Development of Inhibitory Pathways in the Rat Spinal Cord

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Development of inhibitory pathways in the lumbar spinal cord was investigated electrophysiologically using an isolated spinal cord preparation in fetal and new-born rats.1. Development of inhibition in the reflex pathway : Intracellular recordings from lumbal motoneurones at different ages revealed that inhibitory postsynaptic potentials (IPSPs) following stimulation of the dorsal root first appeared at embryonic days (E) 16.5, 5 days before birth. The frequency of occurrence of IPSPs were less than that of excitatory postsynaptic potentials (EPSPs) in individual motoneurones at this stage, indicating that inhibitory pathways would develop later than excitatory ones in the reflex arc. Chronic transection of the lower thoracic cord at E15.5 could not affect the later development of the inhibitory pathways.2. Development of inhibition in the rhythm generator : A neuronal circuit generating rhythmic motor activity was activated by bath application of N-methyl-D, L-aspartate (NMA). Rhythmic activities in the lumbar ventral roots first appeared at E15.5, though srychnine-sensitive inhibitoty action began toqfunction at E17.5 in this neuronal ciruit. The inhibition changed the pattern of activities in the ventral root on two sides from synchronized to alternating, as is the pattern of locomotion.