PERINATAL-DEVELOPMENT OF ACTION-POTENTIAL PROPAGATION IN CAT RUBROSPINAL AXONS

1. The development of action potential conduction was studied by intra cellular recording of antidromic spices in cat rubrospinal cells. 2. T he distance between the C1 and L1 spinal segments increased linearly f rom 5.6 cm at embryonic day (E) 59 to 9.8 cm at postnatal day (P) 30. 3. The conduction time from the C1 segment to the rubrospinal neuron s oma, estimated from antidromic spike latency evoked by stimulation of the C1 segment, decreased rapidly prior to birth and then slowly there after. This coincided with a reduction in conduction time variation be tween cells. 4. The conduction time from the red nucleus to the L1 seg ment followed a similar time course during development. The conduction time reached the adult value by P30, at which time the spinal cord is only half the adult length. 5. The conduction velocity between the C1 and L1 segments increased monotonically between E59 and P30, from a l ow of 1 m s(-1) to a maximum of 34 m s(-1). 6. The rise time of rubros pinal neuron somadendritic spikes followed a developmental time course similar to that for conduction time. 7. Myelination of rubrospinal ax ons, as judged by the presence of myelinated segment spikes, began to occur prior to E59. 8. These findings suggest that development of acti on potential propagation in rubrospinal cells can be divided into an e arly and a late stage: conduction time reaches the adult value during the early stage, i.e. by the first postnatal month, and is maintained during the late stage. We propose that myelination, axon diameter incr ease and maturation of membrane properties act to reduce conduction ti me to adult values during the early stage, while a proportional increa se in fibre diameter with axonal length results in a constant conducti on time during the late stage.