1. GASTROINTESTINAL PHYSIOLOGY

Size: px

Start display at page:

Download "1. GASTROINTESTINAL PHYSIOLOGY"

Curtis Riley
6 years ago
Views:

1 1. GASTROINTESTINAL PHYSIOLOGY FOOD INGESTION Ingested fdstuff cntributes t an increase in the verall weight f an animal, but it des nt becme an integral part f the structure r metablic activities f the animal, until it is absrbed smewhere thrugh the digestive tract. What is nt absrbed is discarded in the faeces as waste. The gastrintestinal tract prvides a cnduit thrugh which all fd has t pass. The first step in the use f such a cnduit is t acquire, chew and swallw the fd (Fig. 1-1). Prehensin Fd ingestin Masticatin / Chewing Deglutitin / Swallwing Figure 1-1. Stages f fd ingestin Prehensin Animals achieve this basic step thrugh the prcess f prehensin. Depending n the species and age, animals use different mdalities t ingest fd. There are specific anatmical adaptatins which permit an animal t ingest fd in a given manner (Fig. 1-2). Prehensin Mechanisms vary greatly with species Hrses and sheep use lips t manipulate and teeth t rip Cattle use tngue t manipulate, hard palate and lwer teeth t rip Carnivres use incisrs t cut and canines t rip Pultry uses beak r bill Figure 1-2. Mechanisms f prehensin On the website, vides 1-3 and 1-4 shw that bth hrses and sheep use their lips t manipulate the grass and their teeth t cut it. On the ther hand, cattle use their tngue t bring grass int their muth and the lwer teeth against the upper palate t cut the grass as shwn in Fig. 1-5 and vide 1-6). Prehensin by cattle Use the tngue t manipulate fd int the muth Use mbile lwer teeth and hard palate t press and tear Figure 1-5. Mechanisms f prehensin by cattle Avian species use their beak r bill t pick up fd. They d nt masticate the fd, but they d cver it with a mucus layer in rder t facilitate swallwing (Fig. 1-7). Masticatin Sme species require significant masticatin prir t swallwing, while thers, such as mst ruminants, masticate very little prir t swallwing. This strategy permits the cllectin f a significant vlume f fd in the rumen, and then, they start a prcess f ruminatin. Ruminatin cnsists f regurgitating prtins f the ingested fd t thrughly masticate it, t reduce the particle size. This activity is nrmally cnducted while the animal is resting in a recumbent psitin. Carnivres d nt significantly masticate their fd; instead, they depend n a very aggressive enzymatic digestin t reduce particle size. They nly masticate their fd enugh t reduce it t a swallwable size. Avian species d nt have teeth t masticate (Fig. 1-8). Masticatin Very imprtant in herbivres Nt imprtant in carnivres Nn existing in avian species Figure 1-8. Rle f masticatin in different species V BS 122 Physilgy II 1 Class f 2014

$They depend n the prductin f mucus t cat the particles prir t swallwing, and then, they macerate the fd within a specialized rgan (the gizzard) that may cntain a variety f hard materials,\which the$

2 They depend n the prductin f mucus t cat the particles prir t swallwing, and then, they macerate the fd within a specialized rgan (the gizzard) that may cntain a variety f hard materials,\which the animal has purpsely ingested. Cmmnly ingested pebbles help, thrugh a grinding actin, in the reductin f the particle size. Herbivres masticate their fd nt nly t reduce the size f the particles but als t break the cellular wall f the fd material, thus permitting the access f digestive enzymes. Smaller particles als reduce the incidence f damage t the walls f the gastrintestinal tract (GIT) (Fig. 1-9). Deglutitin Once the fd has been masticated, it has t be swallwed. This prcess requires an initial vluntary stage which is t mve the blus twards the back f the ral cavity. Then, an invluntary reflex, which permits the passage f the blus frm the ral cavity t the esphagus withut entering the trachea and the respiratry system, is activated (Fig. 1-10). Mst f the time, an animal maintains an pen passage between the nse r muth t the trachea, thus permitting Masticatin in herbivres Reduce particle size Increases surface area Breaks cellulse membrane Lubricate fr swallwing Mix with saliva Expse t digestive enzymes Reduce excriatin f GIT Figure Anatmical cmpnents invlved in deglutitin Figure 1-9. Rle f masticatin in herbivres Deglutitin One vluntary initial stage Mvement f blus t back f ral cavity A pharyngeal stage (invluntary) Passage frm ral cavity t esphagus Esphageal stage (invluntary) Passage f fd int stmach Figure Steps invlved in deglutitin Figure Sequence f anatmical changes taking place during respiratin the passage f air int the lungs and minimizing the entrance f air int the digestive system (Figs. 1-11, 1-12). When there is fd t be swallwed, a passage frm the muth t the esphagus has t be created t permit V BS 122 Physilgy II 2 Class f 2014

