ACCESSORY NERVE (XI)DR. SAMIA SHAHBAZ

The spinal accessory nerve is a motor nerve.

It is the 11th pair out of the total 12 pairs of cranial nerves.

Two components: Cranial part Spinal part

It is a unique cranial nerve as it originates extracranially i.e.

arises from motor neurons of the

upper five segments [c1-c5] of the

cervical spinal cord.

CRANIAL ROOTIt is formed from the axons of nerve cells of

nucleus ambiguus.

It runs lateral in the posterior cranial fossa

And joins the spinal root before exiting through

the foramen magnum

After exiting the foramen magnum,

The cranial root joins with the vagus nerve.

SPINAL PARTThe spinal part is formed from axons of nerve cells in the spinal nucleus.

The spinal portion arises from neurons of the upper spinal cord, specifically C1-C5 spinal nerve roots. These fibres coalesce to form the spinal part of the accessory nerve, 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).

After a short distance, the spinal root separates from the cranial root and 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.

So,

It supplies

STERNOCLEIDOMASTOID MUSCLE

TRAPEZIUS MUSCLE

ACCESSORY NERVE PALSYSTERNOCLEIDOMASTOID MUSCLE:the sternocleidomastoid muscle is tested by asking the patient to turn their head to the left or right against resistance. weakness in head-turning suggests injury to the contralateral spinal accessory nerve: a weak leftward turn is indicative of a weak right sternocleidomastoid muscle (and thus right spinal accessory nerve injury) and vice versa.

TRAPEZIUS MUSCLE

The trapezius muscle is tested by asking the patient to shrug their shoulders with and without resistance. A one-sided weakness is indicative of an injury to the spinal accessory nerve on the same side (ipsilateral) of the body being assessed.

DROOPED shoulders will be indicative of injury.