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Contribution of postural muscle tone to full expression of posture and locomotor movements: multi-faceted analyses of its setting brainstem-spinal cord mechanisms in the cat.

In this minireview, I have summarized and discussed our results concerning the neuronal mechanisms of postural suppression, and our thoughts about contribution of postural muscle tone to the full expression of locomotion. The identified brainstem-spinal cord system is schematically illustrated in Fig. 1 with its neuronal constituents (A). When cells of origin of this system (cells in the nucleus reticularis pontis oralis), or the pathway descending from these cells (DTF area) are tonically activated, cells in the nucleus reticularis gigantocellularis (gc cells) are also tonically activated, and with this process the system as a whole exerts generalized or non-reciprocal tonic inhibitory effects upon alpha-motoneurons innervating hindlimb extensor and flexor muscles. In consequence, postural muscle tone, especially that of the hindlimbs, is tonically suppressed and results in sustained suppression of postural support both in acute decerebrate cats (locomotor preparation) and in awake, freely moving cats, terminating locomotor movements. Since postural control and locomotor control are not separate phenomena, our results indicate that interaction of the above-described two control systems should be studied at each level of the brainstem and the spinal cord. In fact, contribution of brainstem neurons and of spinal interneurons has already been studied. It may also be interesting to elucidate supra-pontine structures which activate the identified brainstem-spinal cord inhibitory system. Such a study may elucidate the neuronal structures related not only to the termination of on going movements but also to the muscular atonia observed during active sleep. As demonstrated, even for the understanding of the brainstem-spinal cord system involved in postural suppression, multi-faceted and mutally related analytical approaches have been necessary as suggested by Brookhart, and these approaches allowed us to approach a much better understanding of the problem one faces. They were neuroanatomical analyses of plausible neuronal structures, electrophysiological analyses of their synaptic organizations, the neuropharmacological analyses of their putative neurotransmitters, and finally the most important, behavioral analyses. For a thorough understanding of the dynamics of postural and locomotor control systems, the problems we are now concerned with are to identify both the "hardware" and the "software" of these two systems. Understanding of the hardware requires an elucidation of the neuronal circuits including synaptic organizations, and that of the software an identification of the putative neurotransmitters that set the identified neuronal circuit to work, so that the systems as a whole exhibit a meaningful external manifestation.(ABSTRACT TRUNCATED AT 400 WORDS)