Excitability of the monosynaptic reflex pathway in the child from birth to four years of age.

The excitability of the monosynaptic reflex pathway was tested in infants and children from birth to four years of age during waking and sleeping. The H reflex, evoked in the soleus muscle, was used to calculate the ratio, maximal H to maximal M, construct recruitment curves and test reflex excitability by the paired stimulus method. The excitability of the monosynaptic reflex pathway during the waking state is greatest at birth; it diminishes gradually during the first years of life without reaching the average level observed in adults by the end of the fourth year of age. From birth, regardless of age, sleep always depresses excitability. The mechanism that underlie this change with age, and with waking or sleeping, are discussed.