The red nucleus, located in the mesencephalon, functions in close association with the corticospinal tract. As shown in Figure 1, 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 mesencephalon. These fibers synapse in the lower portion of the red nucleus, the mag nocellular portion, which contains 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.
Fig1. The corticorubrospinal pathway for motor control, also showing the relation of this pathway to the cerebellum.
The rubrospinal fibers terminate mostly on the inter neurons 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 magnocellular portion of the red nucleus has a somatographic representation of all the muscles of the body, as does the motor cortex. Therefore, stimulation of a single point in this portion of the red nucleus causes contraction 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, especially in human beings, who have relatively small red nuclei.
The corticorubrospinal pathway serves as an accessory route for transmission of relatively discrete signals from the motor cortex to the spinal cord. When the corticospinal fibers are destroyed but the corticorubrospinal 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 corticospinal 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 the lateral motor system of the cord, in contradistinction to a vestibuloreticulospinal system, which lies mainly medially in the cord and is called the medial motor system of the cord, as discussed later in this chapter.
“Extrapyramidal” System
The term extrapyramidal motor system has been used in clinical circles to denote all the portions of the brain and brain stem that contribute to motor control but are not part of the direct corticospinal-pyramidal system. These portions include pathways through the basal ganglia, the reticular formation of the brain stem, the vestibular nuclei, and often the red nuclei. This group of motor control areas is such an all-inclusive and diverse group that it is difficult to ascribe specific neurophysiological functions to the extra pyramidal system as a whole. In fact, the pyramidal and extrapyramidal systems are extensively interconnected and interact to control movement. For these reasons, the term “extrapyramidal” is being used less often both clinically and physiologically.
