242 Neuroanatomy: Draw It to Know It
Central Auditory Pathways
Here, we will create a simplifi ed diagram of the central
auditory pathways. First, draw a coronal view of the
brainstem. On the left side, label the ventral cochlear
nucleus at the pontomedullary junction and then, just
below it, in the upper medulla, label the dorsal cochlear
nucleus. Note that the ventral cochlear nucleus is oft en
further subdivided into its anterior and posterior subnu-
clei. Next draw an axial view of the posterolateral wedge
of the upper medulla/pontomedullary junction to illus-
trate the anterior–posterior relationship of the ventral
and dorsal cochlear nuclei. Draw the restiform body of
the inferior cerebellar peduncle and then show that the
ventral cochlear nucleus lies along its ventrolateral sur-
face and that the dorsal cochlear nucleus lies along its
dorsolateral surface.^5
Now, in the coronal diagram, label the bilateral
superior olivary complexes in the mid-pons. Each supe-
rior olivary complex comprises three separate nuclei: a
nucleus of the trapezoid body, medial superior olivary
nucleus, and lateral superior olivary nucleus. Surrounding
the superior olivary complexes are peri-olivary cells.
Next, in the inferior midbrain, label the bilateral inferior
colliculi. Th en, show projection fi bers from the bilateral
superior olivary complexes to their respective inferior
colliculi and label these projections as the lateral lemnis-
cus tracts. Next, label the left medial geniculate nucleus,
which is part of the metathalamus, and then, label the
left transverse temporal g yri (aka Heschl’s g yri) continu-
ous with the superior temporal g yrus in the Sylvian fi s-
sure. Now, draw a representative projection from the left
inferior colliculus to the left medial geniculate nucleus
via the brachium of the inferior colliculus. And fi nally,
draw a projection from the left medial geniculate nucleus
to the ipsilateral transverse temporal g yri (Heschl’s
g yri) — the primary auditory cortex. Note that the same
projections exist on the right side, as well.
Next, let’s show the projection fi bers from the ventral
and dorsal cochlear nuclei. Th ree acoustic striae exist:
ventral, dorsal, and intermediate. First, let’s draw the
ventral stria. To begin, show projection fi bers from the
left ventral cochlear nucleus to the bilateral superior oli-
vary complexes. Th en, add another pathway that projects
from the ventral cochlear nucleus to the contralateral
inferior colliculus via the lateral lemniscus without syn-
apsing in the superior olivary complex. Label the ventral
cochlear to bilateral superior olivary complex and ven-
tral cochlear to contralateral inferior colliculus projec-
tion fi bers as the ventral acoustic stria, and indicate that
they are also referred to as the trapezoid body. Next, in
our axial diagram show that the ventral acoustic stria
passes ventral to the restiform body.
Now, return to the coronal diagram and show that
the dorsal cochlear nucleus projects to the contralateral
inferior colliculus via the lateral lemniscus without form-
ing a synapse in the superior olivary complex, and label
this projection as the dorsal acoustic stria. In the axial
diagram, show that the dorsal acoustic stria emerges
from the dorsal cochlear nucleus and passes dorsally
around the restiform body.
Next, back in the coronal diagram, show that the ven-
tral cochlear nucleus sends additional projection fi bers
to the contralateral inferior colliculus, which ascend via
the lateral lemniscus tract without synapsing in the supe-
rior olivary complex. Label these fi bers as the intermedi-
ate acoustic stria. In our axial diagram, show that the
intermediate acoustic stria emerges from the dorsal por-
tion of the ventral cochlear nucleus and passes dorsally
around the restiform body.
A simple acronym for the central auditory pathways
is “S-L-I-M,” which denotes a few of the key steps in
the pathway: superior olivary complex to lateral lemnis-
cus to inferior colliculus to medial geniculate body. A
common electrophysiologic test of the auditory system
is the brainstem auditory evoked response, which helps
localize central auditory pathway lesions and helps detect
asymptomatic central nervous system lesions in the diag-
nostic evaluation of multiple sclerosis.^3 , 6