GI Motility

GI Smooth Muscle

- SM are found of tubular structures and hollow organs such as blood vessels, stomach, intestines, uterus, and bladder. - The function of SM is to change the volume of the organ/structure they surround. - SM function is regulated by two distinct layers of SM. An outer layer that is oriented in long axis of the organ and an inner circular layer. - Autonomic Nervous System innervates smooth muscle - There are no t-tubules - There are no Z lines. Actin Filaments are attached to dense bodies Sheets of single-unit muscle cells contract as a single unit because they are electrically linked by gap junctions. Dense bodies are scattered through the cytoplasm of smooth muscle fibers and they are points of attachment for myofilaments taking the place of Z lines in striated muscle. Contraction modulated by: 1. neurotransmitters released by ANS 2. hormones 3. intrinsic properties that produce spontaneous electrical activity 4. changes in local chemical composition 5. stretch The myenteric plexus increases the tone of the gut and the velocity and intensity of contractions. The submucosal plexus is involved with local conditions and controls local secretion, absorption, and muscle movements. The interstitial cells of Cajal (ICCs) are a specialized group of cells in the intestinal wall that are involved in the transmission of information from enteric neurons to smooth muscle cells. It is also thought that ICCs are "pacemaker" cells that have the capacity to generate the basic electrical rhythm, or "slow wave" activity, that is a consistent feature of GI smooth muscle. Slow waves are generated by interstitial cells (ICCs). These cells are located in a thin layer between the longitudinal and circular layers of the muscularis externa and in other places in the wall of the GI tract. Interstitial cells have properties of both fibroblasts and smooth muscle cells. Their long processes form gap junctions with the longitudinal and circular smooth muscle cells; the gap junctions enable the slow waves to be conducted rapidly to both muscle layers. Because gap junctions electrically and chemically couple the smooth muscle cells of both longitudinal and circular layers, the slow wave spreads throughout the smooth muscle of each segment of the GI tract.

Large Intestine

Contractions of the large intestine are organized to allow for optimum absorption of water and electrolytes, and net aboral movement of contents, and orderly evacuation of feces. The muscular layers of the large intestine are composed of both longitudinally and circularly arranged fibers. • Longitudinal muscle fibers are concentrated into three flat bands called the teniae coli • Circular layer of muscle fibers is continuous from the cecum to the anal canal Cecum and Ascending Colon - Flow of contents from the small intestine into the large intestine is intermittent and regulated partly by a sphincter mechanism at the ileocecal junction. • Segmental/haustral contractions • "Mass movement" - sweeps intraluminal contents along the colon. These occur 1-3 times daily Descending and Sigmoid Colon - When material reaches this portion of the colon it has changed from a liquid to a semisolid state • Segmental contractions • "Mass movement" The primary movements seen in the large intestine include haustral contractions and mass movements, which occur mostly in the ascending and transverse colon. Haustral contractions are slow contractions that occur about every 30 minutes and last approximately 1 minute. Segmentation contractions mechanically break down large deposits into finer particles. They are stimulated by stretch when food remnants fill the haustra. Mass movements are long, slow moving, powerful contractions that move over the colon 3 or 4 times per day, typically after meals. Moves forward then relaxes for hours. Move things in mass in colon, walking/activity and gastrin from stomach that is released when there is food can increase mass movements. This gastrocolic reflex accompanies the gastroileal reflex stimulated by gastrin release when the stomach receives food.

Peristalsis

Esophageal peristalsis, which can be triggered by either swallowing or local esophageal distention, serves to propel esophageal contents into the stomach. This is orchestrated by a complicated interaction between the central nervous system and the myenteric plexus, with the latter predominating in the smooth muscle esophagus. Upon swallowing (stimulus), Pharyngeal contraction coincides with relaxation of the upper esophageal sphincter (UES). The inhibitory pathway neurons in the caudal Dorsal Motor Nucleus of the Vagus (cDMN) are activated first, which causes simultaneous inhibition of all parts of the esophagus leading to relaxation of the smooth muscle. This inhibition lasts longer in the lower than in the upper parts. The inhibitory pathway includes pre-ganglionic vagal fibers that are located in the caudal part of the DMN. These fibers project onto postganglionic inhibitory neurons that contain nitric oxide (NO), vasoactive intestinal polypeptide (VIP) and adenosine triphosphate (ATP) to cause relaxation of the smooth muscle aboral (in front) of the food bolus. As the inhibition ends, sequential activation of excitatory (including cholinergic) neurons in the rostral DMN (rDMN) elicits a contraction wave that is peristaltic in nature. The innervations of the esophageal body and LES are similar. The excitatory pathway includes vagal preganglionic neurons that are located in the rostral part of the dorsal motor nucleus (DMN) in the brainstem. These fibers project onto the excitatory postganglionic neurons that contain acetylcholine (ACh) and substance P. These help stimulate contraction of the smooth muscle behind the bolus to propel it further.

