The Brainstem Is the Origin of Three Descending Tracts That Influence Movement

Three brainstem nuclear groups give rise to descending motor tracts that influence motor neurons and their associated interneurons. These consist of the red nucleus, the vestibular nuclear complex, and the reticular formation (Fig. 5.11). The other major descending influence on the motor neurons is the corticospinal tract, the only volitional control pathway in the motor system. In most cases, the de-

Rubrospinal Tract

^MflfflREHn^^ Brainstem nuclei of descending motor path-

^tmmmU^^mw ways. The magnocellular portion of the red nucleus is the origin of the rubrospinal tract. The lateral vestibular nucleus is the source of the vestibulospinal tract. The reticular formation is the source of two tracts, one from the pontine portion and one from the medulla. Structures illustrated are from the monkey. SC, superior colliculus, ca, cerebral aqueduct; IV v., fourth ventricle, Red nucleus mc, red nucleus magnocellular area.

^MflfflREHn^^ Brainstem nuclei of descending motor path-

^tmmmU^^mw ways. The magnocellular portion of the red nucleus is the origin of the rubrospinal tract. The lateral vestibular nucleus is the source of the vestibulospinal tract. The reticular formation is the source of two tracts, one from the pontine portion and one from the medulla. Structures illustrated are from the monkey. SC, superior colliculus, ca, cerebral aqueduct; IV v., fourth ventricle, Red nucleus mc, red nucleus magnocellular area.

scending pathways act through synaptic connections on in-terneurons. The connection is less commonly made directly with motor neurons.

The Rubrospinal Tract. The red nucleus of the mesen-cephalon receives major input from both the cerebellum and the cerebral cortical motor areas. Output via the rubrospinal tract is directed predominantly to contralateral spinal motor neurons that are involved with movements of the distal limbs. The axons of the rubrospinal tract are located in the lateral spinal white matter, just anterior to the corticospinal tract. Rubrospinal action enhances the function of motor neurons innervating limb flexor muscles while inhibiting extensors. This tract may also influence gamma motor neuron function.

Electrophysiological studies reveal that many rubrospinal neurons are active during locomotion, with more than half showing increased activity during the swing phase of stepping, when the flexors are most active. This system appears to be important for the production of movement, especially in the distal limbs. Experimental lesions that interrupt rubrospinal axons produce deficits in distal limb flexion, with little change in more proximal muscles. In higher animals, the corticospinal tract supersedes some of the function of the rubrospinal tract.

The Vestibulospinal Tract. The vestibular system regulates muscular function for the maintenance of posture in response to changes in the position of the head in space and accelerations of the body. There are four major nuclei in the vestibular complex: the superior, lateral, medial, and inferior vestibular nuclei. These nuclei, located in the pons and medulla, receive afferent action potentials from the vestibular portion of the ear, which includes the semicircular canals, the utricle, and the saccule (see Chapter 4). Information about rotatory and linear motions of the head and body are conveyed by this system. The vestibular nuclei are reciprocally connected with the superior colliculus on the dorsal surface of the mesencephalon. Input from the retina is received there and is utilized in adjusting eye position during movement of the head. Reciprocal connections to the vestibular nuclei are also made with the cerebellum and reticular formation.

The chief output to the spinal cord is the vestibu-lospinal tract, which originates predominantly from the lateral vestibular nucleus. The tract's axons are located in the anterior-lateral white matter and carry excitatory action potentials to ipsilateral extensor motor neuron pools, both alpha and gamma. The extensor motor neurons and their musculature are important in the maintenance of posture. Lesions in the brainstem secondary to stroke or trauma may abnormally enhance the influence of the vestibulospinal tract and produce dramatic clinical manifestations (see Clinical Focus Box 5.1).

The Reticulospinal Tract. The reticular formation in the central gray matter core of the brainstem contains many axon bundles interwoven with cells of various shapes and sizes. A prominent characteristic of reticular formation neurons is that their axons project widely in ascending and descending pathways, making multiple synaptic connections throughout the neuraxis. The medial region of the reticular formation contains large neurons that project upward to the thalamus, as well as downward to the spinal cord. Afferent input to the reticular formation comes from the spinal cord, vestibular nuclei, cerebellum, lateral hypothalamus, globus pallidus, tectum, and sensorimotor cortex.

Two areas of the reticular formation are important in the control of motor neurons. The descending tracts arise from the nucleus reticularis pontis oralis and nucleus reticularis pontis caudalis in the pons, and from the nucleus reticularis gigantocellularis in the medulla. The pontine reticular area gives rise to the ipsilateral pontine reticulospinal tract, whose axons descend in the medial spinal cord white matter. These axons carry excitatory action potentials to interneurons that influence alpha and gamma motor neuron pools of axial muscles. The medullary area gives rise to the medullary reticulospinal tract, whose axons descend mostly ipsilateral in the anterior spinal white matter. These axons have inhibitory influences on interneurons that modulate extensor motor neurons.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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