374 Neuroanatomy: Draw It to Know It
Olfactory Cortex & Basal Forebrain, Part 1
Here, we will draw two coronal sections through the
basal frontal and medial temporal lobes to learn the
anatomy of the olfactory cortex and basal forebrain.
First, let’s establish the anteroposterior positions of our
two coronal sections: indicate that one section lies along
the plane of the optic chiasm, which is the more anterior
diagram, and the other lies along the plane of the optic
tract, just posterior to the plane of the optic chiasm. In
the optic chiasm diagram, draw the putamen, caudate,
and their ventral connection: the nucleus accumbens.
Th en, label the intervening internal capsule. Also, for
reference, include the frontal horn of the lateral ventricle
and the corpus callosum. In the optic tract diagram,
again draw the caudate and putamen, but here show that
they are completely separated by the internal capsule
(the nucleus accumbens lies anterior to this plane). Next,
medial to the putamen, draw the globus pallidus. Now,
again include the frontal horn of the lateral ventricle and
the corpus callosum. Next, in each diagram, draw the
border of the basal frontal and medial temporal lobes
and draw the inferior border of the insula.
Now, let’s complete the diagram through the optic
chiasm. First, for further reference, show that the medial
hypothalamus connects the optic chiasm to the basal
frontal lobe. Next, along the basal perimeter, from medial
to lateral, label the subcallosal g yrus, olfactory tubercle,
and piriform cortex. Th en, draw the periamygdaloid
cortex in between the piriform cortex, laterally, and the
entorhinal cortex, medially. Now, in the center of the
medial temporal lobe, draw the amygdala; separate it
into corticomedial and basolateral divisions. Finally, in
between the subcallosal g yrus and the nucleus accum-
bens, label the diagonal band of Broca.
Next, let’s complete the more posterior diagram: the
one through the optic tract. In the middle of the dia-
gram, inferiorly, label the hypothalamus. Superiorly, in
midline, label the septal nuclei and the bed nucleus of
the stria terminalis below them. Next, in the basal fron-
tal lobe, lateral to the hypothalamus and beneath the
basal ganglia, label the substantia innominata. Above
the substantia innominata and below the basal ganglia,
label the anterior commissure. Within the substantia
innominata, label the basal nucleus of Meynert. Note
that the basal nucleus of Meynert is variably considered
to be either synonymous with the substantia innominata
or to reside within this “unnamed substance” as it is
drawn, here. Th en, just below the anterior commissure,
label the ventral pallidum. Now, in the medial temporal
lobe, draw the amygdala, but show that in this more pos-
terior section, the amygdala extends to the surface of the
temporal lobe. Again, divide the amygdala into cortico-
medial and basolateral divisions. Finally, along the
medial rim of the temporal lobe, medial to the amygdala,
label the entorhinal cortex.
Certain basal forebrain structures share important
cholinergic properties that are understood to play a sub-
stantive role in memory. Of the listed structures, those
with the most notable cholinergic properties are the
medial septal nuclei, the diagonal band of Broca, and the
basal nucleus of Meynert. As a clinical corollary, acetyl-
cholinesterase inhibitors, which prevent the breakdown
of acetylcholine, were developed to promote cholinergic
health in the basal forebrain with the hope of slowing
the progression of Alzheimer’s disease.^23 Unfortunately,
although these medications are widely used, they have
limited clinical effi cacy.^6 , 9 , 10 , 24