PBCP 20 GIT1 Motility Handout 2016 PDF

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PBCP 20 GIT1 Motility Handout 2016...

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Physiological Basis of Clinical Practice MED1017

Dr David Grieve

Lecture 20: GIT Motility OVERVIEW OF GIT FUNCTION Gut motility: propels and mixes food, involves swallowing and movement through stomach, small and large intestines. Digestion: exocrine secretion from salivary glands, stomach, pancreas and liver → enzymatic degradation of complex molecules. Absorption: process by which simple molecules pass through intestinal epithelium into circulation. 1. MOUTH AND OESOPHAGUS

Chewing is essentially a activity involving of the mouth and jaw, although there is a reflex component. Intra-oral pressure due to the presence of food causes reflex inhibition of the muscles of mastication leading to jaw drop, activation of the stretch reflex and rebound contraction, thus aiding and of the mouth. Mastication has 3 main functions: (1) to by large lumps of food; (2) and digestion; (3) to for exposure to GI secretions. The process of chewing is co-ordinated by the of the and brain stem nuclei. Swallowing is dependent on co-ordination by the There are 3 stages: . After the oral phase swallowing becomes an . Stimulation of around the opening activates the brain stem via of mainly the . The early events of swallowing are geared to protecting the : the main mechanism is approximation of the vocal cords with acting as a safeguard. Movement of the larynx , the and a of contraction ( is initiated by the . The pharyngeal stage last < 6 seconds so does not interrupt respiration. The peristaltic wave continues into the oesophagus (primary peristalsis) and is aided by . If this is inadequate to propel food into the stomach, then is initiated by the Motor pathways involve both (glossopharyngeal) and (vagus) nerves since the upper oesophagus is (voluntary) muscle while the lower oesophagus is (involuntary) muscle. When the peristaltic wave approaches the stomach cause relaxation of the and stomach (receptive relaxation). 2. STOMACH The stomach serves 3 functions: (1) by receptive relaxation via the

of food after meals, which is assisted reflex; (2) which is mediated by (controlled by the nervous system) spreading from the body to antrum where they are strongest. The sphincter remains forcing the bulk of the stomach contents backwards; (3) produces which is propelled into the duodenum in , depending on the degree of sphincter . Although external nerves are not necessary for spontaneous activity, the rate of gastric contraction/emptying can be modified by reflexes. 1

Physiological Basis of Clinical Practice MED1017

Dr David Grieve Food in the stomach initiates regular peristaltic waves at a rate of 3-4 per minute. This spontaneous activity occurs in 2 forms: (1) , which are due to changes in resting membrane potential. There is growing evidence that slow waves may be mediated by specialised cells known as , acting as the electrical pacemaker; (2) are true actions potentials which occur when resting membrane potential reaches threshold (~ -40 mV). The current is carried by sodium and calcium, and their frequency progressively increases from the fundus to antrum and may be influenced by both in a positive manner, whilst being inhibited by noradrenaline. The frequency of is so high at the pylorus that it results in the sphincter, thus regulating emptying.

The rate of gastric emptying is not regulated by increased pressure within the stomach, rather the balance between strength of , which are controlled by . Receptors in the duodenum and jejunum sense various factors, including acid, fat digestion products, hyperosmotic chyme and amino acids, stimulating release of such as secretin, GIP and which decrease gastric emptying. Similarly central sympathetic stimulation can decrease gastric emptying, whereas it is increased by parasympathetic activation. Vomiting results in expulsion of the gastric contents from the mouth, and involves a complex set of motor functions co-ordinated by the vomiting centre in the medulla. Vomiting is usually associated with nausea and is preceded by , due to increased activity. The vomiting centre receives various afferent inputs. It should be noted that the propulsive force is generated by an increase in pressure which , with little contribution from gastric contraction. Loss of NaCl, H2O and H+ may lead to dehydration and metabolic alkalosis may result. 3. SMALL INTESTINE The small intestine is comprised of the . The majority of digestion and absorption occurs within the proximal 50 %. Spontaneous activity of intestinal smooth muscle leads to 3 types of movement: (1) , which are responsible for mixing of the intestinal contents; (2) which propels chyme along the intestine; (3) , which stimulates of the intestine. Segmentation contractions are the dominant type of motility and are initiated by They occur in different patterns, but typically as one set of contractions disappears another set is generated between the previous contractions resulting in a chopping/mixing action. The frequency of contraction decreases from the duodenum (12 per minute) to ileum (9 per minute), which is dictated by , favouring slow movement in that direction. are required for effective and the , which may be mediated by Sympathetic stimulation decreases the force of activity.

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Localised peristalsis is mainly initiated by which results in contraction proximal to the point of stimulation. This sets up a peristaltic wave which travels a small distance along the intestine before dying out, only to start up again more distally. This helps to slowly propel the and to . Unidirectional tranmission of the peristaltic wave is aided by receptive relaxation, the mechanism for which is unknown. This type of motility is mediated by the . 2

Physiological Basis of Clinical Practice MED1017

Dr David Grieve

Several hours post-prandially, when , a much more powerful propulsive wave occurs, known as the . This is initiated in the stomach by intense bursts of vagal activity due to the release of and causes forceful contraction towards the ileum. The effect is to sweep the intestinal residue into the colon via the , thus inhibiting migration of colonic bacteria and limiting bacterial overgrowth. This only occurs in the fasted state and is , with a return to segmentation. Emptying of the small intestine is controlled by the , which protrudes into the colon. It’s principal function is to and is closed by . The narrow opening provides resistance to emptying of the ileum, and allows chyme to pass into the colon at a rate at which it can be absorbed. Opening of the ileocaecal valve may be promoted by local peristalsis and the gastrocolic reflex after a meal and inhibited by pressure and chemical irritation in the colon, such as that causes by inflammation of the appendix. 4. LARGE INTESTINE The functions of the colon are twofold: (1) in the proximal portion; (2) in the distal portion. Movements in the large intestine are than in the small intestine, typically taking 8-15 hours, allowing stasis and bacterial overgrowth. However, the pattern is similar. Sustained , due to stimulation of the , resulting in characteristic haustrations. This type of activity facilitates mixing of the colon contents with minor forward propulsion and favours reabsorption by gradual exposure to the mucosa...