Abstract
In order for locomotion to occur, a complex pattern of muscle activation is required. For more than a century, it has been known that the timing and pattern of stepping movements in mammals are generated by neural networks known as central pattern generators (CPGs), which comprise multiple interneuron cell types located entirely within the spinal cord. A genetic approach has recently been successful in identifying several populations of spinal neurons that make up this neural network, as well as the specific role they play during stepping. In spite of this progress, the identity of the neurons responsible for generating the locomotor rhythm and the manner in which they are interconnected have yet to be deciphered. In this review, we summarize key features considered to be expressed by locomotor rhythm-generating neurons and describe the different genetically defined classes of interneurons which have been proposed to be involved.
Highlights
Central pattern generators (CPGs) are neural networks that are capable of generating intrinsic patterns of rhythmic activity independent of descending or sensory inputs.CPGs were first postulated to exist in the spinal cord and control locomotor behaviour a century ago when it was demonstrated that rhythmic central circuits in the spinal cord of decerebrate, deafferented cats were capable of producing alternating contractions of hindlimb flexor and extensor muscles [1]
There has been a great deal of investigation into the detailed structure and mechanism of function of locomotor CPGs in the years since, and it is accepted that these circuits, which are located in the caudal spinal cord, are capable of generating rhythmic, locomotor-like motor patterns in all mammals, including humans [2,3,4,5,6,7,8]
While many of the mechanisms involved in locomotor rhythm generation, as well as the identity of many of the cells remain unclear, we do know that excitatory neurons, which use the neurotransmitter glutamate, are an essential component of the rhythmogenic core of the mammalian locomotor CPG [37]
Summary
Central pattern generators (CPGs) are neural networks that are capable of generating intrinsic patterns of rhythmic activity independent of descending or sensory inputs. We can identify the specific populations that are responsible for such key functions as left–right [16,17] and flexor–extensor [18,19] coordination Despite this progress, a single genetically defined population of spinal interneurons that is necessary and sufficient for generating rhythmic locomotor activity in mammals has yet to be identified. A single genetically defined population of spinal interneurons that is necessary and sufficient for generating rhythmic locomotor activity in mammals has yet to be identified Since these neurons are responsible for the basic rhythmicity in the spinal cord and their level of excitability dictates the speed of stepping, their identification and anatomical/electrophysiological characterization are necessary if we are to comprehensively understand how mammalian locomotor activity is generated and if we hope to develop effective strategies to activate these neural circuits in a controlled fashion following spinal cord injury. We describe the different classes of interneurons which have been proposed to be involved in this function and focus on identifying the specific features that they possess
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