Gastrointestinal physiology.General principles.

Dr.M.A.M.Shaikhani.Assistant professor

Consultant physicianCABM

Arabic board in internal medicine Subspecialty in gastroenterology & endoscopy

The main functions of GIT:

• Motility by neurohormones

• Secretion

• Digestion

• Absorption

Characteristics of GIT wall:

5 layers from outside towards the lumen:

1.Serosa.

2.Longitudinal muscle layer.

3.Circular muscle layer.

4.Submucosa.

5.Mucosa with muscularis

mucosal in its deeper

layer.

Meisner

Aurbach

Circular layer

Longitudinal layer

Meisner

Aurbach

Neural activity in one plexus can stimulate/inhibit activityin the other plexus.In general: the submucosal plexus influences secretory activity the myenteric plexus influences smooth muscle activity

 GIT longitudinal & circular muscle layers Smooth muscles, form a syncyctium of 2 types: 1.Longitudinal extends down the whole length of intestinal tract 2.Circular extends around the gut .Connection between the longitudinal muscle fibers themselves &few other connections between the longitudinal & circular onesSo the electrical signals can travel readily from one fiber to the next depending on the their exitability traveling few mms or many cms or along the entire length of GIT exitation of the long. Layer exits the circular one.

 Electrical activity of GIT smooth muscles:

2 types :1.Slow waves: not the action potential but slow undulating

changes in the resting membrane action potential(-50 to –60 mv) in a frequency of 3-12/min.

It isa slow undulation in the activity of Na-K pump. involve only Na+ ions.

Do not directly cause muscle contraction but they control the appearance of intermittent real contractions spikes

 2.Spike potential: true action potential of –40 mv of 1-10

spike /sec.lasting for 19-20 m.secs. caused by Ca++- Na+ Channels which are more slower to

open / close & Ca++ ions stemulate smooth muscles contraction by calmodulen control mechanism causing attraction between actin & myosin muscle filaments.

Excitable by:A.stretching. B.ACH.C.parasympathetics.D.specific GIT hormones.They become less excitable by:A.Norepinephrine & epinephrineB. Sympathetics.

-70

-60

-50

-40

-30

-20

Slow wave

Resting

Spike action potentialDepolarizationStretchAchParasympathetic.

Hyperpolarization:NE, Sympathetics

 Tonic contractions are continuous lasting( mins. - several hs. ), repetitive series of

spike potential produced by continuous entry of Ca++ ions to the interior of the cell ,facilitated by some hormones or other factors.

 Nervous control of GIT movements.Controlled by the (enteric nervous system) lying in the gut

wall consisting of > 100 million neurons connecting the gut with the the CNS explaining the major effect of emotions & psychology on the gut movements & functions.

It consists of 2 plexuses:

1.Myenteric or Aurbach or outer plexus Lie between longitudinal & circular muscle fibers extends along the entire length of the GIT mainly excitatory causes: tonic contractions,intensifies

rhythmical contrsctions,increases rate of contraction , velocity of contractions & peristaltic waves.

It is not entirely excitatory as some of its neurons are inhibitory secreting inhibitory neurotransmitors(NTMS) as vasoactive intestinal peptides (VIP) inhibiting some intestinal sphincters as pyeloric,GE& IC.

2.Miesner or submucosal or inner plexus lies in submucosa ,only within the inner wall of each minute segment of

intestine ,Controls local intestinal contraction by submucosal muscles , local intestinal secretion, absorbtion & local blood flow.

There are nervous connections within & between both plexuses & with the autonomic nervous system(sympathetic & parasympathetic nerve fibers).The enteric NS can function independently or in cooperation with AUT NS.

         NTMS secreted by enteric NS:Acetylcholine(ACH),norepinephrine,ATP,serotonin,dopamine,substance

P,VIP,Somatostatin,leu-enkephalin,met-enkephalin & bombesin .ACH most often is excitatory while noreepinephrine is most often

inhibitory & the others are mixture of both.

Autonomic control of GIT:Of 2 types:1.Parasympathetics: Of 2 types:A.Cranial:through the vagus to the oeso,stomach,pancreas & the 1st

half of the large intestine with little T o SI.B.Sacral from the 2,3& 4th sacral segments of SC passing to the

pelvic nerves & to the distal half of the large intestine serving defecation reflexes.

2.Sympathetic innervations: From T5-L2 segments to the celiac ganglia & various mesenteric ganglia .The postganglionic neurons spread along the blood vessels to all parts of the gut & connect with the enteric NS secreting noreepinephrine which is inhibitory.

Sympathetic ParaSympathetic

Celiac ganglia

Sup mesenteric ganglia

Inf mesenteric ganglia

vagus

medulla

Pelvic nerves

        Functional types of GIT movements: Two types;1.propulsive peristaltic movements: is a contractile ring appear around the gut & moves forward the

intestinal contents in anal direction called (law of the gut).The stimulus is distention of the gut or irritation of the gut wall & extrinsic nerve signals particularly parasympathetic nerves .An effective peristaltic movement needs active myenteric plexus.

2.Mixing movements:this occurs when forward peristaltic movement is opposed by a sphincteric contraction so the peristaltic wave compresses the gut contents rather than pushing it forward causing mixing the food as in stomach when pyeloric sphincter is blocked.

Two basic types of motility are involved:peristalsis: propels contents through the GIT.

Segmentation: mixes contents.

–Segmentation: mixes contents.

Intestinal motility

Peristalsis:Slow

Segmentation:Major Contraction of circular smooth muscle.Mixes chyme.

        GIT blood flow: Is part of more extensive blood system ( splanlic circulation ) include the

blood supply to the GIT,Spleen,liver & pancrease .The blood passes to the liver to remove the bacteria & the other harmfull substances to prevent its passage to the general systemic circulation.The gut blood supply is through superior,inferior &celiac plexuses supplying small,large & stomach respectively.

The intestinal villi are small projections from small intestine to increase the absorptive surface area & its microvasculature including arterioles,venioles & central lymohatic lacteals are arranged in a way to form a countercurrent for the optimum effective absorption of nutrients.

 

       Countercurrent flow in villi:

• In villus, arterial flow into & venous flow out of villus are opposite to each other & in close apposition.

• Much O2 diffuses from arterioles into adjacent venules without reaching villus tip (short circuit) & may not be available for local metabolic function of villi.

• Under normal conditions this shunting is not harmful, but in disease conditions in which flow to gut is already compromised, such as circulatory shock, the O2 deficit can be great that the villus tip will become ischemic & disintegrate.

• This is reason why in many GI diseases villi are blunted & lead to greatly decreased GI absorptive capacity.