THE DIGESTIVE SYSTEM: Introduction and upper GI Dalay Olson Ph.D Office: Jackson Hall 3-120 Office hours: Monday 2-3pm INTRODUCTION TO GI LEARNING OBJECTIVES 3 Distinguish between the wall layers of the esophagus and those classically represented by the small intestine. ESOPHAGUS STRUCTURE UPPER THIRD ESOPHAGUS STRUCTURE LOWER TWO-THIRDS 4 Describe the voluntary and reflex components of swallowing. PHASES OF SWALLOWING Swallowing is integrated in the medulla oblongata. Voluntary Phase Pharyngeal Phase Esophageal Phase The swallowing reflex requires input from sensory afferent nerves, somatic motor nerves and autonomic nerves. VOLUNTARY PHASE The tongue pushes the bolus of food back and upwards towards the back of the mouth. Once the food touches the soft palate and the back of the mouth it triggers the swallowing reflex. This is the stimulus! PHARYNGEAL PHASE • Once the food touches the soft palate and the back of the mouth it triggers the swallowing reflex. This is the stimulus! • The medulla oblongata (control center) then initiates the swallowing reflex causing the soft palate to elevate, closing of the glottis and opening of the esophageal sphincter (response). • Once the food moves into the esophagus the sphincter closes once again. The glottis then opens again and breathing resumes. ESOPHAGEAL PHASE 1. Food moves along the esophagus by peristalsis (waves of smooth muscle contractions) • If the food gets stuck, short reflexes will continue peristalsis. • Myogenic reflex (you will learn about this later) produces contractions that move food forward. 2. As the bolus of food moves toward the stomach the lower esophageal sphincter relaxes and opens allowing the food to move into the stomach. Why is the swallowing reflex a long reflex? 5 Describe the local peristaltic reflexes that occur in the esophagus in response to stretch. Two distinct pathways! FIRST WAY: MYOGENIC REFLEX ALL HAPPENING WITHIN ONE CELL! The Myogenic Reflex (Everything happening within one cell!) Stretch! CELLULAR LEVEL SUBCELLULAR LEVEL Smooth muscles themselves can sense stretch and respond by contracting. The Second Way 1. A cell expressing mechanoreceptors sensitive to stretch (the stimulus) actovate the sensory neuron (“afferent fibers”). 2. Sensory neruons send a signal to the enteric nervous system (integrating center) 3. Myenteric plexus serves as the (“efferent fibers”) innervating smooth muscle (effector). 4. Smooth muscle (effector) contracts in response to the ENS input. Multiple cell types involved!! 6 Describe how the closing of the LES is aided by positive abdominal pressure. Predict how the closing of the LES is altered with displacement into the thoracic cavity. Make the connection between LES function and gastric reflux disease. LOWER ESOPHAGEAL SPHINCTER (LES) The LES is located outside of the thoracic cavity. Why does that matter? Because the LES is located outside of the thoracic cavity it experiences normal atmospheric pressure which helps hold the LES closed forming a barrier between the acid and the esophagus. What is GERD? Who is susceptible? INTRODUCTION TO GI LEARNING OBJECTIVES.