Morphological and physiological studies of development of the monosynaptic reflex pathway in the rat lumbar spinal cord.

1. The developmental process of the monosynaptic reflex pathway was investigated morphologically and electrophysiologically in isolated lumbar spinal cords of new-born and fetal rats. 2. Dorsal root fibres were stained with horseradish peroxidase in the fourth lumbar (L4) segment at different ages ranging from embryonic day (E) 15.5 to post-natal day (P) 0. At E15.5, several collaterals issued from axons in the dorsal funiculus and reached the dorsal part of the dorsal horn. At E16.5, the number of collaterals entering the grey matter increased. Also, a group of collaterals extended ventralwards forming a bundle, and reached the intermediate region. At E17.5, a small number of collaterals reached the motor nuclei. The number of collaterals entering the motor nuclei increased almost linearly with age: 0 at E15.5 and at E16.5, 27 at E17.5, 184 at E18.5, 432 at E19.5 and 746 at P0. 3. The tips of collaterals and their branches had growth cones, boutons (round or oval varicosities) or other varicosities. The mean number of branches with these structures per collateral in the motor nuclei was 1.2 at E17.5, 2.5 at E18.5, 3.6 at E19.5 and 5.8 at E20.5. 4. The percentage of collaterals having growth cones in the motor nuclei was 75% at E17.5, 70% at E18.5, 38% at E19.5 and 15% at E20.5. 5. The mean number of boutons per collateral in the motor nuclei was 0.6 at E17.5, 3.2 at E18.5, 4.9 at E19.5 and 10.7 at E20.5. This increase with age was caused by both branching of collaterals and the increase in the number of boutons of the en passant type. The estimated total number of boutons in the motor nuclei of the L4 segment steeply increased after E17.5: 16 at E17.5, ca. 600 at E18.5, ca. 2000 at E19.5 and ca. 8000 at E20.5-P0. 6. Stimulation of the L4 dorsal root evoked a reflex response in the L4 ventral root, recorded as the ventral root potential, in two out of nine preparations at E15.5 and in all preparations at and after E16.5. The onset of the ventral root potential indicated the onset of excitatory post-synaptic potentials in motoneurones. The segmental latency of the ventral root potentials was markedly shortened between E17.5 and E18.5 (from 12.5 to 7.2 ms) and essentially unchanged at the later stages. 7. The magnitude of monosynaptic reflex responses in the L4 segment gradually increased with age during prenatal stages, becoming maximal at P2-3 and then decreased at the following stages (P4-P8).(ABSTRACT TRUNCATED AT 400 WORDS)