Single motor unit (SMU) spike trains were recorded from the masseter muscles of human subjects while they isometrically produced interocclusal force. The majority of SMUs increased their firing rate as interocclusal force was increased, but the rate changes declined in magnitude at each successive force level and eventually reached constant values. This indicated a progressive decline in the ability of motoneurons to increase firing rate in response to increases in excitatory drive, and the evidence suggests that this is due to temporal summation of afterhyperpolarization conductances. There was a strong tendency for SMUs with low recruitment thresholds to show the largest rate changes per kilogram change in force, which may be a consequence of the decreasing input resistances associated with increases in cell size. There were also monotonic decreases in standard deviations and skewness of the interspike interval (ISI) distributions as firing rate increased, which indicates that synaptic drive to the motoneurons was increasing monotonically. The graph of standard deviation mean ISI revealed less tendency for masseter motoneurons to maintain minimum discharge variability as compared to spinal motoneurons; the lack of recurrent inhibition to masseter motoneurons may be responsible.

ArticlesRelationship among recruitment order, spike amplitude, and twitch tension of single motor units in human masseter muscleL. J. Goldberg, and B. DerflerL. J. Goldberg, and B. DerflerPublished Online:01 Jul 1977https://doi.org/10.1152/jn.1977.40.4.879MoreSectionsPDF (2 MB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat Previous Back to Top Next Download PDF FiguresReferencesRelatedInformation Cited ByPreparatory activity links the frontal eye field response with small amplitude motor unit recruitment of neck muscles during gaze planningSatya Rungta, Debaleena Basu, Naveen Sendhilnathan, and Aditya Murthy31 July 2021 | Journal of Neurophysiology, Vol. 126, No. 2Sex‐related differences in muscle size explained by amplitudes of higher‐threshold motor unit action potentials and muscle fibre typing27 June 2018 | Acta Physiologica, Vol. 225, No. 4Mouse incising central pattern generator: Characteristics and modulation by painPhysiology & Behavior, Vol. 196Age-related differences in the motor unit action potential size in relation to recruitment threshold24 July 2017 | Clinical Physiology and Functional Imaging, Vol. 38, No. 4Dynamic Musculoskeletal Functional Morphology: Integrating diceCT and XROMM13 January 2018 | The Anatomical Record, Vol. 301, No. 2Flexible adaptation to an artificial recurrent connection from muscle to peripheral nerve in manKenji Kato, Syusaku Sasada, and Yukio Nishimura19 February 2016 | Journal of Neurophysiology, Vol. 115, No. 2Activation properties of trigeminal motoneurons in participants with and without bruxismJessica M. 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