The terms pyramidal, corticospinal, and upper motor neuron are often used interchangeably, although they are not altogether synonymous. The pyramidal tract, strictly speaking, designates only those fibers that course longitudinally in the pyramid of the medulla oblongata. Of all the fiber bundles in the brain, the pyramidal tract has been known for the longest time, the first accurate description having been given by Türck in 1851. It descends from the cerebral cortex; traverses the subcortical white matter (corona radiata), internal capsule, cerebral peduncle, basis pontis (ventral pons), and pyramid of the upper medulla; decussates in the lower medulla; and continues its caudal course in the lateral funiculus (column) of the spinal cord—hence the alternative name corticospinal tract (Fig. 3-2). This is the only direct long-fiber connection between the cerebral cortex and the spinal cord. The indirect pathways through which the cortex influences spinal motor neurons are the rubrospinal, reticulospinal, vestibulospinal, and tectospinal; these tracts do not run in the pyramid. All these pathways, direct and indirect, are embraced by the term upper motor neuron or supranuclear, meaning above the anterior horn cells.
A major source of confusion about the pyramidal tract stems from the traditional view, formulated at the turn of the twentieth century, that it originates entirely from the large motor cells of Betz in the fifth layer of the precentral convolution (the primary motor cortex, or area 4 of Brodmann1) (Figs. 3-3 and 21-1). However, there are only some 25,000 to 35,000 Betz cells, whereas the medullary pyramid contains about 1 million axons (Lassek). Thus most of the fibers of the pyramidal tract arise from cortical neurons other than Betz cells, particularly in Brodmann areas 4 and 6 (the frontal cortex immediately rostral to area 4, including the medial portion of the superior frontal gyrus, that is, the supplementary motor area); in the primary somatosensory cortex (Brodmann areas 3, 1, and 2); and in the superior parietal lobule (areas 5 and 7).
Data concerning the origin of the pyramidal tract in humans are less robust than for animals. In the monkey, by counting the pyramidal axons that remained after cortical excisions and long survival periods, Russell and DeMyer found that 40 percent of the descending axons arose in the parietal lobe, 31 percent in motor area 4, and the remaining 29 percent in premotor area 6.
Fibers from the motor and premotor cortices (Brodmann areas 4 and 6, Fig. 21-2), supplementary motor cortex, and portions of parietal cortex (areas 1, 3, 5, and 7) converge in the corona radiata and descend through the posterior limb of the internal capsule, basis pedunculi, basis pontis, and medulla. As the corticospinal tracts descend in the cerebrum and brainstem, they send collaterals to the striatum, thalamus, red nucleus, cerebellum, and reticular formations. Accompanying the corticospinal tracts in the brainstem are the corticobulbar tracts, which are distributed to motor nuclei of the cranial nerves ipsilaterally and contralaterally (see Fig. 3-2). It has been possible to trace the direct projection of axons of cortical neurons to the trigeminal, facial, ambiguus, and hypoglossal nuclei (Iwatsubo et al). It appears that no axons terminate directly in the oculomotor, trochlear, abducens, or vagal nuclei. Insofar as the corticobulbar and corticospinal fibers have a similar origin and the motor nuclei of the brainstem are the homologues of the motor neurons of the spinal cord, the term upper motor neuron may suitably be applied to both these systems of fibers.