312 Neuroanatomy: Draw It to Know It
Direct & Indirect Pathways: Anatomy (Advanced )
Here, in coronal view, we will draw the anatomy of the
direct and indirect pathways and the anatomy of the
fi elds of Forel. Note that we exclude the related corticos-
triatal projections. First, draw the lentiform nucleus and
divide it into the putamen and globus pallidus; then, fur-
ther subdivide the globus pallidus into its internal and
external segments. Next, draw the thalamus, and infero-
lateral to it, draw the zona incerta. Th en, inferior to the
zona incerta, draw the subthalamic nucleus, and then
inferior to it, draw the substantia nigra. Finally, through
the middle of the diagram draw the posterior limb of the
internal capsule and indicate that it becomes the cerebral
peduncle as it descends through the brainstem.
Now, we are able to label the individual fi elds of Forel,
which are Fields H, H1, and H2. First, indicate that
Field H lies medial to the subthalamic nucleus and infe-
rior to the zona incerta; then, show that Field H1 lies in
between the thalamus and zona incerta (medial to the
zona incerta and inferolateral to the thalamus); and
fi nally, indicate that Field H2 lies lateral to the zona
incerta and medial to the internal capsule.
Next, we will draw the direct pathway fi bers, which
comprise the pallidothalamic pathways, and which ema-
nate from the internal segment of the globus pallidus.
Start with the ansa lenticularis. Show that it projects inf-
eromedially from the globus pallidus internal segment,
courses beneath the subthalamic nucleus, then turns
superiorly to pass through Field H and Field H1 to enter
the thalamus. Next, show that the lenticular fasciculus
projects from the internal segment of the globus pallidus
across the internal capsule, through Field H2 and then
Field H, and then show that it courses superiorly through
Field H1 to enter the thalamus. Now, indicate that where
the ansa lenticularis and the lenticular fasciculus run
together in Field H1, they are collectively called the
thalamic fasciculus; note that this is the most limited
defi nition of the thalamic fasciculus. Additional fi ber
pathways pass through Field H and H1 in their ascent
into the thalamus; they include the cerebellothalamic
fi bers from the corticopontocerebellar pathway, the
medial lemniscus, the nigrothalamic fi bers, and the spi-
nothalamic fi bers of the anterolateral system pathway.
Th e term thalamic fasciculus is sometimes broadened to
include the cerebellothalamic fi bers and it is also some-
times used synonymously with the term Field H1 , just as
the term lenticular fasciculus is sometimes used synony-
mously with term Field H2. Finally, note that the thal-
amic fasciculus projects to multiple thalamic nuclei,
including the ventroanterior nucleus, which most nota-
bly communicates with the globus pallidus; the ventro-
lateral nucleus, which most notably communicates with
the cerebellum; the dorsomedial nucleus, which most
notably communicates with the prefrontal cortex and
basal ganglia; and the centromedian and parafascicular
nuclei (the main intralaminar nuclei), which most nota-
bly communicate with the striatum and frontal lobes.
Now, we will draw the globus pallidus external seg-
ment projections, which subserve the indirect pathway,
and which involve the subthalamic nucleus and substan-
tia nigra. First, show that the external segment of the
globus pallidus projects to the subthalamic nucleus, and
then show that the subthalamic nucleus projects back to
the external segment of the globus pallidus and also to the
internal segment of the globus pallidus. Indicate that the
reciprocal pallidosubthalamic and subthalamopallidal
projections are collectively called the subthalamic fascicu-
lus. Next, show that the subthalamic nucleus also projects
to the substantia nigra and that the substantia nigra proj-
ects back to the subthalamic nucleus. Finally, show that
the globus pallidus external segment projects to the sub-
stantia nigra and then that the substantia nigra projects
fi bers through Fields H and H1 that enter the thalamus.