Cranial Nerve Xi Nucleus

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The spinal accessory nerve (CN XI) is classified as a special visceral efferent (SVE) nerve because it innervates striated muscles that arise embryologically from the branchial arches.^ CN XI consists of two distinct portions, the cranial (or accessory) portion and the spinal portion ( .Fig 14-1 ).[y] These are sometimes referred to as the ramus internus and ramus externus, respectively, y each with distinctive embryological developmental

The cranial and spinal portions of CN XI arise from separate nuclei. The nucleus for the cranial portion of the

Figure 14-1 Schematic of the cranial and spinal portions of CN XI.

nerve is the most caudal part of the nucleus ambiguus, which is the longitudinal column of cells that is dorsal to the inferior olivary nucleus in the medulla. y The nucleus ambiguus is composed of cells controlling several functions outside of the domain of CN XI: (1) motor neurons supplying the muscles of the soft palate, pharynx, and larynx; (2) preganglionic parasympathetic neurons involved with control of heart rate; and (3) afferent input from brain stem sensory nuclei, especially the nucleus of the spinal tract of the trigeminal nerve and the nucleus of the tractus solitarius. The nucleus for the spinal portion of CN XI is the accessory nucleus, which is a longitudinal column of cells in the anterior horn of the upper five cervical segments of the spinal cord. M , y As is too often the case in the biological sciences, nomenclature is somewhat muddled, and it is important not to confuse (1) the accessory nucleus in the cervical spinal cord, which gives rise to the spinal portion of CN XI; and (2) the cranial or accessory portion of CN XI, arising from the nucleus ambiguus in the medulla.


The eleventh cranial nerve fibers extend peripherally from two distinct nuclei: the caudal portion of the nucleus ambiguus in the medulla and the accessory nucleus in the cervical spinal cord. Fibers from the caudal portion of the nucleus ambiguus leave the brain stem caudal to the vagus nerve, as the cranial root of CN XI. M , [14] Fibers from the accessory nucleus exit the cervical spinal cord as a series of rootlets. These unite to form a common trunk. They then ascend as the spinal root of CN XI, passing upward through the foramen magnum, and uniting with the cranial root of CN XI near the medulla. The united cranial and spinal roots of the accessory nerve exit the cranium through the jugular foramen.M , y , [14] , y At this point, the spinal accessory nerve lies within the same dural sheath as the vagus nerve, separated from it by a fold of arachnoid. y

After exiting the cranium through the jugular foramen,



Anatomical Site of Damage

CN XI or XII Finding

Other Neurological and Medical Findings

Common Etiologies

Supranuclear (cerebral hemisphere)

Neck weakness, tongue deviation

Hemiparesis with spasticity and extensor plantar responses, pseudobulbar palsy, cognitive deficits, personality changes, urinary incontinence, gait apraxia, primitive reflexes (e.g., suck, snout, glabellar), paratonic rigidity (gegenhalten)

Stroke, brain tumor, multiple sclerosis, brain abscess

Supranuclear (pontine)

Dissociated weakness (ipsilateral SCM and contralateral trapezius), contralateral weakness with spasticity, and deviation of the eyes toward the weakness

Facial numbness (CN V), weakness of the palate (CN X), or autonomic dysfunction. Contralateral loss of pain and temperature in the body and ipsilateral loss of pain and temperature in the face

Stroke, multiple sclerosis, brain abscess, brain tumor


Bilateral lesions may result in diminished ability to rotate the neck, inability to protrude the tongue, slurred and indistinct speech, impaired swallowing, and possibly some respiratory difficulty. The medial medullary syndrome results in ipsilateral tongue weakness, and contralateral weakness and impaired tactile and proprioceptive sense of the arm and leg

With vascular lesions, bilateral pyramidal signs, and diminished vibration and position sensation can occur

Motor neuron disease, poliomyelitis, brain stem abscess, syringobulbia, strokes

Within the cranium or in the region of the foramen magnum

Neck and tongue weakness

Cranial nerve IX and X involvement (depressed gag, palatal paresis, and vocal cord paralysis)

Platybasia, Paget's disease, glomus tumors, acoustic neuromas, meningiomas, metastases to the skull base, syphilis, tubercular meningitis, sarcoidosis

Within the region of the jugular foramen

Vernet's syndrome (ipsilateral weakness of the SCM and trapezius)

Vernet's syndrome (ipsilateral loss of taste and depressed sensation over the posterior one third of the tongue, ipsilateral depressed gag reflex and palatal weakness, ipsilateral vocal cord paralysis, and ipsilateral weakness of the SCM and trapezius muscles)

Tumor, basal skull fracture

In the retroparotid or retropharyngeal spaces, and distal peripheral nerve lesions

Collet-Sicard syndrome (ipsilateral weakness of the SCM, trapezius, and tongue)

Collet-Sicard syndrome (same as Vernet's syndrome, with the addition of weakness of the ipsilateral tongue)

Surgical trauma, local infections, neck irradiation or local tumors

the spinal accessory nerve divides into two portions: (1) the fibers from the nucleus ambiguus join the vagus nerve to form the inferior laryngeal nerve; this nerve innervates the intrinsic muscles of the larynx; (2) the fibers from the accessory nucleus first innervate the SCM muscle and then innervate the rostral portion of the trapezius muscle. The middle and lower portions of the trapezius muscle are innervated by cervical roots. 71 , [14] , y

A number of connections exist between the eleventh cranial and other nerves. In particular, in the neck, the eleventh cranial nerve unites at several points with the first through fourth cervical nerves. y

Some investigators have soundly suggested that the cranial portion of CN XI is actually no more than a mislabeled inferior branch of CN X. [9] First, the cranial portion of CN XI innervates the intrinsic muscles of the larynx rather than the SCM or trapezeii. Second, the cranial portion of CN XI only travels with the spinal portion for a short distance before joining CN X.y When the function of CN XI is assessed in the standard neurological examination, only the spinal portion is tested. y


The nature of the supranuclear input to each of the paired eleventh cranial nerve nuclei is not well understood. It is generally believed that these nuclei receive bilateral supranuclear input, and that this input arises from the lower portion of the precentral gyrus. Although the input for the trapezius muscle is believed to be derived primarily from the contralateral hemisphere, y the input for the SCM is less well understood but is thought to be derived predominantly from the ipsilateral hemisphere. Although a minority have argued that the supranuclear fibers for the SCM do not decussate, there is a body of evidence, from isolated case reports, that there is a double decussation. y , y y The precise location of these decussations is not entirely known, although the first decussation may occur in the midbrain or pons, and the second may occur in the medulla or cervical spinal cord below the level of the first cervical root. Not surprisingly, the supranuclear input for the trapezius muscle travels separately in the brain stem

from the supranuclear input for the SCM muscle, and there have been a number of reports of the pathological dissociation of the two. y , y

The primary supranuclear input for the trapezius muscle is located ventrally in the brain stem and crosses the brain stem in either the midbrain or pons. The supranuclear input for the SCM muscle is located in the brain stem tegmentum, and it decussates twice. Some investigators have suggested that the sternal head of the SCM (responsible for rotation of the head in the contralateral direction) is innervated by the ipsilateral hemisphere, but the clavicular head of the SCM (responsible for tilting the head to the ipsilateral side) is innervated by the contralateral hemisphere. y Teleologically, this may be understood as an example of the concept that each cerebral hemisphere controls movement in the contralateral hemispace rather than just in contralateral muscle groups. y

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