Tracts descending from motor control centers in the brainstem and cortex target motor interneurons and in select cases motoneurons. The mechanisms and constraints of postural control are elaborated and the effect of body mass on posture discussed. Feed-forward reflexes that maintain posture during standing and other conditions of self-motion are described. The role of descending tracts in postural control and the pathological posturing is described. Pyramidal (corticospinal and corticobulbar) and extrapyramidal control of body and face movements is contrasted. Special emphasis is placed on cortical regions and tracts involved in deliberate control of facial expression; these pathways are contrasted with mechanisms for generating emotional facial expressions. The signs associated with lesions of either motoneurons or motor control centers are clearly detailed. The mechanisms and presentation of cerebral palsy are described. Finally, understanding how pre-motor cortical regions generate actions is used to introduce apraxia, a disorder of action.
