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Gastrointestinal physiology II.

62. Functions of the upper GI tract: chewing, salivation,

swallowing.

63. Motor functions of the stomach. Vomiting (emesis).

Prof. Gyula Sáry

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Motor functions of the mouth and the oral cavity

Food intake:

Sucking (brainstem motor reflex in the newborn)

Biting and initiation of chewing is VOLUNTARY, but the process itself is

more or less automated.

CHEWING (MASTICATION)

Chewing deficit does not compromise digestion but the process is greatly

prolonged and large particles may hurt the gastric mucosa.

Chewing is a unilateral motor reflex:

• Opening the mouth activates a stretch reflex and the jaw rises.

• Stimulation of mechanoreceptors in the mucosa inhibits

stretch reflex and the jaw drops. Then the cycle starts over…

• Movements of the tongue keep the food between the teeth!

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The stretch reflex(myotatic reflex)

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SWALLOWING (DEGLUTITION) – a vital BRAINSTEM reflex

1. From the mouth to the pharynx.....voluntary phase

2. Pharyngeal stage

3. Esophageal stage }.....................reflex phases

• The tongue pushes back the bolus into the pharynx (initiation)

• All passages that are otherwise open must be closed (airways): soft palate closes nasal cavity,

larynx is elevated, glottis closes, breathing stops.

• The muscles of the pharynx and upper third of the esophagus are striated muscle: peristaltic

activity in the upper portion of the alimentary canal is controlled by somatomotor innervation.

•The peristaltic wave starts from the pharynx, the UES relaxes. Here the peristalsis is caused by

SEQUENTIAL FIRING of motor units

•The esophageal smooth muscles are controlled by parasympathethic activity of the vagus

nerve : smooth muscle is actively RELAXED followed by rebound CONTRACTION

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SWALLOWING (DEGLUTITION)

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Primary peristalsis:

Peristaltic wave initiated

from the pharynx.

Secondary peristalsis:

residual bolus left in the

esophagus initiates new

peristalsis.

(e.g., during vomiting)

ESOPHAGUS

upper esophageal sphincter.

lower esophageal sphincter

Neurogenic tone inhibited!

Myogenic tone inhibited!

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Boron, Boulpaep, Medicap

Physiology, Elsevier Saunders, 2012

PRESSURE WAVES IN THE ESOPHAGUS DURING

SWALLOWING (esophagometry)

Different mechanisms – still smooth peristaltic wave!

„Volume and pH clearance”

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ACHALASIA: when the LES does not open

Dysphagia

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FUNCTIONS OF THE SALIVA

• Digestion (amylase, lipase)

• Excretion

• Protection of the mucosa

• Bacteriostatic action

• Lubrication, solvent for taste

• Alkalic pH (fresh saliva) – preserves teeth, hypotonic

• Articulated speech

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Saliva secretion (1,5 liters daily)

A cooperation of acinus and ductal cells

acinus:

primarysecretionISOTONIC

ductus:

secondarysecretionHYPOTONIC

Acinus cell

Myoepithel

cell

ductus

intercalaris

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acinus secondary active transport

Na+ and water

follow passivelyprimary saliva

final saliva secretion

resorption

duct

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Transport in the salivary ducts

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COMPOSITION OF THE SALIVA VARIES WITH FLOW

SALIVA PLASMA

Co

nce

ntr

ati

on

(m

M/L

)

Flow (ml/min)

Organic components: enzymes (lipase, amylase), mucus, IgA,

lysozime, lactoferrin, EGF (epidermal growth factor)

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Parasympathetic:

large volume,

serous

Sympathetic:

small volume,

viscous

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Saliva secretion

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Reflectory control

0.5 l/day basal secretion

Psychic effects

Smell, taste: chemical signals

Contact with the oral mucosa

Composition varies with symp./parasymp. stimulation

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Distal (caudad)

portion

Proximal (orad)

portion

Oxyntic (HCl producing)

portion

Gastrin producing

portion

MOTILITY SECRETION

FUNCTIONAL DIVISONS OF THE STOMACH

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FUNCTIONS OF THE STOMACH

• Storage and emptying in small portions

• Grinding, milling

• Production of intrinsic factor --- VITAL FUNCTION!!!

