The basal ganglia may have direct input to the brain stem nuclei controlling posture and balance via the pedunculopontine nucleus (midbrain locomotor region) and the superior colliculus (tectospinal tract). [5 The majority of the basal ganglia output is directed back to the frontal cortex, and this influence is presumably responsible for the hallmarks of disease of the basal ganglia, hypokinesia and hyperkinesia (see ChapterJ6) . The execution of locomotor and postural responses, like other movements, may also be hypokinetic (parkinsonian) or hyperkinetic (choreic, dystonic). Cortex
The cortex is completely unnecessary for routine walking in the cat, as evidenced by the performance of decorticate cats (animals in which the cortex has been removed but the basal ganglia and thalamus are left intact). However, the cat with frontal cortex or medullary pyramid lesions cannot perform stepping that requires precise placement of the feet, for example, walking on the rungs of a horizontal ladder. This indicates that "skilled walking" requires input from the motor cortex. The motor cortex has neurons that fire in phase with stepping and, therefore, like the reticulospinal, vestibulospinal, and rubrospinal systems, it probably facilitates stepping. Stimulation of the medullary pyramid can reset the step cycle. This indicates that the corticospinal tract can affect the central pattern generators and may have a role in initiating and modifying the locomotor pattern produced by the central pattern generators. [3 The sensorimotor cortex also has connections to the midbrain locomotor region and may affect the central pattern generators through this link. Stimulation of the motor cortex can induce discrete movements of a limb, preceded by the appropriate anticipatory postural responses, indicating that the cortex has a role in anticipatory postural responses. Lesions of the frontal cortex may also affect anticipatory reflexes. [iii
Other anatomical areas are undoubtedly important in balance and locomotion. The areas reviewed here are responsible for the proper mechanical execution of balance and locomotion in a relatively stereotyped fashion. They do not explain the adaptation of gait to the individual's goals, the perception of the environmental dangers to balance and upright position, and the development of appropriate strategies to cope with environmental hazards. It is important to realize that the posture and gait observed in an individual are a product of the more or less hard-wired locomotor and postural responses modified by the individual at conscious and subconscious levels to adapt these responses to his or her needs. Posture and gait patterns are not fixed and stereotyped neurological signs as are deep tendon reflexes.
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