Static and dynamic analyses of reverberating neuronal within the brainstem.

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In this study, we have tried to elucidate the presence of reverberating neuronal circuits within the brainstem and the function of them on the control of postural muscle tone in the cats. Our immunohistochemical studies by using a PHA-L technique have demonstrated that there are neuronal connections between the left and the right nucleus reticularis pontis oralis (NRPo), and the NRPo and bilateral nucleus reticularis gigantocellularis (NRGc). Descending axons from the NRGc coursed bilaterally through the ventral and ventrolateral funiculi of the cervical cord. Single axon collaterals arising from the descending axons terminated mainly in the Rexed's laminae VII and VIII, where interneurons are located. Microinjections of carbachol into the unilateral NRPo resulted in tonic activation of a group of cells in the NRPo and in the NRGc. Cells in the NRGc were considered to be activated synaptically by the increased neuronal activity descending from the NRPo. Descending axons of these NRGc cells reached as far caudal as the lumbosacral cord. These NRGc cells were found to exert non-reciprocal postsynaptic inhibitory effects upon hindlimb alpha-motoneurons innervating extensor and flexor muscles. All these results suggested that there are reverberating neuronal circuits within the brainstem, and the NRGc cells are the output cells of the brainstem relaying suprabulbar sustained neuronal activity to the spinal cord. Tonic activation of these NRGc cells results in tonic inhibition of alpha-motoneurons, and consequently tonic suppression of postural muscle tone.