Читать книгу The Esophagus - Группа авторов - Страница 127

The esophagus is a 20–22 cm long muscular tube with sphincters at either end. Measured manometrically, the total length can vary between 17 and 30 cm with a mean of 23 cm [121]. The proximal 5%, including the UES, is composed of striated muscle. Transition from striated to smooth muscle occurs progressively in the middle 35–40%, and the distal 50–60% is composed entirely of smooth muscle [122]. This transition occurs more proximally in the inner circular muscle layer. The outer longitudinal muscle originates anteriorly from the cricoid cartilage along with some fibers from the cricopharyngeus muscle. As the esophagus passes distally and posteriorly, there is a triangle at the top end that is free of this longitudinal layer, called Laimer’s triangle (see Figure 5.8). The longitudinal layer is present through the remainder of the esophageal body to the level of the LES, changing from striated to smooth muscle more distally compared to the circular muscle. There is no serosa covering the esophagus, with only a thin layer of connective tissue allowing the esophagus to move more freely within the mediastinum.

There is a myenteric plexus of ganglion cells and nerve fibers in both smooth and striated muscle sections between the circular and longitudinal muscle layers (Auerbach’s plexus), more prominent in the smooth muscle section, which in turn release neurotransmitters to the esophageal smooth muscle. Additionally, there is a submucosal plexus (Meissner’s plexus), which is sparse compared to the myenteric plexus [123]. In the smooth muscle portion, ganglion cells in the myenteric plexus receive efferent vagal preganglionic motor fibers from the DMNV, which provide excitatory and inhibitory innervation to the muscle layers. Thus, pre‐ and postganglionic motor neurons can be excitatory or inhibitory [124]. The excitatory pathway arises from the rostral part of the DMN, whereas the inhibitory pathway arises from the caudal part. Activation of inhibitory nerves occurs simultaneously in the entire esophagus at the onset of swallowing (deglutitive inhibition), followed by activation of excitatory neurons resulting in peristaltic contraction [125, 126]. Through their connectivity within the enteric nervous system, these ganglia provide a local neural mechanism for peristalsis and distal inhibition. Sympathetic motor input is directed primarily to the myenteric plexus from intermediolateral columns of thoracic spine through sympathetic ganglia, which are denser in the smooth muscle esophagus compared to the striated muscle [127, 128].

Esophageal vagal afferent nerve endings are found in the mucosa, the muscularis propria (intramuscular array endings, IMAs), and the intraganglionic laminar endings (IGLEs). Vagal sensory information is transmitted centrally by nerves with their cell bodies in the nodose ganglia in the neck and is responsible for the sensory–motor regulation of the SPG for peristalsis and for esophagus‐initiated reflexes [129]. Sensory receptors include IGLEs [130] and potentially interstitial cells of Cajal (ICCs) [131] and IMAs [132, 133]. IGLEs are a specialized laminar structure that cover myenteric ganglia in the muscularis propria and perceive passive and active tension [132, 134]. ICCs have been found in esophageal smooth muscle and the LES (intramuscular ICC; ICC‐IM) [135, 136]. ICC‐IMs may influence the release of neurotransmitters from nerve endings to the smooth muscle and thus participate in esophageal peristalsis [136, 137]. Additionally, the network of ICC‐IMs and IMAs may function as stretch receptors [132, 133]. IMAs consist of a branching array of vagal axons along smooth muscle, including the LES. Mucosal afferent endings can respond to various stimuli, including mechanical and chemical stimuli. While these nerves are not sensitive to intraluminal distention, light touch from bolus passage can be detected. Calcitonin gene‐related peptide (CGRP) positive afferent nerves have been demonstrated in esophageal mucosa [138, 139]. Esophageal sensory information also passes to the spinal cord and centrally via the sympathetics through the C1 to L2 dorsal root ganglia [23] and provides cognitive sensation and nociception rather than motor function [20].