IN-VIVO INTRACELLULAR CHARACTERISTICS OF INFERIOR COLLICULUS NEURONS IN GUINEA-PIGS

Intracellular in vivo recordings of physiologically identified inferio r colliculus central nucleus (ICc) auditory neurons (n = 71) were carr ied out in anesthetized guinea pigs, The neuronal membrane characteris tics are described showing mainly quantitative differences with a prev ious report [Nelson, P.G, and Erulkar, S,D., J. Neurophysiol., 26 (196 3) 908-923], The spontaneous spike activity was consistent with the di scharge pattern of most extracellularly recorded units, The action pot entials showed different spike durations, short and long, and some of them exhibited hyperpolarizing post-potentials, There were also differ ences in firing rate. The ICc neurons exhibited irregular activity pro ducing spike trains as well as long silent periods (without spikes). I ntracellular current injection revealed membrane potential adaptation and shifts that outlasted the electrical stimuli by 20-30 ms. Both evo ked synaptic potentials and the spike activity in response to click an d tone-burst stimulation were analyzed. Depolarizing-hyperpolarizing s ynaptic potentials were found in response to contralateral and binaura l sound stimulation that far outlasted the stimulus (up to 90 ms). Whe n ipsilaterally stimulated, inhibitory responses and no-responses were also recorded, Although few cells were studied, a similar phenomenon was observed using tone-burst stimulation; moreover, a good correlatio n was obtained between membrane potential shirts and the triggered spi kes (input-output relationship). These in vivo results demonstrate the synaptic activity underlying many of the extracellularly recorded dis charge patterns. The data are consistent with the known multi-synaptic ascending pathway by which signals arrive at the ICc as well as the d escending corticofugal input that may contribute to the generation of long duration post-synaptic potentials.