Vagal IGLEs are special terminal structures that are located within the myenteric plexus throughout the gastrointestinal tract of a variety of species including rats, mice, and guinea pigs. IGLEs, traced from nodose ganglia are distributed throughout the entire gastrointestinal
Figure 2 (See color insert) Several different classes of mechanoreceptor within the gastrointestinal tract. Mucosal afferents have been extensively documented throughout the upper gastrointestinal tract in addition to the distal colon and perianal mucosa. Mucosal fibers are silent at rest; respond to fine stroking of the luminal surface with rapidly adapting responses, and are unresponsive to distension. Intramuscular arrays (IMAs) have been documented anatomically and are special terminal structures that have a parent axon that branches several times before terminating within the circular and longitudinal muscle layers. IMAs have been suggested to be in-series tension receptors that serve as stretch or length detectors that possibly respond to both passive stretch and active contraction of the muscle. Intraganglionic laminar endings (IGLEs) have been documented anatomically and are typically characterized as single axons entering a myenteric ganglion that have ramifying endings on the surfaces of the ganglia that are flattened, highly arborizing "leaf-like" processes. IGLEs have been recently shown to be the specialized transduction sites of mechanosensitive tension-sensitive vagal afferent neurons and tension-sensitive rectal mechanoreceptors (rIGLES). Mesenteric and serosal afferents have endings that are located close to or on blood vessels or at branching points of capillaries supplying the serosa. They are classified by their response to probing or distortion of the colon but not to low-intensity circular stretch or fine mucosal stroking. However, these afferents can be activated briefly by intense stretch or distortion of the mesenteric attachment and high intensity colonic distension.
tract, with greatest densities in the stomach, in particular the corpus (6.3 IGLEs/mm2), the antrum (3.8 IGLEs/mm2), and the forestomach (2.8 IGLEs/mm2). The density of IGLEs in other regions of the gastrointestinal tract is highest in the proximal duodenum (3.3 IGLEs/ mm ) with very few IGLEs in the distal colon (0.2 IGLEs/mm2) (44-47). IGLEs are typically characterized as single axons entering a myenteric ganglion that have ramifying endings on the surfaces of the ganglia that are flattened, highly arborizing "leaf-like" processes (44-46, 48-52). In many cases, a single axon gives rise to several IGLEs of various sizes in different ganglia (48,49). These endings have been hypothesized to detect mechanical shearing forces between the orthogonal muscle layers (51,53). It has recently been demonstrated, using a combination of rapid anterograde tracing and in vitro electrophysiology, that IGLEs are specialized transduction sites of mechanosensitive vagal afferent neurons in the guinea pig esophagus (48-50). These studies show that morphologically identified IGLEs in the esophagus correspond with the receptive fields or ''hot spots'' of distension-sensitive afferents (48-50).
Spinal IGLEs, with special terminal structures that are located within the myenteric plexus in the guinea pig rectum, have been classified as rectal IGLEs (rIGLEs). These rIGLEs specifically innervate the rectum but not the distal colon, probably via the pelvic nerves from the sacral DRG (54,55). These rIGLEs share characteristics of vagal IGLEs in the upper gastrointestinal tract in that they display branched, flattened, lamellar endings with multiple clusters. However, rIGLEs are approximately 10-fold smaller (~630 mm2) than vagal IGLEs innervating the guinea pig esophagus (6900 mm2) and stomach (6100 mm2) with fewer leaflets and less-extensive branching patterns (48,49,54,56,57). Using a combination of anterograde tracing and electrophysiological techniques, it has been demonstrated that rIGLEs are functionally similar to vagal IGLEs in the upper gut as are mechanotransduction sites, which are sensitive to both distension and muscle contraction (48,49,54-56). These morphologically identified rIGLEs in the rectum correspond with the receptive fields or ''hot spots'' of low threshold, slowly adapting, distension-sensitive mechanoreceptors (54-56).
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