50 Neuroanatomy: Draw It to Know It
Cervical Plexus (Advanced)
Here, we will draw the cervical plexus; it is constituted
by ventral rami from C1–C4, which emerge from under-
neath the sternocleidomastoid muscle and innervate
structures of the anterior and posterior cervical triangles
and the fl oor of the mouth. Begin our diagram with a
few key anatomic structures: fi rst, draw the clavicle;
then, the trapezius muscle; and then, the inferior attach-
ment of the sternocleidomastoid muscle to the anterior
clavicle; and fi nally, the superior attachment of the
sternocleidomastoid muscle to the mastoid bone. We
cut out the sternocleidomastoid muscle belly because it
would obstruct the view of our diagram.
Next, in midline of the page, draw seven small foram-
ina: the jugular foramen, the hypoglossal canal, and the
foramina of C1 through C5. As mentioned, the cervical
plexus is formed from the C1–C4 spinal nerves, but we
include the jugular foramen, hypoglossal canal, and C5
foramen, here, because they are the exit sites of related
anatomic structures.
First, let’s draw the four major sensory nerves of the
cervical plexus, three of which we show by fi rst drawing
an anastomosis between C2 and C3. Th is connection
forms the lesser occipital, greater auricular, and transverse
cervical nerves. Show that the lesser occipital nerve pro-
vides sensory coverage to the superior pole of the pinna
and posterolateral head; the greater auricular nerve pro-
vides sensory coverage to the inferior pole of the pinna
and the angle of the mandible; and the transverse cervical
nerve (aka the anterior cutaneous nerve of the neck) pro-
vides sensory coverage to the anterolateral neck. Next,
for the fourth sensory nerve, draw an anastomosis
between C3 and C4 and show that it derives the supra-
clavicular nerve, which provides sensory coverage to the
posterolateral neck, upper chest, and shoulder. Note that
minor C3 and C4 sensory branches to the trapezius
muscle also exist, which we leave out of our diagram for
simplicity.
Next, let’s draw the motor nerves of the cervical
plexus. For this, we begin with the phrenic nerve. Show
that it originates from C3 and C4 from the cervical
plexus and C5 from the brachial plexus. It descends
through the thoracic cavity to innervate the diaphragm.
Although not part of the cervical plexus, for regional
purposes also include the hypoglossal nerve, which tra-
verses the hypoglossal canal. It innervates all of the
intrinsic tongue muscles and the majority of the extrin-
sic tongue muscles, shown in Drawing 12-6.
Next, show that for a portion of its course, C1 joins
cranial nerve 12 as part of the hypoglossal nerve. Th en,
show that C1 innervates both the geniohyoid and
thyrohyoid muscles and that it forms the superior (aka
descending ) root of the ansa cervicalis, which innervates
the superior belly of the omohyoid muscle.
Now, show that C2 and C3 together form the infe-
rior root of the ansa cervicalis, and show that where the
superior and inferior roots meet, the ansa cervicalis
innervates sternohyoid, sternothyroid, and the inferior
belly of omohyoid.
We can group the aforementioned muscles based on
their anatomic compartments. Sternohyoid, sternothy-
roid, omohyoid, and thyrohyoid are all infrahyoid mus-
cles (meaning they lie below the hyoid bone) and they
are collectively referred to as strap muscles. On the con-
trary, geniohyoid is a suprahyoid muscle (meaning it lies
above the hyoid bone); the other suprahyoid muscles are
mylohyoid, stylohyoid, and the digastric. Th e digastric
and stylohyoid muscles lie within the anterior triangle of
the neck whereas geniohyoid and mylohyoid lie within
the fl oor of the mouth.