500
UNIT 3
Regulation and Integration of the Body
13
XI Accessory Nerves
Origin and course:
Unique in that they form from rootlets that emerge
from the spinal cord, not the brain stem. These rootlets arise laterally
from superior region (C
1
–C
5
) of spinal cord, pass upward along spinal
cord, and enter the skull as the accessory nerves via foramen magnum.
The accessory nerves exit from skull through
jugular foramen
together
with the vagus nerves, and supply two large neck muscles.
Until recently, was considered to have both a cranial and spinal portion,
but the cranial rootlets are actually part of the vagus nerves. This raises
an interesting question: Should the accessory nerves still be considered
cranial nerves? Some anatomists say “no” because they don't arise
from the brain. Others say “yes” because their origin is different from a
typical spinal nerve and they pass through the skull. Stay tuned!
Function:
Mixed nerves, but primarily motor in function. Supply motor
fibers to trapezius and sternocleidomastoid muscles, which together
move head and neck, and convey proprioceptor impulses from same
muscles.
Clinical testing:
Check strength of sternocleidomastoid and trapezius
muscles by asking person to rotate head and shrug shoulders against
resistance.
Homeostatic Imbalance
Injury to one accessory
nerve causes head to turn toward injury side as result of sterno-
cleidomastoid muscle paralysis. Shrugging that shoulder (role of
trapezius muscle) becomes difficult.
XII Hypoglossal Nerves
(hi
0
po-glos
9
al)
Origin and course:
As their name implies (
hypo
5
below;
glossal
5
tongue), hypoglossal nerves mainly serve the tongue. Fibers arise by a
series of roots from medulla and exit from skull via
hypoglossal canal
to travel to tongue. See also Figure 13.6.
Function:
Mixed nerves, but primarily motor in function. Carry
somatic motor fibers to intrinsic and extrinsic muscles of tongue, and
proprioceptor fibers from same muscles to brain stem. Hypoglossal
nerve control allows tongue movements that mix and manipulate
food during chewing, and contribute to swallowing and speech.
Clinical testing:
Ask subject to protrude and retract tongue. Note any
deviations in position.
Homeostatic Imbalance
Damage to hypoglossal
nerves causes difficulties in speech and swallowing. If both nerves
are impaired, the person cannot protrude tongue. If only one side is
affected, tongue deviates (points) toward affected side; eventually
paralyzed side begins to atrophy.
Jugular foramen
Foramen magnum
Accessory nerve (XI)
Sternocleidomastoid
muscle
Trapezius muscle
Rootlets of accessory
nerve emerging from
spinal cord (C
1
–C
5
)
Table 13.2
Cranial Nerves
(continued)
IX.
Glossopharyngeal.
Te name
glossopharyngeal
means
“tongue and pharynx,” the structures that this nerve helps to
innervate.
X.
Vagus.
Tis nerve’s name means “wanderer” or “vagabond,”
and it is the only cranial nerve to extend beyond the head and
neck to the thorax and abdomen.
XI.
Accessory.
Considered an
accessory
part of the vagus
nerve, this nerve was formerly called the
spinal accessory nerve
.
XII.
Hypoglossal.
Te name
hypoglossal
means under the
tongue. Tis nerve runs inferior to the tongue and innervates
the tongue muscles.
You might make up your own saying to remember the first
letters of the cranial nerves in order, or use the following mem-
ory jog sent by a student: “
O
n
o
ccasion,
o
ur
t
rusty
t
ruck
a
cts
f
unny—
v
ery
g
ood
v
ehicle
a
ny
h
ow.”
Medulla oblongata
Intrinsic muscles
of the tongue
Hypoglossal
canal
Hypoglossal
nerve (XII)
Extrinsic muscles
of the tongue
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