Here, we will learn a consolidated view of the major descending (motor) pathways from the cerebrum through the brainstem into the spinal cord.
Start a table, specify that we will learn about the following major pathway projections:
Motor
Corticospinal tract (CST)
Corticobulbar (aka corticonuclear) tract
Motor Pathways
Anatomical Structures
Begin with the motor pathways.
Label the superior/inferior axes.
Draw a coronal view of the right brain.
Then, the brainstem.
The upper cervical spinal cord.
And the lumbar cord.
Innervation
Start another table.
Write that for the motor fibers:
The corticospinal tract fibers innervate the body via spinal motor neurons.
Specify that the lateral CST innervates distal musculature for fine motor movements
Whereas the anterior CST innervates proximal musculature for gross motor movements.
The corticobulbar fibers (aka corticonuclear fibers) innervate the face via the CNs.
Projections
Now, demarcate the internal capsule deep within the cerebrum – the motor fibers consolidate here before entering the ipsilateral cerebral peduncle in the midbrain.
Next, draw the twisting descent of each fiber group through the subcortical white matter.
Show that the facial fibers [RED] emerge from the lateral convexity, descend medially and, generally, decussate to synapse on different cranial nerve nuclei throughout their descent.
Then, show that the leg fibers [GREEN] emerge paracentrally, descend laterally through the brainstem into the ipsilateral medullary pyramid.
Show that the arm fibers [BLUE] emerge from the upper convexity, descend in between the facial and leg fibers, medial to the leg fibers through the brainstem into the ipsilateral medullary pyramid.
We see that they decussate at the cervicomedullary junction.
Key Anatomical Landmarks
Now, stop and mark two key points:
One within the subcortical white matter/internal capsule – here the leg fibers switch medial/lateral position with the arm fibers.
Another within the medullary pyramid – again, here, the leg fibers switch medial/lateral position with the arm fibers.
Decussation
To show this in detail, draw axial sections of the medulla.
And the spinal cord.
Label the posterior/anterior axes.
As the lateral corticospinal tracts decussate and descend through the medullo-cervical junction, show them shift posterolaterally to enter the lateral funiculus of the cervical spinal cord.
Indicate that the arms remain medial to the legs.
75 to 90% percent of the corticospinal fibers undergo the aforementioned decussation as the lateral CST fibers due to their position in the lateral spinal cord.
Show that the remaining arm fibers travel ipsilaterally through the anteromedial spinal cord as the anterior CST remaining uncrossed until they decussate in the cervical spinal cord.
Then, show the leg fibers do the same within the lumbar cord.
Show them descend the spinal cord through the anterior funiculus in axial view, as well.