152 Neuroanatomy: Draw It to Know It
The Midbrain (Cont.)
Periaqueductal gray area functions include far-reaching
modulation of sympathetic responses (ie, pupillary dilation
and cardiovascular responses); parasympathetic-induced
micturition; modulation of reproductive behavior; and
even aff ect locomotion and vocalization. However, its
most widely recognized function is in pain modulation.
Next, in front of the periaqueductal gray area, label
the reticular formation. Initially, the indistinct histolog y
of the reticular formation led people to believe it was
simply a “diff use arousal network,” but now the func-
tional specialization of the reticular formation is well
recognized. Th e reticular formation divides into lateral,
medial, and median zones, and the raphe nuclei popu-
late the last — the median zone. Th e raphe nuclei are pri-
marily serotinergic and are heavily modulated by
psychotropic medications. Th ey aff ect sleep–wake cycles,
pain management, and motor activity but are most com-
monly referenced for their role in mood disorders and
the hallucinatory eff ects of illicit drugs. Th e raphe nuclei
lie along much of the height of the midline brainstem as
six separate subnuclei, which divide into rostral and caudal
nuclear groups based on whether they lie above or below
the mid-pons. Th e rostral raphe group (aka oral raphe
group) comprises the upper pontine and midbrain raphe
nuclei: the caudal linear, dorsal raphe, and median raphe
nuclei. Th e caudal raphe group comprises the lower pon-
tine and medullary raphe nuclei: the raphe magnus, raphe
obscurus, and raphe pallidus nuclei. Note that additional
serotinergic reticular formation areas are also categorized
as part of the raphe nuclei. Eff erent projections from the
rostral raphe group mostly ascend into the upper brain-
stem and forebrain, whereas projections from the caudal
raphe group primarily descend into the lower brainstem
and spinal cord. Aff erents to the raphe nuclei also exist,
which generally originate from behavioral brain areas.^7
Now, label the presence of the cranial nerve 3 and 4
nuclei and also a portion of the cranial nerve 5 nuclei —
subnuclei of cranial nerve 5 lie along the height of the
brainstem. Note that although we show cranial nerves 3
and 4 in the lateral midbrain, here, this is only because of
the constraints of our diagram; they actually lie in the
midline midbrain, as shown more accurately in Chapters
11 and 12.
Next, let’s address the supplementary motor and sen-
sory fi ber tracts. In midline, just in front of the periaque-
ductal gray area, label the medial longitudinal fasciculus,
which plays an important role in conjugate horizontal
eye movements (see Drawing 23-1). Th en, in the central,
dorsal tegmentum, label the central tegmental tract. It
carries ascending reticular fi bers to the rostral intralami-
nar nuclei of the thalamus as part of the ascending arousal
system and descending fi bers from the red nucleus to the
inferior olive as part of the triangle of Guillain-Mollaret.
Along the posterior wall of the central tegmental tract, label
the posterior trigeminothalamic pathway: it originates in
the upper pons and ascends through the midbrain.
Now, let’s draw the key contents of the tectum. First,
draw the colliculi, which divide into paired bilateral
superior and inferior colliculi. Th e superior colliculi lie
in the upper midbrain and are involved in visual func-
tion and the inferior colliculi lie in the lower midbrain
and are involved in auditory function. Th en, draw the
posterior commissure. Th e pathway for the pupillary
light refl ex passes through the posterior commissure and
the nucleus of the posterior commissure helps control
vertical eye movements.
Next, label the tectospinal tract just anterior to the
medial longitudinal fasciculus. Th e tectospinal tract
originates in the superior colliculus and decussates in the
midbrain tegmentum and descends in front of the medial
longitudinal fasciculus. Both the medial longitudinal
fasciculus and the tectospinal tract maintain their poste-
rior, midline position throughout the height of the
brainstem. Regional stimulation of the superior collicu-
lus stimulates eff erent impulses through the tectobulbar
tract to the brainstem for eye movements and through
the tectospinal tract to the upper cervical nuclei for visu-
ally directed neck and head movements.^1 – 11