Physiology of Mastication, Swallowing and G.I. Tract Motility

Professor John PetersE-mail: j.a.peters@dundee.ac.uk

Following this lecture, students should be able to:

List the major functions associated with the mouth

List the 3 main pairs of salivary glands and the nature of their secretions

Describe the concept of primary secretion and its modification by ductal cells

State the functions of saliva

Describe the control(s) of salivary secretion

Describe swallowing in terms of its 'two' phases

Discuss peristalsis in the oesophagus

Discuss the function of the lower oesophageal sphincter

Outline the functions of the stomach

Learning Objectives

The Digestive System and Accessory Structures

The Basic Digestive Processes (1)

Motility

Mechanical activity mostly involving smooth muscle (skeletal at mouth, pharynx, upper oesophagus and external anal sphincter)

Propulsive movements

Mixing movements

Tonic contractionsSecretion

Into the lumen of the digestive tract occurs from digestive tract and accessory structures in response to hormonal and neural signals. Required for: (i) digestion (ii) protection

Digestive secretions contain:

Water (large volume extracted from plasma)

Electrolytes

Organic compounds (enzymes, bile salts, mucus etc.)

The Basic Digestive Processes (2)

Digestion

Biochemical breakdown (enzymatic hydrolysis) of complex foodstuffs to smaller, absorbable, units

Carbohydrates [mostly polysaccharides (e.g. starch, glycogen) and some

disaccharides (e.g. sucrose, lactose)]: converted to monosaccharides

(mainly glucose, some galactose and fructose)

Proteins: converted to constituent amino acids

Fats [mostly triglycerides]: converted to monoglycerides and free fatty

acids

Absorption

Transfer of the absorbable products of digestion (with water, electrolytes and vitamins) from the digestive tract to the blood or lymph

Structure of the Digestive Tract Wall

Mucosa comprises: epithelial cells (absorption)exocrine cells (secrete digestive juices)endocrine gland cells (secrete hormones)lamina propriamuscularis mucosa

Submucosa contains: connective tissueblood and lymph vesselsnerve network – submucous plexus

Muscularis externa comprises: circular musclenerve network – myenteric plexuslongitudinal muscle

Serosa contains: connective tissue

The Mouth

Lips Procurement/containment of food Speech

Teeth Chewing (mastication), mediated by the

masseteric and diagastric reflexes: (i) breaks down food and mixes it with saliva, (ii) stimulates taste buds and (iii) by reflexes following taste bud stimulation increases salivary, gastric, pancreatic and bile secretion

Palate Separates mouth from nasal

passages: allows breathing and chewing simultaneously

Uvula Helps seal off nasal passages

during swallowing

Tongue Guides food, important in

speech, and swallowing; major location of taste buds

Pharynx Common passageway for the

respiratory and digestive systems, tonsils on side walls are lymphoid tissues

Salivary Secretion and Function Saliva is secreted by three major pairs of salivary glands: parotid,

submandibular and sublingual

Parotid - below ear and over the masseter25% of total

Sublingual in floor of mouth under tongue5% of total

Submandibular - under lower edge of mandible70% of total

Functions of Saliva Lubrication (salivary water

and mucus) mouth and food – important for speech and swallowing)

Solvent (important for taste)

Antibacterial (contains lysozyme, lactoferrin and immunoglobulins – important in preventing dental carries )

Digestion of complex carbohydrate (contains amylase)

Neutralization of acid (contains bicarbonate)

Facilitates sucking by infants (fluid seal)

Formation of Saliva (1) Occurs in two stages – primary secretion (by the acinus) – secondary

modification (by duct cells)

Multiple transporters and ion channels participate in the formation of saliva (which requires ATP) – exact details may differ between glands – only general principles are illustrated schematically below

AcinusCells produce a primary

secretion with Na+, K+, Cl- and HCO3

- content similar to plasma

Na+, K+, Cl-, HCO3

-, H2O

Duct cellsCells modify secretion by removing Na+ and Cl- and to a lesser extent

adding K+ and HCO3-, no movement

of H20 – hence diluting

K+HCO3-

Na+ Cl-

H2O

Formation of Saliva (2)

0.05 ml/min – sleep

0.5 ml/min – resting, awake state

5 ml/min – “actively” salivating

Rate

varies according to flow rate

NaCl content much lower than plasma (detection of salty taste?)

HCO3- content: high flow rate ; low flow rate

no glucose (detection of sweet taste?)

Composition

Reflex regulation (neuronal control)

Rate of formation of saliva is increased by:

1. Simple (unconditioned) reflex

2. Conditioned (acquired) reflex (Pavlov’s dog)

Simple (unconditioned)Chemo / pressure

receptors in mouth activated in presence of food (or other stimulus)

Impulses sent via afferent

nervesSalivary centre in medulla

Impulses via extrinsic autonomic nerves -Both sympathetic &

parasympathetic stimulation

Salivary glands increase

production

Cerebral cortex

Acquired (conditioned) Think about, smell, see,

hear preparation of appetising dish

Control of Salivary Secretion by Reflexes

Role of the Autonomic Nerves in Saliva Secretion

Sympathetic stimulation(Dominant at stressful times-

dry mouth when nervous?)

