EMG Power Spectrum and Motor Unit Characteristics in the Masseter Muscle of the Rabbit

Abstract

Masticatory muscles contain a large variety of motor units with different physiological and morphological properties. In this study, we tested the hypothesis that a relationship exists between the mechanical and myo-electric properties of single motor units in the masseter muscle of the rabbit. It was expected that faster-contracting motor units, which usually have a relatively large number of fibers with large diameters, should have faster action potentials with larger amplitudes than slower motor units. Single motor units were stimulated. A two-dimensional force transducer registered mechanical parameters of the units. EMG electrodes were used to determine amplitude and frequency parameters of the action potentials of the same units. The results showed that faster-contracting motor units indeed produced action potentials with higher conduction velocities. However, faster motor units had no significant larger amplitude of the action potential. Small but significant positive correlations were found between the tetanic peak force and the amplitude of the action potentials. Little difference was found among the various frequency and amplitude parameters, respectively, making them equally suitable to describe the action potential. Surprisingly, a negative correlation between the amplitude and frequency parameters of the action potential was found, which may result from variability in arrival times of action potentials at the electrode site. Regional differences in the frequency parameters were found between the anterior and posterior parts of the superficial masseter.

Morphology and Physiology of Masticatory Muscle Motor Units

“…Motor unit action potential amplitude, twitch tension, and recruitment threshold have been reported to be mutually proportional (Goldberg and Derfler, 1977;Clark et al, 1978;Scutter and Turker, 1998). In accordance with these observations are the results of Turkawski and Van Eijden (2000) that revealed a positive correlation between tetanic peak force and action potential amplitude upon single motor unit stimulation of rabbit masseter motor units. The recruitment threshold of motor units appears not to be fixed but depends on the motor task.…”

“…Nordstrom and Miles, 1990). Information on the contractile properties of motor units of masticatory muscles in animals is scarce, As described above, characteristics of rabbit masseter motor units have been examined by extracellular stimulation of motoneurons in the brain stem (Weijs et al, 1993;Kwa et al, 1995a,b, 2001, Turkawski et al, 1996 ponent transducer attached to the teeth (Weijs et al, 1993;Kwa et al, 1995aKwa et al, ,b, 2001), or directly, via a twocomponent force transducer attached to the zygomatic arch (Turkawski et al, 1996(Turkawski et al, , 1998Turkawski and Van Eijden, 2000). A major advantage of the latter method is that the position and direction of motor unit force in the masseter muscle could be registered, and that no transformation was needed for the force registered at the teeth.…”

“…Also, the distance between active fibers and electrodes plays an important role (Fuglevand et al, 1992). It (Turkawski and Van Eijden, 2000).…”

“…Recently, extracellular stimulation of motoneurons in the rabbit trigeminal motor nucleus enabled a large number of masticatory motor unit properties to be documented (Weijs et al, 1993;Kwa et al, 1995a,b, 2001, Turkawski et al, 1996Turkawski and Van Eijden, 2000; ). In the human, external stimulation of masticatory motor units through the skin-such as, for example, stimulation of the median nerve to stimulate motor units of the thenar muscles (Chan et al, 1998)-is not possible.…”

Action potential shape of rabbit masseter motor units and jaw angle

“…For an extensive description of the methods used for anesthesia and recording of action potentials and twitch forces, we refer to earlier work. [10][11][12][13] In brief, preanesthesia was effected with a mixture of rompune and ketamine, applied intramuscularly, and atropine, applied subcutaneously. Further anesthesia was effected through halothane in a 1:3 oxygen-nitrous oxide mixture, provided through a tracheal cannula.…”