Electrophysiological properties of trochlear motoneurons as revealed by IVth nerve stimulation.

Abstract

Stimulation of the IVth cranial nerve evokes a triphasic field potential in the trochlear nucleus (TN) (sharp, positive-negative field succeeded by a slow positivity). This field potential was interpreted in terms of successive compartmental activation of single trochlear motoneurons (TMns) and, subsequently, it was utilized to study motoneuronal organization within the TN. Laminar field potential profiles indicate the TN to be spherical (about 1 mm in diameter) and to contain motoneurons of varying size which show, predominantly, a dorsal to ventral axo-soma-dendritic orientation. Smaller motoneurons were located in the dorso-medial part of the TN. Intracellular recordings from the TMns demonstrated an absence of ortho- and antidromically evoked synaptic potentials following stimulation of the IVth nerve or the opthalmic division of the Vth nerve. On the other hand, each somadendritic spike was followed by an after-potential sequence of “depolarization-hyperpolarization” which could produce marked excitability changes in the TN (as determined by stimulation of the IVth nerve at different condition-test intervals). Similar excitability changes (but of different magnitude) were found in the abducens nucleus (ABN). The origin of this after-potential sequence was examined in relationship to spike propagation throughout the soma-dendritic compartments. The effects of polarizing current on the antidromic spike, membrane resistance and rectification, firing frequency and depolarization-hyperpolarization sequence was also studied. Physiological implications of these findings are discussed in terms of motoneuronal response within the oculomotor system.