Carotid sheath sternum. The four major structures contained in the carotid sheath are: The carotid artery lies medial to the internal jugular vein, and the vagus nerve is situated posteriorly between the two vessels. In the upper part, the carotid sheath also contains the glossopharyngeal nerve (IX), the accessory nerve (XI), and the hypoglossal nerve (XII), which pierce the fascia of the carotid sheath. The ansa cervicalis is embedded in the anterior wall of sheath. It is formed by "descendens hypoglossi" (C1) and "descendens cervicalis" (C2-C3). The three major fascial layers in the neck contribute to the carotid sheath: the investing fascia,
Carotid body acid-base transporters (e.g. Na-H) which raise intracellular pH, and activates transporters (e.g. Cl-HCO) which decrease it. Changes in proton concentration caused by acidosis (or the opposite from alkalosis) inside the cell stimulates the same pathways involved in P sensing. Another mechanism is through oxygen sensitive potassium channels. A drop in dissolved oxygen lead to closing of these channels which results in depolarization. This leads to release of the neurotransmitter dopamine in the glossopharyngeal and vagus afferente to the vasomotor area. The type I (glomus) cells in the carotid (and aortic bodies) are derived from neuroectoderm and are thus electrically excitable.