Action potential shape of rabbit masseter motor units and jaw angle

Action potentials of rabbit masseter motor units (n = 42) were registered at different jaw angles to examine whether the shape of the action potential is related to length of muscle fibers in motor units and depends on the intramuscular location of the motor unit. Twitches were elicited by stimulating motoneurons in the trigeminal motor nucleus. During jaw opening (0-21 degrees), the duration of the action potentials increased by about 10%. Anteriorly located motor units showed an increase in duration larger than that of more posteriorly located units, which was probably due to a larger stretching of the more anteriorly located units.