214 Neuroanatomy: Draw It to Know It
Cranial Nerve 5: Peripheral Innervation
Here, we will draw the peripheral sensory coverage of the
trigeminal nerve and the trigeminal motor innervation
of the muscles of mastication. Th e peripheral divisions of
the trigeminal nerve are division 1 — the ophthalmic
division, which covers the eyes; division 2 — the maxil-
lary division, which covers the cheeks; and division 3 —
the mandibular division, which covers the jaw. First,
draw a head. Next, mark a dot on the superior-posterior
curvature of the head, then mark one at the corner of the
eye, and then the tip of the nose. Now, connect these
dots and label the region supero-anterior to this line as
division 1 — the ophthalmic division. Next, midway
along that line make another dot, then make a dot at the
maxilla, and then the corner of the mouth. Join these
dots and label the region between this line and division
1 as division 2 — the maxillary division. Finally, mark a
dot at the original superior-posterior point, then at the
tragus, and then at the mentum. Connect these dots and
label the region between this line and division 2 as divi-
sion 3 — the mandibular division. Now tap these points
on your own face for sense-memory.
Two key details of facial sensory coverage are that
division 3 of the trigeminal nerve covers neither the
outer ear nor the angle of the mandible. Th e coverage of
the outer ear is complex: it involves the great auricular
nerve (supplied by C2, C3) and cranial nerves 7, 9, and
10; the angle of the mandible is covered by the great
auricular nerve. Th us, sensory examination of the outer
ear and angle of the mandible should be normal in iso-
lated peripheral trigeminal nerve injury. Lastly, note that
the coverage of the trigeminal nerve extends posteriorly
past the superior pole of the head, and thus the sensory
loss in peripheral trigeminal nerve injury should con-
tinue posterior to the superior pole of the head.
Th e main function of the trigeminal motor system is
mastication (chewing ). Mastication requires the trigem-
inally innervated medial and lateral pterygoids, masseter,
and temporalis muscles. Feel just above and in front of
the angle of your mandible and clench and relax your
jaw. Th e contracting muscles under your fi ngertips are
the masseters. Next, look at your temples and cheeks; the
temporalis muscles fi ll out your facial contour. Atrophy
to these muscles is an important potential clue of trigem-
inal neuronal degeneration. Th e medial pterygoids, tem-
poralis, and masseter muscles elevate the mandible to
close the jaw, whereas the lateral pterygoids help to open
it (although much of the work in jaw opening actually
relies on the digastric muscle, which is partially inner-
vated by the trigeminal nerve, and the geniohyoid
muscle). Open your jaw and extend your mandible for-
ward to activate your lateral pterygoids.
Next, draw the lateral pterygoids at an angle to one
another; show that they push the mandible forward.
Th en, point your index fi ngers towards one another
to represent the lateral pterygoids. Drop one of your
fi ngers to represent a weakened lateral pterygoid; the
intact fi nger points toward the weak side. In a lower
motor neuron lesion (eg , when a trigeminal motor
nucleus or nerve is injured or when a lateral pterygoid is
damaged), the jaw deviates toward the injured side.
Cortical innervation of the trigeminal motor nuclei is
bilateral with contralateral predominance; thus, in corti-
cal injury, if jaw deviation occurs, it is directed away from
the injured side. For instance, in a right hemispheric
stroke, the left trigeminal nucleus is unable to activate
the left lateral pterygoid and so the jaw may deviate to
the left (away from the right cerebral hemispheric
injury).^1 – 8