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Co-contraction of antagonist muscles is characteristic of spasticity arising from perinatal brain damage but not in spasticity occurring after brain damage in adulthood. Such co-contraction is a normal feature of early post-natal motor development. Heteronymous, monosynaptic Group Ia projections from biceps brachii to both the antagonist triceps brachii and to other synergist and non-synergist muscles of the upper limb occur in the newborn baby and become restricted during the first 4 years to motor neurons of primarily synergistic muscles. Longitudinal and cross-sectional studies have been performed to test the hypothesis that inappropriate heteronymous excitatory projections persist in children with perinatal brain damage who develop spasticity. Subjects with spasticity, from brain damage acquired in adulthood were also studied to determine if these projections simply become unmasked as part of spasticity, independent of the age of occurrence of the brain damage. Twenty-nine healthy newborn babies and 29 at high risk for cerebral palsy, 12 of whom developed spastic quadriparesis, were studied longitudinally for 4 years. Thirty-eight subjects, aged 8-30 years, with spasticity of perinatal origin (11 hemiplegic, 11 quadriplegic, 16 with Rett syndrome) and 11 subjects with stroke in adulthood and spastic hemiplegia were also studied. The results were compared with those obtained in 372 normal subjects aged from birth to 55 years. Small taps were delivered to the tendon of biceps brachii using an electromechanical tapper. Surface EMG was recorded from biceps and triceps brachii, pectoralis major and deltoid. In the longitudinal study, those developing spastic quadriparesis showed persistent low thresholds for the homonymous phasic stretch reflex, which had abnormally short onset latencies. There was persistence of short onset heteronymous excitatory responses in triceps brachii, while a normal pattern of restriction of heteronymous responses to pectoralis major and deltoid occurred. The same pattern was observed in older subject groups with spasticity of perinatal origin. In adults with hemiplegia following stroke the threshold of the homonymous phasic stretch reflex was low, but it had a normal onset latency. There was no evidence of abnormal heteronymous excitatory responses. In conclusion, exaggerated excitatory responses to primary muscle afferent input were observed in the homonymous (biceps brachii) and antagonist (triceps brachii) motor neurons in subjects with spasticity arising from perinatal brain damage. They are likely to play an important role in the predominant co-contraction of agonist/antagonist muscles during voluntary movement observed in subjects with spastic cerebral palsy.

brain

Abnormal development of biceps brachii phasic stretch reflex and persistence of short latency heteronymous reflexes from biceps to triceps brachii in spastic cerebral palsy