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Dr. P here talking about nerve number eleven,

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the spinal accessory nerve.

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But the term accessory nerve really refers only

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to the nerve supplying the sternocleidomastoid,

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the trapezius muscles,

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and therefore it's known as the spinal accessory nerve.

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Let's draw off the sagittal projection.

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First, the rostral or cranial component,

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which is formed by the spinal nucleus of eleven.

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It sits higher up than you would think.

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It's pretty elongated,

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and it's not so anterior in its position

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as the true accessory nerve.

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So this gives rise to multiple rootlets inside the upper

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medulla oblongata and then they converge

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as a nerve that leaves off to the side.

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Now, we're in the midline, but just for education's sake,

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the nerve is coming out laterally,

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coursing anterolaterally,

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then kind of dips down a little bit before it

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merges with branches of the vagus nerve.

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Now, the one we're really fixed on right now is the true

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accessory nerve component of nerve number eleven.

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It's further caudal, it's pretty elongated.

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It's also more anterior,

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and it dips down into the upper cervical region.

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So, it's medulla oblongata and upper cervical.

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And this time, the roots come together very,

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in a very short fashion, and then they coalesce as the

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apparent origin of this component of nerve number eleven.

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And then it loops upward.

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It has a pretty steep upward course before it kind of

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bounces off the craniad component, and then dips down and

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courses lateral and posterior to innervate the trapezius.

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Now, injury to the spinal accessory nerve is most commonly

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caused by medical procedures that involve the head and neck.

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Injury can cause wasting of the shoulder muscles,

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winging of the scapula, and weakness of shoulder,

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abduction and external rotation.

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The accessory nerve is derived from the basal plate

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of embryonic spinal segments C1 through C6.

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So if we look at the coronal projection,

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we'll take our arrows off now and we scroll off to the

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side and try and locate the position of the accessory

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component of nerve number eleven.

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You can see a little wisp of it right there

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corresponding to this locus right here.

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Not so easy to see in the coronal projection

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because of its diminutive size.

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You can see its companion on the contralateral right side.

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And as we said, it loops upward.

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You can see this one looping upward on the right,

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where it will join the unseen cranial

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division of nerve number eleven.