Point
9
Intro
Corticobulbar fibers (you can voluntarily
swallow!) to nucleus ambiguus are BILATERAL (both
crossed and uncrossed). Therefore, muscles supplied by the
nucleus ambiguus are NOT noticeably weakened in the
event of unilateral lesions of the corticobulbar system
(i.e., in the motor cortex). This means that there is
NO deviation of the uvula following cortical lesions.
Don't confuse the results of lesions of the corticobulbar
projection to nucleus ambiguus with lesions of nucleus
ambiguus! Also don't confuse the bilateral corticobulbar
input to nucleus ambiguus with the primarily CROSSED
corticobulbar input to the HYPOGLOSSAL nucleus. What
it boils down to is that BILATERAL corticobulbar
input is GREAT for you as students, since you don't
have to remember which way something deviates following its
interruption. It is only those corticobulbar projections
that are not equally bilateral (so far only that to the
HYPOGLOSSAL, but more to come) that you need to worry
about.
Motor fibers of C.N. XI that arise from the
nucleus ambiguus join the vagus outside of the skull and
innervate muscles of the larynx. These fibers comprise the
CRANIAL branch of C.N. XI. REMEMBER: CRANIAL
XI=AMBIGUUS. In contrast, the SPINAL portion of
C.N. XI consists of motor axons whose cell bodies lie
in the lateral part of the ventral horn of the first five or
six cervical SPINAL CORD segments. The axons of these
cells pass dorsal and laterally (that is they do not
exit via the ventral root), leave the spinal cord between
the dorsal and ventral roots and unite to ascend in the
spinal canal to enter the skull via the foramen magnum. They
then exit the skull via the jugular foramen along with
cranial nerves IX and X and eventually innervate the
sternocleidomastoid and the upper fibers of the
trapezius. REMEMBER: CAUDAL XI=SPINAL CORD.
Lesions involving C.N. XI fibers to these two muscles result in atrophy of the muscles. Since the RIGHT sternocleidomastoid rotates the head to the LEFT (opposite), a lesion of the RIGHT C.N. XI will result in the chin being turned slightly to the RIGHT (paralyzed) side, especially when the head is flexed. The same RIGHT side lesion will result in paralysis of the RIGHT upper trapezius and slight sagging of the RIGHT shoulder.
As for cortical input to the cells of origin of the spinal part of XI, you are lucky since it is bilateral.