Red Nucleus Serves as an Alternative Pathway for Transmitting Cortical Signals to the Spinal Cord
The red nucleus, located in the mesencephalon, func-tions in close association with the corticospinal tract.
As shown in Figure 55–5, it receives a large number of direct fibers from the primary motor cortex through the corticorubral tract, as well as branching fibers from the corticospinal tract as it passes through the mesen-cephalon. These fibers synapse in the lower portion of the red nucleus, the magnocellular portion, which con-tains large neurons similar in size to the Betz cells in the motor cortex. These large neurons then give rise to the rubrospinal tract, which crosses to the opposite side in the lower brain stem and follows a course immediately adjacent and anterior to the corticospinal tract into the lateral columns of the spinal cord.
The rubrospinal fibers terminate mostly on the interneurons of the intermediate areas of the cord gray matter, along with the corticospinal fibers, but some of the rubrospinal fibers terminate directly on anterior motor neurons, along with some corticospinal fibers. The red nucleus also has close connections with the cerebellum, similar to the connections between the motor cortex and the cerebellum.
Function of the Corticorubrospinal System. The magnocel-lular portion of the red nucleus has a somatographic representation of all the muscles of the body, as is true of the motor cortex. Therefore, stimulation of a single point in this portion of the red nucleus causes con-traction of either a single muscle or a small group of muscles. However, the fineness of representation of the different muscles is far less developed than in the motor cortex. This is especially true in human beings, who have relatively small red nuclei.
The corticorubrospinal pathway serves as an acces-sory route for transmission of relatively discrete signals from the motor cortex to the spinal cord. When the corticospinal fibers are destroyed but the corti-corubrospinal pathway is intact, discrete movements can still occur, except that the movements for fine control of the fingers and hands are considerably impaired. Wrist movements are still functional, which is not the case when the corticorubrospinal pathway is also blocked.
Therefore, the pathway through the red nucleus to the spinal cord is associated with the corticospinal system. Further, the rubrospinal tract lies in the lateral columns of the spinal cord, along with the corti-cospinal tract, and terminates on the interneurons and motor neurons that control the more distal muscles of the limbs. Therefore, the corticospinal and rubrospinal tracts together are called thelateral motor systemof the cord, in contradistinction to a vestibuloreticu-lospinal system, which lies mainly medially in the cord and is called the medial motor system of the cord.