The cerebral hemispheres are where the motor pathways originate, and where the somatosensory pathways terminate. The left cerebral hemisphere controls the motor functions of the right side of the body, and the right cerebral hemisphere controls the motor functions of the left side of the body. This crossed system is maintained in other modalities including the somatosensory and visual pathways: If the left hemisphere controls the right side of the body, it makes sense that it would need somatosensory information about the right side of the body and visual information about the right side of the world.
There are three main clinically relevant tracts for motor and somatosensory function for the body—one motor and two sensory—that span the brain, brainstem, and spinal cord:
Motor: The corticospinal tracts send motor information from the cortex to the spinal cord as the name suggests.
Sensory: The anterolateral (or spinothalamic) tracts and dorsal (or posterior) column pathways bring sensory input from the spinal cord to the brain by way of the brainstem. The names of these pathways refer to their anatomic positions within the spinal cord.
In this chapter, the anatomy of these pathways will be described, providing a foundation for localizing symptoms of weakness and sensory changes, and also laying out anatomic landmarks in the brain, brainstem, and spinal cord that will serve as points of orientation as additional pathways are described in subsequent chapters.
The corticospinal tracts convey the motor plan from the cerebral cortex to the spinal cord—specifically to the alpha motor neurons of the anterior (ventral) horns of the spinal cord (Fig. 4–1). These alpha motor neurons exit the spinal cord as ventral roots that then travel through plexuses and peripheral nerves to relay impulses to the muscles. The corticospinal tracts are sometimes referred to as the pyramidal system (since they travel for part of their course in the medullary pyramids). The extrapyramidal system includes the basal ganglia and cerebellum, which participate in circuits with the motor cortex and are involved in action initiation and coordination (see Chs. 7 and 8).
Each corticospinal tract begins in the motor cortex, which is located in the precentral gyrus (immediately anterior to the central sulcus). The motor cortex is organized by the region of the body it controls: face lateral, hand and arm superior to this, and leg and foot most medial (with arm and leg representations connected at the shoulder/hip such that the hand is most lateral and the foot most medial with the arm and leg between). This cortical map of the regions of the body is referred to as the homunculus...