GASTROINTESTINAL TRACT PHYSIOLOGY - PowerPoint Presentation

Slide1

GASTROINTESTINAL TRACT PHYSIOLOGY

(PHG 222)

by

ADEJARE, A. A.

Department of Physiology

Faculty of Basic Medical Sciences

College of Medicine

University of Lagos

Slide2

OUTLINE

General organization/functional anatomy of the GIT

Review of smooth muscle function

GIT motility

GIT secretions and hormones

Digestion and absorption of food substances

Liver and its functions

Nutrition and metabolism

Slide3

General organization/functional anatomy of the GIT

GIT is a

tube that stretches from the mouth to the anus

Function:

serve as a portal for nutrients and water absorption into the body.

GIT: regulatory mechanisms act locally to coordinate the function of the gut

Slide4

Alimentary tract

Slide5

Physiologic anatomy of the wall

cross section of the intestinal wall:

(1) the

serosa

,

(2) a

longitudinal muscle

layer

,

(3) a

circular muscle layer

,

(4) the

submucosa

,

and

(5) the

mucosa

Slide6

Typical cross section of the gut

Slide7

Slide8

Review of smooth muscle function

Smooth muscle

fibres

are:

Length 200-500

μ

m, diameter 2-10

μ

m

connected with one another through

gap junctions

that allow low-resistance movement

of ions from one muscle cell to the next.

Seperated

by loose connective tissues

each muscle layer functions as a

syncytium

;

Slide9

Types of smooth muscle

Multi-unit smooth muscle:

ciliary

muscle, iris muscle,

the

piloerector

muscles.

Unitary or single-unit smooth muscle: contract together as a single unit.

visceral smooth muscle

Slide10

GIT smooth muscle

There are two types of electrical waves.

slow waves

and

Spikes

Slow Waves.

Responsible for rhythmical contractions

not action potentials

slow, undulating changes in the resting membrane potential. 

Their intensity varies between 5 -15mv

their frequency: 3 to 12

per minute

:

Slide11

Slow Waves

RMP: -65 to -45 mV

caused by the

interstitial cells of

Cajal

,

electrical pacemakers

for smooth muscle cells.

Stomach & small intestine: located in the

outer circular muscle layer

Colon:

submucosal

border of the circular muscle layer

The interstitial cells of

Cajal

undergo cyclic changes in membrane potential due to unique ion channels that periodically open and produce inward (pacemaker) currents that may generate slow wave activity.

The slow waves

mainly excite the appearance of intermittent spike potentials,

and the spike potentials in turn actually excite the muscle contraction.

The function of the BER is to

coordinate peristaltic and other motor activity

Slide12

Slow waves and spikes

Slide13

Spike Potentials

Spike Potentials.

are

true action potentials.

They occur when the RMP becomes more positive than about -40 millivolts

The higher the slow wave potential rises, the greater the frequency of the spike potentials,

between 1 and 10 spikes per second.

The spike potentials last longer

The AP is caused by calcium and sodium ions moving thro

calcium-sodium channels

. K+ for repolarization

These channels are much

slower to open and close

than are the rapid sodium channels of large nerve fibers.

accounts for the

long duration of the AP

.

causes the intestinal muscle fibers to

contract

Slide14

Factors that depolarize the membrane

(1)

stretching

of the muscle,

(2) stimulation by

ach

(3) stimulation by

parasympathetic nerves

and

(4) stimulation by

GI

hormones

.

Important factors that hyperpolarize the membrane

and make the muscle fibers less excitable—are

(1) the effect of

norepinephrine

or

epinephrine

(2) stimulation of the sympathetic nerves

Slide15

Tonic Contraction of Some Gastrointestinal Smooth Muscle.

