Developmental changes in rhythmic spinal neuronal activity in the rat fetus.

In the developing rat spinal cord, formation and differentiation of the central pattern generator for locomotion occur during the prenatal period. Early on, excitatory synaptic transmission mediated by glycine receptors plays a leading role for rhythmogenesis, at a later stage, followed by glutamate-receptor-mediated synaptic transmission becoming dominant. The maturation of inhibitory circuitry in the spinal cord, mediated largely by glycinergic synapses, is crucial for the generation of alternating activity between left/right limbs and flexor/extensor muscles. Formation of left/right alternation is presumably due to developmental changes in the properties of the postsynaptic neurons, themselves, whereas flexor/extensor alternation requires the additional emergence of inhibitory synaptic functions in the spinal cord.