ACCESSORY NERVE (Cranial Nerve XI) - KUHS Essay Note

INTRODUCTION

1. Cranial root (bulbar root)
2. Spinal root

UNIQUE FEATURES

• Only cranial nerve with two distinct roots (cranial and spinal) that temporarily unite and then separate again.
• The spinal root arises from the spinal cord (not the brainstem), making it unusual among cranial nerves.
• It passes into the cranial cavity through the foramen magnum before exiting through the jugular foramen.

FUNCTIONAL COMPONENTS AND NUCLEI

1. Special Visceral Efferent (SVE) Fibres

• Provide motor supply to the muscles of the soft palate, pharynx, and larynx.
• Arise from the nucleus ambiguus in the medulla oblongata.
• Form the cranial root of the accessory nerve.

2. General Somatic Efferent (GSE) Fibres

• Provide motor supply to the sternocleidomastoid (SCM) and trapezius muscles.
• Arise from the spinal nucleus of the accessory nerve, located in the ventral horns of the upper 5 cervical spinal segments (C1-C5).
• Form the spinal root of the accessory nerve.

COURSE, RELATIONS, AND DISTRIBUTION

A. Cranial Root

• Arises by 4-5 rootlets from the posterolateral sulcus of the medulla, between the olive and the inferior cerebellar peduncle.
• The rootlets are in line with the rootlets of the vagus nerve above.
• These rootlets unite to form a single trunk which runs laterally along with the 9th, 10th cranial nerves to reach the jugular foramen, where it is joined by the spinal root.

B. Spinal Root

• Arises by a series of rootlets from the lateral aspect of the spinal cord (upper 5 cervical segments), along a vertical line between the ventral and dorsal roots of spinal nerves.
• Rootlets unite to form a single trunk which ascends in the vertebral canal and enters the cranial cavity through the foramen magnum (posterior to the vertebral artery).
• The spinal root joins the cranial root at the jugular foramen.

C. At the Jugular Foramen

• The combined trunk exits the cranial cavity through the middle compartment of the jugular foramen, enclosed in a dural sheath along with the vagus nerve.
• Immediately after exiting, the two roots separate again.

D. Course of the Cranial Root (After Separation)

• Joins the vagus nerve just below its inferior ganglion.
• Distributed through the branches of the vagus to supply muscles of the palate, pharynx, and larynx.

E. Course of the Spinal Root (After Separation)

1. Descends vertically between the internal jugular vein and the internal carotid artery.
2. At a point midway between the angle of the mandible and the mastoid process, it turns downward and backward across the internal jugular vein (may be superficial or deep) toward the carotid triangle.
3. Crosses in front of the transverse process of the atlas, under the posterior belly of digastric and the occipital artery.
4. Accompanied here by the upper sternomastoid branch of the occipital artery.
5. Pierces the sternocleidomastoid at the junction of its upper 1/4th with the lower 3/4th and supplies it.
6. Passes through SCM and emerges through its posterior border a little above its middle to enter the posterior triangle of the neck.
7. Runs downward and backward under the fascial roof of the posterior triangle, parallel to the fibres of levator scapulae.
8. Leaves the posterior triangle by passing deep to the anterior border of the trapezius, about 5 cm above the clavicle, and supplies the trapezius.

F. Communications

• C2 and C3 spinal nerves - carry proprioceptive fibres from the SCM
• C3 and C4 spinal nerves - carry proprioceptive fibres from the trapezius

DISTRIBUTION (Supply)

Cranial Root (via Vagus and Pharyngeal Plexus):

1. All muscles of the soft palate - EXCEPT tensor palati (supplied by mandibular nerve/V3)
2. All muscles of the pharynx - EXCEPT stylopharyngeus (supplied by glossopharyngeal nerve/CN IX)
3. All intrinsic muscles of the larynx

Memory tip: "ALL except" - Accessory root supplies ALL except the two exceptions (tensor palati and stylopharyngeus).

Spinal Root:

1. Sternocleidomastoid - along with C2 and C3
2. Trapezius - along with C3 and C4

CLINICAL CORRELATION

Causes of Lesions

• Fracture of the base of skull through the jugular foramen
• Stab wounds in the neck
• Surgical removal of cervical lymph nodes (most common iatrogenic cause)

Effects of Unilateral Lesion of the Spinal Accessory Nerve (Proximal to SCM)

1. Ipsilateral Paralysis of Sternocleidomastoid

• Patient is unable to tilt head toward the ipsilateral shoulder
• Patient is unable to turn the face toward the opposite side
• (SCM turns the face to the contralateral side; loss of SCM = inability to rotate face away from the side of the lesion)

2. Paralysis of Trapezius

• Patient is unable to shrug the shoulder against resistance
• Drooping of the ipsilateral shoulder
• Weakness of shoulder elevation and retraction of the scapula

3. Spasmodic Torticollis (Irritative Lesion)

• Results from irritative central lesions of the spinal accessory nerve
• Characterized by clonic spasms of the sternocleidomastoid muscle
• Causes rhythmic or sustained turning/tilting of the head

CLINICAL TESTING OF THE SPINAL ACCESSORY NERVE

SUMMARY TABLE

IMPORTANT EXAM POINTS (KUHS Focus)

1. CN XI is purely motor - no sensory component.
2. The spinal nucleus is located in ventral horns of C1-C5 (not in the brainstem).
3. The spinal root enters the cranial cavity through the foramen magnum before exiting through the jugular foramen - this is unique.
4. Cranial root joins vagus just below the inferior ganglion of vagus - hence it is "accessory to the vagus."
5. Tensor palati (V3) and stylopharyngeus (IX) are the two notable exceptions to the cranial root supply.
6. The nerve pierces SCM at the junction of upper 1/4th and lower 3/4th - a surgically relevant landmark.
7. Proprioception from SCM = C2, C3; from Trapezius = C3, C4.
8. Damage during neck dissection/lymph node removal is the classic clinical scenario in exams.
9. Spasmodic torticollis = irritative lesion of CN XI (clonic SCM spasms).
10. Test SCM by turning face to opposite side; test Trapezius by shrugging against resistance.