Accessory nerve

Anatomy

Traditionally the accessory nerve has been described as having both spinal and cranial roots. Consequently, the nerve is commonly discussed according to the two divisions (i.e. cranial and spinal roots). The cranial division (internal ramus) emerges from the medulla oblongata of the brainstem at the level of the nucleus ambiguus. The nucleus ambiguus is a large motor neuron that is embedded in the reticular formation. The caudal part of the motor neuron is also continuous with the nucleus of the accessory nerve.

The spinal part (external ramus) of the accessory nerve leaves the anterior surface of the upper five segments of the spinal cord between the dorsal and ventral sulci. 

Revise the location of the 12 cranial nerves with the following quiz.

Function

The cranial accessory nerve has a similar function to the vagus nerve. It carries motor innervation to the pharyngeal constrictors, larynx, and muscles of the soft palate (except tensor veli palatini).

The spinal accessory nerve is a purely motor entity. It innervates both the trapezius and the sternocleidomastoid muscles. Note that the sternocleidomastoid’s attachment to the atlantoaxial joint (sternomastoid head) and its connection between the clavicle and the mastoid process (cleidomastoid head) facilitates turning of the head to the contralateral side and flexion of the neck to the ipsilateral side (respectively). Additionally, activation of the trapezius muscle causes extension of the neck. It also allows retraction, elevation, and rotation of the scapula.

Examination of the accessory nerve

With all clinical exams, be sure to introduce yourself to the patient and explain the examination procedure that will be done. Assessment of the accessory nerve in the clinical setting can be quickly conducted by checking the muscle bulk, tone, and power. With the patient sitting upright, adequately expose the patient to visualize the sternocleidomastoid and trapezius muscles. Stand in front of the patient to inspect for signs of muscle hypertrophy or wasting of the sternocleidomastoid. Also, quickly palpate the sternocleidomastoid muscle and compare the findings with that of the contralateral muscle. Repeat the process of inspection and alternate palpation of the trapezius muscle while standing behind the patient.

Also while standing behind the patient, ask that they shrug their shoulders as the examining hands apply opposing downwards pressure. This assesses the power of the trapezius muscles. When facing the patient, ask them to turn their head to the left while the examining hand applies a continuous opposing for to the left chin. This assesses the power of the sternocleidomastoid muscle. Test the other sternocleidomastoid by reversing the test.

Is there a cranial root of the accessory nerve?

Several researchers have published data that contradicts the existence of the cranial nuclei of the accessory nerve. Some studies demonstrate that the rootlets that originate from the caudal part of the nucleus ambiguus do not join those of the spinal nucleus. Instead, these fibers go on to join the upper fibers from the nucleus ambiguus that form CN X. In contrast to these observations, a relatively large cadaveric study makes several compelling arguments. After identifying a cranial accessory nerve in 76% of their specimen, the researchers proposed the following theories:

• The presence of a cranial accessory nerve may explain the preservation of trapezius function following radical neck dissections where both the spinal accessory and cervical nerves have been deliberately sacrificed.
• The cranial rootlets are extremely small and the use of surgical microscopes greatly increases the rate of detection.
• In keeping with the size of the rootlets, if the brain is removed for examination, the rootlets may be severed. Consequently, it will give a false impression that they do not exist.

Embryology

The upper five spinal cord segments house the nucleus of the accessory nerve. The development of the spinal cord begins with closure of the neural tube and proliferation of the dorsally located somites during week 5 of gestation. This process involves a series of inductive events associated with progressive proliferation, differentiation, and migration of primitive nerve tissue. Between the 9th to 10th gestational weeks, the primitive grey matter continues to proliferate. It then separates into ventral and dorsal thickened areas known as basal and alar plates. The basal plates eventually give rise to the ventral motor horn of the spinal cord, from which the fibers of the spinal accessory nerve arise.

The most caudal part of the developing medulla oblongata has significant similarities to the developing spinal cord. Like those found in the spinal cord, the neuroblasts found in the basal plates of medulla oblongata also give rise to three motor nuclei. The nucleus ambiguus arises from the most lateral of the three columns of neuroblasts. For completion, the other two columns form the nucleus of the hypoglossal nerve (general somatic efferent; CN XII) medially, and the special somatic efferent nuclei that innervate the derivatives of the derivatives of the pharyngeal arches.

During weeks 4 to 6 of gestation, the primitive nucleus ambiguus is medially located. Subsequently, they move laterally during the 6th week of gestation to form the nucleus ambiguus from which the general visceral efferent components of the glossopharyngeal, vagus and part of the accessory cranial nerves (CN IX, X, and part of XI, respectively) arise. The development of the nucleus follows a craniocaudal pattern between weeks 7th to 12th weeks of gestation, where the cranial part of the nucleus becomes more organized initially and the caudal portion of the nucleus is the last to be organized. It is the caudal third of the nucleus ambiguus that gives rise to the few rootlets of the cranial accessory nerve.