Morphologic characteristics of physiologically defined neurons in the cat trigeminal nucleus principalis

Although the principalis nucleus (Vp) contains trigeminothalamic and internuclear tract cells, the functional and morphologic differences between the two kinds of neurons have remained unsettled. The present study was aimed to address these problems by using the intracellular horseradish peroxidase injection technique in the cat. Of 20 neurons stained, 7 and 13 were located in the dorsomedial subnucleus (Vpd) and ventrolateral subnucleus (Vpv) of Vp, respectively. The Vpd neurons received input from the intraoral structures only but the Vpv neurons from the intraoral or facial structures. Nineteen neurons could be divided as class I and class II, based on the branching pattern of their stem axons. Class I (eight neurons) had an ascending stem axon without branching. Class II was divided into two subclasses (IIa and IIb). Class IIa (eight neurons) had an ascending stem axon from which branches were given off. Their branches formed a local-circuit restricted to the lower brainstem. Class IIb (three neurons) had a stem axon that formed the local-circuit only. The dendritic morphology was indistinguishable between different classes of neurons and between the subdivisions. Although the dendritic arborization pattern was governed by the location of the somata, it was suggested to be also important elements for determining primary afferent arborizations. In the brainstem nuclei, the jaw-closing motor nucleus received the highest density of projections from class II neurons with the receptive field involving the periodontal ligaments. The present study provides new findings that Vp neurons could be divided into three distinct populations and suggests that each population exerts a distinct function with respect to sensory discrimination, sensorimotor reflexes, or both.