Rubrospinal Tract

As shown in the illustration, rubrospinal fibers decussate almost immediately on leaving the red nucleus to descend through the lateral part of the brainstem to the spinal cord. In the cord, the tract lies in the posterolateral funiculus, just anterior to the lateral corticospinal tract. The distal branches of the rubrospinal fibers terminate in the intermediate regions and anterior horn (laminae V, VI, and VII) of the spinal gray matter (see Plate 2-12).

The rubrospinal tract influences the motor neurons in the anterior horns, primarily through its action on inhibitory or excitatory interneurons, but in primates some fibers end directly on anterior horn motor neurons. The predominant pattern of rubrospinal action is to facilitate flexor motor neurons and thus excite limb flexor muscles and to inhibit the corresponding extensor muscles via interneurons. However, a number of rubrospinal fibers have the opposite action. This allows a wide variety of movements to be executed by the selective activation of appropriate groups of rubrospinal neurons. The rubrospinal tract may thus be responsible for much of the relatively fine control of the extremities—discriminative movement that is retained when the pyramidal tract is damaged. In animals, lesions involving both the pyramidal and rubrospinal tracts result in a much greater deficit in distal movement than that obtained from a lesion of either tract alone.

Rubrospinal control of afferent input to the spinal cord takes the form of presynaptic inhibition acting at the central posterior horn terminals of fibers from Golgi tendon organs and cutaneous receptors.

The two major sources of the input that controls the activity of rubrospinal neurons are the cerebellum and the cerebral cortex. The cerebellar projection to the red nucleus consists primarily of fibers from the interposed (emboliform and globose) nuclei, which cross in the decussation of the superior cerebellar peduncle (brachium conjunctivum) to excite the red nucleus neurons of the opposite side. Neurons of the red nucleus are also excited by branches of small pyramidal cells from the ipsilateral motor cortex (see Plate 2-8). Afferents from the motor and premotor cerebral cortex synapse on their distal dendrites and from the cerebellum on their proximal dendrites and cell bodies. In addition, activity in pyramidal tract axons from giant neurons in the same cortical region exerts an opposite, inhibitory effect on rubrospinal neurons via inhibitory interneurons. The input as well as the output of the red nucleus is somatotopically organized. Thus rubrospinal fibers projecting to the lumbar part of the spinal cord originate from neurons in the lateral part of the nucleus. This same region receives input from regions of the cerebellar deep nuclei and motor cortex related to control of the lower limbs. Conversely, the medial part of the red nucleus, which contains neurons projecting to cervical levels of the spinal cord, receives input from cerebellar and cerebral regions responsible for control of the arms. This pattern of organization allows for the selective activation of individual extremities by different groups of rubrospinal neurons.

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