W1 - Ingestion

What are Salivary Glands?

A typical exocrine gland consisting of acini and ducts that secrete saliva into the oral cavity which is part of the GIT Lumen

How do Exocrine glands work?

produces secretion inside acini and delivers it via ducts onto surfaces or cavities.

Exocrine glands include salivary glands and are accessory structures to digestion —> not in direct contact with food but secrete their substance into GIT which then interacts with food

What happens to fluid from exocrine glands?

Fluid from exocrine glands is taken from the extracellular fluid compartment and then modified by the duct cells to be hypotonic to allow better taste perception.

What structures are involved in salivation?

1. Acini (secretory)

2. Ducts (transport)

3. Myoepithelial cell (contractile)

What are Acini? and what are there function in salivation?

What: Sac of secretory cells (mucous or serous type)

Function: Produce isotonic secretion from ECF by modifying the fluid with their own unique constituents and then send it through the ducts.

What are the type of secretory cells that make up the acini?

You can have mucous type or serous type secretory cells

• Acini of the Mucous gland = musin glycoprotein

• Acini of the Submandibular gland = mixed gland

• Acini of the Parotid gland = mainly serous

Why do Acini produce their isotonic secretions from ECF?

because the ECF has a rich blood supply so it borrows blood plasma

How do Acini produce isotonic secretion from ECF?

by modifying the fluid with their own unique constituents and then send it through the ducts.

This allows the external solution to have the same solute concentration and water concentration as the body/ intracellular fluids.

What are Ducts? What is their main purpose/ function?

What: a tube or vessel of the body through which fluids pass.
Function: transport saliva to oral cavity

What is the function and the ductal cells in salivation? and how is this achieved

Function: makes fluid fit for purpose
How: Modify the isotonic fluid secreted by the acini into a hypotonic secretion (saliva) by reabsorbing Na and Cl

How do the ducts keep the fluid hypotonic?

They have little permeability to water and therefore exchange of ions can occur but not water.

Why is it important for the duct cells to modify the isotonic solution to become hypotonic?

Hypotonic solutions have a lower osmotic pressure than the body/ intracellular fluids. This allows better taste perception as it prevents the ions in the saliva from overpowering the taste of some of the food molecules.

What are myoepithelial cells and what are their functions?

What: Contractile cells
Function: Push secretions (saliva) out of the acini and proximal ducts —> into the distal ducts —> then into the opening where they need to be delivered

where are myoepithelial cells located

Mostly around acinic and proximal parts of the duct

What are the constituents of Saliva?

1. Water (>99%)

2. Electrolytes

3. Buffers

4. Mucous Glycoproteins

5. Antimicrobials (endogenous)

6. Digestive Enzymes

7. Nitrogenous Wastes

What is the function of water in saliva?

• Keeps oral cavity clean by moistening it - nothing sticking to teeth for too long

• Allows swallowing by lubricating food

• Dissolving and distributing food particles for taste

What happens when you have a dry mouth?

• Can’t taste food properly - water is needed to dissolve and distribute the food particles

• Mouth becomes dirty - needs to be constantly flushed to be kept clean

• Cannot swallow effectively - foods need to be lubricated

What is the function of electrolytes in saliva?

Maintain tonicity

Examples = Na, K, Cl

What is the function of buffers in saliva?

Buffers are alkaline —> Maintain pH of 7 by neutralizing acid from oral microbes (ferment them)

Examples = HCO3 (bicarbonate)

What is the function of mucous glycoproteins in saliva?

• Mucins lubricate the oral cavity

• More sticky - provide protective barrier in oral cavity

Why are buffers important in saliva?

Buffers are alkaline substances and help to neutralise acids from oral microbes in order to maintain a pH of 7. If the pH becomes too acidic, the enamel of the teeth will be dissolved.

What is the function of Antimicrobials in saliva?

Control oral microbes - prevents over-colonization

Example: lysozyme, IgA

What is the function of digestive enzymes in saliva?

Digestion of macronutrients - smaller they are allows us to taste them more

Types of digestive enzymes

1. α amylase (ptyalin) digests complex carbs e.g. starch (amylose)

2. Lingual lipase digests lipids e.g. triglycerides

What is the function of nitrogenous wastes in saliva?

