A distinct functional distribution of alpha and gamma motoneurons in the rat trigeminal motor nucleus

摘要

Gamma-motoneurons (gamma MNs) play a crucial role in regulating isometric muscle contraction. The slow jaw-closing during mastication is one of the most functional isometric contractions, which is developed by the rank-order recruitment of alpha-motoneurons (alpha MNs) in a manner that reflects the size distribution of alpha MNs. In a mouse spinal motor nucleus, there are two populations of small and large MNs; the former was identified as a population of gamma MNs based on the positive expression of the transcription factor estrogen-related receptor 3 (Err3) and negative expression of the neuronal DNA-binding protein NeuN, and the latter as that of alpha MNs based on the opposite pattern of immunoreactivity. However, the differential identification of alpha MNs and gamma MNs in the trigeminal motor nucleus (TMN) remains an assumption based on the size of cell bodies that were retrogradely stained with HRP. We here examined the size distributions of alpha MNs and gamma MNs in the dorsolateral TMN (dl-TMN) by performing immunohistochemistry using anti-Err3 and anti-NeuN antibodies. The dl-TMN was identified by immunopositivity for vesicular glutamate transporter-1. Immunostaining for choline acetyltransferase and Err3/NeuN revealed that the dl-TMN is composed of 65% alpha MNs and 35% gamma MNs. The size distribution of alpha MNs was bimodal, while that of gamma MNs was almost the same as that of the population of small alpha MNs , suggesting the presence of alpha MNs as small as gamma MNs. Consistent with the size concept of motor units, the presence of smaller jaw-closing alpha MNs was coherent with the inclusion of jaw-closing muscle fibers with smaller diameters compared to limb muscle fibers.