296 Neuroanatomy: Draw It to Know It
Internal Capsule
Here, we will draw the anatomy of the internal capsule in
both axial and sagittal views. Th e internal capsule com-
prises fi ber bundles that originate from widespread brain
regions and it carries fi bers of many disparate functional
modalities, including motor, sensory, cognitive, and
emotional fi ber pathways. We will begin by drawing an
axial view of the key basal ganglia structures (and the
thalamus) because the internal capsule forms a wedge in
between them. For orientational purposes, fi rst draw the
posterior aspect of the lateral ventricle and then the fron-
tal horn of the lateral ventricle. Next, draw the head of
the caudate; then, the thalamus; and next, the globus
pallidus and putamen, which together form the lenti-
form nucleus. Now, show that the internal capsule fi lls
the V-shaped wedge in between these nuclei.
Next, re-draw an axial view of the internal capsule so
we can label its individual limbs. Indicate that the ante-
rior portion, between the head of the caudate and the
lentiform nucleus, is called the anterior limb; then, show
that the middle (bend) of the internal capsule, which
occurs along the medial–lateral plane of the anterior
thalamus, is called the genu; next, indicate that the por-
tion of internal capsule between the thalamus and the
lentiform nucleus is called the posterior limb; then, show
that the retrolenticular limb lies posterior to the poste-
rior limb — its fi bers run posterior to the lentiform
nucleus; and fi nally, label the sublenticular limb in
parentheses — its fi bers run underneath the lentiform
nucleus and lie inferior to the plane of this diagram.
Note that the relative anterior–posterior positions of the
sublenticular and retrolenticular limbs are inconsistently
defi ned; therefore, we consider them both together as
lying posterior to the posterior limb.
In sagittal view, we will now show the position of
a few fi ber bundles that pass through the internal cap-
sule; bear in mind that the fi bers we will draw represent
only a few key clinical highlights. Most important to
clinical neurolog y is the position of the motor fi bers
because infarction of the genu and posterior limb of the
internal capsule produces a classic and common pure
motor stroke. Th e motor fi bers are arranged somatotopi-
cally, meaning the facial fi bers pass through the genu, the
arm fi bers lie posterior to the facial fi bers in the anterior
portion of the posterior limb, and the leg fi bers descend
posterior to the arm fi bers in the posterior portion of the
posterior limb. Importantly, the motor fi bers crowd
together inferiorly within the internal capsule; thus, the
more inferior the infarct, the more broad the motor defi -
cit relative to the size of the infarct.
Draw a sagittal view of the internal capsule as a broad-
based triangle sitting on a pointed edge; the internal cap-
sule gradually dips down along the anterior–posterior
course of the anterior limb and then rises again along the
anterior–posterior course of the posterior limb. From
anterior to posterior, label the anterior limb, genu, poste-
rior limb, and sublenticular and retrolenticular limbs.
Indicate that the anterior limb consists of fi bers from the
prefrontal cortex, predominantly anterior thalamic radi-
ation fi bers that connect the frontal cortex with the thal-
amus and also frontopontine fi bers. Th en, show that the
genu and posterior limbs comprise the motor fi bers.
We commonly associate the facial fi bers purely with
the genu, but show that the facial fi bers actually begin
in the anterior limb, descend through the genu, and enter
the posterior limb. Next, show that the arm fi bers descend
through the anterior portion of the posterior limb. Th en,
show that the leg fi bers originate posterior to the arm
fi bers and pass anteriorly during their descent through
the posterior limb. Finally, indicate that the sublenticu-
lar limb carries auditory fi bers from the medial genicu-
late body, and then show that both the sublenticular and
retrolenticular limbs carry visual projection fi bers.^3 – 9