GI Physiology II: GI Physiology II: Cephalic Phase of Cephalic Phase of DigestionDigestionIDP/DPT GI course, Fall 2011IDP/DPT GI course, Fall 2011

Jerome W. Breslin, PhD, Dept. of Jerome W. Breslin, PhD, Dept. of PhysiologyPhysiology

jbresl@lsuhsc.edu, 504-568-2669, 504-568-2669

MEB 7208MEB 7208

Lecture II OutlineLecture II OutlineCephalic Phase of DigestionCephalic Phase of Digestion

Initial Stimuli and ResponsesInitial Stimuli and Responses

Salivary SecretionsSalivary Secretions

Chewing and SwallowingChewing and Swallowing

Esophageal MotilityEsophageal Motility

Esophageal Sensation and PainEsophageal Sensation and Pain

Lower Esophageal Sphincter OpeningLower Esophageal Sphincter Opening

Initiation of Gastric SecretionsInitiation of Gastric Secretions

Required Reading:Required Reading:

• Barrett, Barrett, Gastrointestinal PhysiologyGastrointestinal Physiology

• Chapter 4 - section on salivary Chapter 4 - section on salivary secretionsecretion

• Chapter 7Chapter 7

Phases of Phases of DigestionDigestionCephalicCephalic

GastricGastric

Early IntestinalEarly Intestinal

Late IntestinalLate Intestinal

Cephalic PhaseStimuli:Auditory (Pavlov’s Dinner Bell), Cognitive, Visual, Olfactory, Taste

Higher Brain Centers

Dorsal Vagal Complex

Increased Parasympathetic

Signaling(Vagus Nerve)

Effector Response:Saliva SecretionGastric SecretionPancreatic Juice SecretionMild Contraction of the Gallbladder, however theSphincter of Oddi remains closed

Gustation and Gustation and OlfactionOlfactionPossible to distinguish up to Possible to distinguish up to 10,000 chemicals by taste (aided 10,000 chemicals by taste (aided by sense of smell).by sense of smell).

Five types of taste qualities: Five types of taste qualities: BitterBitter

SaltSalt

SweetSweet

SourSour

Umami (savoriness - detection of glutamate Umami (savoriness - detection of glutamate -- why MSG makes ordinary food taste good!)why MSG makes ordinary food taste good!)

Taste Transduction:Taste Transduction:Taste receptors usually can detect 2 Taste receptors usually can detect 2 or more basic tastes, and some can or more basic tastes, and some can detect all. There are variations based detect all. There are variations based on the cells’ sensitivity to different on the cells’ sensitivity to different compounds.compounds.

Smell is an important component (e.g. Smell is an important component (e.g. sensing the “nose” of a wine in wine sensing the “nose” of a wine in wine tasting).tasting).

““Good palate” is a detailed Good palate” is a detailed understanding of what things should understanding of what things should taste like and identify foods, taste like and identify foods, flavors, or ingredients by taste (and flavors, or ingredients by taste (and smell) alone.smell) alone.

Cephalic Phase of Cephalic Phase of Digestion:Digestion:• STAGES:

• Biting and chewing (mastication)• Stimulation of olfactory epithelia• Stimulation of gustatory receptors• Salivary secretions occur

• Food mixes with saliva• Softens and lubricates• Facilitates swallowing• Reduces food particle size

• (Gastric & pancreatic secretions)• Bolus Formation• Swallowing• Initiation of esophageal peristalsis• Relaxation of LES

Mastication Mastication (Chewing)(Chewing)

Incisors exert 30-80 pounds of pressure.Incisors exert 30-80 pounds of pressure.

Molars exert 100-160 pounds of pressure.Molars exert 100-160 pounds of pressure.

Mixing with saliva in the mouth:Mixing with saliva in the mouth:

Saliva acts as a lubricating agent.Saliva acts as a lubricating agent.

Saliva contains Saliva contains amylaseamylase for digestion of for digestion of carbohydrates, and may contain carbohydrates, and may contain lingual lipaselingual lipase for digestion of fats.for digestion of fats.

Convert chunks of food into a bolus for Convert chunks of food into a bolus for swallowing.swallowing.

