fig3

Figure 3. Control of posture and gait by the brainstem and spinal cord. (A) The functional organization of the core posture-gait control system. The red dashed line encircles crucial structures in the mesopontine tegmentum in posture-gait control. They are the PPN, MLR, and SC. (a) Signals from the MLR activate both the excitatory RST (Exit-RST) and inhibitory RST (Inh-RST) in addition to monoaminergic pathways including the NA coerulospinal tract from the LC and serotonergic (5-HT) raphespinal tract from the RN; (b) PPN-ACh neurons activate the Inh-RST descending from the PMRF; (c) The balance between inhibitory and excitatory RSTs’ activities regulates postural muscle tone. RST also contributes to the alignment of the trunk and upper and lower limbs, as represented by the tonic neck reflex, which generates the so-called postural figures; (d) The VST from the VN contributes to balance control and extending the body; (e) The tectospinal tract from the SC is involved in gazing (saccade) by acting to the PGC in the paramedian pontine reticular formation. Various sensory inputs acting on the PMRF, VN, and SC evoke postural reactions (postural reflexes); (f) Supraspinal inputs to the spinal locomotor network, including the CPGs, generate locomotor rhythm, regulate locomotor pattern, and activate motoneurons innervating extensor (Ex) and flexor (Fx) muscles to elicit locomotion. Proprioceptive sensation from muscle spindles, tendon organs, joints, and skin modulates locomotion by acting on the locomotor network; (g) Monoaminergic pathways from the LC and RN contribute to modulating pain sensation and CPG activity; (h-i) Nigrostriatal DA projection (i) modulates activity of the intrinsic basal ganglia pathways (h) to alter GABAergic output from the internal segment of the GPi and SNr. GABAergic output from GPi/SNr to SC, MLR, and PPN modulates saccade, locomotion, and postural muscle tone, respectively; (B) Effects of skin afferent depend on step cycles. (a) During the swing phase, CPGs’ output facilitates the skin afferent’s transmission to flexor motoneurons and inhibits it to extensor motoneurons; (b) During the stance (support) phase, CPGs’ output facilitates the skin afferent’s transmission to extensor motoneurons and inhibits it to flexor motoneurons; (C) Stumble-corrective reaction. Stimuli to toe, indicated by filled arrows, during the stance phase, elicits leg flexion (a) but facilitates the extension of the leg, i.e., presses foot sole firmly against the grand during stance phase (b). PPN: Pedunculopontine tegmental nucleus; MLR: mesencephalic (midbrain) locomotor region; SC: superior colliculus; RST: reticulospinal tract; NA: noradrenaline; LC: locus coeruleus; 5-HT: serotonin (5-hydroxytryptamine); RN: raphe nuclei; ACh: acetylcholine; PMRF: pontomedullary reticular formation; VST: vestibulospinal tract; VN: vestibular nuclei; PGC: pontine gaze center; CPGs: central pattern generators; LC: locus coeruleus; DA: dopamine; GPi: globus pallidus; SNr: substantia nigra pars reticulata; INs: interneurons.