Cat medial pontine reticular neurons related to vestibular nystagmus: firing pattern, location and projection.

In alert cats, extracellular spikes of neurons in the medial pontine tegmentum were recorded simultaneously with whole nerve discharges of the abducens and medial rectus nerves during horizontal vestibular nystagmus. Nystagmus-related neurons were classified by their firing patterns in relation to the abrupt cessation of the slow phase nerve activity of abducens or medial rectus nerves. The ipsilateral abducens nucleus was electrically stimulated to examine the axonal projections of physiologically identified examples of each category of neurons. Anatomically, pause units clustered near the midline at the rostral pole of the abducens nucleus. Long- and medium-lead burst units were 1-4 mm rostral to the area for pause units. Most burst-tonic units, clearly distinguished from nearby axons of passage, were found close to the MLF. Physiologically, it was concluded that: (1) some long-lead burst units terminate in the abducens nucleus and may excite motoneurons and/or internuclear neurons; (2) pause units directly inhibit burst inhibitory neurons which terminate slow phase activities of contralateral abducens motoneurons; (3) burst-tonic units fire in a manner very similar to contralateral abducens motoneurons; and (4) some medium-lead burst, long-lead burst and burst-tonic neurons (but not pause neurons) project to the cerebellar flocculus.