Physiological and morphological characteristics of cat masticatory motoneurons—intracellular injection of HRP

Abstract

The physiology and morphology of masticatory motoneurons of adult cats were examined by the methods of intracellular recording and intracellular injection of horseradish peroxidase. Masseter and jaw-opening motoneurons were identified by intracellular recordings of the antidromic response following stimulation of the masseter and mylohyoid nerves, respectively. An excitatory postsynaptic potential (EPSP) was recorded from masseter neurons by stimulation of the masseter nerve with stimulus intensity below threshold for antidromic response. In contrast, the EPSP was not recorded from jaw-opening motoneurons by stimulation of the mylohyoid nerve with stimulus intensity below threshold for antidromic response. Patterns of postsynaptic potentials (PSPs) in the masseter motoneurons following stimulation of the tooth pulp or periodontal afferents were classified into 4 types: hyperpolarization ( n = 40), depolarization-hyperpolarization ( n = 9), hyperpolarization-depolarization ( n = 5), and depolarization with spike potentials ( n = 10). On the other hand, patterns of the PSPs in the jaw-opening motoneurons following stimulation of the same afferents were classified into two types: depolarization with spike potentials ( n = 19), and hyperpolarization ( n = 5). Twenty-five masseter and 7 jaw-opening motoneurons and an intranuclear neuron were reconstructed from serial sections in the transverse plane. On the basis of dendritic morphology, the masseter motoneurons could be classified into two major groups, type I ( n = 15) and type II ( n = 9), whereas two neurons were found to constitute a separate category of the masseter motoneuron. The dendritic distributions of all the jaw-opening motoneurons examined were generally similar and there was no indication of the existence of subtypes, whereas there were 2 or 3 subgroups in type I and type II masseter motoneurons. Type I masseter neurons had primary dendrites which extended radially in all directions, and the whole profile of their dendritic trees presented a spherical and an egg-shaped appearance. In type II masseter neurons, the origin of primary dendrites was bipolar or tripolar, and the whole profile of their dendritic trees presented a hemispherical and mirror-imaged, funnel-shaped appearance. The other two masseter motoneurons had a particular dendritic tree which was much simpler in configuration, with less tapering or branching than those of other neurons examined. In contrast, the dendritic profiles of all the jaw-opening motoneurons were similarly organized and showed vertically oriented dendritic trees which were more developed in the dorsomedial than in the ventrolateral direction. Dendrites from all groups extended beyond the boundaries of the motor nucleus into the reticular formation. The border between the dorsolateral and ventromedial subdivisions was crossed by some dendrites but the overlap was small. Among neurons examined in the present study, one intranuclear neuron identified had the simplest dendritic tree and smalles soma size, with axon collaterals projecting to the reticular formation. No significant differences in several soma-dendritic parameters were found between morphologically distinct motoneuronal subpopulations and between the jaw-opening and masseter motoneurons. There were no instances in which axon collaterals were observed for the masticatory motoneurons.