Tonic contraction is caused by

continuous repetitive spike potentials—the greater the frequency, the greater the degree of contraction.

hormones

continuous entry of

Ca2+

into the cell

Slide16

GIT MOTILITY

Ingestion of food is determined by

Hunger: intrinsic desire for food

Appetite: desire for a particular type of food

Mechanics of ingestion

1. mastication (chewing)

2. swallowing

Slide17

Mastication

Chewing

breaks up large food particles and mixes the food with the secretions of the salivary glands.

chewing process is caused by a

chewing reflex

,

The

presence of a bolus of food

in the mouth

first initiates

reflex inhibition

of the muscles of mastication: lower jaw drops.

initiates a

stretch reflex

of the jaw muscles that leads to

rebound

contraction.

This automatically raises the jaw to cause

closure of the teeth

and

compresses the bolus again against the linings of the mouth,

which inhibits the jaw muscles once

again

,

allowing the jaw to drop and rebound another time

Slide18

I

mportance

Chewing is important

for

breaking the food

.

chewing aids the digestion increases

surface area

exposed to the digestive secretions.

prevents excoriation of the

GIT

and

increases the ease with which food is

emptied from the stomach into the small intestine

Slide19

Regulation

Most of the muscles of chewing are innervated by the motor branch of the

fifth cranial nerve, and the chewing process is controlled by

nuclei in the brain stem

:

Stimulation of specific reticular areas in the brain stem taste centers will

cause rhythmical chewing movements.

stimulation of areas in the

hypothalamus

,

amygdala

the cerebral cortex

Slide20

Deglutition

Swallowing can be divided into

a

voluntary stage

, which initiates the swallowing process;

a

pharyngeal stage

, constitutes

passage of food through the pharynx into the

esophagus; and

an

esophageal stage

, transports food from the pharynx to

the stomach.

Slide21

Voluntary Stage of Swallowing.

When the food is ready for swallowing, it is “voluntarily” squeezed or rolled posteriorly

into the pharynx by pressure of the tongue

upward and backward against the palate

Slide22

Swallowing mechanism

Slide23

Pharyngeal Stage of Swallowing.

As the food enters the pharynx, it stimulates

epithelial swallowing receptor areas

and impulses from these pass to the brain stem to initiate a series of automatic pharyngeal muscle contractions as follows:

1. The soft palate is pulled upward to

close the posterior nares

2. The

palatopharyngeal

folds approximate each other.

3. The vocal cords

a

re strongly approximated.

These cause

epiglottis to swing backward over the opening of the larynx

: to prevent passage of food into the

trachea

.

Slide24

Stages of deglutition

Slide25

4. The upward movement of the larynx also

enlarges the opening to the esophagus,

upper esophageal sphincter

relaxes

.

5. The pharynx contracts,

a fast peristaltic wave initiated by the nervous system of the pharynx forces the bolus of food into the upper esophagus which propels the food by peristalsis into the esophagus.

Slide26

Slide27

Reflex

inhibition:

The swallowing center specifically

inhibits the respiratory center of the medulla during

this time, halting respiration at any point in its cycle to

allow swallowing to proceed.

Slide28

Esophageal Stage of Swallowing.

Esophagus

functions to conduct food from the pharynx to the stomach,

exhibits two types of peristaltic movements:

Primary peristalsis

:

continuation of the peristaltic wave that begins in the pharynx 8 to 10 seconds.

S

econdary

peristaltis

:

results from distention of the esophagus itself by the retained food.  

Slide29

Peristalsis

Slide30

Regulation

The musculature of the

pharyngeal wall and upper third of the esophagus

is controlled by

skeletal nerve impulses from the glossopharyngeal and

vagus

nerves

.

In the lower two thirds of the esophagus, the musculature is

smooth muscle

, but this portion of

the esophagus is also strongly controlled by the

vagus

nerves

acting through connections with the esophageal

myenteric

nervous system.

Slide31

Function of the Lower Esophageal Sphincter

remains

tonically

constricted

“

receptive relaxation

” for easy propulsion of the swallowed food

into the stomach.

prevent significant reflux of stomach contents into the esophagus

except under very abnormal conditions.

