Spinal Accessory Nerve: Function & Anatomy

Instructor: Sarah Phenix
In this lesson, we will explore the spinal accessory nerve. We'll look at where it is located and which muscles it stimulates, as well as what movements those muscles enable you to complete. Then you can test your new knowledge with a quiz.

Move your Head

Have you ever wondered where the signal comes from that causes you to rotate your head right and left? As you're shaking your head 'no' right now, you're actually engaging that very signal as we speak! The signal comes from your spinal accessory cranial nerve (XI), which is one of 12 pairs of nerve bundles that exit directly from your brain, hence the name 'cranial' nerve.

Cranial Nerves

Cranial nerves emerge from the base of your brain through openings in your skull, just like spinal nerves come out through your spine from your spinal cord. You have 12 pairs of cranial nerves in total, which together are responsible for the transmission of neural signals that result in a wide variety of functions and bodily responses. In general, nerves work something like telephone wires connecting different areas of the body to the 'management' of our brains. These 'wires' enable our brains to essentially 'call' (or signal) different areas of our bodies and give them orders to perform specific functions.

Cranial nerves are numbered using Roman numerals I - XII, and in this lesson, we will explore the 11th (XI) cranial nerve, the spinal accessory cranial nerve.

What's In a Name

The Spinal Accessory Cranial Nerve
Cranial Nerves and the Spinal Accessory Nerve

The spinal accessory (SA) nerve used to be called the accessory nerve (XI) because of the nerve rootlets that it's made of. The SA nerve is a combination of 4-5 little rootlets that exit from the medulla of the brainstem and 5-6 rootlets that exit from the first five cerebral (C) vertebrae of the neck (C1-C5/6). These rootlets merge into one bundle of fibers that runs up the base of your brain stem and into your skull.

Because this nerve bundle contains rootlets stemming from the medulla (a structure belonging to the larger organ of your brain) the bundle was originally thought to belong to the cranial nerve group. However, it was later found that the SA nerve actually loses its cranial nerve rootlets to the vagus nerve (X), the tenth cranial nerve, located just above it. Which means that the SA nerve's cranial rootlets that branch from the medulla actually belong to the vagus nerve. As a result, the nerve bundle name was changed to the spinal accessory nerve (XI), reflecting the fact that this nerve only has sole control over its spinal branch functions.


So, as we discussed above, the SA nerve is that wonderful little nerve that gives you control over two different muscles that allow you expressive functions such as shaking your head 'no' and shrugging your shoulders. These muscles are your sternocelidomastoid and your trapezius muscles.

The Sternocleidomastoid Muscle

Muscles of SA Nerve Innervation

While this first muscle name is quite a mouthful, it actually makes perfect sense once you know what it stands for. The sternocleidomastoid is a muscle that has two heads, or connecting points, of origination. One head connects to the top of your sternum (sterno) while the second connects to your clavicle (cleido).These fibers merge into one major bundle that runs up your neck to a bone that protrudes from the base of your skull, behind your ear, called the mastoid process (mastoid). Therefore, sterno-cleido-mastoid refers to its two origination points and the structure it connects to.

If you were to turn your head to one side, the sternocleidomastoid is the muscle that becomes most prominent from the front of your neck. If you turn your head in the opposite direction you will find its paired mate and, where the two join in front of your throat is the sternal connecting point. This muscle also allows you to draw your rib cage up and, if you turn your head and breathe in deeply, you should feel the muscle contract as your chest rises.

The Trapezius Muscle

Trapezius Muscle
Trapezius Muscle

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