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The brain stem contains motor neurons that send their axons through certain cranial nerves, primarily to muscles of the tongue, face, and eyes. Like the spinal motor pools, many of these cranial motor nuclei receive direct input from sensory neurons and less direct influences from proprioceptive interneurons. Brain stem CPGs generate rhythmic movements, such as those underlying breathing and chewing.

Some parts of the brain stem interact with spinal CPGs and other components of the spinal motor system. One such region has been termed the midbrain locomotion region, which is thought to trigger the

Figure 1 Major components of the motor system. Corticofugal projections are depicted as gray arrows. Projections to effectors are depicted as open arrows and arrowheads. The basal ganglia are contained within the gray box. Preganglionic autonomic motor nuclei are shown as stippled ovals. GPi, internal segment of globus pallidus; GPe, external segment of globus pallidus; STN, subthalamic nucleus; SNr, substantia nigra pars reticulata.

Figure 1 Major components of the motor system. Corticofugal projections are depicted as gray arrows. Projections to effectors are depicted as open arrows and arrowheads. The basal ganglia are contained within the gray box. Preganglionic autonomic motor nuclei are shown as stippled ovals. GPi, internal segment of globus pallidus; GPe, external segment of globus pallidus; STN, subthalamic nucleus; SNr, substantia nigra pars reticulata.

activity of spinal CPGs and thereby initiate locomotion. However, higher order networks, akin to CPGs but having more complex output patterns, have an important role in a number of instinctive behaviors, including aggressive posturing (a form of "body language'') and inarticulate vocalization (such as crying, laughing, and screaming). Some of these networks are located in and near a midbrain structure called the periaqueductal gray.

a. Reticulospinal System Cells in the brain stem reticular formation that project to the spinal cord make up the reticulospinal system, which extends through medullary, pontine, and midbrain levels. The reticulospinal system performs a diverse set of functions, including the regulation of muscle tone, control of posture and locomotion, and integration of lower order motor signals with those emanating from the cerebellum and cerebral cortex. Different reticulospinal pathways exert influences on flexor versus extensor muscles and on proximal versus distal parts of the limb.

Part of the reticulospinal system serves as a fast transmission route to postural motor neurons and helps prevent movements from destabilizing balance.

For example, when a person lifts a heavy object, the leg muscles need to stiffen before the elbow flexes. This postural adjustment prevents the object's weight from pulling the person off balance. The reticulospinal system activates leg muscles to stiffen them and help preserve balance. On the whole, while people are awake the reticulospinal system has a predominantly facilatory influence on motor pools. However, this effect changes dramatically during sleep. Then, re-ticulospinal neurons exert a strong inhibitory influence that, for example, prevents the performance of imagined actions during dreams.

Important influences over the reticulospinal system come from other systems, including vestibular afferents, which signal movements of the head and its orientation with respect to the earth's gravitational field, and the motor cortex, which provides information otherwise unavailable at brain stem levels. Through vestibulospinal projections, the vestibular system can contribute directly to various reflexes that adjust head position, posture, and limb movements. However, the vestibular afferents also provide inputs to the reticulospinal system. Consider the role of the reti-culospinal system as a person runs through a field of obstacles. The signals conveyed by the reticulospinal system to CPGs and spinal motor pools adjust posture and movement based primarily on vestibular and proprioceptive inputs. However, cortical and other higher order inputs supply the information needed for dynamic motor adjustments that allow people to step over and around visible obstacles.

b. Cerebellum and Red Nucleus The largest component of the brain stem motor system is the cerebellum. The medial cerebellum controls posture, whereas the lateral cerebellum participates more in voluntary movement. Accordingly, vestibular and propriospinal inputs predominate in the medial cerebellum, and inputs to the lateral cerebellum arise mainly from the cerebral cortex, relayed through mossy fibers originating in the basilar pontine nuclei. In addition, the cerebellum receives mossy fiber input from the red nucleus via the lateral reticular nucleus (which also has major spinal inputs) and from other sources. Mossy fibers terminate on the output nuclei of the cerebellum (the deep cerebellar nuclei) as well as on neurons in the cerebellar cortex. Another type of input, conveyed by climbing fibers originating in the inferior olivary complex, is thought to signal motor error or discoordination. These signals may play a central role in motor learning (see Section IV).

The output of the cerebellar cortex comes from GABAergic Purkinje cells, which inhibit neurons in the deep cerebellar nuclei and in one of the vestibular nuclei. The deep cerebellar nuclei send excitatory outputs to a variety of structures. Their largest projections terminate in the thalamus (Fig. 1), but other efferents reach the reticulospinal system, red nucleus, superior colliculus, and spinal cord. Cerebel-lar outputs to many of its targets are accompanied by return projections through a variety of direct and indirect pathways. One example is the cerebellar projection to the motor cortex (via the thalamus), which is returned by a cortical projection to the cerebellum (via the basilar pontine nuclei). Recurrent, excitatory circuits such as this are thought to form functional networks termed cortical-cerebellar modules.

The red nucleus plays an enigmatic role in motor control, especially in the human brain, but appears to be intimately related to cerebellar function. It receives a major projection from the deep cerebellar nuclei as well as from the motor cortex, and the largest part of the red nucleus (its parvocellular, or small-cell, component) projects predominantly to the inferior olivary complex, the source of cerebellar climbing fibers. The magnocellular (large-cell) red nucleus sends its axons directly to the spinal cord through the rubrospinal tract, which might be particularly important in stabilizing the limb by coactivating agonist and antagonist muscles. However, the magnocellular red nucleus is said to be relatively small in the human brain, which may reflect a dominant role of cortical motor control in our species.

c. Superior Colliculus The superior colliculus, although typically discussed in terms of eye-movement control, also has an important role in the control of head movements. Generally stated, its function involves the orientation of the retina and other receptors on the head, which the superior colliculus guides through its interaction with the reticulospinal system, premotor neurons in the brain stem reticular formation, and direct projections to the spinal cord (the tectospinal system).

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