362 Neuroanatomy: Draw It to Know It
Limbic System, Part 1
Here, we will draw the anatomy of the limbic system.
Generally, the limbic system is meant to encompass the
limbic lobe and functionally related structures, most
notably the cingulate and parahippocampal g yri, hip-
pocampus, and amygdala. First, we will draw a sagittal
view of the major gray matter structures of the limbic
system. Th e limbic system forms outer and inner rings
around the diencephalon. Draw the following important
anatomic landmarks: the corpus callosum, basal frontal
lobe, thalamus, and the septum pellucidum. Next, draw
the outer ring : the limbic g yrus. To do so, fi rst, above the
corpus callosum, label the cingulate g yrus. Th e cingulate
g yrus is commonly divided into anterior and posterior
divisions. Th e anterior cingulate g yrus is part of the ante-
rior network of motivation, attention, and behavior and
is associated with the amygdala, whereas the posterior
cingulate g yrus is part of the posterior network of learn-
ing and memory and is associated with the hippocam-
pus. Next, posterior to the rostrum of the corpus callosum
and underneath the septum pellucidum, label the sub-
callosal area; show that it extends anteriorly directly
underneath the corpus callosum — we draw this region
in detail at the end. Th en, show that the parahippocam-
pal g yrus forms the inferior aspect of the limbic g yrus,
and label the g yral fold at its anterior tip as the uncus.
Th e parahippocampal g yrus channels information to
and from the hippocampus to help in memory consoli-
dation; an effi cient parahippocampal g yrus is required
for eff ective memory processing. Now, label the poste-
rior transitional zone between the cingulate and para-
hippocampal g yri as the isthmus of the cingulate g yrus.
Next, let’s draw the inner limbic ring : the intralimbic
g yrus. Along the superior surface of the corpus callo-
sum, beneath the cingulate g yrus, draw the indusium
griseum (aka the supracallosal g yrus). Th e indusium gri-
seum, along with the anatomically associated medial and
longitudinal striae, are considered vestigial remnants of
the hippocampal formation.^1 Now, along the bulk of
the anteroposterior length of the superior aspect of the
parahippocampal g yrus, draw the hippocampus. Th e
hippocampus is the center for memory processing ; we
draw it in detail in Drawing 21-3. Next, just beneath the
splenium of the corpus callosum, draw the fasciolar
g yrus, which is the transitional zone between the hip-
pocampus and the indusium griseum.
Now, just anterior to the hippocampus, deep to the
posterior aspect of the uncus, draw the almond-shaped
amygdala (aka amygdaloid body). Th e simplest way to
understand the amygdala is to divide it into corticome-
dial and basolateral groups. Both groups connect to a
wide variety of brain regions, but the corticomedial
group connects, most notably, to the olfactory system
and hypothalamus for autonomic function and the baso-
lateral group connects, most notably, to the thalamus
and prefrontal cortex for more conscious-related pro-
cesses such as sensory processing of visual, auditory, and
somatosensory stimuli.^2 , 3 Bilateral amygdala destruction
results in Klüver Bucy syndrome, which involves, most
prominently, social tameness and loss of avoidance man-
ifesting with hypermetamorphosis (the incessant explo-
ration of objects within the environment), hyperorality,
visual agnosia, and hypersexuality. Klüver Bucy syn-
drome was fi rst described from the eff ects of bilateral
amygdala and hippocampal destruction in monkeys, but
it is a rare complication of a variety of naturally occur-
ring illnesses in humans, including herpes simplex
encephalitis, frontotemporal lobar dementia, and anoxic-
ischemic lesions in the bilateral anterior medial tempo-
ral lobes.^4
Next, include the following diencephalic structures:
the anterior nuclei of the thalamus and the mammillary
body of the hypothalamus, both of which are fundamen-
tal to the Papez circuit. Th e mammillary bodies (along
with other midline periventricular structures: the dorso-
medial thalamus, periaqueductal region, and fl oor of the
fourth ventricle) are aff ected in Wernicke-Korsakoff
syndrome, which manifests with confusion, memory
loss, ophthalmoplegia, and ataxia.^3 , 5 – 10