Neurocircuitry of gait: Brainstem integration of rhythmicity and goal-directed behavior in normal and pathological states.

This review explores the pivotal role of the brainstem in regulating gait, highlighting its function in modulating the balance between rhythmic motor patterns and goal-directed behaviors. Gait is a voluntary motor behavior that involves the continuous interplay of rhythmic motor control and higher-order cognitive processes. Key brainstem structures, including the pedunculopontine nucleus (PPN), dorsal raphe nucleus (DRN), and gigantocellular nucleus (GiN), integrate sensory inputs, motivational states, and contextual cues to modulate locomotion. These nuclei maintain efficient motor adaptation and conserve cognitive resources by coordinating brainstem and spinal cord circuitry (i.e., central pattern generators) which "automate" the control of muscles that produce the rhythmical locomotor behaviors. While this review primarily focuses on these three key nuclei, it also briefly discusses additional brainstem structures involved in gait and postural control, such as the vestibular nuclei, red nucleus, and inferior olive. Disruptions in the brainstem networks due to neurodegenerative diseases like Parkinson's disease and aging impair the ability to adjust the level of cognitive control required for gait, necessitating increased cognitive effort for motor and balance regulation. By presenting a hierarchical model of gait control, this review provides insight into the dynamic interplay between brainstem, spinal pattern generators, and cortical systems in locomotion control.