Brain Stem

The brain stem has three important functions in balance and locomotion: (1) setting the activity of the spinal central pattern generators to determine the initiation and speed of locomotion; (2) setting postural tone; and (3) modulating the force generated by the muscles activated by the central pattern generators.

Control of the spinal cord central pattern generators is exerted by the subthalamic and midbrain locomotor regions and relayed to the spinal cord by the reticulospinal tracts. The subthalamic locomotor region is defined physiologically as an area in the posterior lateral hypothalamus from which locomotion can be evoked by stimulation in the decerebrate animal (an animal whose brain stem has been transected just below the thalamus). The subthalamic locomotor region is not in the subthalamic nucleus but is medial and dorsal to it. This region is not associated with any particular cell group, and stimulation of it may excite axons passing through the area. A cat with an intact thalamus and basal ganglia but with the cortex removed walks continuously in a nondirected manner, termed obstinate progression. This behavior has been interpreted as disinhibition of the subthalamic locomotor region or of the influences that pass through the area. Stimulation of the subthalamic locomotor region in an intact cat induces locomotion and searching behavior. [4

The midbrain locomotor region is also defined physiologically but appears to overlap at least partially the pedunculopontine nucleus. This fact is important because the pedunculopontine nucleus receives a major input from the basal ganglia and from the sensorimotor and limbic cortex. This area may represent one way in which the cortex and basal ganglia affect the initiation of locomotion. y Progressive increases in electrical stimulation of the midbrain locomotor region of decerebrate monkeys induce stepping first in the contralateral limb, then in both limbs, and finally in a gallop. The midbrain locomotor region can also be stimulated by injections of gamma-aminobutyric acid (GABA) antagonists that are thought to disinhibit the midbrain locomotor region from tonic GABAergic input from the basal ganglia.

Stimulation of the midbrain locomotor region in the neurologically intact cat induces locomotion that appears to be initiated to avoid a painful or frightening stimulus. y

Output for the locomotor regions descends by way of the medullary reticulospinal tract, which arises from the ventral medial reticularis gigantocellularis and magnocellularis nuclei. This tract traverses the ventral medial spinal cord, and lesions of the tract prevent locomotion and impair balance.. y ,[7]

The role of the brain stem in postural control is evident by the postural responses that can be elicited in decerebrate animals. Vestibular, neck, and righting reflexes are present in decerebrate animals that have an intact brain stem. [8] Stimulation in the dorsal tegmental fields of the pons causes an intact cat to cease walking and reduces postural tone, so that the cat sits and then lies down. Conversely, more inferior stimulation in the ventral tegmental fields of the pons increases postural tone and induces locomotion, so that a supine cat rises and then begins to walk. These stimulation sites indicate that postural and locomotor responses may be mediated, at least partially, by the same brain stem structures. [4

Brain stem systems do not simply turn on the spinal central pattern generators, as is done experimentally by electrical stimulation of the locomotor regions. Recordings from reticulospinal neurons that activate the central pattern generators as well as from vestibulospinal and rubrospinal neurons demonstrate that activity in these pathways is phase-locked to the step cycle. This phasic activity is largely abolished if the cerebellum is ablated. The phasic activity in these descending tracts does not determine the frequency of stepping but enhances its force and rhythmicity. y , [?] The tectospinal tract influences the posture of the head and neck and its orientation to visual targets. This tract arises from the superior colliculus, a structure that receives a major input from the substantia nigra pars reticulata, hence providing another connection between the basal ganglia and the postural and locomotor systems.

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