The spinal accessory nerve is a unique cranial nerve that has both a cranial and spinal component. It originates from motor neurons in the upper cervical spinal cord and nucleus ambiguus. The cranial component joins with the vagus nerve after exiting the skull through the jugular foramen. The spinal component separates and innervates the sternocleidomastoid and trapezius muscles, supplying motor function to the neck and shoulder regions. Injury to the spinal accessory nerve can result in weakness of the sternocleidomastoid or trapezius muscles on the same side, causing issues with head and shoulder movement.

1. ACCESSORY NERVE (XI)
DR. SAMIA SHAHBAZ

3. 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

4. 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.

5. CRANIAL ROOT
It 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.

6. SPINAL PART
The 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.

7. 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).

10. 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

12. ACCESSORY NERVE PALSY
STERNOCLEIDOMASTOID 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.

13. 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.