Neural substrate of locomotor central pattern generators in mammals

Central pattern generators are neural networks organized to produce any rhythmic output without requiring a rhythmic input. In mammals, locomotor CPGs are organized in the lumbar and cervical segments of the spinal cord, and are used to control rhythmic muscle output in the arms and legs. Certain areas of the brain initiate the descending neural pathways that ultimately control and modulate the CPG signals. In addition to this direct control, there exist different feedback loops that coordinate the limbs for efficient locomotion and allow for the switching of gaits under appropriate circumstances.

Locomotion can be initiated by pathways that originate in the caudal hindbrain and brainstem and descend to the spinal cord. These descending pathways originate in the Mesencephalic Locomotor Region (MLR) and the lateral hypothalamus. These areas project to reticulospinal neurons in the pons and medulla, which themselves project throughout the spinal cord to activate the CPGs involved in locomotion. In particular, the reticular formation in the pons plays an important role in inducing locomotion. The parapyramidal region (PPR) in the ventral medulla is also known to produce locomotion when stimulated in neonatal rats. Several different command pathways play a role in the initiation of locomotion. Neurons involved in each of these include glutamatergic, noradrenergic (NA), dopaminergic (DA), and serotonergic (5-HT) neurons.

The central pattern generators responsible for locomotion in vertebrates reside as half-center modules in the cervical and lumbar region of the spinal cord. Each CPG generates a basic motor output pattern that is responsible for the rhythmic contractions of flexor-extensor muscles that correspond to the forelimbs and hindlimbs. There exist neural substrates that control bilateral limb coordination, particularly in the lumbar spinal circuits controlling the hindlimbs of quadruped mammals. Rhythmogenic potential is highest in the center of the spinal cord and decreases in a mediolateral direction. The ability to generate fast and regular rhythmic activity decreases in the caudal direction, but the rhythm-generating networks extends from the lumbar region into the caudal thoracic region of the spinal cord. Both lateral and ventral funiculi are able to coordinate activity in the rostral and caudal regions. Although CPGs do exist in humans, supraspinal structures are also important for the additional demands of bipedal locomotion. There are different functional networks controlling forward and backward movement, as well as different circuits controlling each leg. These networks are largely nonoverlapping, although some feedback does exist between them.

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