Unitary activity of vestibular and oculomotor neurons in the course of nystagmus

Abstract

Extracellular recordings of single cells around the abducens nucleus show that certain cells discharge rythmically during nystagmus. The period of their firing and the period of nystagmus are the same but are slightly out of phase. From this, we assume that interneurones also play a part in producing nystagmic discharges. 1. By antidromic stimulation, it was possible to identify ocular motoneurones (single shock antidromic stimulation) and interneurones of motor reverberating circuits (repetitive antidromic stimulation). 2. Interneurones of the vestibulo-ocular circuit are triggered by stimulation of the ampullar nerve from the horizontal semi-circular canal. The stimulation can produce either a phasic response (single shock) or a nystagmus (repetitive stimulation). 3. Oculomotor nystagmic cells may be discribed by comparing the latencies of their responses and of vestibulo-ocular reflex and also by comparing their firing pattern in relation to the nystagmic discharges. 4. Only one group of interneurones is exclusively concerned with nystagmic activity. They are never activated by antidromic stimulation or by single shock ampullar nerve stimulation. Their rythmic firing during nystagmus is in opposite phase to the motor discharges. They are found in the dorso-medial area of the medial vestibular nucleus. 5. The other groups of interneurones are activated in vestibulo-ocular reflex responses and produces rythmic firing synchronised with the nystagmic discharges. 6. We suggest that those interneurones which are exclusively concerned with nystagmus have an inhibitory function. The other nystagmic interneurones would then have a re-excitatory function. Their opposing functions could explain the periodicity of the nystagmic discharges and also their progressive increase in amplitude by invoking the action of short closed loops of interneurones.