INTERACTION BETWEEN THE CAUDAL BRAIN-STEM AND THE LAMPREY CENTRAL PATTERN GENERATOR FOR LOCOMOTION

Because of its remarkable simplicity and the robustness of the isolate d preparation, the lamprey has been used as a model system to study lo comotion and its central pattern generator. The function of the spinal cord is relatively well understood in this context, but the role of t he brain or even the caudal brainstem remains less so. We here present a study of the interaction between the caudal brainstem and the spina l pattern generator for locomotion. We show that the interaction is hi ghly complex, with both feedforward input from the brainstem to spinal cord and feedback input from the spinal cord to brainstem playing a s ignificant role in the motor output during locomotion. The brainstem, when diffusely stimulated pharmacologically, can initiate fictive loco motion, or it can disrupt or alter the ongoing D-glutamate initiated m otor output. The nature of the disruptions vary greatly, and can induc e generalized irregularity, while the alterations can include accelera ting or decelerating of the bursting. All behaviors are displayed with spectrograms of the motor nerve discharge. We also show that the unst imulated brainstem can disrupt as well as slow the bursting, but in a complex fashion. Finally, a slow episodic behavior initiated from the caudal brainstem is also described. This can be elicited either by D-g lutamate to the brainstem or by ascending activity from the spinal cor d pattern generator. Thus, we demonstrate that the interaction between the brainstem and the spinal cord during the production of locomotion is highly complex. The locomotion that is exhibited by the combined b rainstem-spinal cord preparation is extremely variable. This is in str iking contrast to the variability of the locomotor output pharmacologi cally induced in the spinal cord alone. The latter preparation exhibit s remarkable regularity, or upon occasion, irregularity, but not the r outine irregularity or the systematic up and down changes in frequency seen with the brainstem present. However, the pattern of frequency ch anges induced by the brainstem is not predictable, and remains to be u nderstood. Copyright (C) 1996 IBRO. Published by Elsevier Science Ltd.