Salivary Salivary Gland Gland

Structure:Structure:3 pairs of 3 pairs of large exocrine large exocrine glands:glands:1. Parotid1. Parotid2. Sublingual2. Sublingual3. Submaxillary 3. Submaxillary

Innervated by Innervated by the ANSthe ANSSecretes saliva into the buccal Secretes saliva into the buccal cavity in response to stimulation cavity in response to stimulation from the CNS.from the CNS.

Berne & Levy, Fig 32-1Berne & Levy, Fig 32-1

Regulation of Saliva Regulation of Saliva SecretionSecretionControlled by nerve impulses, Controlled by nerve impulses, originating from the salivary center originating from the salivary center of the medulla.of the medulla.

Most sensory afferents to the medulla Most sensory afferents to the medulla salivary center come from the mouth.salivary center come from the mouth.

Can be activated from GI tract or Can be activated from GI tract or eyes as a voluntary reflex.eyes as a voluntary reflex.

Both parasympathetic and sympathetic Both parasympathetic and sympathetic NS are activeNS are active

Parasympathetic > SympatheticParasympathetic > Sympathetic

Constituent Functions

Water Facilitates taste and dissolution of nutrients; aids in swallowing and speech

Bicarbonate Neutralizes refluxed gastric acid

Mucins Lubrication

Amylase Starch digestions

Lysozyme, Lactoferrin, IgA Innate and acquired immune protection

Epidermal and Nerve Growth Factors Assumed to contribute to mucosal growth and protection

Table 4-2 in Barrett: Constituents of saliva and their functions.

MAJOR ORGANIC COMPONENTS OF SALIVAComponent Region Gland Function

Proline-rich proteins Acinus P, SM LubricationAntimicrobialEnamel formation

Mucins Acinus SL, SM LubricationAntimicrobialProtease protective

Enzymes α−Amylase Lipase Ribonuclease Kallikrein

AcinusAcinus

DuctDuct

P, SMSLSMAll

Starch DigestionFat DigestionRNA digestion?

Other Lactoperoxidase Lactoferrin Lysozyme sec IgA Growth Factors

AcinusAcinus

DuctDuctDuct

SM?

SM?

SM`

AntimicrobialAntimicrobualAntimicrobialAntimicrobial?

Ionic Composition of Saliva:Ionic Composition of Saliva:

1. In humans, saliva is always 1. In humans, saliva is always hypotonic to plasma.hypotonic to plasma.2. Tonicity increases with flow rate.2. Tonicity increases with flow rate.3. Maximum tonicity is about 70% of 3. Maximum tonicity is about 70% of plasma.plasma.

Berne & Berne & Levy Fig. Levy Fig.

32-332-3

Similar to Similar to Fig. 4-9 Fig. 4-9

in Barrettin Barrett

Two stage model of saliva secretion:Two stage model of saliva secretion:

Berne & Berne & Levy Fig. Levy Fig.

32-432-4

Control of Salivary Secretion: Control of Salivary Secretion: Cellular Cellular MechanismsMechanisms

Berne & Berne & Levy, Fig. Levy, Fig.

32-732-7

Swallowing Swallowing (Deglutition)(Deglutition)Process by which material is Process by which material is transported from the mouth transported from the mouth through the pharynx and through the pharynx and esophagous to the stomach.esophagous to the stomach.

Stages based on region where Stages based on region where bolus passes:bolus passes:

• 1. Oral (Mouth)1. Oral (Mouth)

• 2. Pharyngeal (Pharynx)2. Pharyngeal (Pharynx)

• 3. Esophageal (Esophagous)3. Esophageal (Esophagous)

A normal swallow A normal swallow consists of three consists of three integrated phases:integrated phases:

PhasePhase MuscleMuscle Neural Neural ControlControl

Voluntary Voluntary ControlControl

OralOral StriatedStriated Cortex/Cortex/MedullaMedulla FullFull

PharyngealPharyngeal StriatedStriated MedullaMedulla SomeSome

EsophagealEsophageal Striated/Striated/SmoothSmooth Medulla/ENSMedulla/ENS NoneNone

The swallowing reflex is coordinated by the medulla oblongata, which stimulates the appropriate sequence of contraction and relaxation in the participating skeletal muscle, sphincters, and smooth muscle groups.

