The ansa cervicalis, often mistakenly referred to as the "gan nerve," is a nerve loop located in the anterior neck. It is formed by contributions from the cervical plexus and is positioned anterior to the carotid sheath.
Here's a more detailed breakdown of its location and course:
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Origin and Formation: The ansa cervicalis is comprised of two roots:
- Superior Root (Descendens Hypoglossi): This root contains fibers from the C1 spinal nerve, sometimes with contributions from C2. It travels with the hypoglossal nerve (CN XII) for a short distance before branching off to form part of the ansa.
- Inferior Root (Descendens Cervicalis): This root contains fibers from the C2 and C3 spinal nerves.
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Course and Position:
- The superior root descends superficially, either within or superficial to the carotid sheath, which contains the common carotid artery, internal jugular vein, and vagus nerve.
- The inferior root ascends to meet the superior root.
- The two roots join to form a loop (the "ansa") typically at the level of the intermediate tendon of the omohyoid muscle. This loop lies superficially on the carotid sheath.
- The ansa cervicalis leaves the cervical plexus at the posterior border of the sternocleidomastoid muscle (Erb's point) and courses anteriorly over the lateral surface of this muscle. It then courses superiorly, dividing into anterior and posterior branches.
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Targets: From the loop, branches extend to innervate the infrahyoid muscles (sternohyoid, sternothyroid, and omohyoid), which are important for swallowing and speech. It does not innervate the thyrohyoid muscle, which is innervated directly by fibers from C1 traveling with the hypoglossal nerve.
In summary, the ansa cervicalis (what is often meant by "gan nerve") is located in the anterior neck, superficial to the carotid sheath, and forms a loop around the intermediate tendon of the omohyoid muscle. It's derived from the cervical plexus (C1-C3) and innervates most of the infrahyoid muscles.
