Brainstem and spinal cord mechanisms that control locomotor activity in larval lamprey.

摘要

In vitro brain/spinal cord preparations from larval lamprey were used to examine several aspects of locomotor systems: (a) organization of brain locomotor command systems that initiate locomotor behavior; (b) mechanisms responsible for turning maneuvers; (c) pharmacology of descending activation of spinal locomotor networks; and the contributions of (d) local coupling, (e) long distance coupling, and (f) oscillator frequency gradients to coordination of spinal locomotor activity. In addition, computer modeling was used to test hypotheses generated by the biological data.; Chemical microstimulation in the brain with excitatory amino acids (EAA) or their agonists indicated that several areas of the brain were effective in initiating coordinated spinal locomotor activity. These results suggest that trigeminal sensory systems have inputs to higher order locomotor control centers which then project to descending brain neurons that activate spinal locomotor networks and initiate locomotor behavior.; Results from whole animals and in vitro preparations indicate that turning maneuvers involve changes in intensity as well as timing of locomotor activity. These results, in conjunction with computer model simulations, suggest that this adaptation of locomotor behavior is mediated by descending modulatory inputs primarily to the spinal oscillator networks involved with the timing of motor activity, but possibly also to motoneurons.; Application of general EAA receptor blockers to the spinal cord abolished brainstem-initiated locomotor activity, while specific EAA receptor blockers only attenuated the activity. The results suggest that these receptors are not concentrated in areas of the spinal locomotor networks that control cycle time.; Application of strychnine, a glycinergic receptor blocker, to the spinal cord converted left-right alternation of burst activity to synchronous bursting, indicating that left and right oscillators are connected by strong reciprocal inhibition in parallel with weaker reciprocal excitation. Results also suggest that short distance coupling is stronger in the descending direction than in the ascending direction.; When short distance coupling was blocked in the middle spinal cord, long distance coupling could coordinate rostral and caudal burst activity for at least 40 segments. However, long distance coupling is relatively weak, suggesting that short distance coupling is the main mechanism for coordinating locomotor activity along the spinal cord.; Finally, evidence for oscillator frequency gradients along the spinal cord was obtained. This mechanism may contribute to coordination of locomotor activity, but frequency gradients do not appear to be necessary for normal locomotor coordination.