ISSN:
1432-1106
Keywords:
Ballistic contraction
;
Gating
;
H reflex
;
Muscle afferents
;
Reaction time
;
Human
Source:
Springer Online Journal Archives 1860-2000
Topics:
Medicine
Notes:
Summary The monosynaptic reflex (H reflex) is facilitated before movement onset in human subjects who are performing a conditioned plantar flexion of the ankle in a reaction time task. The aim of this study was to investigate how tightly this gating of Ia spindle input is coupled with the conditioned muscle contraction. Test H reflexes were elicited at various times during the reaction time (RT) in order to test the efficacy of Ia volleys on the soleus motoneurons. Tactile, auditory and visual go stimuli were used. The RT to a tactile stimulus was about the same as the RT to an auditory stimulus although distance and therefore conduction time from the site of stimulation to the cerebral cortex was much larger for the tactile than for the auditory modality. The RT to visual stimulation was about 20 ms longer than to the other two modalities. Although central latencies depended clearly on the stimulus modality the duration of the H reflex facilitation, i.e. the interval between the onset of the facilitation and the onset of the voluntary muscle contraction, was always the same. Similarly, the reflex facilitation was insensitive to the succeeding contrast of a visual go stimulus. The subjects were also examined in visual RT tasks in which different advance information about the laterality and the execution of the contraction was given. By combination the following four RT situations were realized: (1) simple, go, (2) choice, go, (3) simple, go — no go and (4) choice, go — no go. RT was shortest in the simple go and increased by about 65 ms in the choice, go and the simple, go — no go situation. It lengthened, however, less than 130 ms in the choice, go — no go situation indicating an interaction between the factors laterality and execution of the contraction. As with various stimulus modalities and succeeding contrasts, the premovement H reflex facilitation remained constant irrespective of the complexity of the RT task. This property was however not observed if data obtained within one RT task (constant modality, simple, go) were processed. There was a highly significant positive correlation between the duration of the facilitation and RT. The duration of the EMG burst of the conditioned contraction was, however, independent of RT. These results were interpreted as indicating that RT depended on attention which affects most central components of RT and that the interval between the onset of the gating process and the onset of the contraction (duration of the H reflex facilitation) might depend on several supraspinal motor centers whereas the duration of the EMG burst might be computed locally. Several studies including the present one provide a possible explanation for the lag between the onset of the H reflex facilitation and the movement onset. The relatively slow speed at which presynaptic inhibition at Ia afferents can be removed necessitates an early onset such that a fully operative spinal reflex for assistance of muscle contractions and correction of perturbances is assured at movement onset.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00230000
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