On the bilateral innervation of masticatory muscles: a study with retrograde tracers.

Using horseradish peroxidase and fluorescent compounds as retrograde tracers, the myotopical arrangement and axon trajectories of the motoneurons innervating the supramandibular and the suprahyoidal muscles were studied in the rat. Only motoneurons ipsilateral to the injection site were labelled. In the trigeminal motor nucleus, motoneurons of the superficial masseter muscle were found laterally, of the anterior deep masseter muscle centrally, and of the temporalis muscle dorsally and medially. Motoneurons of the anterior and posterior digastric muscle were located in the accessory motor nucleus of the trigeminal and facial nerve respectively. Evidence is provided that jaw-opening muscles are innervated from a separate motor complex.