Development of the monosynaptic reflex pathway in the human spinal cord.

Development of the monosynaptic reflex pathway of the spinal cord was investigated in 96 neurologically normal infants with ages ranging from 25 weeks in postconceptional age (PCA) to 24 months after full-term delivery (PDA) by examining H-reflexes from the triceps surae and hypothenar muscles in terms of their incidence, latency and maximal size in reference to the maximal M-wave. The triceps H-reflex was evoked in all cases, and the latency was longest (26 ms) in the youngest case of 25 weeks (PCA). It gradually shortened until full-term gestation, reaching the shortest value of 17 ms (mean). The H-reflex size initially increased until full-term gestation, reaching the maximum value of 70% and then reducing gradually to the plateau level of about 30% at 12 months (PDA). The hypothenar H-reflex could not be elicited until 32 weeks (PCA). The time course of changes in its latency and size was similar to those of the triceps H-reflex, except that it could not be elicited after 12 months (PDA). Thus, the monosynaptic reflex pathway is already functioning at the age of 25 weeks (PCA) in man. The significance of the systematic change in latency and excitability of the H-reflex with age is discussed.