The postnatal functional development of muscle stretch receptors in the rat.

The response to a 5-sec stretch of the triceps muscle was studied in dorsal root filaments L5 of 72 infant rats (1-19 days old) under urethane anesthesia. More than 50% of all units in 1-day-old rats responded by repetitive firing until the end of the 5-sec stretch (slowly adapting or SA receptors), while the rest ceased to fire earlier (relatively rapidly adapting or 1/2 SA receptors), or gave an "on" response only. The number of units exhibiting an SA response increased with age and attained 80% in 5-day-old rats. By the 10th day of life, almost 90% of endings behaved as SA receptors. During development, the maximal discharge frequencies at the peak of stretch increased markedly, and their values in 18-day-old rats were comparable to those in adult rats. The phasic component of the response to stretch, although less well defined in the younger animals, was already present even in 1-day-old rats. Adaptation of the static response during maintained stretch was relatively steep in all the age groups studied. The results indicate that, in the rat, large numbers of muscle stretch receptors are capable of responding to sustained stretch as SA receptors, even at an age when their morphological and ultrastructural maturation is not yet fully accomplished.