1 iii gastric motility. 2 major function of gastric motility to serve as a reservoir to break food...
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III GASTRIC MOTILITY
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Major Function of Gastric Motility
To serve as a reservoir
To break food into small particles and
mix food with gastric secretions
To empty gastric contents into the
duodenum at a controlled rate
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1. Anatomy and innervation of the Stomach
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The stomach can be divided into three anatomic regions (A)
and two functional regions (B)
Gastric reservoirTonic contractions Gastric reservoirTonic contractions
Gastric pumpPhasic contractionsGastric pumpPhasic contractions
BBFundusFundus
CorpusCorpusAntrumAntrum
PylorusPylorus
AA
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OesophagusLower OesophagealSphincter Fundus
Body
Antrum
DuodenumPylorus
Functional Anatomy of Stomach
Fundus
Body
Antrum
•Storage
• Storage• Mucus• HCl• Pepsinogen• Intrinsic factor
• Mixing/Grinding• Gastrin
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AnatomyAnatomy
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Innervation of the GI tract
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2. Responses to Gastric Filling –
Receptive Relaxation
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Receptive relaxation During chewing and swallowing food,
the stimulation of food to the receptors in mouth, pharynx, and esophagus
reflexly causes the smooth muscle of the fundus and body of the stomach to relax,
This process allows the stomach to accommodate a large amounts of food and fluid.
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The relaxation of the gastric reservoir is mainly regulated by reflexes. Three kinds of relaxation can be differentiated: the receptive, adaptive and feedback-relaxation
Inhibitoryvagal fibre(NANC-inhibition)
Nutrients
CCKRelaxation of
gastric reservoir
ACH
Vaguscentre
1. Receptive relaxationMechanical
stimuli in the pharynx
3. Feedback relaxation
2. Adap tive relax ation
NutrientsTensionreceptors
Distension
NO + VIP et al.
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3. Peristalsis of the Gut and Gastric Emptying
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Gastric MotilityPeristaltic waves: Body
Antrum
BodyThin muscle weak contraction No mixing
AntrumThick muscle powerful
contractionA Mixing
B Contraction of pyloric sphincter
1 Only small quantity of gastric content (chyme) entering duodenum
2 Further mixing as antral contents forced back towards body
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What produces gastric peristaltic waves?• Peristaltic rhythm (~3/min) generated by pacemaker
cells (longitudinal muscle layer)
• Slow waves – spontaneous depolarisation/ repolarisation
• Slow wave rhythm = basic electrical rhythm (BER)
• Slow waves conducted through gap junctions along longitudinal muscle layer
• Slow wave depolarisation sub-threshold - require further depolarisation to induce action potentials contraction
• Number of APs/wave determines strength of contraction
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Gastric Slow Waves and Motility
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peristalsis
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Peristalsis
Figure 24.4
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Phase of propulsion Phase of retropulsionPhase of emptying
Bulge
Rapid flow of liquids withsuspended small particlesand delayed flow of largeparticles towards pylorus
Emptying of liquids withsmall particles whereaslarge particles are retainedin the buldge of the terminalantrum
Antrum
The contraction of the gastric pump can be differentiated into three phases:A: phase of propulsion, B: phase of emptying,C: phase of retropulsion and grinding
Retropulsion of largeparticles and clearingof the terminal antrum
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Control of gastric motility
Vagovagal reflex – fundal relaxation
Myenteric plexus – slow waves – contraction
Parasympathetic and Gastrin – increase contraction force and frequency
Sympathetic – decrease contraction force and frequency
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Gastric emptying
1. Def.
The process by which the chyme is expelled from the stomac
h into the duodenum is called the gastric emptying.
2. Control
1) stomach: stimulating factor, neuronal and hormonal
2) duodenum: inhibiting factor
entero-gastric reflex, hormones
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Control of Gastric emptying
Stimulating factors in stomach
Presence of food
Gastrin
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Control of gastric emptyingControl of gastric emptying
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Balance between gastric reservoir and antral pump
Gastro-gastric reflexes
Enhanced and prolonged relaxation of
reservoir
Distension
Disten-sion
Excitatory reflex
Antral pump switched on and intensified
Inhibitory reflex
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Inhibitory effects in duodenum and jejunum – throug refelxes and hormones
Inhibitory reflexes – direct – myenteric plexus
indirect – via extrinsic nerves
Neural reflexes stimulated by:
Distension, irritation, acidity, high osmolarity, protein/fat
Fats and acids also stimulate release of humoral factors which reduce gastric emptying
Cholecystokinin (CCK), stimulated by fats
Secretin, stimulated by acids
Control of Gastric emptying
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Enterogastric Reflexregulates the rate at which chyme leaves the stomach
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The feedback regulation of gastric emptying is performed by entero-gastric reflexes and release of intestinal hormones
Ehrlein Figure 14Ehrlein Figure 14
It causes enhanced relaxation of the gastric reservoir, inhibition of the antral pump, and reduced opening of the pyloric sphincter.
Vagalcenter
Inhibitoryvagal fibers
NO, VIP et al.
Senso
ric
affe
rent
fiber
s
CCK
ACHEnhancedrelaxation
andstorage
Stimulating cholinergicvagal fibers
Nutrients Long chain fatty acids Amino acids Dipeptids GlucoseOsmolalityHydrochloric acid
Reduced openingof pyloric sphincter
Reduced contraction
Backflow
+
+_
ACH
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non-digestible spheres
“Quality” of food regulates gastric emtying
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Solids and liquids of the gastric chyme are emptied with different velocities.
Lag phase
Time (min)
Viscouscontent
Liquid content
Solids100
80
60
40
20
00 20 40 60 80 100 120
Gas
tric
vo
lum
e ( %
)
Emptying of liquids is exponential.Emptying of large solid particles only begins after sufficient grinding (lag phase). Afterwards the viscous chyme is mainly emptied in a linear fashion
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4. Vomiting
• Emesis
• Stretching, toxins, alcohol, spicy foods, and drugs may
stimulate this.
• Emetic Center of the Medulla
• Diaphragm and abdominal wall contract
• Cardiac sphincter relaxes.
• Soft palate rises