Additional Prevention of Esophageal Reflux by

Valvelike

Closure of the Distal End of the Esophagus.

Slide32

LES

Slide33

APPLIED PHYSIOLOGY

1. Achalasia: failure of the esophagus to relax

due to

increased resting LES tone

and incomplete relaxation on

swallowing

Treatment:

pneumatic dilation of the

sphincter

incision

of the esophageal muscle (

myotomy

)

Use of

botulinum

toxin to inhibit further Ach release

Slide34

2.

Gastroesophageal

reflux

disease (GERD): due to LES incompetence

There is

reflux of acid gastric contents into the esophagus

effects: heartburn, esophagitis, ulceration

Treatment: omeprazole, surgery

Slide35

Gastric motility

Anatomical division of the stomach

Anatomically, the stomach is usually divided

into two major parts:

(1) the

body

and

(2) the

antrum

.

Physiologically, it is more appropriately divided into

(1) the “

orad

” portion: the first two

thirds of the body, and

(2) the “

caudad

”

portion:the remainder of the body plus the antrum.

Slide36

Physiologic anatomy

Slide37

Motor Functions of the Stomach

threefold:

storage

of large quantities of food

mixing

of this food with gastric secretions until it forms a semifluid mixture called

chyme

; and

slow emptying

of the

chyme

from the stomach into the small intestine

Slide38

Storage Function of the Stomach

Food is

temporarily stored

in the stomach

ff

receptive relaxation

a “

vagovagal

reflex

” from the stomach to the brain

stem and then back to the stomach

reduces the tone

in the muscular wall of the body of the stomach (0.8 to 1.5 liters)

Slide39

Mixing function of the stomach

As long as food is in the stomach, weak peristaltic

constrictor waves

, called

mixing waves

, begin in the

mid- to upper portions

of the stomach wall and move toward the

antrum

about once every 15 to 20 seconds.

These waves are initiated by the gut wall

basic electrical rhythm

, consisting of electrical “slow waves” that occur spontaneously in the stomach wall.

Slide40

Stomach emptying- 3

rd

fxn

promoted by intense peristaltic ring-like

contractions in the stomach

antrum

50-70 cmH

2

O pressure

each strong

peristaltic wave

forces up to several milliliters of

chyme

into the

duodenum

“pyloric pump.”

The pyloric sphinter regulates flow into the duodenum

Slide41

Regulation of stomach emptying

Gastric Factors That Promote Emptying

Increased

food volume

in the stomach promotes increased emptying

from the stomach.

Increased production of

gastrin from

antral

mucosa

caused by

stomach wall stretch and the presence

digestive products of meat

Fat> protein>CHO

Slide42

Regulation (

contd

)

Duodenal Factors

1.

Enterogastric

Inhibitory Reflexes: inhibit pyloric pump, increase tone of pyloric sphincter

(1) directly from the duodenum to the stomach through the enteric nervous system in the gut wall,

(2) through extrinsic nerves that go to the

prevertebral

sympathetic ganglia and then back through inhibitory sympathetic nerve fibers to the stomach, and

(3) probably to a slight extent through the

vagus

nerves

Slide43

The

enterogastric

inhibitory reflexes are stimulated by

1. distention of the duodenum

2. presence of irritation of the duodenal mucosa

3. acidity of the duodenal

chyme

(< 3.5 - 4)

4. osmolality of the

chyme

5. presence of breakdown products of proteins and fats

Slide44

Duodenal Factors That Inhibit Stomach Emptying

2. Hormonal feedback from the duodenum: stimulated by

presence of fat

cholecystokinin (CCK) from jejunum mucosa

Secretin from duodenal mucosa

Gastric inhibitory peptide (GIP) from duodenum and jejunum

Slide45

APPLIED PHYSIO

Vomiting:

starts

with salivation and the sensation of

nausea

Reverse peristalsis empties material from the upper part of the small intestine into the stomach.

