This is a purely motor nerve, of spinal rather than cranial origin. Its fibers arise from the anterior horn cells of the upper 4 or 5 cervical segments and enter the skull through the foramen magnum. Intracranially, the accessory nerve travels for a short distance with the part of the tenth nerve that is derived from the caudalmost cells of the nucleus ambiguus; together, the two roots are referred to as the vagal-accessory nerve or the cranial root of the accessory nerve. The 2 roots together leave the skull through the jugular foramen. The vagus fibers then rejoin the main trunk of the vagus. The motor fibers derived from the upper cervical segments of the spinal cord form an “external ramus” and innervate the ipsilateral sternocleidomastoid and trapezius muscles. Only the somatic motor fibers constitute the accessory nerve in the strict sense. In patients with torticollis, however, division of the upper cervical motor roots or the spinal accessory nerve has often failed to ablate completely the contraction of the sternocleidomastoid muscle. This suggests a wider innervation of the muscle, perhaps by fibers of apparent vagal origin that join the accessory nerve for passage through the jugular foramen. The supranuclear innervation of the spinal accessory nuclei is apparently mainly ipsilateral as evidenced by contraversive turning of the head during a convulsion, the result of contraction of the ipsilateral sternocleidomastoid muscle. Whether this is attributable to a direct ipsilateral tract, or to double crossing of the supranuclear tracts, is not known.
A complete lesion of the accessory nerve results in weakness of the sternocleidomastoid muscle and upper part of the trapezius (the lower part of the trapezius is innervated by the third and fourth cervical roots through the cervical plexus). Weakness of the sternocleidomastoid can be demonstrated by having the patient forcibly turn the head in the opposite direction against the examiner’s hand. When it is weak, little force will be generated and the examiner will feel that the sternocleidomastoid does not contract firmly beneath the fingers. This muscle can be further tested by having the patient press his head forward against resistance or lift his head from the pillow. Weakness of the trapezius can be demonstrated by asking the patient to shrug his shoulders; the affected side will be found to be weaker, and there will often be evident atrophy of the upper part of the trapezius. With the arms at the sides, the shoulder on the affected side droops and the scapula is slightly winged; the latter defect is accentuated with lateral movement of the arm (with serratus anterior weakness, winging of the scapula is more prominent and occurs on forward elevation of the arm).
Motor neuron disease, poliomyelitis, syringomyelia, and spinal cord tumors may involve neurons of the spinal accessory nerve. In its intracranial portion, the nerve is usually affected along with the ninth and tenth cranial nerves by herpes zoster or by lesions of the jugular foramen (glomus tumors, neurofibromas, metastatic carcinoma, internal jugular vein thrombosis). Tumors at the foramen magnum may also damage the nerve. In the posterior triangle of the neck, the eleventh nerve can be damaged during surgical operations and by external compression or injury. Compressive and invasive lesions of the nerve may be visualized by CT or MRI of the posterior cervical space.
A benign disorder of the eleventh nerve, akin to Bell’s palsy, has been described by Spillane and by Eisen and Bertrand. It begins with pain in the low lateral neck that subsides in a few days and is followed by weakness and atrophy in the distribution of the nerve. Also, a recurrent form of spontaneous accessory neuropathy has been described (Chalk and Isaacs). About one-quarter to one-third of eleventh nerve lesions are estimated to be of this idiopathic type; most, but not all, of the patients recover.