Postnatal development of the cutaneous flexor reflex: comparative study of preterm infants and newborn rat pups

Cutaneous flexor reflex thresholds have been used as a measure of somatosensory function: the authors measured threshold in preterm infants and newborn rat pups. Preterm infants of less than 30 weeks postconceptional age (PCA) had very low thresholds of less than 1.0 g, but by 37.5+ weeks PCA thresholds were equivalent to those in normal term babies, although these are still well below adult levels. Newborn rat pups also have very low flexor reflex thresholds, which do not approach adult levels until four postnatal weeks. Repeated stimulation of the foot in preterm babies resulted in sensitization of the flexion reflex up to about 32 weeks PCA. After that age repeated stimulation resulted in habituation, as is observed in the adult. Sensitization also occurred in newborn rat pups, which changed at four postnatal weeks to habituation. The results demonstrate the sensitivity of spinal reflexes to cutaneous inputs in the neonate and it is argued that this results from lack of inhibitory control in the immature spinal cord.