What is acid reflux?

Prevention of retrograde flow of gastrointestinal contents is a major function of the lower esophageal sphincter (LES) . Retrograde flow is prevented by the lower esophageal sphincters, which remain closed between swallows.
The lower esophageal sphincter is composed of smooth muscle and is innervated by parallel sets of parasympathetic excitatory and inhibitory pathways. It remains closed because of its intrinsic myogenic tone, which is modulated by the excitatory and inhibitory nerves. It opens in response to the activity of the inhibitory nerves. The neurotransmitters of the excitatory nerves are acetylcholine and substance P, and those of the inhibitory nerves are vasoactive intestinal peptide (VIP) and nitric oxide. The function of the LES is supplemented by the striated muscle of the diaphragmatic crura, which surrounds the LES and acts as an external LES. Relaxation of the LES without esophageal contraction occurs during belching and gastric distention. Gastric distention-evoked transient lower esophageal sphincter relaxation (tLESR) is a vasovagal reflex. Fatty meals, smoking, and beverages with a high xanthine content (tea, coffee, cola) also cause a reduction in sphincter pressure. Many hormones and neurotransmitters can modify LES pressure. Muscarinic M2 and M3receptor agonists, a-adrenergic agonists, gastrin, substance P, and prostaglandin F2acause contraction. Nicotine, b-adrenergic agonists, dopamine, cholecystokinin, secretin, VIP, calcitonin gene-related peptide (CGRP), adenosine, prostaglandin E, and nitric oxide donors such as nitrates reduce sphincter pressure.