216 Neuroanatomy: Draw It to Know It
Cranial Nerve 5: Nuclei
Here, we will draw the trigeminal nuclei in sagittal view.
First, draw the trifurcated sensory ganglion of the
trigeminal nerve, called the trigeminal ganglion, which
means the “three twins.” It lies in a low depression known
as the trigeminal impression at the apex of the petrous
temporal bone. Th e trigeminal ganglion is enveloped in
Meckel’s cave: a dural-based, cerebrospinal fl uid-fi lled
cavern that lies adjacent to the posterolateral aspect of
the cavernous sinus. Now show that from rostral to
caudal, the divisions of the trigeminal ganglion are the
ophthalmic, maxillary, and mandibular divisions — their
central projections terminate in the various trigeminal
nuclei. Next, show that the ophthalmic division traverses
the superior orbital fi ssure; the maxillary division tra-
verses foramen rotundum; and the mandibular division
traverses foramen ovale. As they proceed centrally, the
ophthalmic and maxillary divisions (divisions 1 and 2,
respectively) pass through the lateral wall of the cavern-
ous sinus; they then merge together along with the man-
dibular division to form the trigeminal ganglion within
Meckel’s cave.^9
Now, let’s draw a sagittal section through the brain-
stem in order to show the four diff erent trigeminal
nuclei: one motor, three sensory. We will use the upper
pons as the central point of the trigeminal nuclei, so also
draw an axial cross-section through the upper pons for
reference. Next, establish the position of the trigeminal
motor nucleus and the principal sensory nucleus in the
upper pons. Th ey lie in parallel just above the abducens
nucleus. Show that the trigeminal motor nucleus lies
medial to the principal sensory nucleus — their positions
follow the general organization of the cranial nerves (see
Drawings 11-2 and 11-4). Whereas the principal sensory
and motor trigeminal nuclei are restricted to this upper
pontine level, the two other trigeminal sensory nuclei
combined span the height of the brainstem and upper
cervical spinal cord. Return, now, to the sagittal diagram
and show that the mesencephalic nucleus spans from the
upper pons to the level of the superior colliculus (in the
midbrain), and then show that the spinal trigeminal
nucleus and tract span from the upper pons to the upper
cervical spinal cord (their termination is variably listed
as anywhere from C2 to C4). Trigeminal sensory aff er-
ents descend the spinal trigeminal tract and then synapse
in the adjacent spinal trigeminal nucleus in a specifi c
somatotopic pattern (see Drawing 13-3 ). Th e caudal end
of the spinal trigeminal nucleus is continuous with the
substantia gelatinosa, which lies within the dorsal horn
of the spinal cord, and the spinal trigeminal tract is con-
tinuous with the posterolateral fasciculus (aka Lissauer's
tract), which lies along the dorsal edge of the dorsal horn
of the spinal cord (see Drawings 7-1 and 7-2).
Next, show that the motor division of the trigeminal
nerve passes ventrolaterally through the cerebellopon-
tine angle cistern to exit the brainstem (all of the pha-
ryngeal arch derivatives leave the brainstem along a
ventrolateral path) and joins the mandibular division as
it exits the middle cranial fossa through foramen ovale to
innervate the muscles of mastication.
Th e mesencephalic nucleus contains the primary sen-
sory neurons for proprioceptive aff erents from the mus-
cles of mastication, which ascend through the mandibular
division of the trigeminal nerve. Th us, the mesenceph-
alic nucleus is the only central nervous system nucleus to
house primary sensory neurons (the primary sensory
neurons of the mesencephalic sensory aff erents do not lie
within a peripheral ganglion). Th e mesencephalic
nucleus projects to other sensory nuclei and also to the
trigeminal motor nucleus to produce the masseter refl ex
(ie, the jaw jerk). When the muscles of mastication are
stretched, they activate the mesencephalic nucleus,
which triggers the jaw jerk. A brisk jaw jerk can suggest
pathologic disinhibition of suppressive corticopontine
fi bers to cranial nerve 5.
We discuss the principal sensory and spinal trigemi-
nal nuclei next.^1 – 6 , 10