Principles of Secretion and Motility
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- Created by: amazingemilyjones
- Created on: 23-04-19 11:17
Principles of Secretion and Motility
Principles of Secretion and Motility
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Enteric Nervous System
- May be considered a third division of autonomic nervous sytem
- Communicates with sympathetic and parasympathetic divisions but can function completely independently
Enteric Nervous System
- The enteric nervous system has two major networks:
- Myenteric plexus (Auerbach's plexus)
- Muscularis mucosa
- Control of smooth muscle tone (motility)
- Submucosal plexus (Meissner's plexus)
- Submucosa of vessel
- Control of secretion
- Blood flow
- Afferent sensory information
- Chemical, stretch
- Myenteric plexus (Auerbach's plexus)
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GI Tract Secretion and Motility
- GI smooth muscle contracts and relaxes
- Peristalsis, mass movement, segmentation
- GI secretions are essential for specific nutrient digestion
- saliva, gastric juice, pancreatic juice
- Three principle mechanisms regulate motility and secretion in the GI tract:
- endocrine hormones
- paracrine mediators
- neural transmission ('neurocrines')
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Endocrine Mechanism
- Mediator (hormone) secreted directly into the circulation - action on cells distant to the site of production
- Five major GI peptide hormones:
- secretin - decreases gastric acid secretion, increases bicarbonate from pancreas and bile from liver
- gastrin - increases gastric acid secretion, motility
- cholecystokinin (CCK) - increases pancreatic juice and bile, decreases gastric acid secretion
- gastric inhibitory peptide (GIP; glucose-dependent insulinotropic peptide) - increases insulin secretion
- motilin - increases gastric emptying, peristalsis, pepsin
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Endocrine Hormones Production
Endocrine Hormones Production
- GI tract is the largest endocrine organ but secretory cells are diffusely distributed
- Principal endocrine cells of the GI tract are APUD cells (amine precursor uptake decarboxylase)
- Located in basal portion of gastric glands
- Enteroendocrine cells
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Cells of the Small Intestine
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Endocrine Hormones Production
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Physiological Stimuli for Hormone Release
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Paracrine Mediators
- Mediator diffuses locally with action on neighbouring cells of different type
- Derived from various cell types:
- vascular endothelium, mast cells, enterochromaffin cells, macrophages
- Non-peptide: histamine, nitrogen monoxide, serotonin (5-HT)
- Peptide: bradykinin, somatostatin
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Example of Paracrine Regulation
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Neurotransmitters
- Mediator released from neurones to act locally
- May be a classical neurotransmitter or co-transmitter (more than one released at a time)
- Mainly under autonomic control
- independent of central nervous system and voluntary control
- Parasympathetic and sympathetic nervous systems - extrinsic
- Enteric nervous system - intrinsic
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Enteric Nervous System
- Principal neurotransmitters of the enteric nervous system:
- Excitatory
- Acetylcholine - contraction of smooth muscle via intrinsic nerve
- 5-HT (serotonin) - increase contraction of the gut
- substance P - increase saliva production, muscle contraction
- cholecystokinin (CCK) - stimulates pancreatic enzyme secretion and gallbladder contraction
- Inhibitory
- Enkephalins (opioid peptides) - modulates smooth muscle activity
- vasoactive intestinal polypeptide (VIP) - inhibits gastrin release and acid secretion
- somatostatin - modulates release of acid, pepsin, gastrin and insulin
- NO - modulated motility
- ATP - modulates motility
- Excitatory
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Parasympathetic and Sympathetic Innervation
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Arrangement of Muscle in GI Tract
- Muscle in the gastrointestinal tract is arranged mainly in two layers
- outer longitudinal layer
- inner circular layer
- Muscle is predominantly smooth muscle (non-striated) therefore is not under voluntary control
- Skeletal muscle found in:
- pharynx
- upper part of oesophagus
- external anal sphincter
- Involved in voluntary control of swallowing and defaecatioj
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Electrical Activity of Smooth Muscle
- Slow wave activity is seen in most phasic smooth muscle
- Do not cause contraction
- Action potential is required
- Spontaneous, regular oscillations of membrane potential = basal electrical rhythm of visceral smooth muscle
- Origin of slow waves are specialised pacemaker cells - interstitial cells of Cajal
- Basal rate of electrical firing of cells of Cajal varies from 3-20 cycles per minute
- Amplitude and rate of slow waves may be modulated by neurotransmitters and hormones
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Oesophageal Peristalsis
- Propulsion of food from pharnx to stomach by coordinated contraction of oesophageal muscle
- wave of contraction followed by wave of relaxation
- Oesophagus contains both skeletal and smooth muscle
- Oesophageal peristalsis is controlled by autonomic nerves, co-ordniated by the swallowing centre of the medulla
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Perceptive Relaxation of the Stomach
- Proximal region of stomach relaxes to accommodate passage of food with minor changes in intragastric pressure
- Peristaltic contractions arising in the mid-stomach move food towards the antrum and pylorus, and some food into the body of the stomach
- Wave of peristalsis moves caudad and increases in intensity, the distal antrum is intensely constricted
- Chyme passes through the pylorus with each contraction (3ml)
- Most chyme is squirted back into the body by retropulsion
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Receptive Relaxation of the Stomach
- Stomach emptying is determined by the tone of the pyloric muscle (pyloric pump)
- Stomach emptying is regulated by
- myenteric reflexes in stomah due to stretch (increase)
- gastrin (increase)
- duodenal distension (enterogastric reflex) (decrease)
- fats in duodenum via CCK release (decrease)
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Small Intestine Contraction
- Mixing movement (segmentation)
- sustained contraction of rings of circular muscle
- followed by contractions of adjacent segment
- promotes mixing of digestive secretions with chyme
- Propulsive peristalsis
- migrating peristaltic waves moving caudad
- slow and interrupted by episodes of segmentation
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Migrating Myoelectric Complex
- Between meals (interdigestive phase), powerful regular cycles of peristalsis occur (~80-90 mins)
- Spontaneous contractions originate in distal stomach, possibly regulated by motilin
- Migrating myoelectric complex migrates down the duodenum and ileum at rate of the slow waves
- Migrating myoelectric complex finished in terminal ileum and another begins
- Important for preventing accumulation of debris and microbial overgrowth
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Colon Function
- Four principal functions
- storage
- absorption of water and electrolytes
- bacterial synthesis of vitamin K
- regulation of chyme entry (ileo-caecal sphincter)
- Motility generally slower than in ileum with 2 major patters:
- Haustration (formation of sacs that store and propel contents)
- Peristalsis movement of contents caudad and backward (anti-peristalsis or retropulsion)
- 'mass movements' are periods of intense caudad peristalsis
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