Abstract
Microelectrodes were inserted into the third cranial nucleus of anesthetized rabbits. The third nucleus was identified by observing the field potentials evoked antidromically upon stimulation of the third cranial nerve trunk or its branch to the inferior oblique muscle. The antidromic field potentials consisted of two large negativities, i. e., an early and a late negative field potential. Extracellular recording from the third nucleus motoneurons revealed two distinct types of unit firing patterns; type I and type II. Type I units were characterized by relatively regular discharges whereas type II units showed burst discharges. When stimulated antidromically, type I units initiated an action potential with a short latency. Type II units responded with a long latency to an antidromic stimulation. Calorically induced nystagmus caused corresponding responses from type I units and their firing patterns could be classified into two groups, phasic and tonic. Type II units showed no relationship to the caloric nystagmus. Histologically it was found that the type I units were localized chiefly in the central portion of the nucleus, whereas most of the type II units were found in the periphery of the nucleus. None of these units responded to photic stimulation. It was considered that the type I and type II units would subserve fast and slow fibers, respectively, in the extraocular muscles of the rabbit.
