MEMBRANE-PROPERTIES UNDERLYING THE FIRING OF NEURONS IN THE AVIAN COCHLEAR NUCLEUS

Neurons of the avian nucleus magnocellularis (NM) relay auditory infor mation from the VIIIth nerve to other parts of the auditory system. To examine the cellular properties that permit NM neurons to transmit re liably the temporal characteristics of the acoustic stimulus, we perfo rmed whole-cell recordings in neurons of the chick NM using an in vitr o thin slice preparation. NM neurons exhibited strong outward rectific ation near resting potential; the voltage responses to depolarizing cu rrent steps were substantially smaller than to equivalent hyperpolariz ing steps. Suprathreshold current steps evoked only a single action po tential at the start of the step. In contrast, stimulation with trains of brief current pulses evoked repetitive firing that was phase-locke d to the stimulus cycle. The number of action potentials evoked by the pulses during the train decreased with increasing stimulus rate. Volt age-clamp experiments revealed a rapidly activating, slowly inactivati ng, outward current with a threshold near -65 mV. During depolarizing voltage steps, the outward current rose sigmoidally to a peak and then decayed slowly, reaching steady state within 5 sec. Application of 20 0 mu M 4-aminopyridine (4-AP) reduced the peak of the outward current by 84%, leaving a small, persistent component. Under current clamp, ap plication of 200 mu M 4-AP reduced the outward rectification and incre ased the amplitude and duration of the action potentials. Moreover, NM neurons could no longer sustain firing during high rates of stimulati on with the current pulses: increased temporal summation of the potent ials caused sufficient depolarization to inactivate the sodium conduct ance underlying the action potential. These results suggest that the o utward current is necessary for NM neurons to transmit well-timed even ts reliably for the duration of an acoustic stimulus.