Physiology of Mastication, Swallowing and G.I. Tract Motility
Professor John PetersE-mail: [email protected]
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 contractions Secretion
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
acidsAbsorption
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 WallMucosa 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 ReflexTissue/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
bran
e po
tent
ial (
mV
)
Threshold Sm
ooth
mus
cle
actio
n po
tent
ials
Forc
e (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