch-evoked short-latency response. 4. With the flexor engaged and the extensors disengaged both stretch and release evoked their normal types of response. In control experiments, surface EMG recordings from the interosseus muscles confirmed that the procedure used for extensor disconnection was effective. These findings exclude the possibility that the reduction of EMG activity of the unloading response of FDP might be attributable to an inhibition evoked by the concomitant stretch of its antagonists. 5. The long-latency unloading response (whether with the extensors engaged or disengaged) remained when the sensory receptors in the finger itself were inactivated, confirming that these were not responsible.(ABSTRACT TRUNCATED AT 400 WORDS)

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