Increases in excitability of neurons of the motor cortex of cats after rapid acquisition of eye blink conditioning

Measurements were made of resting potentials, input resistance, and excitability to intracellularly applied, depolarizing current pulses in neurons of the pericruciate cortex of conscious cats before and after acquisition of a rapidly conditioned eye blink reflex (CR). Neuronal excitability increased after conditioning, and an increased input resistance was found to be correlated with the increased level of excitability. No associated changes were found in resting potentials as a consequence of conditioning. When cells were divided into groups according to the latency of spike activity elicited by a click conditioned stimulus (CS) in relation to four separate excitatory EMG components of the compound blink CR, excitability increases were found in cells with increased spike activity at alpha 1 (8–40 msec), alpha 2 (40–72 msec), beta (72–112 msec), and gamma (112–160 msec) latencies after delivery of the CS. Also, the proportion of cells with high excitability (less than 0.7 nA required for spike elicitation) was increased at each latency period after conditioning. Increases in later components of spike discharge could also be found in the cells with increases in earlier components of discharge and increased excitability. The findings suggested that excitability increases facilitated a responsiveness to the CS that supported production of long- as well as short-latency components of the blink CR. Many of the changes in neuronal properties found after rapid eye blink conditioning, such as the increases in excitability and resistance and in the proportion of CS-excitable cells, resembled changes found earlier after acquisition of a slowly developing Pavlovian blink CR, using the same click CS and tap unconditioned stimulus without addition of a hypothalamic stimulus. The possibility should be considered that the (10–100 times) more rapidly acquired form of eye blink conditioning does not represent a different form of conditioning, but instead a change in the rate of conditioning supported by the more rapid production of increases in neural excitability.