SYNAPTIC PHENOMENA OF NYSTAGMUS

In a previous work (Horcholle and Tyč-Dumont 1968), sets of nystagmic interneurones have been described in the region of the abducens nucleus of the cat. One set - the inhibitory nystagmic interneurones (INI) -is triggered by nystagmogenic stimulation and its firing during nystagmus is in opposite phase to the motor nystagmic discharges. In this study, intracellular recordings from ocular motoneurones and from INIs have been obtained during nystagmus to analyse the synaptic mechanisms of the periodic activity. Motoneurones are identified by antidromic invasion following single shock stimulation of the abducens nerve. Their intracellular record during nystagmus has shown a periodic depolarization of the membrane which is synchronous with the motor nystagmic discharges. Between the phases of depolarization, the motoneurone is not excitable by orthodromic inputs of vestibular origin although it can be triggered by antidromic invasion. A reduction in amplitude of the evoked EPSPs, which is synchronous with the silent period of the motoneurones, could suggest a presynaptic inhibitory mechanism. INIs are identified by their rhythmic firing during nystagmus which is in opposite phase to the nystagmic discharges recorded simultaneously from the abducens nerve. Intracellular records from INIs have shown a periodic hyperpolarization of their membrane which is synchronous with the nystagmic motor discharges. INIs phase of activity coincides with the silent period of the abducens nerve and is synchronous with the inhibitory phase of the inputs of vestibular origin. A functional relation between the motoneurones and the INIs is highly probable and is considered to play a role in the elaboration of the rhythmic activity of nystagmus. © 1969 Springer-Verlag.