Action potentials and twitch forces of rabbit masseter motor units at optimum jaw angle

This study examines mutual correlations between electrical and contractile motor-unit properties. Action potentials and twitch force responses of 42 masseter motor units were recorded in 14 rabbits. Motor units were excited by stimulating motoneurones in the trigeminal motor nucleus. Action potentials and twitches were measured at different jaw gapes between 0 and 21°, in steps of 3°. For each motor unit, the jaw angle–active force interrelation was determined and variables for action potential and force were compared at the jaw angle at which the motor unit produced the largest force. The results showed a large variation in variables for action potential and force, possibly related to the variation in motor-unit morphology. A weak correlation was found between the variables for action-potential amplitude and the magnitude of optimum force, indicating that motor units producing larger forces tended to have action potentials with larger amplitudes. Twitch-contraction time and the moment arm of the motor unit correlated positively with both the median frequency and the duration of the action potential. This indicates that slower contracting motor units had longer action potentials and is in accord with the earlier observation that slower motor units are preferentially located in the anterior regions of the masseter.