XI - Accessory Nerve Flashcards
Which muscles does CN XI innervate?
The accessory nerve is the eleventh paired cranial nerve.
It has a purely somatic motor function, innervating the sternocleidomastoid and trapezius muscles.
v accessory nerve divide it into a spinal part and a cranial part
The spinal accessory nerve innervates 2 muscles: The sternocleidomastoid and Trapezius.
Sternocleidomastoid: has 2 connecting points of origination. One head connects to the top of the sternum(sterno) while the second connects to the clavicle(cleido). These fibers merge into one major bundle that runs up the neck to the base of the skull behind the ear, called the mastoid process (mastoid).
Action: Lateral flexion and rotation of the neck when acting unilaterally, and extension of the neck at the atlanto-occipital joints when acting bilaterally.
Trapezius: Runs from the base of the skull and the spinous processes of C7-T12 vertebrae to lateral third of the clavicle and the acromion of the scapula.
Action: It is made up of upper, middle and lower fibers. The upper fibers of the trapezius elevate the scapula and rotate it during abduction of the arm. The middle fibers retract the scapula and the lower fibers pull the scapula inferiorly.
Describe the nuclei of CN XI
the spinal accessory nucleus, is located in the lateral aspect of the anterior horn of the spinal cord, and stretches from where the spinal cord begins (at the junction with the medulla) through to the level of about C6
* the cranial accessory nucleus derives from the nucleus ambiguus of the medulla oblongata
Describe the spinal component of CN XI
The spinal portion arises specifically from C1-C5/C6 spinal nerve roots. which then runs superiorly to enter the cranial cavity via the foramen magnum.
The nerve traverses the posterior cranial fossa to reach the jugular foramen. It briefly meets the cranial portion of the accessory nerve, before exiting the skull (along with the glossopharyngeal and vagus nerves)..
/ Outside the cranium, the spinal part descends along the internal carotid artery to reach the sternocleidomastoid muscle, which it innervates. It then moves across the posterior triangle of the neck to supply motor fibres to the trapezius.
Note: The extracranial course of the accessory nerve is relatively superficial (it runs between the investing and prevertebral layers of fascia), and thus leaves it vulnerable to damage
Describe the cranial component of CN XI
The cranial portion is much smaller, and arises from the lateral aspect of the medulla oblongata. It leaves the cranium via the jugular foramen, where it briefly contacts the spinal part of the accessory nerve.
* Immediately after leaving the skull, cranial part combines with the vagus nerve (CN X) at the inferior ganglion of vagus nerve. The fibres from the cranial part are then distributed through the vagus nerve. For this reason, the cranial part of the accessory nerve is considered as part of the vagus nerve
Describe the palsy of CN XI
The most common cause of accessory nerve damage is iatrogenic (i.e. due to a medical procedure). In particular, operations such as cervical lymph node biopsy or cannulation of the internal jugular vein can cause trauma to the nerve.
* Clinical features include muscle wasting and partial paralysis of the sternocleidomastoid, resulting in the inability to rotate the head or weakness in shrugging the shoulders. Damage to the muscles may also result in an asymmetrical neckline.
How do you asses for CN XI innervation to the trapezius muscle?
Assessing the Trapezius Muscle (Upper Part):
Begin by asking the patient to sit or stand comfortably.
Instruct the patient to elevate their shoulders as if they are shrugging.
Place your hands on top of the patient’s shoulders and gently push down while they attempt to elevate their shoulders.
Resistance should be applied equally on both sides.
Signs of Dysfunction:
Weakness in elevating the shoulders on one or both sides.
Asymmetrical muscle contraction or a lack of response when resistance is applied.
Clinical Implications:
Weakness or lack of movement in the trapezius muscle on one side may suggest a nerve damage, compression
How do you assess for CN XI innervation to the Sternocleidomastoid Muscle?
Assessing the Sternocleidomastoid Muscle (Neck Turning):
Have the patient sit or stand with their head in a neutral position.
Instruct the patient to turn their head to one side against resistance.
Place your hand against the patient’s cheek or forehead to provide resistance while they attempt to turn their head.
Repeat the procedure for the opposite side.
Signs of Dysfunction:
Weakness or limited range of motion when turning the head against resistance on one side or both.
Inability to maintain head position against resistance.
Clinical Implications:
Impaired function of the sternocleidomastoid muscle, typically due to CN XI dysfunction, can affect the ability to turn the head and may be indicative of neurological or muscular dysfunctions
