164 Neuroanatomy: Draw It to Know It
Major Motor Projections
Here, let’s draw the corticonuclear and corticospinal motor
fi ber descent through the brainstem. Most of the motor
fi bers originate in parallel to the sensory fi bers just in front
of the central sulcus in the precentral g yrus and anterior
paracentral g yrus, from lateral to medial as the face, arm,
and leg fi bers. Show the origins of the diff erent somatomo-
tor fi bers: the facial fi bers in the lateral cortex, the arm
fi bers in the upper convexity, and the leg fi bers in the para-
central frontal lobe. Draw the twisting descent of each
fi ber group through the subcortical white matter. Th e
facial fi bers descend medially, the leg fi bers descend later-
ally, and the arm fi bers descend in between the two. Show
these descending fi bers bundle in the internal capsule and
pass into the ipsilateral cerebral peduncle in the midbrain.
Next, show that the motor fi ber arrangement in the
midbrain, from medial to lateral, is as follows: face, arm,
and leg. Th e facial fi bers are called corticonuclear fi bers
(aka corticobulbar fi bers) and they synapse on diff erent
cranial nerve nuclei throughout their descent. Th e arm
and leg fi bers form the corticospinal tract. In the pons, as
the motor fi bers descend through the pontine nuclei, the
face, arm, and leg fi bers maintain their same orientation.
Show that they are positioned as follows, from medial to
lateral: face, arm, and leg. At the most inferior level of
the medulla, by defi nition, the corticonuclear fi bers have
completed their descent, so only the arm and leg corti-
cospinal fi bers are found; they descend through the base
of the medulla in the ipsilateral medullary pyramid:
indicate that the arms are still medial to the legs.
As the corticospinal tract descends through the
medullo-cervical junction, show it decussate and shift
posterolaterally to enter the lateral funiculus of the cervi-
cal spinal cord. Indicate that during the decussation, the
arm and leg fi bers twist so that the arms remain medial
to the legs. Seventy-fi ve to ninety percent of the corti-
cospinal fi bers undergo the aforementioned decussation;
they are called lateral corticospinal tract fi bers due to
their position in the lateral spinal cord. Th e remaining
fi bers travel ipsilaterally through the anteromedial spinal
cord as the anterior corticospinal tract and remain
uncrossed as they leave the brainstem. Th ese fi bers
descend the spinal cord through the anterior funiculus,
and when they reach the level of their target neuron,
they cross within the anterior commissure.^1 G e n e r a l l y ,
lateral corticospinal tract fi bers innervate distal muscula-
ture for fi ne motor movements, whereas anterior corti-
cospinal tract fi bers innervate proximal musculature for
gross motor movements.
With our drawing complete, let’s demonstrate the
rotation and descent of the motor fi bers with our hand.
During the sensory fi ber demonstration, we used our
thumb as the leg fi bers and our little fi nger as the arm
fi bers; let’s use the same representation here. We begin
where we ended our sensory fi ber rotation — in the cere-
bral cortex with our hand turned palm towards us and
our arm across our body. To demonstrate the twist and
bundling of the motor fi bers as they descend through
the subcortical white matter and into the internal cap-
sule, lower your hand and turn it over (palm away
from you) as you bring your fi ngers together. Your pinky
(the arm fi bers) should be medial to your thumb (the
leg fi bers). To demonstrate the motor fi ber descent
through the brainstem, simply continue to drop your
hand. Now, we need to demonstrate the pyramidal
decussation at the medullo-cervical junction and the
second twist of the corticospinal fi bers. Th is rotation is
important because it keeps the leg fi bers lateral to the
arm fi bers when the corticospinal tract crosses midline
in the upper cervical cord. For this step, bring your hand
across midline and turn your palm back towards you so
that your thumb is lateral and your little fi nger is medial
when your hand crosses midline.^1 – 6