Abstract
The vertebrate control of locomotion involves all levels of the nervous system from cortex to the spinal cord. Here, we aim to cover all main aspects of this complex behavior, from the operation of the microcircuits in the spinal cord to the systems and behavioral levels and extend from mammalian locomotion to the basic undulatory movements of lamprey and fish. The cellular basis of propulsion represents the core of the control system, and it involves the spinal central pattern generator networks (CPGs) controlling the timing of different muscles, the sensory compensation for perturbations, and the brain stem command systems controlling the level of activity of the CPGs and the speed of locomotion. The forebrain and in particular the basal ganglia are involved in determining which motor programs should be recruited at a given point of time and can both initiate and stop locomotor activity. The propulsive control system needs to be integrated with the postural control system to maintain body orientation. Moreover, the locomotor movements need to be steered so that the subject approaches the goal of the locomotor episode, or avoids colliding with elements in the environment or simply escapes at high speed. These different aspects will all be covered in the review.
Highlights
THE BUILDING BLOCKS OF THE...LOCOMOTION IN HUMANS AND...SELECTION OF BEHAVIOR: ROLE OF...THE LOCOMOTOR COMMAND SYSTEMS... 281DOWNSTREAM PROJECTIONS FROM... 282TRANSITION FROM STANDING TO...THE SPINAL COORDINATION OF...CENTRAL PATTERN GENERATION
This review summarizes the logics of the neural control of motion extending from the basal ganglia mechanisms responsible for selection of behavior and cortex for precision walking to the cellular and molecular design of the central pattern generator networks in the brain stem-spinal cord
Specific neural circuits are dedicated to serve each of these functions, often referred to as central pattern generator circuits (CPGs) or motor programs
Summary
“To move things is all that mankind can do, and for this the sole executant is a muscle, whether it be whispering a syllable or felling a forest.” This quote from Charles S. CPG circuits in the spinal cord are essential for the propulsive aspect of locomotor movements as they produce the complex motor pattern corresponding to swimming, flying, or walking [201] They act as a processing interface to integrate sensory feedback and instructive signals from the brain to optimize the execution of locomotor movements [393]. The analyses of the organization and function of locomotor circuits have traditionally relied mostly on electrophysiological and anatomical approaches to define sets of interneurons essential for generating the motor pattern and map their connectivity In this regard, accessible vertebrate preparations such as the lamprey and Xenopus tadpole have unraveled many of the fundamental principles of organization at the spinal level with ipsilateral excitatory drive combined with reciprocal inhibition [70, 387] that represents the backbone of the spinal locomotor circuits in all vertebrates. Clearly of critical importance, we will only marginally include biomechanical aspects [94, 160, 163, 202, 213, 354, 451] and only very briefly the area of spinal cord injury
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