Have no function, hence wastes —> they are secreted into mouth and then excreted through faeces ( similar to excretion at kidneys)

What is Xerostomia

a condition of dry mouth due to inadequate salivation. This can be caused by a number of etiologies:

• general conditions e.g. dehydration, emotional stress (effects NS which regulates salivation)

• diseases (pathological) e.g. Sjögren’s syndrome, diabetes, cystic fibrosis

• therapies (iatrogenic) e.g. opioids, radiation -- based on stimulation of NS

What are consequences of Xerostomia?

Xerostomia = dry mouth due to inadequate salivation

Based on the composition of saliva, some consequences of xerostomia are;

1. Teeth rot - not cleaned

2. Incomplete digestion - lack of digestive enzymes

How is Salivation controlled?

salivation consists of secretomotor activity. Secretomotor activity is controlled by the parasympathetic nervous system.

This produces 2 types of salivary secretions (serous, mucous)

What are the two types of salivary secretions and how are they controlled?

Secretomotor activity is controlled by the parasympathetic nervous system.

Two types of secretions:

1. Serous – contains enzymes, electrolytes, antibodies, etc

2. Mucus – contains mucins for lubrication, little bit of protection

What do the 2 different types of salivary secretions contain?

1. Serous – contains enzymes, electrolytes, antibodies, etc

2. Mucus – contains mucins for lubrication, little bit of protection

How much do each glands contribute to the secretomotor activity of saliva?

1. Parotid Gland - 20 - 25%

2. Submandibular Gland - 70 - 75%

3. Sublingual Gland - 5%

Which types of salivary secretions does each gland produce?

1. Parotid Gland - Serous only

2. Submandibular Gland - serous and mucus

3. Sublingual Gland - serous and mucous

Which gland is the main contributor to salivary excretions?

Parotid gland —> responsible for 75% of secretions

What innervates the parotid gland (PSNS)?

Glossopharyngeal nerve (CN IX)

What innervates the submandibular gland (PSNS)?

Chorda Tympani of Facial Nerve (CNVII)

What innervates the sublingual gland (PSNS)?

Chorda Tympani of Facial Nerve (CNVII)

What is the brain stem responsible for (in terms of reflex)

responsible for unconscious control of basal physiological functions that contribute to homeostasis and general daily regulations in out body.

What is the cerebral cortex responsible for (in terms of reflex)

conscious control over body movements

Stimuli of the Unconditioned (innate) Salivation reflex

• mechanical (chewing, texture of food)

• chemical (taste of food)

Salivation Reflex - Unconditioned (innate) Reflex

Stimulus → brain stem (salivatory nucleus) → glands

Sensory Input of the Unconditioned (innate) Salivation reflex

Comes from different cranial nn. with varying levels of contribution. Delivers signals to the salivatory nucleus of the brainstem.

Motor Output of the Unconditioned (innate) Salivation reflex

All PSNS. Nerve involved is dependent on the gland:

• glossopharyngeal nn. = parotid gland

• facial nn. = submandibular and sublingual gland

Salivation Reflex - Conditioned (learned) Reflex

Stimulus → cerebral cortex → brain stem → glands

Stimuli of the Conditioned (learned) Salivation reflex

• sensory (sight, smell, sound of food)

• cognitive (thought of food). e.g. ice cream truck

Classical conditioning

when a controllable (external) conditioned stimulus is used to hijack the reflex.

What is this?

the salivatory reflex

Compared to the remaining GIT, why are pharyngeal muscles unique?

Motility in the oral cavity and pharynx is controlled by skeletal muscle, whereas the rest of the GIT (excl. anus) is controlled by smooth muscle. THEREFORE… pharyngeal muscle activity can be voluntary or involuntary

What are the palatal mm. used for?

Together with the tongue, these assist with the voluntary phase of swallowing.

What are the pharyngeal mm. used for?

Pharyngeal mm. assist with the involuntary phase of swallowing/ deglutition.

These include the pharyngeal constrictors and laryngeal elevators

• ext. pharyngeal constrictors (circular mm.) - contract and constrict the lumen above the bolus of food

• int. laryngeal elevators (longitudinal elevator mm.) – pull the pharyngeal wall up and over the bolus of food

What is deglutition?

the action or process of swallowing

What are the 3 phases involved in deglutition?

