Saccade-Related Inhibitory Input to Pontine Omnipause Neurons: An Intracellular Study in Alert Cats

Abstract

Omnipause neurons (OPNs) are midline pontine neurons that are thought to control a number of oculomotor behaviors, especially saccades. Intracellular recordings were made from OPNs in alert cats to elucidate saccade-associated postsynaptic events in OPNs and thereby determine what patterns of afferent discharge impinge on OPNs to cause their saccadic inhibition. The membrane potential of impaled OPNs exhibited steep hyperpolarization before each saccade that lasted for the whole period of the saccade. The hyperpolarization was reversed to depolarization by intracellular injection of Cl⁻ ions, indicating it consisted of temporal summation of inhibitory postsynaptic potentials (IPSPs). The duration of the saccade-related hyperpolarization was almost equal to the duration of the concurrent saccades. The time course of the hyperpolarization was similar to that of the radial eye velocity except for the initial phase. During the falling phase of eye velocity, the correlation between the instantaneous amplitude of hyperpolarization and the instantaneous eye velocity was highly significant. The amplitude of hyperpolarization at the eye velocity peak was correlated significantly with the peak eye velocity. The time integral of the hyperpolarization was correlated with the radial amplitude of saccades. The initial phase disparity between the hyperpolarization and eye velocity was due to the relative constancy of peak time (∼20 ms) of the initial steep hyperpolarization regardless of the later potential profile that covaried with the eye velocity. The initial steep hyperpolarization led the beginning of saccades by 15.9 ± 3.8 (SD) ms, which is longer than the lead time for medium-lead burst neurons. These results demonstrate that the pause of activity in OPNs is caused by IPSPs initiated by an abrupt, intense input and maintained, for the whole duration of the saccade, by afferents conveying eye velocity signals. We suggest that the initial sudden inhibition originates from central structures such as the superior colliculus and frontal eye fields and that the eye velocity-related inhibition originates from the burst generator in the brain stem.