Spinal reflex organization in early development: Electrophysiological measures and proposed motor pathways

Abstract

The delay in developmental milestones in cerebral palsy (CP) is presumed to result from a nonprogressive insult to the developing central nervous system (CNS). In CP, the site(s), type, duration, and magnitude of CNS insult(s) may never be defined, and a single focus of damage may not be revealed by standard brain imaging methods. The neurological examination (i.e., assessment of reflexes, tone, and motor coordination) generally does not define the insult with precision, nor can it predict the delays in motor skill development. The CNS damage may only be suspected from the medical history during the prenatal or perinatal period; confirmation of CNS insult may be deferred until the child misses developmental milestones in the first few months or years of life. In CP, patterns of motor incoordination and delay emerge gradually over time during development. While the original CNS injury is presumed to be a nonprogressive insult to higher motor centers (e.g., cerebral motor cortex), the damage probably causes significant modulation of functional neural connections during maturation of the brain and spinal cord. Such aberrant communication in motor pathways may be caused by, or result in, motor incoordination. In turn, repetition of uncoordinated movements may reinforce aberrant neural connections with, or at, the spinal cord. In this paper, we review neural responses to stimulation of sensory and motor systems and compare data for healthy neonates and children, and children and adults with CP. Age-related differences are identified in measured onset latencies of responses, computed central conduction delays, and relative levels of excitatory drive. Differences in muscle responses are shown among subject groups for myotatic reflex activation of agonist, antagonist, and more distant limb muscles. From these data, inferences have been made that, in CP, impairments in CNS circuitry may include descending corticospinal pathways and/or spinal reflex pathways (e.g., “reciprocal excitation”). Data from a number of investigators support the theory that changes in spinal reflex pathways are likely to play a fundamental role in the development of motor control. MRDD Research Reviews 3:175-183, 1997. © 1997 Wiley-Liss, Inc.