Locomotor synergies are present in the spinal cord. Spinally transected vertebrates, including humans, can produce coordinated stepping movements without any input from supraspinal neural structures. The spinal networks that produce patterned muscle activation, termed central pattern generators, are distributed throughout the spinal cord. The rhythmical muscle activation that is a part of coordinated stepping can be generated from even a few isolated segments of the hemitransected spinal cord. The central pattern generator has been deduced to consist of excitatory and inhibitory interneurons using glycine, glutamate, and acetylcholine as neurotransmitters. Interneurons and collaterals connect the ipsilateral and contralateral central pattern generators to produce reciprocal movements of the limbs. Propriospinal neurons connect different levels to produce coordination between the hind limbs and axial muscles and the arms or front legs. The pattern of central pattern generator activation can determine the speed of
locomotion and the direction of locomotion (forward or backward). The output of the central pattern generator is the motoneuron. Coordinated, sequential activation of the limbs and axial muscles involved in stepping is produced by the rhythmical excitation and inhibition of the motoneurons of various muscles.
Input to central pattern generators includes local sensory input and descending input from the brain stem via the reticulospinal pathways. Locomotion in spinal animals or fictive locomotion (rhythmical firing of motor neurons innervating limbs in the isolated spinal cord in vitro) can be induced by brain stem stimulation, sensory input, and treatment with monoamines (L-dopa and clonidine) in vivo and by treatment with excitatory amino acids in vitro. Spinal locomotion can be modified by sensory input and level of electrical or chemical stimulation. For example, progression from a slow walk to a trot to a gallop can be induced by increasing the speed of the treadmill on which the animal stands or by increasing the amount of electrical excitation of brain stem locomotor regions. In the isolated spinal cord, central pattern generators are adaptable to a limited degree based on peripheral sensory input. The limb that encounters an obstacle is reflexly lifted higher to clear the obstacle. However, the locomotor pattern is stereotyped and does not anticipate environmental constraints on locomotion or the needs of the animal. Nor does the spinal cord have the balance synergies that are essential for successful locomotion. The important point is that the actual programming of the muscle activation required for gait is present at the spinal level. Higher centers initiate gait and adapt it to individual needs. For straightforward walking, the higher centers need not specify the individual muscle activations necessary to produce locomotion but instead activate the central pattern generators. [1 , y This arrangement does not preclude supraspinal control of individual muscles during locomotion when precise foot placement is necessary. y
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