Physiology and morphology of the masticatory alpha and gamma motoneurons -intracellular recording and HRP injection technique-

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In order to identify alpha and gamma types in the masticatory motoneurons, intracellular recording and HRP injection technique was applied to cat masseter motoneurons in vivo. The distribution of latency of antidromically evoked spikes in the masseter motoneurons was not divided into two distinct groups, suggesting that the latency can not be used as an criterion to identify the motoneurons. A small number of masseter motoneurons with monosynaptic EPSP evoked after stimulations of the lingual nerve or without monosynaptic EPSP after stimulations of the masseter nerve showed shorter latency of the antidromically evoked spikes. All of these electrophysiological data indicated that the alpha and gamma masseter motoneurons differ from alpha and gamma spinal motoneurons, respectively, which can be distinguished by several electrophysiological criteria. On the other hand, morpholoy of the HRP-labeled masseter motoneurons were reexamined and showed that a few masseter motoneurons have very simply branched dendrites. Thus these neurons were further analyzed three dimensionally and showed smaller dendritic volume, shorter total length of dendrites and longer first-order dendrites than those of the other masseter motoneurons, suggesting that gamma masseter motoneurons exist and should be distingushed on the basis of both their morphology and physiology. Therefore we tried to study morphological characteristics of synapses between a masseter motoneuron and a functionally identified spindle afferent by means of electron microscope in addition to light microscope. These frontier studies clarified that several different types of synapses exist between the two neurons, suggesting that these differences may be involved in the functional distinction between alpha and gamma types.