trachea and pens the passage twards the esphagus.

3 mvement f the fd. This passage is achieved by the elevatin f the sft palate, which prevents the fd frm entering int the nasal cavity, and by the mvement f the epiglttis backwards, which blcks the entrance t the trachea and pens the passage twards the esphagus. All these changes, which take place in a very rapid sequence are invluntary and cnstitute the prcess f DEGLUTITION THE DIGESTIVE TRACT Mst related species have similar anatmical cmpnents in the digestive tract, with slight differences in size, shape and smetimes regulatin f the functin f each cmpnent. Basic anatmy The gastrintestinal wall is made up f several layers. The utermst sersa is lined by a muscular layer which lies lngitudinally t the tract and then the innermst layer f muscle cnsist f circular fibres. Twards the center is fund the submucsa and the mucsal layers (Fig. 1-14). Within the mucsa, there are a series f smth muscle fibres, which frm the muscularis mucsa. The muscle fibres are gruped in individual parallel bundles, cnnected t thers by multiple gap junctins alng its length. Each bundle is surrunded by cnnective tissue with a fusin Smth muscle Figure Sequence f anatmical changes taking place during deglutitin deglutitin (Fig. 1-13) at which time the respiratry prcess is temprarily suspended. Arranged in bundles cntaining ~ 1000 fibers Fibers cnnected electrically by gap junctins Each bundle is separated by cnnective tissue but they cntact each ther at discrete pints Functin as a syncytium Electric impulse travels in all directins Sme cnnectins between lngitudinal and circular smth muscle layers Figure Characteristic f the smth muscle in the digestive system pint in between, frming a netwrk f muscles functining as a syncytium (Fig. 1-15). Figure Main cmpnents fund thrughut the digestive tract. The figure represents a transversal cut f the small intestine Physilgy f the GIT The syncytium permits that an actin ptential, which is generated at sme pint, can mve in all directins. Furthermre, there are cnnectins between the lngitudinal and circulatry layers which permit the V BS 122 Physilgy II 3 Class f 2014

4 synchrnizatin f mvements t either mix r prpel the intestinal cntent. T cntrl the tne and the mtr activity f the gut, the membrane f the smth muscle is cntinuusly stimulated by intrinsic electrical activity in the frm f slw waves r spikes. Variatin in the changes f frequency f the membrane ptential creates the rhythm f the slw waves (range frm 3-12/min). What ultimately changes t create the slw waves is the membrane resting ptential (Fig. 1-16). Electrical stimulatin Cntinus slw intrinsic activity Slw waves Spikes Slw waves (3-12/min) result by changes in resting membrane ptential (range 5-15 mv) (frm -50 t - 35 mv) Cntrls spike ptential that initiates cntractin Last msec Spike ptentials True actin ptential Triggered when mre psitive than - 40 mv The higher the slw waves rise, the mre frequent spike ptentials ccur (1-10 /sec) ++ Triggered by CaP P influx thrugh calcium-sdium channels Slwer than sdium channels f nerve fibres Figure Cnditins required t trigger a spike ptential Figure Characteristic f the electrical activity in the intestinal tissue The cunterparts f this prcess are the factrs that trigger a spike. The resting membrane ptential f the smth muscle has t be larger than -40 mv. These spikes trigger a true actin ptential, which in turn cntrls cntractile activity. The spike ptential increases in frequency when the slw wave resting membrane ptential is higher (Figs. 1-17, 1-18). The mre deplarized the membrane becmes, the mre excitable it is and the mre chances it has f cntractile activity. The mechanism fr achieving this higher membrane ptential is presumed t be thrugh an influx f CaP++P thrugh calcium-sdium channels as ppsed t the faster sdium channels used by nerve fibres. Several factrs cntribute t the deplarizatin f the fibre s membranes. Physical distensin (stretching) f the muscle, usually due t the presence f fd in the tract; acetylchline stimulatin as well as parasympathetic Figure Characteristics f different membrane ptentials stimulatin by neurnes secreting acetylchline; and finally, several gastrintestinal hrmnes (Fig. 1-19). The cunterparts f this prcess are the factrs that cntribute t the hyper-plarizatin f the membranes. These are the effects f nerepinephrine r epinephrine n the membranes and the sympathetic stimulatin whse nerves secrete nrepinephrine (Fig. 1-20). V BS 122 Physilgy II 4 Class f 2014

