370 Neuroanatomy: Draw It to Know It
Olfactory System, Part 1
Here, we will draw the olfactory system; we will focus on
the olfactory nerve, bulb, tract, and striae, here, and learn
the olfactory cortex in Drawings 21-7 and 21-8. Begin
with a sagittal view of the midline nasal cavity; include
the following anatomic landmarks: the cribriform plate,
which separates the cranial vault from the nasal cavity;
the medial anterior frontal lobe and temporal lobe; and
the anterior corpus callosum. Note that fracture to the
cribriform plate (or more commonly to the ethmoid air
cells posterolateral to the cribriform plate) is a common
cause of rhinorrhea — a cerebrospinal fl uid leak from the
nasal cavity. Now, along the upper nasal cavity, draw the
olfactory epithelium. Next, just above the cribriform
plate, draw an olfactory bulb, and underneath the fron-
tal lobe, draw the olfactory tract. Th en, within the olfac-
tory epithelium, draw a bipolar primary olfactory neuron
and show that its centrally mediated axon, the olfactory
nerve, extends through the cribriform plate to innervate
the olfactory bulb. Note that this is the extent of cranial
nerve 1, the olfactory nerve. Th e olfactory bulb and tract
are telencephalic extensions of the cerebrum, itself. Next,
within the olfactory bulb, draw a bipolar secondary
olfactory cell, and show that it connects with the olfac-
tory nerve in the inferior olfactory bulb and also sends
an axon down the olfactory tract. Th en, at the posterior
end of the olfactory tract, at the olfactory trigone, show
that the olfactory tract divides into a medial olfactory
stria, which innervates the medial olfactory area in the
subcallosal (aka septal) region, and a lateral olfactory
stria, which innervates the primary olfactory cortex in
the basal frontal and anteromedial temporal lobes. Note
that (as we will draw later) olfactory impulses also extend
across the anterior commissure to the opposite side of
the cerebrum.^14 – 17
Next, let’s draw an expanded view of the olfactory
nerve and bulb. Draw the olfactory epithelium, cribri-
form plate, olfactory bulb, and the anterior segment of
the connected olfactory tract. Next, draw a representa-
tive primary olfactory neuron (aka olfactory receptor
cell), which is bipolar. Show that at one end, it projects
an apical dendrite to the epithelial surface. Indicate that
cilia from the apical dendrite interact with the mucus
layer of the olfactory epithelial surface. Next, indicate
that an unmyelinated nerve bundle, which is the olfac-
tory nerve, itself, projects from the other end of the pri-
mary olfactory neuron through the cribriform plate to
the olfactory bulb. As we will soon show, it interacts with
dendrites from the secondary olfactory neuron at the
glomerular layer of the olfactory bulb. Other constitu-
ents of the olfactory epithelium include the sustentacular
cells, which are olfactory supporting cells; the basal cells,
which renew the primary olfactory neurons and susten-
tacular cells; and the Bowman’s glands, which secrete a
serous, watery fl uid that serves as an odor dissolvent.^18
Now, within the olfactory bulb, draw a representative
bipolar secondary olfactory neuron. Two principal forms
of secondary olfactory neuron exist: tuft ed cells and mitral
cells; less notable interneurons (eg, periglomerular and
granule cells) also exist within the olfactory bulb. Indicate
that dendrites from the secondary olfactory neurons com-
municate with the primary olfactory axons within the
inferior olfactory bulb. Th ese nerve processes intermingle
within spherical glomeruli — thus, this layer is called the
glomerular layer. Lastly, show that the secondary olfac-
tory neurons project axons that travel either directly down
the olfactory tract to synapse in the olfactory cortex or
fi rst to the anterior olfactory nucleus, which then projects
down the olfactory tract to the olfactory cortex.^10 , 14 , 17 , 18