This review argues neuronal mechanisms of controlling posture and locomotion. Volitional gait behaviors are composed of “automatic control of posture and rhythmic limb movements” that operate in conjunction with “intentional control of precise movements of trunk, limbs and feet”. The automatic control of posture and rhythmic limb movements are mediated by the ventromedial descending motor system from the brainstem structures. Descending signals in this system activate spinal interneuronal circuits, which are termed as central pattern generator (CPG), automatically generate locomotor rhythm and pattern in conjunction with sensory afferents from muscles, joints and skins. On the other hand, when locomoting subject requires intentional gait modification, an activation of the dorsolateral descending motor system, or the lateral corticospinal tract, arising from the motor cortical areas is critically involved. An intentional gait control is always preceded by appropriate postural adjustments which are achieved by the motor programs in motor cortical areas including the supplementary motor area and premotor area. Information of bodily function such as body schema, which is always updated at the temporoparietal cortex on the basis of proprioceptive, visual, vestibular and auditory sensations, can be critically required to generate and develop the motor programs. The motor programs of postural control may be propagated to the brainstem, via the cortico-reticular projection, so that anticipatory postural adjustment can be performed by the activation of the ventromedial systems immediately before the onset of the intentional gait modification. On the other hand, motor programs of intentional gait control are transmitted to the primary motor cortex so that precise limb trajectory is generated, and accurate foot placement is achieved by the activation of the corticospinal tract. Signals from the basal ganglia and the cerebellum regulate “automatic process” by acting on the brainstem and “intentional process” by acting on the cerebral cortex. Operation of the basal ganglia largely affected by dopamine, and cerebellar function depends on feed-forward and feedback signals acting on the cerebellum. Dysfunction of either mechanism of the above processes may result in postural disturbance and/or gait failure.