The

glottis closes, preventing aspiration of vomitus into the trachea.

The

breath is held in mid inspiration.

The

muscles of the abdominal wall contract, and because the chest is held in a fixed position, the contraction increases intra-abdominal pressure.

The

lower esophageal sphincter and the esophagus relax, and the gastric contents are ejected.

Slide46

Vomiting pathways

Slide47

Intestinal motility

can be divided into

mixing contractions

and

Propulsive contractions

.

mixing

contractions

:

stretching of the intestinal wall

elicits localized concentric contractions.

The contractions cause “

segmentation

” of the small intestine

Slide48

Contractions in the small intestine

Slide49

The maximum frequency of the

segmentation contractions

in the small intestine is determined by the

frequency of

electrical slow waves

in the intestinal wall

12 contractions/minute in the duodenum/jejunum

8-9

contractions/minute

in

the ileum

Slide50

Propulsive contractions

1. peristaltic waves from pylorus to

ileocaecal

valve (3-5 hours, peristaltic rush in

diarrhoea

/ protective):

Control:

Presence of meal

Gastroenteric

reflex

Gastrin

CCK increase

Insulin

Motilin

Serotonin

Secretin

Glucagon decrease

Slide51

On reaching the

ileocecal

valve, the

chyme

is sometimes blocked for several

hours until the person eats another meal; at that time,

a

gastroileal

reflex intensifies peristalsis in the ileum

and forces the remaining

chyme

through the

ileocecal

valve into the cecum of the large intestine

Slide52

Ileocecal

valve

Slide53

Function of

ileocecal

valve

Prevent

backflow of fecal contents from the colon into the small intestine

Has a thickened part,

ileocecal

sphincter which

slows emptying of

ileal

contents into

the cecum

Control

:

distension of cecum increases contraction,

irritation in the cecum delays emptying from ileum

Slide54

Colonic motility

The

principal functions

of the colon are

(1) absorption of water and electrolytes from the

chyme

to form solid feces and

(2) storage of fecal matter until it can be expelled.

Slide55

Colonic motility

mixing movements

There is combined

contractions of the circular and longitudinal strips of

muscle -

haustrations

.

Haustrations

move slowly to the anus during contraction

propulsive

movements

(8-15hrs)

Also called mass movements

Results from

haustral

contractions

15 minutes after taking breakfast

Slide56

Regulation of mass movement

Gastrocolic

reflex

Duodenocolic

reflex. Both under ANS control

Irritation in the colon

eg

in ulcerative colitis

Applied

Physio

Constipation:

refers to a pathological decrease in bowel

movements

Symptoms:

slight

anorexia

mild

abdominal discomfort

distention

Slide57

Defecation

Occurs

w

hen a mass movement forces

feces into the rectum

There is reflex contraction of the rectum and

Relaxation of the anal sphincters

Continual dribble of fecal matter through the anus is prevented by tonic constriction of

(1) an

internal anal sphincter

of smooth muscle and

(2) an

external anal sphincter

, composed of striated voluntary muscle that both surrounds the internal sphincter and extends distal to it.

Slide58

Control

The external sphincter is controlled by the

pudendal

nerve

, which is part of the somatic nervous system and therefore is under

voluntary

,

conscious control

; subconsciously, the external sphincter is usually kept

continuously constricted

unless conscious signals inhibit the constriction.

Slide59

Defecation Reflexes (intrinsic reflex)

Slide60

This process is usually fortified by a parasympathetic defecation reflex

1

2

3

4

4

4

4

Slide61

Parasympathetic defecation reflex

Slide62

Distention of the stomach by food initiates contractions of the rectum and, frequently, a desire to defecate. The response is called the

gastrocolic

reflex

.

Because

of the response, defecation after meals is the rule in children. In adults, habit and cultural factors play a large role in determining when defecation occurs.