Regulation of Gastric Emptying

Gastric emptying into the duodenum is regulated to maximize optimum time for digestion and absorption of food stuffs. The transfer rate of material from the stomach to the duodenum depends on the physical nature and composition of the gastric contents. The following 3 conditions of stomach content slow gastric emptying. • high lipids • high [ H+] • hypertonicity and hypotonicity Regulation of gastric emptying is brought about by alterations in motility of the stomach, gastroduodenal junction and duodenum. Once the nutrients of the food are digested the pattern of gastric motility changes with respect to undigested items left in the stomach. These undigested items (i.e. pennies, a diamond ring, or kernels of corn) are swept into the duodenum by a burst of peristaltic contractions. The contractions begin in the orad region and sweep the entire length of the stomach. The pylorus dilates during each sweep and the duodenum relaxes so the resistance to emptying is minimal.

Esophageal Diseases

Gastroesophageal reflux disease (GERD) - occurs when stomach acid flows back into the esophagus. GERD (heartburn) occurs when lower esophageal sphincter does not close correctly leading to splash back of stomach acid into esophagus **Know this one, probably the only one needed Barrett's esophagus - precancerous condition that develops as a result of GERD. Esophageal achalasia - motor disorder characterized by a complete loss of contraction and relaxation of muscles used to move contents down the esophagus. Paraesophageal hernia - condition where the stomach protrudes through the diaphragm into the chest alongside the esophagus. Esophageal diverticulum - is a pouch that protrudes outward in a weak portion of the esophageal lining Cancers of the esophagus Many chemical agents (eg. gastrin, acetylcholine, serotonin, prostaglandin F20, motilin, substance P, histamine and pancreatic polypeptide) decrease esophageal tone. However, their precise roles in normal esophageal function remain unclear.

Gastric Emptying of the Stomach

Ingested foods are stored temporarily in the stomach, where they are mixed with gastric juice and churned to reduce the size of the solid particles. The activity of the smooth muscle cells in the 3 layers of the stomach are important in the emptying process. These layers of smooth muscle cells are richly innervated with both intrinsic and extrinsic nerves. The orad area of the stomach shows little contractile activity during digestion. The caudad area of the stomach shows contractile activity during digestion. Contractions begin in the mid stomach and move toward the gastroduodenal junction. The primary forms of contractions are peristaltic which last for 2 to 20 seconds at a frequency of 2 to 5 per minute. These contractions serve to mix and propel gastric contents.

Esophagus

The cervical esophagus and the small part of the thoracic esophagus that includes the upper esophageal sphincter are composed of striated skeletal muscle. The lower two thirds of the esophagus, including the thoracic and abdominal parts containing the lower esophageal sphincter (which is below the diaphragm), are composed of smooth muscles. The esophagus is innervated by both parasympathetic and sympathetic nerves. The parasympathetics control peristalsis via the vagus nerve. The esophageal smooth muscle is innervated by the extrinsic autonomic nerves and the enteric nervous system. It has a well-developed myenteric plexus located between the circular and longitudinal smooth muscle layers of the lower esophagus. Esophageal myenteric plexus contains cell bodies of motor, sensory, and interneurons. cholinergic neurons. Cholinergic neurons are excitatory in nature as they contract the smooth muscles by releasing acetylcholine. The second set of motor neurons is called nonadrenergic non-cholinergic inhibitory neurons. These neurons release vasoactive intestinal peptide (VIP) and nitric oxide (NO). The esophagus extends from the pharynx to the stomach and is about 25 cm in length. It traverses three regional anatomical areas, the neck, the thorax, and the abdominal cavity. Under physiological conditions the average resting esophageal pressures (pharynx = 0) are: - UES = +100 mmHg (+1.9psi) - Esophagus = -5 mmHg - LES = +20 mmHg (+0.39psi)

