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Mark L. Shik

Microstimulation of the brain stem was performed in the mesencephalic mudpuppy. Repetitive (8-15 pps) stimulation of the lateral mesencephalic tegmentum elicited cyclic side-to-side movements of the tail or the body and stepping of the hindlimbs or forelimbs in different combinations. The association of all four components was most common, followed by the combined movements of the tail and the body; the tail, the hindlimbs, and the body; and the tail and the hindlimbs. Movements of the tail and the hindlimbs also occurred separately, but lateral undulation of the body was observed only in combinations, as was mainly stepping of the forelimbs. The cycle durations varied in different patterns: In particular, a cycle lasted 5 to 7 s for tail movements and 1.3 to 2 s for complete locomotion. Usually only one pattern was evoked by stimulating a particular site in the brain stem, but sometimes increasing the strength or frequency of stimulation involved an additional elementary synergy, thus forming a more complex pattern. Similar patterns could arise from stimulation of different sites. The order of involvement of components in the same complex pattern varied when stimuli were applied to different sites. The combinatorial composition and morphological specification are possible mechanisms of the broad repertoire of elicited locomotor patterns in the mudpuppy.

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Mark L. Shik

The population of reticulospinal neurons of the hindbrain receives input from several brain regions and activates spinal interneurons that assemble stepping. Axons of these reticulospinal neurons descend in the ventral part of the lateral funiculus. Axons of certain spinal neurons form the “stepping strip” in the dorsal part of the lateral funiculus. Stimulation of this strip elicits stepping via particular spinal neurons that send their axons to the ventral part of the lateral funiculus. Neurons of these two types form the propriospinal system capable of activating spinal interneurons that assemble stepping.