![]() Parasympathetic ganglia and various peptides (galanin, cGRP, VIP, neuropeptide Y, substance P, tyrosine hydroxylase) have been found in the CP, but their role in control of the CP is unknown. The GLN may be sensory the sympathetics may innervate the mucosa, blood vessels, and glands but no functional innervation by the RLN has been identified. Only the PE and SLN provide motor fibers to the CP. ![]() The CP is innervated by branches of the vagus nerves: pharyngoesophageal (PE), superior laryngeal (SLN), and recurrent laryngeal (RLN) glossopharyngeal (GPN) and cervical sympathetics. ![]() The high compliance of the CP allows it to be opened by distraction of other muscles (e.g., geniohyoideus) or increased intraluminal pressure. A passive tone in the CP is present and increases through all degrees of stretch. The optimum length of the CP for development of active tension is about 1.7 times resting length therefore, in some respects the CP acts more like cardiac than striated muscle. In humans and rats, but not other animals, the CP has no median raphe. The fibers may attach to the connective tissue framework, forming a muscular net. The CP is a striated muscle composed of variable-sized small (25-35 microm) muscle fibers that are primarily type I (slow twitch), highly oxidative, and contain abundant (40%) endomysial elastic connective tissue. All 3 muscles may at times function to maintain tone in the UES, but only the CP contracts and relaxes in all physiologic states consistent with the UES. The upper esophageal sphincter (UES) is composed of the cricopharyngeus (CP), thyropharyngeus (TP inferior pharyngeal constrictor in humans), and cranial cervical esophagus.
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