1. Oral Phase - Voluntary

2. Pharyngeal Phase - Involuntary

3. Esophageal Phase (in transit) - Involuntary

Steps of Oral Phase of Deglutition

Oral phase (voluntary)

1. Tongue moves the bolus posteriorly and creates a seal - vacuum needed to swallow food

2. Soft palate moves posteriorly and closes the passage to the nasopharynx to avoid bolus moving into nasal cavity (this occurs if do it too quickly)

3. Epiglottis is elevated and the laryngeal inlet is opened to allow breathing

4. Upper esophageal sphincter is closed due to tonic contraction

what does it mean if a sphincter is closed due to tonic contraction?

• tonic = always tone/ tension

• THEREFORE…. Tonic contraction = sphincter only opens when something wants to move from upstream of the sphincter to downstream of the sphincter

Steps of Pharyngeal Phase of Deglutition

Pharyngeal phase (involuntary)

1. Epiglottis is depressed (movement is primarily due to the bolus pushing on it)

2. Laryngeal inlet is closed to avoid bolus entering the respiratory system (Breathing is temporarily inhibited)

3. Pharyngeal muscles contract to move the bolus inferiorly (so food isn't caught at the back of throat)

4. The upper esophageal sphincter relaxes and opens to allow passage of bolus

What stimulates the Pharyngeal Phase of Deglutition? What is the response

Pharynx stretches due to presence of the bolus from the oral phase. Baroreceptors detect the stretch and signal the sphincter to relax and constrictors to contact to allow safe passage of bolus of food

Steps of the Oesophageal Phase of Deglutition

Esophageal phase - in transit (involuntary)

• Esophagus (smooth muscle) undergoes peristalsis to move food inferiorly to the stomach

• Lower esophageal sphincter relaxes and opens to allow entry of bolus into the stomach

What forms the lower esophageal sphincter?

the esophageal hiatus in the muscular part of the diaphragm

Innervation of Pharyngeal Muscles

Innervation to these muscles is via the pharyngeal plexus (parasympathetic).

Deglutition reflex - Voluntary Phase

• Stimulus → cerebral cortex → brain stem → muscles

  • Bolus in mouth, reflex is not stimulated

Deglutition reflex - Involuntary Phase

• Stimulus → brain stem → muscles

• Bolus enters pharynx, reflex is stimulated

Deglutition reflex

1. Stimulus — Distension (activates stretch receptors)

2. Afferent signal to brainstem via CN IX

3. Efferent signal to pharynx via pharyngeal plexus (mainly CN X, also CN IX and XI)

   • pharyngeal muscles contract

   • upper esophageal sphincter relaxes

What is peristalsis?

Peristalsis is a contraction of smooth muscle that propels food unidirectionally through the GIT.

Steps of Peristalsis

1. Initial State — Bolus is formed in the mouth and enters the GIT

2. ext. circular muscles behind the bolus contract

3. int. longitudinal muscles ahead of the bolus contract

4. A wave of contraction in circular muscular layer forces the bolus forward

What do secretions control?

salivation

What does motility control?

deglutition/ motility

Overall explanation of exocrine secretion

• Glands contain secretory cells.

• These borrow contents of ECF, including plasma and then modify the fluid with their unique constituents and send it through the ducts.

• The Ducts modify the contents of secretion and then secrete their contents into the lumen of the GIT

Examples of secretion other than salivation

1. Stomach: gastric pits that secrete protein-digesting enzymes and HCl

2. Liver: bile ducts that secrete lipid-digesting bile, stored in the gallbladder

3. Pancreas: exocrine secretions include macronutrient-digesting enzymes and bicarbonate (pancreas = Car wash of GIT)

What is the major difference between salivary secretion and other GIT secretions?

Endocrine control

Overview of muscles role in motility

Can be smooth or skeletal muscles

• Input = Distension from the bolus of food stimulates stretch receptors

• Output = Muscles contract to propel contents

Overview of sphincters function in motility

• Without stimulus = tonically contracted = permanent tension to close them (sympathetic)

• With stimulus = relaxed and open

Overall control of motility

• Parasympathetic nervous system

• Stimulus: the presence of food in the mouth

• Output: contraction of the constrictor muscles.

How does motility operate in other parts of the GIT?

1. Stomach: propulsion includes peristalsis and churning (multidirectional)

2. Intestines: propulsion includes peristalsis and segmentation (bidirectional)

3. Sphincters: pyloric (smooth muscle), anal (smooth and skeletal muscle)

What are the major differences between pharyngeal motility and other GIT motility?

• muscle type (smooth muscle)

• intrinsic control (enteric nervous system).

Overview