Ansa cervicalis !

The ansa cervicalis is a nerve loop on the side of the neck that supplies the thin strap muscles below the hyoid bone.

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What it is

A nerve loop (C1-C3) on the carotid sheath.

What it does

Supplies the strap muscles below the hyoid.

Key facts

Roots: C1–C3 (superior root with XII, inferior root C2–C3)
Moves: Sternohyoid, sternothyroid, omohyoid
Feeling from: None

Relations

Course: The superior root runs down on the front of the internal carotid artery and common carotid artery; the inferior root winds around the internal jugular vein to meet it, forming a loop at a variable level, often near the cricoid cartilage (texts vary)
Above: Hypoglossal nerve (XII), which the superior root leaves below the posterior belly of the digastric
Superficial: Sternocleidomastoid, with the omohyoid crossing in front lower down
Deep: Carotid sheath with the carotid arteries and internal jugular vein; the loop lies on or within its front wall

How to find it

In neck dissection it is found on the front of the carotid sheath under the sternocleidomastoid. Following the superior root upward leads to the hypoglossal nerve, a safe way to find and protect it. Its branches enter the lateral borders of the sternohyoid, sternothyroid and omohyoid.

On imaging

Too thin for CT or routine MRI. High-frequency ultrasound may show it as a small dark nerve loop on the front of the carotid sheath, over the internal jugular vein and under the sternocleidomastoid; in practice it is found at surgery. On CT and MRI the carotid sheath is the landmark for its position. Long-standing loss of its supply leaves thin infrahyoid muscles.

Clinical notes

Cutting it during neck dissection or carotid endarterectomy usually causes no obvious deficit, so it can be sacrificed when needed
It can also serve as a donor nerve to reinnervate the tongue after hypoglossal nerve injury
In carotid endarterectomy with a high bifurcation, the superior root can be cut so the hypoglossal nerve can be lifted out of the field without stretching it

Anatomical variants

The superior root may seem to come from the vagus nerve, and the inferior root passes medial rather than lateral to the internal jugular vein in some people
The height of the loop varies; a high loop lies close to the carotid bifurcation

Test yourself

Think of the answer, then tap the question to see it.

What are the roots of the ansa cervicalis?

Superior root: C1 fibres travelling with the hypoglossal nerve; inferior root: C2–C3 from the cervical plexus; they join on the carotid sheath.

Which infrahyoid muscle is not supplied through the ansa cervicalis?

The thyrohyoid, which gets C1 fibres directly along the hypoglossal nerve.

Why are the strap muscles divided in their upper part when they must be cut in thyroid surgery?

Branches of the ansa cervicalis enter them low down, so a high cut keeps their nerve supply.

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Educational information, not medical advice. For any symptom, see a health professional.

Its names

Official Latin (TA2): ansa cervicalis TA2 6376
Official Spanish (DeCS): Asa cervical
English (TA2): Ansa cervicalis
Also called: ansa nervi hypoglossi

Sources: Terminologia Anatomica (TA2); Anatomy, Head and Neck: Anterior Cervical Region; Anatomy, Head and Neck: Cervical Nerves.

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Representative model, for educational purposes only.