Tracts of the lateral motor system include the lateral corticospinal tract and rubrospinal tract. Both are located in the lateral column of the spinal cord white matter and synapse on lower motor neuronal cell bodies in the lateral aspect of the anterior horn gray matter.
Clinically, the most important tract is the lateral corticospinal tract, because it is responsible for controlling movement of the upper and lower extremities. Cell bodies of upper motor neurons forming this tract are located in the primary motor cortex (eFig. 9.54). Axons of these upper motor neurons converge in the corona radiata and descend through the posterior limb of the internal capsule to reach the crus cerebri of the midbrain. These axons continue through the anterior aspect of the pons as small bundles to accommodate the transverse pontocerebellar fibers, which are also present in this location. Once the fibers reach the medulla, they are again grouped together and form a large swelling known as the pyramid (eFig. 9.54). At the caudal medulla, before transitioning into the spinal cord, most of the axons decussate over to the contralateral side to form the lateral corticospinal tract (eFig. 9.54). The remaining axons will stay ipsilateral and form the anterior corticospinal tract, a tract included in the medial motor systems.
After decussating and forming the lateral corticospinal tract, the axons descend through the spinal cord to synapse on the cell bodies of lower motor neurons in the lateral portion of the anterior horn gray matter. Axons of these lower motor neurons then exit the spinal cord through the anterior root (eFig. 9.54).
The other lateral motor system pathway is the rubrospinal tract (eFig. 9.55). Cell bodies of upper motor neurons in this pathway begin in the red nucleus of the midbrain. After leaving the red nucleus, the axons cross the midline as the ventral tegmental decussation and descend as the rubrospinal tract through the brainstem and lateral column of the spinal cord white matter (eFig. 9.55). These axons only descend to cervical regions of the spinal cord, and axons synapse with interneurons in the anterior horn gray matter to facilitate flexor muscle activity and inhibit extensor muscle activity of the upper limb.
Tracts of the medial motor system regulate axial or truncal muscles involved in maintaining posture, balance, automatic gait-related movements, and orientating movements of the head and neck. Unlike the lateral motor system, the tracts in this system primarily project bilaterally on interneurons within the spinal cord. This makes it difficult to test each tract individually in the clinical system. We will briefly review the four tracts of the medial motor system beginning with the anterior corticospinal tract.
The anterior corticospinal tract is formed by the remaining descending upper motor neurons that did not decussate in the caudal medulla to form the lateral corticospinal tract. These upper motor neurons, which remain ipsilateral to form the anterior corticospinal tract, descend through the medial aspect of the anterior spinal cord to the level of the upper thoracic region (eFig. 9.54). These axons project bilaterally to synapse on cell bodies of lower motor neurons in the medial portion of the anterior horn gray matter. Axons of these lower motor neurons then exit the spinal cord through the anterior root.
Tectospinal tract axons arise from cell bodies located in the superior colliculus of the dorsal midbrain (eFig. 9.56). These axons decussate in the dorsal tegmental decussation shortly after leaving the nucleus to form the tectospinal tract along the midline of the brainstem. The tectospinal tract continues through the brainstem near the medial longitudinal fasciculus and into cervical regions of the spinal cord near the anterior median fissure. Within the cervical spinal cord, axons project bilaterally to synapse on cell bodies of interneurons in the anterior horn gray matter. As the superior colliculus receives visual input, it is believed that the tectospinal tract modulates reflex postural movements in response to visual stimuli.