Response of the motor unit of the rat to paired stimuli

¹ Department of Neurology Harvard Medical School, and Neurological Unit, Boston City Hospital, Boston, Massachusetts

The response of the single motor unit of the gastrocnemius of the rat to paired stimuli delivered to a ventral root filament was studied. The peak-to-peak potential of the unit in response to the second of paired stimuli was 5–15% lower than the amplitude of a single response, and persisted in many cases at a stimulus interval greater than 30 msec. The absolute refractory period ranged from 0.6 to 2.3 msec. The absolute irresponsive period ranged from 1.1 to 2.75 msec. With decreasing stimulus intervals, the second response declined in peak-to-peak amplitude, increased in duration, and developed multiple peaks with variable latency. It is suggested that variability of response characteristics of subunits, the anatomic nature of which is unknown, allows the motor unit to be fractionated under certain experimental conditions.