Physiology of the GI Tract L1

• Smooth muscles in the body can be stimulated to contract even if the stimulus arrives at only one muscle, as the signal will spread to the rest through gap junctions.

• Muscles like the ciliary muscle and iris of the eye, and the piloerector muscle, are examples of this. Other muscles include those in the uterus and GI tract.

• Muscle contraction can be initiated in response to nerve signals (neurogenic) or intrinsically within the muscle itself (myogenic).

• Myogenic contraction is initiated by self-excitable cells called pacemakers that generate an action potential (AP) regardless of external innervation.

• In the case of smooth muscles in the GI tract, they are divided into two types based on their function: longitudinal and circular.

• Longitudinal muscles shorten the segment of the intestine and expands the lumen, while circular muscles reduce the diameter of the lumen and increase its length.

• The presence of intracellular Ca2+ release and more gap junctions makes myogenic contractions more important in the GI tract.

• The Enteric Nervous System (ENS) is important for the functions of smooth muscles in the GI tract as it contains excitatory and inhibitory motor neurons.

• Smooth muscle in the GI tract functions as a syncytium, meaning it acts as a single unit when contracting.

• The electrical activity of smooth muscles in the GI tract is characterized by slow waves and spike potentials.

• Slow waves are oscillating depolarization and repolarization in the resting membrane potential of the muscle, but they do not generate contractions on their own.

• The frequency and amplitude of slow waves vary between different parts of the GI tract.

• The RMP value of smooth muscle is less stable than that of skeletal or cardiac muscle and is characterized by spontaneous gradual alternating hyperpolarizing and depolarizing swings in potential, called slow wave potentials.

• The ion responsible for the slow waves in smooth muscle is Na+ influx, not Ca2+.

• The Interstitial cells of Cajal (ICC) are specialized non-contractile cells that act as pacemakers and generate APs in smooth muscles.

• ICCs are abundant in the myenteric plexuses and form a network with each other and smooth muscle cells, allowing the GI system to function normally even when disconnected from the CNS.

• The GI wall has five layers: serosa, muscularis, submucosa, mucosa, and mucosal muscle.

• The muscularis layer consists of two types of muscle: longitudinal and circular.

• Longitudinal muscle layers shorten the segment, while circular muscles constrict and decrease the lumen's diameter.

• The mucosa contains smooth muscle fibers and sparse bundles of smooth muscle fibers.

• Smooth muscles are non-striated, spindle-shaped, and have smaller and shorter fibers than skeletal muscles.

• Smooth muscles use cross-bridge cycling between actin and myosin to develop force and calcium ions to initiate contraction.

• Smooth muscles can be classified as phasic (contracts in bursts) or tonic (continuously contracted) depending on their contractile activity and how cytosolic Ca+2 increases.

• Tonic contractions can be caused by repetitive spike potentials, hormones, or continuous entry of Ca+2 ions and are not associated with slow waves.

• Single-unit smooth muscles are composed of discrete, separate muscle fibers that operate independently and do not contract in response to stretch or without neural input.

• Stomach and intestine smooth muscles are classified as multi-unit and tonic, and they contract spontaneously in response to stretch in the absence of neural or hormonal influence.

• The lower esophageal sphincter is the only smooth muscle in the esophagus and is responsible for preventing reflux of stomach contents into the esophagus.

• The GI tract, gastric antrum, small intestine, and esophagus are examples of phasic smooth muscles.

• Blood vessels and airways are examples of tonic smooth muscles.

• Muscles are usually partially contracted at all times and only relax when there's an inhibitory signal coming from the nervous system.

• Smooth muscles also play a role in blood pressure regulation and are connected to each other by gap junctions.

Test your knowledge of the anatomy and physiology of the gastrointestinal (GI) tract, including the sphincters and layers of the GI wall. This quiz covers the control of secretion from accessory organs, the sphincter of Oddi, and the four main layers or five layers of the GI wall.