Tonic cervical influences on forelimb and hindlimb monosynaptic reflexes

The influence of tonic neck reflexes on voluntary fatiguing elbow movements in humans

Tonic neck reflexes (TNR) are often assumed to be included in the building of voluntary motor programmes. The present study was designed to test the importance of TNR in the performance by healthy human adults of high level dynamic exercise. The subjects were placed in an experimental situation similar to the original one used by previous authors, but in a fixed restrained posture. They repeatedly lifted weights by extending the elbow, with the head rotated either towards the side of the active upper limb or towards the opposite side. The rate of movement was 0.5 Hz, and the initial amplitude 60 degrees. Successive series of 15 movements separated by an equivalent period of rest were made by each subject until exhaustion. The mechanical work performed during each series of movements was calculated. Surface electromyograms (EMG) from triceps brachii, anconeus, and biceps brachii muscles were recorded simultaneously with the elbow rotation displacement and then integrated. The EMG from the sterno-cleïdo-mastoidei muscles were also recorded. With successive series of lifting, movement amplitude decreased progressively as a result of fatigue. Thus there was a decrease in the work performed. In agreement with the experimental data of the previous authors as well as with the expected effect of TNR, the decrease in work was less marked when the subjects kept their heads turned towards their moving limb. However, in contrast with the results from other authors, the facilitation was very low. From these results, we concluded that TNR can slightly influence the amount of work that a subject can produce.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of proprioceptor stimulation on postural stability in patients with peripheral or central vestibular lesion

Body sway in upright stance at rest and after inducing proprioceptor stimulation, elicited by vibration applied to the calf or neck muscles, was studied in 11 patients with peripheral lesion and in 17 patients with central vestibular lesion. The responses were compared with those of 20 normal subjects. Vibratory stimulus was applied at five different frequencies, ranging from 32 to 150 Hz, and at a constant amplitude of 2.1 mm. Postural stability was measured with a force platform in terms of average deviation of body position (ADBP) analyzed in relation to the individual maximum support distance in the anterio-posterior direction. In patients with peripheral vestibular lesion ADBP was moderately increased, compared to normal subjects, when the calf muscles were exposed to vibration under eyes closed conditions (i.e. no visual information available); stimulation of neck muscles both under eyes open and eyes closed conditions and stimulation of calf muscles with open eyes produced an ADBP of the same magnitude as in controls. In patients with central vestibular lesion, proprioceptor stimulation of calf and neck muscles caused increased ADBP whether with eyes open or closed. The ADBP induced by stimulation of neck muscles was significantly greater in patients with a central lesion than in those with a peripheral vestibular lesion. The results indicate that patients with peripheral lesion differ from those with central vestibular lesion in their reaction to proprioceptor stimulus; and that in patients with central vestibular lesion proprioceptor stimulation of the neck muscles produces disproportionately powerful cervico-collic reflexes.

Vestibulo-ocular reflex (VOR), cervico-ocular reflex (COR) and its interaction in active head movements

In normal adults the vestibulo-ocular reflex (VOR) and the cervico-ocular reflex (COR) were investigated during passive and active head or body movements, respectively. Sinusoidal rotations around the vertical axis of the body at frequencies of 0.05, 0.1, and 0.2s-1 and total amplitudes of 20 degrees, 40 degrees, 60 degrees, or 80 degrees were employed. The average eye deviations (Schlagfeld) during VOR were directed opposite to the direction of the head turning. During COR, however, slow eye deviations of higher amplitude were anticompensatory relative to the fixed head. During active head turnings the average eye deviations showed the same anticompensatory direction as in COR, but were still larger. The increased with stimulus amplitudes up to 60 degrees. At least a weak cervical nystagmus was elicited in all subjects, with its fast phases beating in the direction of the relative head movement. Its gain reached marked values up to 0.5, but only for peak stimulus velocities below 25%. The nystagmus gain during active head turnings was only slightly higher than during VOR. With higher stimulus velocities, large anticompensatory saccades appeared just before the change of stimulus direction; these are typical for active head movements, but were also found during COR.