• HCl promotes absorption of iron (keeping it as Fe2+)

• Bacteriostatic function

•Trophic influence on the rest of the GI tract (gastrin

promotes growth and development of the mucosal

membrane)

• Digestion of proteins

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MOTILITY OF THE PROXIMAL PORTION OF THE STOMACH

During the filling phase:

receptive relaxation

Mecanisms:

vago-vagal reflex, local reflex,

stress relaxation

X

stretching

During emptying:

Gradual tonic contraction

The proximal portion does

not participate in peristalsis.

Mechanisms of receptive relaxation in the

proximal stomach

• Feeding (mechanical, taste, visual, olfactory stimuli)

stimulates vagal efferents activating ENS neurons that

INHIBIT smooth muscle contraction (using NO, VIP) –

cephalic phase

• Food in the stomach will activate somatosensory STRETCH

receptors that deliver the signal to the brainstem evoking

activation of the same parasympathethic nerves (long

vago-vagal reflex)

• Also ENS sensory neurons are activated that can also elicit

LOCAL reflexes to dilate the stomach

• Finally myogenic stress relaxation (in response to stretch)

can also be demonstrated

These three are responsible for the gastric phase – MOST

important! 20

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Intestinal phase (least

important):

Hormones of the

small intestine dilate

proximal stomach to

promote storage (to

slow emptying)

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MOTILITY OF THE DISTAL PORTION OF THE STOMACH

1. Empty stomach: MMC

2. Filled state: PERISTALSIS

pacemaker region

• The major function of the stomach is storage and grinding.

• The pyloric region contracts before the peristaltic wave reaches the pylorus.

• The pyloric canal serves as a „filter” allowing only small parts <2mm! to pass

• Only 2-4 ml liquid chyme enters the duodenum in each cycle.

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proximal part

of the stomach

distalis

gyomor

distal parts

of the stomach

pacemaker

t

No BER

BER!

• 3 cycles/min

• Most material is

mixed, and

ground against

the pyloric

muscle

• Small „injection”

of acidic cntent

into the

duodenum

Boron, Boulpaep, Medical Physiology,

Elsevier Saunders, 2012

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contraction ring

Control of gastric peristalsis and emptying

• Gastric phase (stimulation): (1) neural control,

stretch and contents of the stomach stimulate

ENS neurons promoting gastric motility. (2)

Hormonal control: Gastrin produced in response

to neuronal and chemical stimuli also promotes

motility.

• Intestinal phase inhibition: acid, nutrients in the

gut evoke (1) Hormonal inhibition – secretin, CCK,

GIP etc, and (2) ENS local „enterogastric reflex”

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CONTROL OF GASTRIC EMPTYING

Gastric mechanisms:

PROMOTE EMPTYING

Distention

Protein

Gastrin

Intestinal mechanisms:

INHIBIT EMPTYING

Neural:HyperosmosisAcidDistention

Ileum

Humoral:

CCK

Secretin

GIP

Neurotensin

ProteinLipidAcid

pyloric sphincter:

intrinsic neurons (enkephalin, VIP)

extrinsic: vagus, sympathicus

2814th century illustrations of vomiting

Vomitus: A complex brainstem protective reflex to empty the

stomach and often proximal small intestinal contents. Multiple

brainstem areas and spinal cord segments are involved.

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VOMITUS

chemosensitive trigger zone

(Area postrema)

no blood-brain barier!!

medullary center

cerebellum

Evoking stimuli:GI tract irritation

Visceral pain

Kinetosis

Drugs (apomorphine)

Visual, olphactory stimuli

emotions stim.

pregnancy

irradiation

…

Reverse filling of the stomach is caused by

giant retrograde intestinal contractions

• Müller maneuver (inspiration effort ,

closed glottis) + opening of the LES and

contraction of the abdominal muscles

(content enters the esophagus)

• Forceful expiration (Valsalva) and opening

of UES –EXPULSION – throwing up

Remnants delivered back to stomach by a

secondary peristalsis – new cycle may start

Nausea: symptoms preceding

vomitus