[THE EXCITATION PATTERN OF SINGLE NEURONS OF THE ABDUCCENT NERVE NUCLEUS IN RABBITS].

Extracellular microelectrede studies were made on 47 units of the abducens nucleus in rabbits. In normal head position, the abducens units showed great differences in their discharged impulses only when the external rectus muscle was more strongly stimulated. Others showed a discharge of up to 60/sec, even when the eye was in normal position. Accordingly the muscles moving the eye were under continuous tonic innervation. Maximal frequencies observed were over 300/sec. There were further characteristic features in vestibular nystagmus: In contradistinction to the neurones of the reticular formation, the discharge frequency was regular. During the slow ipsilateral phase there was a crescendo-like increase in frequency. The ensuing, almost linear, contraction of the muscle was accompanied by a linear increase in discharge frequency of the motoneurone during isotonic contraction. Additionally, an afterrecruitment of new units could be observed in which the sequence of activated neurones was strictly maintained. During the slow contralateral phase of nystagmus, the neurones which did not discharge spontaneously remained silent; the others showed a decrescendo-like discharge pattern. In other words, there was not a complete relaxation of the muscle. During the rapid ipsilateral phase both types of neurones were activated, but, in contrast to the slow phase, the maximal frequency was found at the beginning. The corresponding movement of the bulb in the opposite direction, however, was linked with a cessation of neuronal discharges. The reciprocal innervation of eye muscles was apparent even at microphysiological level. The discharge frequency was also tested during deviations from the normal position, i. e. with the head turned to each side, forwards, and backwards. Increase and decrease in the discharge frequency occured during antagonistic movements. In abnormal positions the rotation activity also was tested. Under such conditions, the tonic and phasic predilection types observed here were not definitely fixed but could be changed from one frequence pattern to the other variations of its central or peripheral drives.