Topographical Representation of Motoneurons Innervating the Transverse Mandibular Muscle in the Trigeminal Motor Nucleus, with Special Reference to Rats

Localization of motoneurons of the transverse mandibular muscle (TM) was examined by a retrograde tracing method (horseradish peroxidase (HRP) and wheat germ agglutinin-HRP (WGA-HRP) ) in the rat (rodent), house musk shrew (Suncus murinus, insectivore) and miniature pig (even-toed, ungulate). In the injection of HRP solution into the TM of the rat, labeled neurons with HRP appeared at the lateral and ventral margins of the dorsolateral division in the rostral two-thirds level of the trigeminal motor nucleus. This distribution area was close to or overlapped that of medial pterygoid and lateral pterygoid motoneurons. TM motoneurons of the shrew and pig were located in the ventromedial part of the dorsolateral division at the rostral half level of the nucleus. The distribution patterns of each masticatory muscle motoneuron in the rat were also confirmed for comparison with those of TM motoneurons. Electromyography during feeding in the rat showed that TM activity almost synchronized with the activities of both jaw-closer and jaw-opener muscles.