Structural and functional maturation of the buccal stretch receptors in rats.

Postnatal functional and structural development of the buccal stretch receptor (BSR) of rats was investigated, using electrophysiological and morphological techniques. For functional analysis, sustained discharges in response to ramp-and-hold stretches were recorded from the BSRs isolated from animals aged 10 days to 10 weeks. The threshold amplitude of stretch for a sustained discharge fell significantly between 10 days and 3 weeks, reaching adult values at 5 weeks of age, while the static sensitivity increased conspicuously between 2 and 4 weeks after birth. On the other hand, between 1 and 4 weeks of age, apparent structural changes in the BSR were observed on the number of preterminal branches in a sensory unit, the size of the varicose-like swellings along the terminal axon, the density of collagen and elastic fibers around the core structure, and the content of the sub-capsular space. From these results, we suggest that the increase in the density of the connective tissue around the core structure is associated with an enhancement in the elasticity of the BSR in the early postnatal stages, decreasing the threshold amplitude of stretch for a sustained discharge. One possible explanation for the maturation of the static sensitivity of this receptor is growth of the sensory axon terminals filled with dense mitochondria.