Overview

Why is it called the "accessory" nerve if it’s so important?

Have you ever imagined not being able to turn your head to one side or another to see what’s going on, or being unable to shrug to express doubt or emotion? Well, all of these actions are controlled by a vital nerve whose name doesn’t do it justice: the accessory nerve.

In this article, we’ll explore the anatomy of this nerve, how it works, and what can happen if it gets injured.

What is the accessory nerve?

The accessory nerve is one of the 12 cranial nerves, which are nerves that emerge directly from the brain rather than the spinal cord. These special nerves control many important functions, from facial expressions to eye movement and swallowing.

In general, nerves work like electrical cables: they carry messages from the brain to the muscles, telling them when and how to move. The accessory nerve is a motor nerve, meaning it only sends signals to muscles to move.

It’s unique because of its dual origin:

• A cranial part, which quickly merges with the vagus nerve
• A spinal part (the clinically important one), which comes from the upper cervical spinal cord (C1–C6)¹²

Where does it go?

Once your accessory nerve leaves the upper part of your spinal cord, it takes a very interesting path through your body:

• It first travels upward into the skull through a hole called the foramen magnum, the same opening through which the spinal cord connects to the brain
• It then comes back out through another hole called the jugular foramen, which it shares with other important nerves and blood vessels
• From there, it moves down into your neck, passing close to the internal jugular vein (a major blood vessel) and the digastric muscle, which helps with jaw movement
• It eventually reaches a part of the neck called the posterior triangle, where it lies close to the skin and becomes easier to access

Here, it sends branches to two key muscles:

• The sternocleidomastoid, which helps you turn your head left or right
• And the trapezius, which helps lift and move your shoulder, like when you shrug, for example

Because it’s located so close to the surface in this area, the accessory nerve is at higher risk of injury during certain neck surgeries or after trauma^3,4

What does it do?

The XI nerve is involved in many everyday movements.

First, it innervates the sternocleidomastoid (try saying that five times without a break). It's a long muscle on the side of your neck, and it helps you:

• Turn your head left and right, like when you’re looking in the rearview mirror
• Or tilt your head slightly, like when you try to touch your ear to your shoulder (which you probably do more often than you think)

The second muscle it innervates is the trapezius, the large muscle at the top of your back and shoulders. It helps you:

• Lift your shoulders, like when you shrug to show you’re unsure about something
• And it also stabilises your shoulder blades while you move your arms

Without this muscle working properly, you wouldn’t be able to brush your hair or turn to check behind you while driving. That’s why, even though it’s called "accessory," this nerve is essential for many everyday movements.

When things go wrong

Because of its position close to the surface of your neck, the accessory nerve (cranial nerve XI) is quite exposed and can be injured during neck surgeries such as lymph node biopsies, parotid gland removal, or cancer-related procedures. ¹⁰¹¹

Unfortunately, it can also be affected by trauma, like a blow to the neck, or more rarely, for no identifiable reason.³

What happens if it’s damaged?

When the accessory nerve is injured, you might notice:

• Shoulder drooping on one side
• Trouble turning your head
• Difficulty lifting your arm or carrying bags on that side

You might also see your shoulder blade sticking out more than usual; this is called scapular winging, and over time, the affected muscles may become weaker or shrink.⁸¹¹

How do doctors find out?

Your doctor will usually start with a clinical exam, including:

• A muscle strength assessment to check for weakness or abnormal movement
• Use an EMG (a test that measures how well your muscles respond to nerve signals), or order imaging tests like an MRI or CT scan to check for compression or damage to the nerve^3,11

Summary

In the end, you can see that the accessory nerve is far from optional, it’s essential for many everyday movements involving your head and shoulders. Understanding its anatomy and function is especially important in the context of neck surgeries or when trying to make sense of the consequences of trauma in this region.

So next time you shrug or turn your head, you’ll know exactly which nerve to thank.