5 Factrs deplarizing membranes (membrane ptential less negative) Stretching Acetylchline Parasympathetic stimulatin Mediated thrugh acetylchline Specific gastrintestinal hrmnes Figure Factrs making membrane ptential less negative Factrs hyperplarizing membranes (membranes ptential mre negative) Nrepinephrine r epinephrine n fibre membrane Sympathetic stimulatin Secreting nrepinephrine Figure Factrs making membrane ptential mre negative Regulatin f gut mtility Enteric nervus system. Regulatin f all cntractile and secretry activity is supprted by an independent nervus system imbedded in the wall f the gut. This system is called the enteric nervus system and it extends the length f the GIT starting in the esphagus. The ttal number f neurns is as large as thse fund in the spinal crd. The enteric nervus system is made up f tw plexuses: ne lcated between the lngitudinal and circulatry muscle layer called the Myenteric r Auerbach s plexus and the ther is fund in the sub mucsal layer and is called the sub mucsal r Meissner s plexus (Figs. 1-14, 1-21, 1-22). These tw are intercnnected in rder t synchrnize GIT functins. Mtility f the gut is primarily cntrlled by the myenteric plexus, while the secretry activity and bld flw are cntrlled mainly by the submucsal plexus. Enteric nervus system Lies in the gut wall Extends frm the esphagus t anus Has as many neurns as the spinal crd Made f tw plexuses Myenteric r Auerbach s plexus Situated between lng. and circ. muscle layer Submucsal r Meissner s plexus Situated in the submucsal layer Figure Characteristics f the enteric nervus system The enteric system is capable f cntrlling gastrintestinal functins independent f ther bdy functins. Hwever, n tp f being cnnected between themselves, the tw plexuses are further innervated by the sympathetic and parasympathetic systems. This innervatin permits verall inhibitin r stimulatin f the gastrintestinal activities (Fig. 1-23). Enteric nervus system Myenteric plexus cntrls gastrintestinal mvement Submucsal plexus cntrls secretins and bld flw Innervated by sensry neurns frm the epithelium Cnnected t parasympathetic and sympathetic divisins Figure Rle f the enteric nervus system V BS 122 Physilgy II 5 Class f 2014

There are multiple mechanisms thrugh which the myenteric plexus cntrls mtr activity thrughut the gut (Fig. 1-24). and, as a result, there is an increase in peristaltic mvement.

