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G.M. Macalusol, G. Pavesi2, and A. De Laat3*
Abstract. An electric stimulation of the masseteric nerve elicits a heteronymous H-reflex in the temporal muscle. The characteristics of this reflex response were investigated by analysis of the firing probability changes of single motor units. Eleven healthy subjects participated in the experiments. The heteronymous H-reflex of the temporal muscle was electrically elicited by stimulation of the masseteric nerve at 120% of the intensity needed for the maximal masseteric M-wave. From 8 to 24 motor units were sampled from the temporal muscle of each subject. Peri-stimulus time histograms of motor unit recordings were built with a 0.5-ms bin width. The mean firing probability was calculated for the 20 ms preceding the stimulus. The firing probability was considered increased when it exceeded the mean by 3 standard deviations. Of 104 sampled motor units, 40 motor units showed a significant increase of the firing probability, which lasted 1 ms or less in 29 of them. In 12 out of 16 motor units, a significant increase of firing probability also persisted at a lower stimulation intensity (120% of the threshold needed to elicit a masseteric M wave). These data indicate that: (1) some temporal muscle motor units are modulated by afferents from the masseter muscle, (2) the heteronymous H-reflex has a monosynaptic component, and (3) there might be a more complex than just monosynaptic organization serving the heteronymous temporal H-reflex. For the latter conclusion regarding synaptic wiring, however, PSTH studies like the present one can offer only indirect evidence, and this question could be better studied in animals.
Key words: H-reflex, peri-stimulus time histogram, temporal muscle, electromyography, masseteric nerve.
Introduction
Electrical stimulation of a nerve serving a muscle results in a reflex contraction of that muscle, which follows the direct motor response. The reflex is called the H-reflex (for review, see Schieppati, 1987) and is mainly due to stimulation of the fibers originating from muscle spindles (Ia fibers). In some instances, the same stimulation also elicits an H-reflex in heteronymous muscles (Cavallari and Katz, 1989; Meunier et al., 1990); in this case, the heteronymous H-reflex is not preceded by a direct motor response.
Electrical shocks via a needle inserted between the condyle and the coronoid process of the...