156 Neuroanatomy: Draw It to Know It
The Medulla
Here, we will draw an anatomic, axial cross-section of
the medulla. First, show the axes of our diagram: indi-
cate that the anterior pole is at the bottom of the page
and that the posterior pole is at the top and then show
the anatomic left –right orientational plane. Next, on
one side, label the nuclei and, on the other, the tracts.
Now, draw an outline of the medulla. Show that the bulk
of the medulla is tegmentum and that the basis is reserved
for the descending corticospinal tract fi bers, which pop-
ulate the medullary pyramids. Most of the classes of cor-
ticofugal fi bers (corticonuclear and corticopontine)
synapse above or at the level of the medulla; the only
corticofugal pathway within the medulla is the corti-
cospinal tract.
Now, let’s draw the large fi ber sensory system. In the
dorsal medulla, label the gracile nucleus, medially, and
the cuneate nucleus, laterally; they receive the ascending
gracile and cuneate posterior column tracts from the
spinal cord. Th en, draw the medial lemniscus tract along
the midline of the medulla and indicate that in the lower
medulla, internal arcuate fi bers decussate from the grac-
ile and cuneate nuclei to the opposite side of the medulla
in what is referred to as the great sensory decussation
(see Drawing 10-1). Next, along the outside of the medial
lemniscus, label the anterior trigeminothalamic tract.
We fi nd anterior trigeminothalamic projections through-
out the brainstem because the anterior trigeminotha-
lamic tract is formed from fi bers of the spinal trigeminal
nucleus, which extends inferiorly into the upper cervical
spinal cord. On the contrary, fi bers from the posterior
trigeminothalamic tracts originate and ascend from the
principal sensory nucleus in the pons, so no posterior
trigeminothalamic tract fi bers are found within the
medulla.
Next, label the inferior olive (aka inferior olivary
complex), which comprises the main inferior olivary
nucleus and the accessory olivary nuclei. Th e inferior
olive receives many diff erent fi ber pathways, including
tracts from the spinal cord (from below) and from the
red nucleus (from above). Th e inferior olive sends climb-
ing fi bers to the contralateral dentate nucleus of the
cerebellum, which projects back to the contralateral red
nucleus to complete the triangle of Guillain-Mollaret.
Now, let’s focus on the lateral wall of the medulla.
First, posterior to the inferior olive, draw the anterolat-
eral system. At the medullary level, the anterolateral
system contains many ascending sensory pathways in
addition to the spinothalamic fi bers, including the spino-
reticular, spinomesencephalic, and spinotectal pathways.
It also carries spino-olivary and spinovestibular fi bers,
which disperse within the medulla, itself. Next, poste-
rior to the anterolateral system, draw the spinocerebellar
tracts: both the anterior and posterior spinocerebellar
pathways are found within the medulla. Th en, medial to
the anterolateral system and spinocerebellar tracts, label
the rubrospinal tract.
Now, draw the other main supplementary pathways:
the medial longitudinal fasciculus in the posterior mid-
line and the tectospinal tract anterior to it (the anterior-
posterior and medial-lateral positions of these pathways
remain roughly unchanged throughout their course
through the brainstem). Next, along the outer, posterior
wall of the medulla, label the inferior cerebellar peduncle.
Now, indicate that the fourth ventricle is the cerebrospi-
nal fl uid space of most of the medulla. A small tissue fold,
called the obex, exists where the fourth ventricle funnels
inferiorly into the central canal. At the rostral–caudal
level of the obex lies the gracile tubercle — the swelling
formed by the gracile nucleus in the posterior wall of the
medulla. Along the anterior border of fourth ventricle
(in its inferior fl oor) label the area postrema, which is an
important chemoreceptor trigger zone for emesis (vom-
iting ). Next, indicate that the central gray area surrounds
the fourth ventricle. Th en, show that within the central
medullary tegmentum lies the reticular formation, in the
median zone of which lies the raphe nuclei.
Lastly, label the nuclei of cranial nerves 5, 8, 9, 10, and
12 in the dorsal medulla. Again, note that our diagram
does not refl ect the medial-lateral position of the cranial
nerve nuclei. Th e cochlear nucleus of cranial nerve 8
represents the medulla’s contribution to the auditory
system.^1 – 11