6 There are multiple mechanisms thrugh which the myenteric plexus cntrls mtr activity thrughut the gut (Fig. 1-24). and, as a result, there is an increase in peristaltic mvement. Althugh mst f the activities f the myenteric system are excitatry in nature, there are als sme inhibitry elements. These elements are believed t be mediated thrugh the release f peptides; the mst plausible candidates are thse with vasactive intestinal plypeptide (VIP). The actin f these peptides appears t be fcused in inhibiting sphincter activity (pylric and ilececal), thus mdifying the rate f fd passage int the dudenum and cecum respectively. The submucsal plexus in turn cntrls secretry, absrptive and cntractile activity in very small discrete sectins f the GIT (Fig. 1-25). Submucsal plexus Figure Innervatins f the intestinal muscle by the enteric nervus system, sympathetic and parasympathetic Myenteric plexus Cntrls mtr activity alng the gut Increases tnic cntractin r tne f gut wall Increases intensity f rhythmical cntractins Increases rhythm Increases velcity, thus mre peristalsis Has sme inhibitry neurns Secrete VIP Cntrls sphincter activity Figure Rle f the myenteric plexus Cntrls the functin f small sectins f the intestine Signal frm epithelium t SMP cntrls Lcal secretin Lcal absrptin Lcal cntractin Regulate flding f gastrintestinal mucsa Figure Rle f the submucsal plexus The apprpriate respnse is usually prepared and implemented by a signal that is generated by sensrs in the epithelium f the gut which then is cnveyed t the submucsal. Neurtransmitters. A large number f neurtransmitters (Fig. 1-26) have been identified as participating in the regulatin f gastrintestinal activity, but their specific rles are yet t be determined. The myenteric plexus can increase the tnic cntractin f the intestinal wall as well as the intensity and the rate with which the rhythmical cntractins take place. Finally, stimulatin f the myenteric system can result in faster cnductin f excitatry waves alng the intestinal wall V BS 122 Physilgy II 6 Class f 2014

7 Acetylchline Nrepinephrine Neurtransmitters Mainly excites GI activity Usually inhibits GI activity Adensine triphsphate (ATP) Sertnin Vasactive intestinal plypeptide (VIP) Smatstatin (SS) Leu-enkephalin Met-enkephalin Bmbensin Figure Neurtransmitters invlved in regulatin f gastrintestinal activity Gastrintestinal reflexes. The verall regulatin f gastrintestinal functin is based n reflexes cntrlled by the enteric, sympathetic and parasympathetic systems. These are reflexes which are integrated within the enteric system and are respnsible fr secretin as well as fd mixing and peristalsis f the GIT (Fig. 1-27). Other types f reflexes transmit infrmatin thrugh lng distances in the GIT. They require the participatin f the gut as the starting pint, integratin in the sympathetic ganglia and respnse by anther sectin f the gut. Amng these we find the gastrclic reflex which is respnsible fr the evacuatin f the cln; the entergastric reflex which invlves the participatin f the small intestine, and the cln which sends signals t reduce stmach mtility and its secretin; and the clnileal reflex which signals frm the cln t reduce r prevent emptying f ileal cntents int the cln. There is still anther type f reflex which invlves the larger trajectry frm the gut t the spinal crd and / r brain and back t the GIT. Thrugh this type f reflex the dudenum, based n the rate at which it receives fd, is capable f cntrlling gastric mtr and secretry activity. In additin, reflexes that invlve pain use these mechanisms t reduce verall gastrintestinal activity. Finally, faeces accumulatin in the cln triggers the defecatin reflex via signals sent t the spinal crd and Gastrintestinal reflexes Integrated cmpletely within the enteric system and cntrls Secretin, peristalsis, lcal mixing Gut-sympathetic ganglia-gut Gastrclic reflex Prmtes cln evacuatin Entergastric reflex Prevents stmach mtility and emptying Clnileal reflex Inhibits ileal emptying Gut-spinal crd / brain-gut Uses vagus nerve Frm stmach and dudenum Cntrls gastric mtr and secretry activity Pain reflexes Inhibits gastrintestinal activity Defecatin reflex Prmtes clnic rectal and abdminal cntractin Figure Reflexes f the gastrintestinal tract back resulting in cntractin f the cln, rectum and abdmen in rder t defecate. As hinted earlier, many hrmnes influence different aspects f mtility f the GIT and sme influence bth mtility and secretin f the GIT (Fig. 1-28). Chlecystkinin is a pwerful cmpund secreted by I cells f the dudenum and jejunum which, in respnse t the presence f fat digestin by-prducts, fatty acids and mnglycerides, triggers cntractin and emptying f the gallbladder. This makes bile salts available in the small intestine which emulsify the fats cming frm the stmach. At the same time, chlecystkinin inhibits stmach mtility, thus reducing the rate f stmach emptying int the dudenum and permitting better digestin. A similar effect is seen when fat and prteins are being emptied int the V BS 122 Physilgy II 7 Class f 2014

Chlecystkinin Endcrine cntrl f mtility Is secreted by I cells f the mucsa f the dudenum and jejunum Respnds t FA, mnglycerides in intestinal cntent Increases cntractility f the gallbladder Secretes

emptying Allws fr prper digestin f intestinal cntent plypeptide which slws dwn the rate f stmach emptying. Functinal mvements within the GIT.

