Classically conditioned alterations in single motor unit activity in the spinal cat

Muscle tension and single motor unit EMG recordings from a flexor muscle of acute spinal cats were obtained during presentation of classical conditioning and control paradigms. Conditioned increases in muscle tension were similar to previously obtained results. Motor unit recordings suggested that this conditioned reflex facilitation is brought about by an increased probability firing of motoneurons initially responsive to the conditioned stimulus, as well as orderly, size-dependent recruitment of initially non-responsive motoneurons. These results indicate that the same physiological mechanisms for grading motor output may apply for conditioned responses as have been demonstrated for reflexive and voluntary movements.