Smooth muscle SM Stomach o SM modulates luminal pressure tension o SM tissue is present in sheets bundles or sheaths around tissues Cardiovascular system Respiratory system Digestive system Urinary system Reproductive system Innervation and stimulation of SM o Primarily under control of ANS involuntary Diffuse branching of nerve fibers Transmitters include ACh and NE Membrane potential 50 to 60 mV o Smooth muscle cell types categorized based on excitation Single unitary visceral Multi unit Unitary SM cells o Walls of the digestive tract gall bladder urinary bladder o May not have direct contact with any motor neuron o Autorhythmic Adjacent cells connect via gap junctions transmission of APs from one fiber to another o Cell membranes adhere to one another transmission of force o Slow synchronized graded contraction o Entire sheet of muscle contracts Functional syncytium Structure of SM o Sarcoplasmic reticulum is poorly developed relative to skeletal muscle o Thick filaments of SM have actin gripping heads along their entire length o Thick and thin filaments are arranged diagonally within the cell contraction results in twisting motion Special features of SM o Capable of sustained contraction without fatigue and at very little energy cost o Maintains a low level of tension or tone even in the absence of action potentials o Stress relaxation response Stretch causes initial increase in tension tension decreases within 1 2 minutes allows SM to change length but maintain ability to contract important for storage organs Length Tension relationship o Smooth muscle operates over a wider range of resting lengths o Generally broader length tension relationship in smooth muscle compared to skeletal muscle Stomach Anatomy o Parts of the Stomach Cardia Fundus Body Pylorus Pyloric Sphincter o Stomach filling triggers secretions and motility o The stomach empties slowing into the duodenum as small squirts of chyme leave through the pyloric sphincter Mechanical digestion o Gentle mixing waves occur in the stomach to mix the bolus of food with gastric juice enzymes acid water and turn it into chyme a thin liquid o More vigorous waves traveling from the body of stomach to the pyloric region move the chyme along o Intense waves near the pylorus lead to opening of the pyloric sphincter squirting 1 2 teaspoons into the duodenum with each wave Chemical Digestion o Protein Digestion begins in stomach HCl denatures proteins HCl activates pepsinogen into pepsin an enzyme that breaks peptide bonds between certain amino acids o Fat digestion continues Gastric lipase splits the triglycerides in milk fat although this is most effective at pH 5 6 infant stomach o HCl kills microbes in food o Mucous cells secrete mucus to protect the stomach walls from being digested Absorption of Nutrients by the Stomach o Water o Electrolytes o Some drugs aspirin and alcohol o Fat content in stomach slows passage of alcohol to the intestine where absorption is more rabid o Alcohol is absorbed slower if taken with a meal Regulation of Digestion o Cephalic Phase anticipation Prepare mouth and stomach for food Cerebral cortex sight smell taste thoughts of food stimulate parasympathetic nervous system to stimulate Salivation facial and glossopharyngeal Gastric glands to secrete gastric juice vagus o Gastric Phase Breakdown of material Neural influences over stomach activity Stretch receptors and chemoreceptors pH signal bolus entry Vigorous peristalsis and gastric gland secretions Chyme periodically released into the duodenum Endocrine influences over stomach activity Distention and presence of food in stomach cause G cells to secrete gastrin into the bloodstream gastrin increases gastric gland secretions and motility and causes pyloric sphincter relaxation Neural and hormonal regulation of gastric emptying Gastric emptying increased as bolus of food enters the stomach o Distention of the stomach and presence of undigested contents increase the secretion of gastrin hormone and vagal nerve impulses o This stimulates contraction of the lower esophageal sphincter and the stomach as well as relaxation of the pyloric sphincter for emptying o Intestinal Phase controlled release of chyme Entry of chyme into duodenum slows gastric activity and increases intestinal activity Neural influences distention of the duodenum and chemical contents of the chyme activate sympathetic nerves which slow gastric activity enterogastric reflex Endocrine influences distention of the duodenum and contents of chyme trigger hormonal release from enteroendocrine cells in the duodenum Secretin hormone decreases stomach secretions Cholecystokinin CKK decreases stomach emptying Neural and hormonal regulation of gastric emptying Enterogastric reflex regulates the amount of chyme released into the duodenum o initiated by distension of duodenum nad contents of the chyme o o sensory impulses sent to the medulla inhibit parasympathetic stimulation of the stomach they also cause cholecystokinin and secretin release from the duodenum and stimulate sympathetic impulses both of which inhibit gastric emptying