368 Neuroanatomy: Draw It to Know It
Hippocampus: Circuitry
Here, we will draw the fundamental extra- and intra-
hippocampal circuitry. We begin with the extra-hip-
pocampal circuitry, namely the Papez circuit. First, in
sagittal view draw the following structures: the corpus
callosum, cingulate g yrus, basal forebrain, thalamus, and
parahippocampal gryus with its anterior g yral fold — the
uncus. Next, let’s show the steps in the Papez circuit.
First, indicate that the entorhinal cortex (in the anterior
parahippocampal g yrus) projects to the hippocampus,
which projects via the fornix to the mammillary nuclei.
Divide the fornix as follows: the crus of the fornix is its
vertical ascent; the body of the fornix is its anterior pro-
jection underneath the corpus callosum; and the column
of the fornix is its descent. Note that the fornix descends
both anterior and posterior to the anterior commissure.
Th e anterior projection is called the precommissural
fornix and the posterior projection through the hypo-
thalamus (shown here) is called the postcommissural
fornix. Next, indicate that the mammillary nuclei proj-
ect to the anterior thalamic nuclei via the mammillotha-
lamic fasciculus (aka the Vicq d’Azyr bundle), and then
show that the anterior thalamic nuclei project to the cin-
gulate g yrus. Finally, show that the cingulate g yrus proj-
ects back to the entorhinal cortex via the cingulum to
close the Papez circuit loop. When James Papez intro-
duced this circuit in 1937, he believed it played a funda-
mental role in emotional processing ; however, now the
Papez circuit is understood to be the cornerstone of
memory consolidation, instead.
Neuronal input reaches the hippocampus through
widespread neocortical, limbic, diencephalic, and brain-
stem neurobehavioral areas. Here, we will show two
examples of how these areas reach the hippocampus.
First, simply show that the cingulate g yrus has reciprocal
connections with the neocortex that project via the cin-
gulum to the entorhinal cortex, which then projects to
the hippocampus. Second, indicate that widespread neo-
cortical areas project to the posterior parahippocampal
g yrus, which then projects to the perirhinal cortex, and
subsequently to the entorhinal cortex and on to the hip-
pocampus. Note that although not shown as such, here,
many areas skip synapsing in the posterior parahip-
pocampal g yrus and directly synapse in the perirhinal or
entorhinal cortices or even directly in the hippocampus,
itself.
Now, let’s draw the intrahippocampal circuitry,
namely the perforant pathway. First, in coronal view,
redraw the medial temporal lobe. Include the dentate
g yrus, cornu ammonis, subiculum, entorhinal cortex,
alveus, and fi mbria. Th en, add the posterior length of
the hippocampus. Show that the fi mbria becomes the
crus of the fornix, which makes its vertical ascent at the
splenium of the corpus callosum. Th e intrahippocampal
circuitry is distinct in that it follows a very specifi c unidi-
rectional progression — unlike most cortical projections,
which are generally bidirectional. Show that the entorhi-
nal cortex projects through the subiculum (it perforates
it) to synapse in the dentate g yrus. Th en, indicate that
the dentate g yrus projects to CA3, which projects to
CA1 via Schaff er collaterals. CA1 then projects to the
subiculum, which projects along the alveus to the fi m-
bria, which passes posteriorly and becomes the crus of
the fornix. Th e fornix ultimately projects, most notably,
to the mammillary nuclei (as shown in the Papez circuit
portion of the diagram). Finally, show that the subicu-
lum also projects back to the entorhinal cortex. Note
that CA3 and CA1 also send direct projections to the
fi mbria that skip the intervening steps in this pathway,
and also note that many other intrahippocampal projec-
tions also exist, including subiculum projections to the
other components of the subicular complex (ie, the pre-
subiculum and parasubiculum).^13 Lastly, consider that
because of the anatomic relationship between the ento-
rhinal cortex, dentate g yrus, cornu ammonis, and subic-
ulum, these structures are oft en collectively referred to as
the hippocampal formation.^3 , 6 – 9 , 12