Swallowing

Steps: Chew it, mucin helps form bolus of food, push it back with tongue and palates into oropharynx (this is all voluntary but after this is all involuntary). Food is pushed down by involuntary movements of oropharynx which has taste receptors which sends impulses to medulla oblongata and lower pons to signal soft palate to shut off nasopharynx and epiglottis to seal off larynx to prevent it from entering respiratory tract. If soft palate is not shut off, food would go up into nasopharynx. Airway is in front of esophagus, it is open when epiglottis is open and esophagus is shut until swallowing pressure and upper esophageal sphincter relaxes. Bolus moves into esophagus which contracts to prevent backflow. Bolus is pushed down via peristalsis until it reaches lower esophageal sphincter and moves into stomach Eating and swallowing are complex behaviors involving voluntary and reflexive activities of more than 30 nerves and muscles. They have two crucial biological features: food passage from the oral cavity to stomach and airway protection. Swallowing (deglutition) is the process by which food is transported from the mouth to the stomach. Functionally, it may be divided into three phases—preparatory, transfer, and transport phases—that follow each other in a sequence. The preparatory phase includes conscious effort to ingest food and reflexes in the oral cavity that help the preparation of the bolus to be swallowed. The transfer phase involves reflex activities in the oral and pharyngeal passages. The transport phase includes transport of the swallowed food bolus through the esophagus into the stomach. **Don't need to know phases Swallowing is initiated by the voluntary action of collecting the oral contents on the tongue and propelling them backward into the pharynx. This starts a wave of involuntary contraction in the pharyngeal muscles that pushes the material into the esophagus. Inhibition of respiration and glottic closure are part of the reflex response by the pons and the medulla. A peristaltic ring contraction of the esophageal muscle forms behind the material, which is then swept down the esophagus at a speed of approximately 4 cm/s. When humans are in an upright position, liquids and semisolid foods generally fall by gravity to the lower esophagus ahead of the peristaltic wave.

Enteric Nervous System

The GI tract has a complex nerve supply, divided into two major groups: extrinsic (parasympathetic and sympathetic innervation), and intrinsic (enteric) innervation. Digestive tract can carry out many of its functions by itself without brain input, but fine-tuning and feedback mechanisms that make it function at top form have input from brain. Sympathetic stimulation causes inhibition of gastrointestinal secretion and motor activity, and contraction of gastrointestinal sphincters and blood vessels. Parasympathetic stimuli typically stimulate these digestive activities The enteric nervous system is located within the wall of the digestive tract, all the way from the esophagus to the anus. It is comprised of two well-organized neural plexuses. The myenteric plexus is located between longitudinal and circular layers of muscle; it is involved in control of digestive tract motility. The submucosal plexus is located in the submucosa between the circular muscle and the luminal mucosa; it senses the environment of the lumen and regulates gastrointestinal blood flow and epithelial cell function. Parasympathetic and sympathetic nerves connect the central nervous system to the enteric nervous system or directly to the digestive tract. Although the enteric nervous system can function autonomously, normal digestive function often requires communication between the central nervous system and the enteric nervous system. Enteric neurons secrete an number of neurotransmitters. 1. Acetylcholine - excitatory, stimulates smooth muscle contraction, increases intestinal secretions, release of enteric hormones and dilation of blood vessels. 2. Norepinephrine - derived from the extrinsic sympathetic neurons and is almost always inhibitory and has the opposite effect of acetylcholine. The enteric nervous system contains three types of neurons, most of which are multipolar. 1. Sensory neurons innervating receptors in the mucosa that respond to mechanical, thermal, osmotic, and chemical stimuli. 2. Motor neurons control motility, secretion, and absorption by acting on smooth muscle and secretory cells. 3. Interneurons integrate information from sensory neurons and feedback ("program") to the enteric motor neurons.

Upper GI Motility

The digestive tract is unique among internal organs because it is exposed to a large variety of physiochemical stimuli from the external world in the form of ingested food. As a consequence, the gastrointestinal system has developed a rich repertoire of coordinated movements of its muscular apparatus to ensure the appropriate mixing and propulsion of contents during digestion, absorption, and excretion.

