DEVELOPMENT OF THE STRETCH REFLEX IN THE NEWBORN - RECIPROCAL EXCITATION AND REFLEX IRRADIATION

The stretch reflex is a spinal cord pathway between a muscle's stretch receptors and its own motor units. This reflex is thought to play an important role in normal motor function, because it is unique as a mon osynaptic pathway, and because its hyperactivity is a hallmark of many motor disorders. We previously reported a difference in the stretch r eflexes in healthy newborn infants and nonambulatory individuals with cerebral palsy (CP): these reflexes are characterized by responses fro m the stretched muscle and from the reciprocal or antagonist muscle. W e proposed the existence of a functional spinal cord pathway that conn ects both agonist and antagonist muscles at a single joint. We hypothe sized that this ''reciprocal excitation'' is a functional pathway of a ll newborn infants, which during the normal course of development of m otor skills in infants is eliminated. if the CNS is damaged at birth, as in CP, the pathway of reciprocal excitation endures and is reinforc ed. In the current study of newborns, we recorded stretch reflex respo nses from all major muscle groups of the lower limb. This ''irradiatio n of reflexes'' is a normal phenomenon of the newborn CNS. This pathwa y becomes suppressed during normal maturation and control of coordinat ed limb movements.