Localization and Morphometric Analysis of Masticatory Muscle Afferent Neurons in the Nucleus of the Mesencephalic Root of the Trigeminal Nerve in the Cat

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Abstract
Injections of horseradish peroxidase (HRP) were made into the ipsilateral temporal muscle and contralateral masseter muscle of 10 cats in order to identify and characterize neurons in the nucleus of the mesencephahc root of the trigeminal nerve that innervate muscle receptors in the orofacial periphery. Neurons labelled by HRP injections and unlabelled cells from 5 control cats were measured with a computer-based image analyzer, and their position was mapped on a stereotaxic graph. Cells that innervate the masseter and temporal muscles were identified throughout the rostrocaudal extent of the nucleus. There was no indication of a somatotopic pattern nor of a specific segregation within the nucleus for cells innervating muscle receptors. The nucleus contained small, rounded unipolar neurons located primarily in the dorsal border of the periaqueductal gray (PAG) matter in the rostral part of the nucleus and larger oval unipolar neurons which were scattered throughout the nucleus, but were predominant in the pontine portion of the nucleus. HRP injections labelled both large and small cells, as well as occasional multipolar cells. The last-mentioned tended to be located in the lateral margins of the PAG. The mean geometric values obtained for the control group were: area 552.7 μm2 perimeter 110.3 μm; maximum diameter 36.0 μm. and diameter of an equivalent circle 26.1 μm. The mean values of the labelled neurons were: area 606.6 μm2; perimeter 100.1 μm; maximum diameter 36.0 μm, and diameter of an equivalent circle 27.2 μm.