As the fd enters the stmach, it starts frming layers, with the deepest layers being the smaller particles, ready t be emptied int the dudenum, and thse clse t the esphagus being the larger particles

8 Chlecystkinin Endcrine cntrl f mtility Is secreted by I cells f the mucsa f the dudenum and jejunum Respnds t FA, mnglycerides in intestinal cntent Increases cntractility f the gallbladder Secretes bile int the small intestine Emulsifies fat Inhibits stmach mtility Slws dwn transit Gastric inhibitry plypeptide Is secreted by mucsa f upper intestine Respnds t fat and prtein Slws dwn stmach emptying Allws fr prper digestin f intestinal cntent plypeptide which slws dwn the rate f stmach emptying. Functinal mvements within the GIT. The stmach carries ut a significant amunt f mvement in rder t mix its cntent and t separate physically large particles int smaller prtins. As the fd enters the stmach, it starts frming layers, with the deepest layers being the smaller particles, ready t be emptied int the dudenum, and thse clse t the esphagus being the larger particles which need significant disintegratin befre mving t lwer layers (Fig. 1-29). Figure Hrmnes influencing mtility f different cmpnents f the GIT PERISTALSIS Figure Relative particle size distributin within the stmach Figure Sequential mvements f a primate's stmach that permits the mixing f ingesta and advancement f digested materials twards the dudenum dudenum and upper jejunum. T this end, the mucsa releases a cmpund knwn as gastric inhibitry V BS 122 Physilgy II 8 Class f 2014

9 Acid and / r enzymes d mst f the digestin chemically, but the mvements f the stmach significantly supprt this prcess. These mvements are perfrmed in a repetitive manner fllwing a similar pattern. This pattern changes frm species t species, and depends n the specific anatmical design f the rgan. Figure 1-30 depicts a stmachs pattern f mvement. Functinal mvements f the GIT Prpulsive mvements Mves cntents alng the tract Rate permitting digestin Mixing mvements Mixes cntent Facilitate enzymatic digestin Increases expsure t epithelial wall Facilitates digestin and absrptin Figure Prpulsive and mixing mvements f the gut permit mixing and displacement f the digesta thrugh the GIT PERISTALSIS PROPULSIVE MOVEMENTS The stmach has significant mvement t mix and initially digest the ingesta. In rder t prperly digest the fd and discard what is indigestible, the GIT has t mve the intestinal cntents alng the tract, but at the same time, prvide pprtunity fr prper digestin. T achieve this, there are tw types f mvements within the GIT, the prpulsive and the mixing mvements (Fig. 1-31). Prpulsive mvements push cntents alng the tract which are als implemented by a crdinated effrt f lngitudinal and circular muscles f the gut (Fig. 1-32). The rate at which the cntents are mved influences the degree f digestin taking place, with a lnger retentin resulting in a mre thrugh digestin prcess. Prpulsive mvements take place in waves in which cnsecutive sectins f the intestine reduce their diameter frcing the intestinal cntents twards the anus. Mixing mvements d nt displace fd alng the tract. They cnsist f the cnstrictin f relatively clse sectins f the intestine frcing a change in psitin f the cntent, as well as t facilitate physical separatin and disintegratin f particles s that they are better expsed t digestive enzymes (Fig. 1-33). PERISTALSIS MIXING MOVEMENTS (Segmentatin) Figure Pattern f prpulsive peristaltic mvements f the GIT Figure Pattern f segmentatin (mixing mvements f the GIT) V BS 122 Physilgy II 9 Class f 2014

Biology 30S Unit Test Review: Digestion

Biology 30S Unit Test Review: Digestion Bilgy 30S Unit Test Review: Digestin Test utline: Multiple Chice: 10 Questins, 1 mark each. Shrt Answer: 5 Questins (answer 3 f them), 5 marks each. Lng Answer: 2 Lng Answer Questins, 10 marks each. Tasks