150 Neuroanatomy: Draw It to Know It
The Midbrain
Here, we will draw an anatomic, axial cross-section
through the midbrain. First, show the anterior–posterior
and left –right axes of our diagram and then label the left
side of the page as nuclei and the right side as tracts. Th en,
label the basis, tegmentum, and tectum. Next, anteriorly,
draw the bilateral crus cerebri. Divide the center of the
crus into the corticonuclear tracts (aka corticobulbar
tracts), medially, and the corticospinal tracts, laterally.
Next, in the most medial portion of the crus, draw the
frontopontine tracts, and in the most lateral portion,
draw the additional corticopontine tracts, which ema-
nate from the occipital, parietal, and temporal cortices.
Now, label the substantia nigra as the long stretch of
dopaminergic nuclei just posterior to the white matter
pathways in the base of the midbrain. Loss of dopamin-
ergic cells in the dark, melanin-rich pars compacta divi-
sion of the substantia nigra results in Parkinson’s disease.
Th e reddish, iron-rich, pars reticulata division of the
substantia nigra plays an important role in the direct and
indirect basal ganglia pathways (see Drawing 18-4). Th e
substantia nigra helps initiate movement, and patholog y
to it, either from degeneration or injury, produces bra-
dykinesia, a slowness and stiff ness of movement.
Next, in the anterior aspect of the midbrain tegmen-
tum, draw the circular red nucleus just off midline. Th e
red nuclei receive fi bers from both the motor cortex and
cerebellum. Each red nucleus connects with the ipsilat-
eral inferior olive as part of the triangle of Guillain-
Mollaret (via the central tegmental tract), and each red
nucleus also sends rubrospinal tract fi bers down the
brainstem and spinal cord to produce fl exion movements
of the upper extremities (see Drawing 7-3). Make a nota-
tion that the red nuclei span the mid and upper mid-
brain. Injury in the vicinity of the red nucleus can
produce a low-frequency, coarse postural and action
tremor on the contralateral side of the body, called a
rubral tremor. Despite its name, which suggests a close
relationship to the red nucleus, rubral tremor can occur
from injury to other brainstem areas, as well, and also
from injury to the cerebellum and thalamus.
Now, show that fi bers from the superior cerebellar
peduncle (the major outfl ow tract of the cerebellum)
decussate in the central midbrain tegmentum, and indi-
cate that this decussation occurs in the inferior midbrain
(below the level of the red nuclei). Injury to these cross-
ing fi bers produces cerebellar ataxia on the side of the
body that the fi bers originated from (regardless of where
they are injured along their path). For instance, whether
it happens pre- or post-decussation, injury to superior
cerebellar fi bers from the right cerebellum produces
ataxia on the right side of the body.
Next, draw a red nucleus on the opposite side of the
midbrain so we can see how its presence forces the
ascending sensory pathways out laterally. Immediately
lateral to the red nuclei, draw the medial lemniscus, and
posterolateral to it, draw the anterolateral system. Next,
along the posterior wall of the medial lemniscus, label
the anterior trigeminothalamic tract (we draw the poste-
rior trigeminothalamic tract later). Th en, along the pos-
terolateral wall of the anterolateral system, label the
lateral lemniscus.
Now, label the cerebral aqueduct as the small cerebro-
spinal fl uid space in the dorsum of the midbrain. Th en,
label the surrounding periaqueductal gray area. Th e
periaqueductal gray area is packed with neuropeptides,
monoamines, and amino acids, but it most notably con-
tains opioids, which help in pain suppression. Electrical
stimulation of the periaqueductal gray area to produce
analgesia was fi rst attempted in the 1970s but has had
mixed results. Of note, the periaqueductal gray area
receives ascending spinomesencephalic fi bers via the
anterolateral system, which play a role in the emotional
aspect of pain, and it receives descending fi bers from the
hypothalamus via the dorsal longitudinal fasciculus.