The uppermotor neurons in thecerebral cortex reside in several adjacent and highly interconnected areas in thefrontal lobe, which together mediate the planning and initiation of complex temporal sequences of voluntary movements. These cortical areas all receive regulatoryinput from thebasal ganglia andcerebellum via relays in the ventrolateralthalamus (see Chapters 18 and 19), as well as inputs from the somaticsensory regions of theparietal lobe (see Chapter 9). Although the phrase “motor cortex” is sometimes used to refer to these frontal areas collectively, more commonly it is restricted to theprimary motor cortex, which is located in theprecentral gyrus (Figure 17.7). The primary motor cortex can be distinguished from the adjacent “premotor” areas both cytoarchitectonically (it is area 4 in Brodmann's nomenclature) and by the low intensity of current necessary to elicit movements by electrical stimulation in this region. The lowthreshold for eliciting movements is an indicator of a relatively large and direct pathway from the primary area to the lower motor neurons of thebrainstem andspinal cord. This section and the next focus on the organization and functions of the primary motor cortex and its descending pathways, whereas the subsequent section addresses the contributions of the adjacent premotor areas.
The primary motor cortex and the premotor area in the human cerebral cortex as seen in lateral (A) and medial (B) views. The primary motor cortex is located in the precentral gyrus; the premotor area is more rostral.
The pyramidal cells of cortical layer V (also called Betz cells) are the uppermotor neurons of theprimary motor cortex. Their axons descend to thebrainstem and spinal motor centers in thecorticobulbar and corticospinal tracts, passing through theinternal capsule of theforebrain to enter the cerebral peduncle at the base of the midbrain (Figure 17.8). They then run through the base of thepons, where they are scattered among the transverse pontine fibers and nuclei of the pontinegray matter, coalescing again on theventral surface of themedulla where they form themedullary pyramids. The components of thisupper motor neuron pathway thatinnervatecranial nerve nuclei, thereticular formation, and the red nucleus (that is, the corticobulbar tract) leave the pathway at the appropriate levels of the brainstem (seeFigure 17.8 andBox A). At thecaudal end of the medulla, most, but not all, of the axons in thepyramidal tract cross (or “decussate”) to enter thelateral columns of thespinal cord, where they form thelateralcorticospinal tract. A smaller number of axons enters the spinal cord without crossing; these axons, which comprise theventral corticospinal tract, terminate either ipsilaterally or contralaterally, after crossing in the midline (via spinal cord commissure). The ventral corticospinal pathway arises primarily from regions of the motor cortex that serve axial andproximal muscles (seeFigure 17.6).
The corticospinal tract. Neurons in the motor cortex give rise to axons that travel through the internal capsule and coalesce on the ventral surface of the midbrain, within the cerebral peduncle. These axons continue through the pons and come to lie on(more...)
Descending Projections to Cranial Nerve Motor Nuclei and Their Importance in Diagnosing the Cause of Motor Deficits. The symptoms pertinent to the cranial nerves and their nuclei are of special importance to physicians seeking to identify the location(more...)
The lateralcorticospinal tract forms the direct pathway from thecortex to thespinal cord and terminates primarily in the lateral portions of theventral horn and intermediategray matter (seeFigures 17.6 and17.8). The indirect pathway to lowermotor neurons in the spinal cord runs, as already described, from themotor cortex to two of the sources of upper motor neurons in thebrainstem: the red nucleus and thereticular formation. In general, the axons to the reticular formation originate from the parts of the motor cortex that project to themedial region of the spinal cord gray matter, whereas the axons to the red nucleus arise from the parts of the motor cortex that project to the lateral region of the spinal cord gray matter (seeFigure 17.6).
