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# Chapter 6: Gastrointestinal Physiology ## Structure and Innervation of the Gastrointestinal Tract The gastrointestinal (GI) tract is composed of different parts that are specialized for secretion or absorption. ### Structure of the gastrointestinal (GI) tract: * **Epithelial cells:** Are specia...

# Chapter 6: Gastrointestinal Physiology ## Structure and Innervation of the Gastrointestinal Tract The gastrointestinal (GI) tract is composed of different parts that are specialized for secretion or absorption. ### Structure of the gastrointestinal (GI) tract: * **Epithelial cells:** Are specialized in different parts of the GI tract for secretion or absorption. * **Muscularis mucosa:** Contraction causes a change in the surface area for secretion or absorption. * **Circular muscle:** Contraction causes a decrease in diameter of the lumen of the GI tract. * **Longitudinal muscle:** Contraction causes shortening of a segment of the GI tract. * **Submucosal plexus (Meissner plexus) and myenteric plexus:** Comprise the enteric nervous system of the GI tract. These two plexuses integrate and coordinate the motility, secretory, and endocrine functions of the GI tract. ## Innervation of the GI tract The autonomic nervous system (ANS) of the GI tract comprises both extrinsic and intrinsic nervous systems. ### Extrinsic innervation (parasympathetic and sympathetic nervous systems) * **Efferent fibers:** Carry information from the brainstem and spinal cord to the GI tract. * **Afferent fibers:** Carry sensory information from chemoreceptors and mechanoreceptors in the GI tract to the brainstem and spinal cord. #### Parasympathetic nervous system * Is usually excitatory on the functions of the GI tract. * Is carried via the vagus and pelvic nerves. * Preganglionic parasympathetic fibers synapse in the myenteric and submucosal plexuses. * Cell bodies in the ganglia of the plexuses then send information to the smooth muscle, secretory cells, and endocrine cells of the GI tract. * The **vagus nerve** innervates the esophagus, stomach, pancreas, and upper large intestine. * Reflexes in which both afferent and efferent pathways are contained in the vagus nerve are called **vagovagal reflexes**. * The **pelvic nerve** innervates the lower large intestine, rectum, and anus.