372 Neuroanatomy: Draw It to Know It
Olfactory System, Part 2 ( Advanced )
Next, let’s show the fl ow of olfactory information from
the olfactory bulb to the olfactory cortex. Note that this
fl ow of information is actually bidirectional, although
we show it one direction, only, here. Also, note that the
olfactory system bypasses the thalamus in its projection
to the cerebral cortex, which is unique. Consider that
auditory, visual, somatosensory, and gustatory sensory
pathways all relay within the thalamus prior to synapsing
in the cerebral cortex. Draw an inferior view of the fron-
tal and anterior-temporal lobes and, for anatomic refer-
ence, include the optic chiasm, pituitary stalk, and
mammillary bodies. Also, in front of the optic chiasm,
draw the diagonal band of Broca, which is a prominent
cholinergic gray and white matter structure (see Drawing
21-7 ). Next, in the frontal lobes, draw the bilateral olfac-
tory bulbs and tracts, which lie within the olfactory sulci.
Note that the olfactory bulb is oft en distinguished as the
main olfactory bulb because the majority of vertebrates
also have an accessory olfactory system.^19 , 20 H o w e v e r , t h e
role and existence of the accessory olfactory system (aka
vomeronasal system) in humans is disputed.
Now, show that at the olfactory trigone the olfactory
tract splits into lateral and medial striae. Th e bilateral
medial striae communicate at the anterior commissure.
In between the olfactory trigone and the diagonal band
of Broca, label the anterior perforated substance; the
perforated appearance of this gross anatomic area gives it
its name. Next, indicate that behind the olfactory trig-
one, overlying the anterior perforated substance, is a gray
matter structure called the olfactory tubercle, which is
innervated by a third, not yet introduced, olfactory stria:
the intermediate olfactory stria. Th e existence and sig-
nifi cance of the intermediate stria in humans is contro-
versial. Next, return to the olfactory tract and show that
the anterior olfactory nucleus lies along its anteroposte-
rior length. Th e anterior olfactory nucleus has cortical
cytoarchitecture and, thus, is oft en referred to as the
anterior olfactory cortex, and it is commonly considered
to be part of the primary olfactory cortex (see Drawing
21-8 ).^8 , 21 , 22
Finally, we’re ready to show the fl ow of information
from the olfactory bulb to the primary olfactory cortex.
First, show an impulse pass from the olfactory bulb
down the olfactory tract into the lateral olfactory stria
to the primary olfactory cortex. Note that we defi ne the
structures of the primary olfactory cortex in Drawing
21-8. From the primary olfactory cortex, information
projects to secondary olfactory regions, which include
the orbitofrontal cortex, lateral hypothalamus, insula,
anterior hippocampus, and indusium griseum. Next,
show an impulse pass from the anterior olfactory nucleus
down the ipsilateral olfactory tract to the ipsilateral
medial stria. Show that it decussates in the anterior
commissure to the contralateral medial stria and that it
then passes anteriorly along the contralateral olfactory
tract to the olfactory bulb. Again, note that olfactory
sensory impulses are bidirectional, which means that the
olfactory cortex also communicates with the olfactory
bulb in a reciprocal manner to what we have drawn,
here.^16 , 17