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