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Accessory nerve: want to learn more about it?

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Accessory nerve

The accessory nerve (CN XI) is probably one of the most controversial of the twelve cranial nerves. There is significant dispute throughout the medical literature regarding whether or not the nerve should truly be classified as a cranial nerve. This discord surrounds the observation that nerve fibers arising from the cranial nucleus are only briefly associated with those coming from the spinal nucleus, before continuing with other cranial nerves. This debate is stimulated even more by the fact that the Galenic description of cranial nerves is those that developed within the brain, and not necessarily those that pass through the cranial vault. Most texts will still refer to the accessory nerve as having two components. The fibers arising from the intracranial component is known as the cranial accessory nerve, while those fibers arising from the spinal nucleus are called the spinal accessory nerve.

While acknowledging the ongoing dispute, this article will discuss the structure of the accessory nerve as was classically described (i.e. two components). Concepts about the course, function, and examination of the nerve will be addressed. Further points regarding lesions of the accessory nerve will also be included here.

Embryology of the accessory nerve

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.

Key facts
Divisions (parts) Cranial division and spinal division
Nuclei Spinal accessory nucleus, cranial nucleus of the accessory nerve (nucleus ambiguus)
Type Motor
Brainstem emergence Posterior border of the olive below the level of the vagus nerve
Associated foramen Jugular foramen (pars vascularis)
Innervation Trapezius muscle, sternocleidomastoid muscle, pharyngeal constrictors, larynx, muscles of the soft palate (motor)
Clinical Extracranial lesions, lesions within the jugular foramen, lesions of the nucleus, lesions above the nucleus

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.

Anatomy of the accessory nerve

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. 

Nuclei of the accessory nerve

Motor neurons from each of the five upper cervical segments give rise to neuronal rootlets that protrude from the anterior surface of the spinal cord. This spinal accessory nucleus (also called the accessory nucleus proper) rarely extends beyond C5 but has been observed as low as C7. The rootlets eventually coalesce to form a solitary nerve fiber known as the spinal accessory nerve.

Spinal accessory nucleus (dorsal view)

The caudal third of the nucleus ambiguus also gives rise to several rootlets that merge to form a single nerve root. This portion of the nucleus ambiguus is referred to as the cranial nucleus of the accessory nerve. The motor neurons that make up these nuclei are classified as branchial lower motor neurons. They utilize acetylcholine (ACh) as their neurotransmitter, have end-organ targets outside of the central nervous system, and result in paresis (paralysis) when they are injured. The nuclei also receive bilateral corticonuclear innervation. Therefore, the end organ motor may not be severely affected by unilateral upper motor neuron lesions.

Nucleus ambiguus (dorsal view)

Accessory nerve pathway

The pathway of the accessory nerve can be subdivided into several components. The nerve has an intracranial course, a brief passage through a foramen, and an extracranial course. The extracranial course can be further subdivided into the course in the anterior, and then in the posterior, triangles of the neck.

Intracranial route of the accessory nerve

The cranial accessory rootlets emerge along the posterior border of the olive (which is located at the ventrolateral surface of the medulla oblongata) below the level of the vagus nerve (CN X). It can be challenging to recognize these rootlets as there are other nearby rootlets that belong to CN XII and CN X. On average, around four rootlets usually arise from this area, measuring roughly 17 mm in length. However, the rootlets tend to be shorter along the upper segment and got longer toward the inferior part of the medulla oblongata. In most cases, the rootlets merge to form a single nerve that travels laterally in the cerebellomedullary cistern toward the vagus nerve within the jugular foramen. However, in some cases the rootlets do not merge and appear indistinguishable from the caudal rootlets of the vagus nerve.

Accessory nerve (ventral view)

The spinal accessory nerve also has a brief intracranial course. After the rootlets fuse, the spinal accessory nerve travels cranially behind the dentate ligaments of the spinal cord. They travel parallel to the spinal cord and enter the cranial vault via the foramen magnum.

Foraminal Route of the Accessory Nerve

Recall that the jugular foramen is a wide, irregular conduit found in the posterior cranial fossa on the basal aspect of the temporal bone. It is a paired structure (with one on either side of the skull) that is limited posteromedially by the occipital bone and anterolaterally by the temporal bone. It is more commonly divided into the pars nervosa and the pars vascularis. The pars nervosa is the smaller than the pars vascularis, located anteriorly toward the medial aspect of the foramen. It conveys the inferior petrosal sinus as well as the glossopharyngeal nerve (CN IX).

The glossopharyngeal, vagus, and accessory nerves, all pass through the pars vascularis which is located posteriorly toward the lateral aspect of the foramen. The jugular bulb also passes through this part of the foramen as well. Within the foramen, the spinal accessory nerve is laterally related to the vagus nerve.

Extracranial route of the accessory nerve

As the spinal accessory nerve leaves the jugular foramen to enter the anterior triangle of the neck, it has a variable relationship to the internal jugular vein. However, it is more common to observe the nerve passing anterolaterally with respect to the internal jugular vein. As the accessory nerve moves across the transverse process of the atlas, it is crossed by the occipital artery. As it moves caudally, it is medially related to the stylohyoid and digastric muscles, as well as the styloid process. It continues along an inferior oblique path until it pierces the sternocleidomastoid between the two heads of the muscle (deep to the cleidomastoid, and superficial to the sternomastoid).

Accessory nerve (ventral view)

The spinal accessory nerve then leaves the posterior border of the sternocleidomastoid to enter the posterior triangle of the neck. It then travels along the posterior edge of the sternocleidomastoid and then travels posteroinferiorly across the posterior triangle. Here, the nerve travels between the investing and prevertebral layers of the deep cervical fascia; the latter separating it from the levator scapulae muscle.

The accessory nerve then travels caudally along the inferior surface of the trapezius muscle, after piercing its anterior border. It enters between the muscle and its investing fascia as it moves parallel to the anterior border. The spinal accessory nerve then takes a medial turn and travels parallel to the spine of the scapula (which is the insertion of the muscle).

Accessory nerve (lateral-right view)

Function of the accessory nerve

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.

Accessory nerve: want to learn more about it?

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