Cutaneous flexion reflex in human neonates: a quantitative study of threshold and stimulus-response characteristics after single and repeated stimuli.

The cutaneous flexion reflex has been used to study spinal sensory processing in 68 infants (37 female, 31 male) aged between 28 and 42 weeks postconceptional age (PCA). Mechanical and electrical stimuli were singularly and repeatedly applied to the foot, and single-surface EMG responses were recorded from the biceps femoris muscle. A clear correlation was demonstrated between the mechanical stimulus intensity and latency and the amplitude of the reflex. Mechanical threshold normally increased with age, but the flexion-reflex threshold was lowered by local limb-tissue damage in the contralateral limb. The incidence of response to repeated mechanical stimulation at 2.8 x threshold decreased significantly with increasing age. Repeated mechanical stimulation at 2.8 x threshold caused a build-up in the size of the response followed by a diminution. The flexion reflex can, therefore, be used to investigate sensory processing in the neonate, and the effects of tissue damage. The importance of using natural rather than electrical stimulation is highlighted.