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

Abstract

Advance information about the direction of a movement to be performed in a reaction time (RT) paradigm is known (I) to improve performance, (2) not to result in any related change in monosynaptic spinal reflexes triggered at the time of the response signal, and (3) to trigger related changes in the activity of a number of neurons in the motor cortex.The aim of this experiment was to compare the evolution of the early (Ml) and late (M2) responses to a muscular overload during the foreperiod of an RT task, according to advance information about movement direction.Four subjects were trained to perform wrist extension and flexion movements in order to point to a visual target on either the right or left of a display panel. One second before the movement signal, a warning signal indicated to the subject at which target he was to point. A hand position perturbation causing a stretch of the wrist flexors was triggered at various unpredictable times during this 1-set foreperiod.The analysis of the surface electromyogram recorded upon wrist flexors showed that the evolution of Ml and M2 response amplitudes differed during the foreperiod. Ml increased up to 600 msec after the warning and then decreased until the response signal, while M2 continued to increase during the whole foreperiod. There was no change, however, in this differential evolution in relation to advance information about movement direction.The results were interpreted as the combination of two preparatory processes: first, a general activation, expressed by M2 increase, which can be viewed as a pre-setting of motor structures for a rapid execution of the motor program, and second, an inhibition of spindle afferent action upon spinal motor structures, expressed by the decrease of Ml response before movement execution, which can be viewed as a mechanism for protecting the motoneurons about to be activated from peripheral control pathways. Lastly, no specific presetting of the motor structures involved in Ml and M2 electromyographic responses according to the movement direction was found.An examination of spinal excitability changes in human subjects by soliciting the monosynaptic reflex either by electrical stimulation of the nerve or by tendinous tap has shown that, during the foreperiod of reaction time (RT) tasks, there is a selective modification in the excitability of the motoneurons controlling the muscle involved in the motor response (Requin, 1969;Gerilovski and Tsekov, 1971;Requin and Paillard, 1971). However, this pre-setting was expressed in a relative depression of reflex pathway reactivity, the depth of which showed no change whether the muscle was acting as agonist or ' To whom correspondence should be addressed

Influence of long-latency reflex modulation on the performance of quick adjustment movements

“…According to Student's paired t-test, there was no signi®cant dierence in latency between the non-action and the target-alignment tasks [M1, t À0X07, df 25, not signi®cant (NS); M2, t À0X98, df 25, NS]. The latencies of these re¯ex components are similar to values reported in previous studies that examined the re¯ex activities of forearm muscles (Lee and Tatton 1975;Bonnet and Requin 1982;Abbruzzese et al 1985).…”

“…The gain of these re¯ex activities, particularly the long-latency components, has been reported to be modi®ed by anticipation of a stimulus and by motor preparation for the direction and type of movement (Nashner 1976;Rothwell et al 1980;Hallett et al 1981;Bonnet and Requin 1982;Al-Falahe and Vallbo 1988;Gielen et al 1988;Yamamoto and Ohtsuki 1989;Johnson et al 1991;Doemges and Rack 1992a,b;Goodin and Amino 1992). Furthermore, some studies have reported that the degree of change in long-latency activity was more remarkable for the subjects with short reaction times than for those with long reaction times, and that the long-latency component was modi®ed by motor preparation for the reaction movement (Bonnet 1983;Woollacott et al 1984).…”

Cognitive tuning of corticospinal excitability during human gait: adaptation to the phase

“…Another important point to note is that, in both conditions, the change in CS excitability involved the muscle that was stretched when a central perturbation occurred during the NINT trials. In fact, muscle stretch (mechanically induced by a rapid joint angular displacement) is well known to trigger a set of EMG responses composed of short‐ and long‐latency components (Rothwell et al ., 1980; Bonnet & Requin, 1982). Using the same instructions to ‘prepare to resist’ vs. ‘to prepare to let go’ when facing such a rapid joint angular displacement, it was shown several times that the long‐latency stretch response was modulated by the type of instruction (Rothwell et al ., 1980; Bonnet & Requin, 1982).…”

Mental imagery in the motor context

“…We know that during preparation inhibition occurs at the segmental spinal level: hence the decrease of spinal reflexes during the preparatory period and their reincrease shortly before the movement starts [6]. The pattern of spinal excitability during motor simulation, with a marked increase in T-reflex, is thus closer to that of real action than that of preparation.…”