Grouped spindle and electromyographic responses to abrupt wrist extension movements in man

Abstract

SUMMARY1. Different techniques were used to generate sudden ramp extension movements of the wrist while the subjects were either relaxed or maintaining a weak voluntary contraction in the wrist flexors. Afferent responses to the displacements were recorded with a tungsten micro-electrode inserted into a fascicle of the median nerve supplying one of the wrist flexor muscles, and e.m.g. responses were recorded with needle electrodes inserted into the same muscle.2. With the wrist flexors either relaxed or contracting, extensions at 100-200'/sec for 60-70 msec (generated by either an hydraulic motor or a torque motor) produced segmented afferent responses with two to four afferent bursts, separated by intervals of 20-30 msec. The successive neural peaks, occuring during the stretch phase, were correlated to mechanical vibrations sensed by a strain gauge and sometimes also by a wrist goniometer. With the flexor muscles contracting, the successive peaks in the neurogram were followed by similar peaks in the e.m.g, the delay between neural and e.m.g. peaks being 20-25 msec.3. Small abrupt extension movements of 1-2°lasting only 10-15 msec often produced segmented afferent responses with one neural burst occuring during the stretch phase and another 15-20 msec later, corresponding to a mechanical oscillatory event succeeding the stretch. The oscillation and the second neural burst were not present with small extension movements of smooth onset and halt. With the flexor muscles contracting, stimuli producing one afferent burst produced only one e.m.g. peak, whereas double-peaked afferent discharges produced double-peaked e.m.g. responses, the delay between individual neural e.m.g. peaks being 20-25 msec.4. Similar segmentation of the neural stretch responses was seen when abrupt displacements were produced by electrically induced muscle twitches, by manual pulls on a spring attached to the hand or by the subject making fast voluntary wrist extensions. This grouping of afferent discharges was seen in both multi-unit and in single-unit recordings from fibres identified as group Ia afferents. 6. For imposed movements with a duration of 60-70 msec the successive e.m.g. peaks caused a fused reflex contraction, appearing as a torque trace deflexion, starting at about the time when the movement ended and reaching its peak within about 40 msec. With longer-lasting movements the mechanical reflex response accompanying the successive e.m.g. bursts, appeared as a decelerative force, starting to oppose the ongoing movement about 60 msec after its start. Mechanical consequences of stretch reflex contractions starting after, rather than during, the stretch movement are discussed.

Tendon organ firing during active muscle lengthening in awake, normally behaving cats.

“…bursts in response to rapid muscle stretch in man (Hagbarth et al 1981), in the spinal cat (Tracey et al 1980) and in the awake cat (Prochazka & Wand, 1981) follow at reflex latency the bursts of Ia discharge. It is tempting to conclude that the e.m.g.…”

Effects of prior instruction and anaesthesia on long‐latency responses to stretch in the long flexor of the human thumb.

“…The mechanism underlying this latter component has interested many: it has been said that it might represent a 'long-loop', C. K. C. LOO AND D. I. McCLOSKEY possibly transcortical (Phillips, 1969) reflex, but other evidence has indicated that it may be based upon the discharges of group II spindle afferents (Matthews, 1984a, b). The possibility of such responses being based on a partitioning of bursts of spindle afferent input has also been raised (Hagbarth, Hagglund, Wallin & Young, 1981). It was not our intention in the present study to attempt to resolve these matters.…”

Long loop and spinal reflexes in man during preparation for intended directional hand movements

“…This result was claimed again by Tracey et al (1980) in the spinal monkey without any clear experimental evidence, whereas North and Tatton (1980) have emphasized the differences observed between species: "In the cat, motor cortical neuron (MCN) activity does not contribute to the generation of the 2nd EMG response peak, in support of the transection studies and in contrast to MCN activity related to the M2 response for monkey distal upper limb musculature." Lastly, from their recent studies on man, Hagbarth et al (1981) have claimed that the segmented EMG responses are almost solely due to segmented afferent burst rather than multiple central reflex pathways.…”