Long-term increases in excitability of facial motoneurons and other neurons in and near the facial nuclei after presentations of stimuli leading to acquisition of a Pavlovian conditioned facial movement.

Levels of neuronal excitability to injected current were measured intracellularly in facial motoneurons and other neurons in and near the facial nuclei of three groups of awake cats: a "Conditioned" group consisting of animals that had previously received sufficient numbers of paired presentations of click CSs and glabella tap USs to produce eyeblink CRs; a "US-only" group that had received presentations of the USs only; and a "Naive" group that had received neither of these stimuli. Thresholds of intracellularly applied, depolarizing pulse currents required to elicit repeatable spike activity were significantly lower in the "Conditioned" and "US-only" groups than in the "Naive" group. The increased levels of neuronal excitability were correlated with increases in neuronal input resistance. Levels of neuronal excitability remained elevated when measured more than a month after presentations of both CSs and USs, whereas the increases in neuronal excitability decayed within a few weeks in animals given USs only. The increases in neuronal excitability and input resistance following repetitive presentations of glabella tap USs alone appeared to support a latent facilitation of motor performance reflected by an absence of a blink CR to click CS after such presentations but an increased rate of acquisition of subsequent eyeblink conditioning using paired click CS and tap US. The rate of eyeblink conditioning was found to be accelerated in a group of cats given repetitive presentations of tap USs seven days prior to conditioning with paired CSs and USs, compared to a group that was not given USs or CSs before similar conditioning. These findings provide direct, in vivo evidence that increases in the excitability and input resistance of neurons in and near the facial nucleus can occur in cats following presentations of the stimuli used for conditioning.