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Figure 2. Neural bases of posture-gait control. (A) Basic signal flows in motor control. Locomotor behaviors are composed of volitional (a), emotional (b), and automatic (c) processes. Whether volitional or emotional, the automatic process by the core locomotor system in the brainstem and spinal cord is required. On the other hand, interaction between the volitional (intentional) and emotional processes is essential in selecting either active avoidance or escaping, such as when facing danger; (B) Role of sensory signals and neurotransmitters. Signals from visual, vestibular, auditory, somatosensory (proprioceptive), and visceral sensations act on various sites in the brain. Signals in these sensory systems acting on the brainstem and spinal cord contribute to the modulation of automatic posture-gait control. On the other hand, those acting on the limbic system and cerebral cortex modulate emotional and volitional processes of posture-gait control, respectively. DA neurons in the SNc project to the basal ganglia. Those in the VTA project to the prefrontal cortex and the limbic regions. DA neurons also project to the SC, PPN, and cerebellum. Cholinergic (ACh) BF neurons innervate to broader regions of the cerebral cortex. PPN-ACh project to the thalamus (Thal.), basal ganglia, brainstem, cerebellum, and spinal cord. Accordingly, each neurotransmitter’s action alters the activity of the automatic, emotional, and volitional posture-gait control systems. In addition, the cholinergic projections to the thalamic nuclei regulate arousal levels and sensory processing as one of the ascending reticular activation systems; (C) Hypothetical neural mechanisms that are essential for bipedal posture-gait control. Parieto-frontal pathways (a) send cognitive information to the PFC (b) and PMC (c). The cortico-brainstem projection from the PMC to the posture-gait relating areas in the midbrain (PPN, MLR, and SC) and the PMRF (cortico-reticular projection) (d) and that from the parietotemporal cortex, such as the PIVC to the VN (cortico-vestibular projection; e) contribute to the biped posture-gait control. Please look at the text for a detailed explanation. DA: Dopaminergic; SNc: substantia nigra pars compacta; VTA: ventral tegmental area; SC: superior colliculus; PPN: pedunculopontine nucleus; ACh: acetylcholine; BF: basal forebrain; PFC: prefrontal cortex; PMC: premotor cortices; MLR: midbrain (mesencephalic) locomotor region; SC: superior colliculus; PMRF: pontomedullary reticular formation; PIVC: perieto-insular vestibular cortex; VN: vestibular nuclei.
