Abstract
I. In the decerebrate (inter-collicular transection) preparation (cat) the reflex contraction of the knee extensor muscle evoked by passive stretch of that muscle (8) exhibits a tonic persistence suggesting likelihood of labyrinthine influence. We have, therefore, examined the reflex subsequent to severance of both nervi octavi. Tonic character is then still retained by the reflex. Co-operation of the labyrinths is, therefore, not an essential condition for the reflex or for its tonic persistence. But in the light of the observations of Magnus and de Kleyn (10), of Magnus and Wolf (11), and of Beritofl (2) in the Utrecht laboratory, we have examined change of space-position of the otolith-organs and of neck-posture as possibly affecting the myotatic reflex. In doing so our examination, which has been conducted in the way described in our previous paper (8), has been hampered by the necessity of keeping the pelvis rigidly fixed to the fall-table in a nearly supine position and vertically under the optical myograph. Starting with the animal fully on its left side, except that its pelvis was nearly supine, the stretch reflex of right knee-extensor was recorded for a given table-fall. Then the position of the animal’s head was changed to the “minimal” (10) otolith position, a change involving neck posture, namely, by rotation of the neck anti-clockwise (as viewed from in front) round its long axis. The extensor’s stretch reflex was then again recorded; the reflex was found to retain its general form and character but to develop less tension (isometric record) than in the reflex with the animal laid in the ordinary left-lateral position. Conversely, when the head had been placed in the “maximal” (10) otolith position, the neck having been rotated clockwise, the myotatic reflex, still retaining its usual general form and character, developed more tension than when the animal lay in the initial ordinary left lateral position. This result is in harmony with the Utrecht observations, and might indeed be predicted from them. The knee-extensor stretch reflex, like the extensor limb-reflexes examined, therefore, is influenced by the Magnus-de Kleyn tonic reflexes. Our experiments have not discriminated between the labyrinthine and cervical influences. II. The amount of tension developed by the quadriceps cruris in its myotatic reflex varies, under otherwise like conditions, directly with the amplitude of the passive stretch evoking it (8). But the tension developed does not in our experience, even with considerable amplitudes of passive stretch, equal that which can be developed by the muscle when activated by stimulation of its motor nerve. The inference is that the myotatic reflex even when maximal does not bring into play at any one time the entirety of the fibres of the muscle.