Small volumeThick

Mucus rich(mediated by 1-adrenoceptors)

Postganglionic fibres from superior cervical

ganglia

Parasympathetic stimulation (Dominant role in ”normal”

saliva production)

Large volumeWatery

Enzyme rich(mediated by M3 muscarinic

acetylcholine receptors)

Glossopharyngeal and facial nerves

Note: muscarinic receptor antagonists (e.g. atropine) and antidepressants that block muscarinic receptors cause a dry mouth

Swallowing (Deglutition) Movement of food from the mouth to the stomach occurs in two phases

1. Oropharyngeal stage (mouth pharynx oesophagus (1 second)

Bolus of food must be directed to the oesophagus and away from the nasal passages and trachea – requires a series of highly co-ordinated responses

Bolus formed in mouth by action of teeth and

tongue

Tongue forces bolus into pharynx

at rear of mouth

Voluntary

Pressure stimulates pharyngeal pressure

receptors

Afferent impulse to swallowing centre in

medulla

Efferents initiate all-or-nothing reflex sequence of

muscle movements(see next slide)

Upper oesophageal sphincter opens

Food passes through pharynx into oesophagus

Involuntary

Stages of the Reflex

Tissue/organ Action Effect – to help prevent food from entering …

Soft palate Raises Nasal passages

Tongue Presses against hard palate Nasal passages or mouth

Uvula Presses against back of throat Nasal passages (part of larger process)

Larynx Elevation Trachea

Epiglottis Tilting Trachea

Vocal cords Close across larynx opening (glottis) Trachea

Swallowing centre (brain) Inhibits respiratory centre (brain) Trachea

Pharyngeal muscles

Contract & force bolus into oesophagus Trachea

Upper oesophageal sphincter

Opens Allows food into oesophagus

Swallowing centre (medulla oblongata) triggers primary peristaltic wave and closure of the upper oesophageal sphicter

Wave mediated by skeletal muscle in upper oesophagus and smooth muscle in distal regions

Peristalis co-ordinated by the enteric nervous system (cholinergic neurones)

Circular fibres behind bolus squeeze bolus down towards stomach

Longitudinal fibres in front of bolus shorten distance of travel

Lower oesophageal sphincter opens within 2-3 s of the initiation of a swallow (closes after passage of bolus to prevent reflux)

Sticky food may become lodged stimulating local pressure receptors that cause:

secondary peristaltic wave - more forceful than primary – locally triggered

increased saliva production

Oesophageal stage [oesophagus stomach (4-10 seconds)]

Absorption and Secretion by the Oesophagus

Secretion is entirely mucus

Lubricates for passage of food

Protects epithelium from attack by acid and enzymes in gastric juice.

Epitheilium itself (stratified squamous) has a rapid turnover

Transit of a bolus of food is far too rapid to permit absorption

The Stomach J – shaped bag: 50 >1000 ml

capacity: relaxes receptively (driven by vagus) to accommodate food from oesophagus

Starting point for digestion of proteins (pepsin and HCl)

Mixes food with gastric secretions to produce chyme

Limited amount of absorption

Stores food before passing it into small intestine as chyme for further digestion and absorption

Secretes approximately 2 litre/day of gastric juice from gastric pits in the gastric mucosa

Gastric Motility Pacemaker cells (interstitial cells of Cajal) in the fundus establish a basal

electrical rhythm (BER) that spreads over the stomach from fundus towards the pyloric sphincter

The BER occurs continuously (3 per minute) and may, or not, generate smooth muscle contraction, dependent upon the excitability of the latter

Mem

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Threshold Sm

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mu

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pote

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For

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g)

Time (s)

Excitatory stimulus

BER

Gastric Emptying and Mixing

Direction of movement of

peristaltic contraction

Oesophagus

Duodenum

Lower oesophageal sphincter

Pyloric sphincter

Peristaltic contraction

Peristaltic contraction

= movement of chyme

Emptying Mixing

Stomach emptying Strength of antral wave determines escape of chyme through pyloric

sphincter

Governed by: Gastric factors• Duodenal factors

Gastric factors

Rate of emptying proportional to volume of chyme in stomach

Distension increases motility due to stretch of smooth muscle, stimulation of intrinsic nerve plexuses, increased vagus nerve activity and gastrin increase

Consistency of chyme

Emptying facilitated by finely divided, thick liquid chyme

Duodenal Factors Duodenum must be ready to receive chyme and can delay emptying by:

Neuronal response – the enterogastric reflex – decreases antral peristalic activity through signals from intrinsic nerve plexuses and the autonomic nervous system

Hormonal response – release of enterogastrones [e.g. secretin and cholecystokinin CCK)] from duodenum inhibits stomach contraction

Stimuli within the duodenum that drive the neuronal and hormone responses include:

Fat – particularly potent – delay in gastric emptying required for digestion and absorption in small intestine

Acid – time is required for neutralization by bicarbonate secreted from the pancreas – important for optimal function of pancreatic digestive enzymes

Hypertonicity – products of carbohydrate and protein digestion are osmotically active and draw water into the small intestine – danger of reduced plasma volume and circulatory disturbances (e.g. ‘dumping syndrome’)

Distension