Lower Esophageal Sphincter

The musculature of the gastroesophageal junction (lower esophageal sphincter; LES) is tonically active but relaxes on swallowing. The tonic activity of the LES between meals prevents reflux of gastric contents into the esophagus. The LES is made up of three components. The esophageal smooth muscle is more prominent at the junction with the stomach (intrinsic sphincter). Fibers of the crural portion of the diaphragm, a skeletal muscle, surround the esophagus at this point (extrinsic sphincter) and exert a pinchcock-like action on the esophagus. In addition, the oblique or sling fibers of the stomach wall create a flap valve that helps close off the esophagogastric junction and prevent regurgitation when intragastric pressure rises. The tone of the LES is under neural control. Release of acetylcholine from vagal endings causes the intrinsic sphincter to contract, and release of NO and VIP from interneurons innervated by other vagal fibers causes it to relax. Contraction of the crural portion of the diaphragm, which is innervated by the phrenic nerves, is coordinated with respiration and contractions of chest and abdominal muscles. Thus, the intrinsic and extrinsic sphincters operate together to permit orderly flow of food into the stomach and to prevent reflux of gastric contents into the esophagus.

Rectum and Anal Canal

The rectum is usually empty but can fill intermittently. Normally the anal canal is closed because of contraction of the internal anal sphincter. When the rectum is distended by fecal material, stretch receptors are activated and send afferent signals to spinal cord that causes the internal sphincter to relax as part of the rectosphincteric reflex. Rectal distention also elicits a sensation that signals the urge for defecation. The rectum can accommodate rather large quantities of material, so it acts as a storage organ. Defecation is accomplished by a series of voluntary and involuntary acts. When the rectal distention is followed by defecation, muscles of the descending and sigmoid colon and the rectum may contract to propel contents toward the anal canal. Then both the internal and external sphincters relax to allow passage of the bolus. However, you don't go to bathroom every time material moves into rectum so rectum stores feces. External anal sphincter is skeletal muscle and can be controlled, get signals that you may need to go to bathroom after initial distension and then the signal/urgency goes away because the receptors have already been stretched so you can ignore it, but next time material moves in get even more intense urgency. If you don't go, your sphincter isn't strong enough to hold everything back and will forcibly go

Small Intestine Motility

The small intestine is ~ 2.9 m (11 ft) in length • Duodenum - 25 cm in length • Jejunum - 1.1 m in length • Ileum - 1.7 m in length Motility of the small intestine is organized to optimize the processes of digestion and absorption of nutrients and the aboral (away from the mouth) propulsion of undigested material. Movement occurs the same way as it does in the esophagus. Contractions of the small intestine perform three functions: • mixing of ingested foodstuffs with digestive secretions and enzymes • circulation of all intestinal contents to facilitate contact with the intestinal mucosa • net propulsion of the intestinal contents in an aboral direction Contractions of the small intestine are caused by activities of two layers of smooth muscle cells: • outer layer of cells are arranged longitudinally • inner layer of cells are arranged circularly These smooth muscle cells are richly innervated by elements of the autonomic nervous system. There are two types of contractions in the small intestine: • segmentation - movements are achieved by sustained contractions of the circular muscles. Most contractions are local events and involve only 1 cm to 4 cm of small intestine at a time. This movement is important in mixing and circulation of the chyme. • peristalsis - movements are achieved by coordinated contraction of the circular and longitudinal muscles moving the chyme toward the large intestine. Movement of Digested Material from the Iieum to the Colon - Iliocecal sphincter controls material moving from small intestine into the large intestine. GI tract controls portions of food that moves through it. Stretch occurs at end of ileum that causes the sphincter to relax so material can move via rhythmic contraction into large intestine. As food moves into large intestine, it causes colonic distension which then contracts the sphincter.

Sphincters of the GI Tract

There are 7 sphincters along the GI tract to assure that chyme flows in the right direction. The first and last of these sphincters are composed of striated skeletal muscle; the rest are smooth muscle. 1. Upper Esophageal - Striated muscle. Separates mouth/pharynx from esophagus 2. Lower Esophageal - Smooth muscle. Separates esophagus from stomach. 3. Pyloric - Smooth muscle. Separates stomach from duodenum. 4. Sphincter of Oddi - Smooth muscle. Separates pancreatobiliary ducts from duodenum. 5. Ileocecal - Smooth muscle. Separates small intestine from large intestine. 6. Internal Anal - Smooth muscle. Controls defecation. 7. External Anal - Striated muscle. Controls defecation