Corticospinal input onto motor neurons projecting to ankle muscles in individuals with cerebral palsy

Cross-correlograms between voluntarily active soleus (SOL) and tibialis anterior (TA) motor units were generated from seven control subjects and six subjects with spastic cerebral palsy (CP). Short-duration central peaks were observed in three subjects with spastic diplegia only. All subjects demonstrated reciprocal inhibition in TA following electrical stimulation of group I afferents to SOL, and all subjects with CP demonstrated strong activation of both TA and SOL in response to transcranial magnetic stimulation. Responses in SOL were stronger than those observed from controls. These data support the existence of abnormal corticospinal projections to soleus motor neurons in individuals with spastic CP. In spastic diplegia, short-term discharge synchrony between SOL and TA motor units may reflect abnormal interneuronal modulation at the spinal level. Abnormal corticospinal projections and/or modulation of spinal interneurons may contribute to the disordered movement patterns and co-activation observed in this population.