Gastrointestinal Physiology Overview

Physiology Study Notes

• Gastrointestinal (GI) Physiology: Four main processes occur along the GI tract: motility, secretion, digestion, and absorption.
• Functional Structures in the GI Tract: Smooth muscle cells, interstitial cells of Cajal, and secretory cells are key components.
• Related Structures: Control systems for GI functions include neural control (enteric and autonomic nervous systems), hormonal control (GI endocrine), and blood flow to the GI.
• Smooth Muscle Cells: Two main layers (longitudinal and circular) are usually present in the GI tract, forming a syncytium for coordinated contraction. A third layer, the muscularis mucosae, is between the mucosa and submucosa.
• Electrical Activity: Smooth muscle cells have slow waves (basic electrical rhythm) and spike potentials; these action potentials trigger calcium influx for contraction.
• Calcium in Contraction: Extracellular calcium and release from the sarcoplasmic reticulum are essential to smooth muscle contraction. Calcium binds to calmodulin, activating myosin and causing contraction.
• Chemical Control: Smooth muscle cells respond to neurotransmitters and hormones, activating or inhibiting contractions depending on the type of receptor and signal transduction.
• Integration of Responses: Smooth muscle responses are integrated by ligand binding, resulting in tonic or rhythmic contractions that regulate GI activity.
• Interstitial Cells of Cajal (ICCs): These cells are found throughout the GI tract and act as pacemaker cells to generate slow waves, controlling electrical activity in smooth muscle. They communicate with other ICCs and smooth muscle via gap junctions.
• ICCs Communication: This cell-cell communication, via gap junctions, synchronizes the contractile activity of smooth muscle cells, forming a syncytial response.
• ICCs Generate Slow Waves: ICCs produce slow waves, electrical activity that regulates and synchronizes rhythmic contractions in the gut. Slow waves are metabolism-dependent.
• ICCs Receive Inputs from the ENS: ICCs receive inputs from the enteric nervous system (ENS), mediating and regulating smooth muscle activity and promoting coordinated contractions.
• Secretory Cells: These cells are crucial for GI secretion. Some are solitary, others form glands and secrete substances like enzymes, hormones, and mucus
• Enteric Nervous System (ENS): The ENS is a network of nerves in the GI tract, controlling GI functions such as movement, secretion, and blood flow. Consists of two plexuses: myenteric and submucosal.
• Neural Connectivity: Neurons within the plexuses are interconnected; sensory neurons transmit signals from the GI epithelium to enteric plexuses, prevertebral ganglia, spinal cord, and brainstem (via the vagus nerve).
• Function of Enteric Neurons: Enteric neurons regulate GI functions through neurotransmitters, having inhibitory or excitatory effects on motility, secretion, and vascular flow.
• Key Neurotransmitters: Acetylcholine (Ach), Substance P (SP), Vasoactive Intestinal Peptide (VIP), Calcitonin Gene-Related Peptide (CGRP), and Gastrin-Releasing Peptide (GRP) are key examples in the ENS.
• Autonomic Nervous System: The parasympathetic division (cranial and sacral) enhances GI functions (motility, secretion, and blood flow) whereas the sympathetic division (T5-L2) reduces GI activity. Sympathetic stimulation decreases GI activity and constricts blood vessels, while parasympathetic stimulation increases activity and blood flow.
• Endocrine Cells and Hormones: Different hormones are secreted by GI endocrine cells and influence various aspects of the GI tract such as motility and secretion. Examples include gastrin, cholecystokinin (CCK), secretin, GIP, and others.
• Blood Flow: Meal-stimulated processes trigger an increase in blood flow to the GI tract, promoting absorption, secretion, and motility. Blood flow is regulated by various substances (vasodilators) and systems (ENS, autonomic nervous system).
• Summary of Pathways Controlling the Digestive System: Digestive activities are controlled by external influences, intrinsic and extrinsic nerves, the ENS and hormonal pathways working together.

This quiz covers fundamental concepts in gastrointestinal physiology, focusing on processes such as motility, secretion, digestion, and absorption. It also explores the structural and functional components of the GI tract, including smooth muscle cells and control systems. Test your knowledge on how these processes are regulated and coordinated.