Figure 15-14

Swallowing

Timeline of events involved in swallowing:Timeline of events involved in swallowing:

Opening of the upper esophageal Opening of the upper esophageal sphincter is dependent upon sphincter is dependent upon multiple factors:multiple factors:

FactorFactor EffectEffect

Hyoid & Laryngeal Hyoid & Laryngeal ElevationElevation Pulls relaxed UES openPulls relaxed UES open

Cricopharyngeal Cricopharyngeal RelaxationRelaxation Allows opening to occurAllows opening to occur

Cricopharyngeal Cricopharyngeal ComplianceCompliance

Controls degree of Controls degree of openingopening

Pharyngeal Pressure WavePharyngeal Pressure Wave Pushes bolus through Pushes bolus through relaxed sphincterrelaxed sphincter

Note: Cricopharyngeal Relaxation is not synonymous with Note: Cricopharyngeal Relaxation is not synonymous with UES opening.UES opening.

Dysphagia:Dysphagia:Difficulty in Difficulty in SwallowingSwallowingCan result from anatomical Can result from anatomical abnormalities that alter the abnormalities that alter the mechanics of swallowing.mechanics of swallowing.

Can also arise secondary to Can also arise secondary to neurological problems (e.g. neurological problems (e.g. patients who have previously patients who have previously suffered a stroke)suffered a stroke)

The coordinated sequence of contraction and relaxation in the upper esophageal sphincter, the esophagus, and the lower esophageal sphincter is necessary to deliver swallowed food to the stomach.

Figure 15-15

1. Peristalsis of bolus to the stomach

2. Opening of LES

3. Receptive relaxation in stomach

Figure 15-15

Striated Muscle

Mixed Smooth and Striated Muscle

Smooth Muscle

Figure 15-15

Atmospheric Pressure

(0 mm Hg)

UES: 100 mm Hg

Intraesophageal Pressure: -5 mm Hg

LES: 20 mm Hg

Sphincters prevent influx of air into the stomach, as well as reflux of gastric contents into the esophagus.

Coordinated, directional contraction of smooth muscle propels ingested food forward (Peristalsis)

ESOPHAGEAL PERISTALSIS

SWALLOWING DISTENSION

PERISTALSISPERISTALSIS

PRIMARY SECONDARY

Distension in the esophagous can Distension in the esophagous can initiate peristalsis through initiate peristalsis through both local and vago-vagal both local and vago-vagal reflexes:reflexes:

Secondary peristaltic waves Secondary peristaltic waves sometimes occur to clear any sometimes occur to clear any

remnants of the bolus.remnants of the bolus.

Regulation of Esophageal PeristalsisRegulation of Esophageal Peristalsis

Esophageal SensationEsophageal Sensation

Esophageal StimuliEsophageal Stimuli

StimulusStimulus ReceptorReceptor ResponseResponse

AcidAcid ChemoreceptorChemoreceptorFeedback Feedback ControlControlPain?Pain?

TemperatureTemperature ThermoreceptorThermoreceptor Non-Painful Non-Painful SensationSensation

DistensionDistension MechanoreceptorMechanoreceptor Burning; PainBurning; Pain

Lee & Mitter, Lee & Mitter, GI MotilityGI Motility, 2006, 2006

Burning sensation in the Burning sensation in the esophagous in the absence of esophagous in the absence of acid:acid:

COMMON FACTORS THAT ALTER LES PRESSURE

• Protein

• Increased intra-abdominal pressure

• Elevated gastric pH

• Fat• Chocolate• Peppermint• Alcohol• Smoking• Gastric distension

(aerophagia)

INCREASE DECREASE

Three Phases of Gastric SecretionSlide number

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

1. The taste or smell of food, tactile sensations of food in the mouth, or even thoughts of food stimulate the medulla oblongata (green arrow).

2. Parasympathetic action potentials are carried by the vagus nerves to the stomach (pink arrow).

3. Preganglionic parasympathetic vagus nerve fibers stimulate postganglionic neurons in the enteric plexus of the stomach.

4. Postganglionic neurons stimulate secretion by parietal and chief cells and stimulate gastrin secretion by endocrine cells.

5. Gastrin is carried through the circulation back to the stomach (purple arrow), where it stimulates secretion by parietal and chief cells.

Cephalic Phase

Gastrin

Circulation

Secretionsstimulated

Taste or smell of foodTactile sensation in mouth

Medulla oblongata

Vagus nerves

Stomach

1

2 3

5

4