LECTURE 4

Secretory Functions

of theAlimentary Tract

Secretions are either

  • Digestive enzymes or
  • Mucous

TheDigestive Enzymes

General properties:

  • Most digestive secretions are formed only inresponse to the presence of food in the alimentary tract,
  • The quantitysecreted in each segment of the tract is almost exactly the amount needed forproper digestion.
  • Furthermore, in some portions of the gastro-intestinal tract,even the types of enzymes and other constituents of the secretions are varied inaccordance with the types offood present.

Neural and Hormonal Regulation

Neural regulation comprises:

  • Enteric Nervous Stimuli
  • Sympathetic
  • Parastmparhetic

Hormonal regulation comprises:

Different hormones e.g. Gastrin, CCk, Secretin, etc.

Enteric Nervous Stimulation

Themechanical presence of food in a particular segment of the gastrointestinal tractusually causes the glands of that region and often of adjacent regions to secretemoderate to large quantities of juices through:

  1. tactile stimulation,
  2. chemical irritation, and
  3. distention of the gutwall.

The resulting nervous reflexes stimulate both themucous cells on the gut epithelial surface and the deepglands in the gut wall to increase their "mucous" secretion.

Parasympathetic Stimulation

Stimulation of theparasympathetic nerves to the alimentary tract almostinvariably increases the rates of alimentary glandularsecretion.

This is especially true of the glands in theupper and lower portions of the tractinnervated by the glossopharyngealand vagus parasympathetic nervessuch as the salivary glands, esophageal glands, gastricglands, pancreas, and Brunner’s glands in the duodenum.It is also true of some glands in the distal portionof the large intestineinnervated by pelvic parasympatheticnerves.

Secretion in the remainder of thesmall intestine and in the first two thirds of the largeintestine occurs mainly in response to local neural andhormonal stimuli in each segment of the gut.

Sympathetic Stimulation

Stimulation of the sympatheticnerves going to the gastrointestinal tractcausesa slight to moderate increase in secretion by someof the local glands.

But sympathetic stimulation alsoresults in constriction of the blood vessels that supplythe glands.

Therefore, sympathetic stimulation canhave a dual effect:

  • First, sympathetic stimulation aloneusually slightly increases secretion.
  • Second, ifparasympathetic or hormonal stimulation is alreadycausing copious secretion by the glands, superimposedsympathetic stimulation usually reduces the secretion,sometimes significantly so, mainly because of vasoconstrictivereduction of the blood supply.

Hormonal regulation

This type of stimulationis particularly valuable to increase the output ofgastric juice and pancreatic juice when food enters thestomach or duodenum.Chemically, the gastrointestinal hormones arepolypeptides or polypeptide derivatives.

Digestive Glands Secretes:

  • Enzymes
  • Water
  • Electrolytes

Secretion of Enzymes

It is the typical process of protein synthesis and secretion.Its mechanism and regulation is different from that of water and electrolytes.

Secretion of Water and Electrolyte

The digestive glands add water and electrolytes, to the enzymes secreted, through the following mechanism:

  1. Nerve stimulation has a specific effect on the basal portion of the cell membrane to cause active transport of chloride ions(chloride channels) to the cell interior.
  1. The resulting increase in electro-negativity induced inside the cell by excess negatively charged chloride ions then causes positive ions such as sodium ionsalso to move through the cell membrane to the interior of the cell.
  1. Now, the new excess of both negative and positive ions inside the cell creates an osmotic force that causes osmosis of water to the interior, thereby increasing cell volume and hydrostatic pressure inside the cell, causing the cell itself toswell.
  1. The pressure in the cell then initiates minute openings of the secretory border of the cell, causing flushing of water, electrolytes, and organic materials "enzymes" out of the secretory end of the glandular cell.

The Mucus

Lubricating and ProtectiveProperties

Mucus is a thick secretion composed mainly of

  1. water,
  2. electrolytes,
  3. mixture of several glycoproteins

Mucus is slightly different in different parts of the gastro-intestinaltract, but everywhere it has several importantcharacteristics that make it both an excellentlubricantand a protectant for the wall of the gut.

  • First,mucus has adherent qualities that make it adhere tightlyto the food or other particles and to spread as a thin filmover the surfaces.
  • Second, it has sufficient body thatit coats the wall of the gut and prevents actual contactof most food particles with the mucosa.
  • Third, mucushas a low resistance for slippage, so that the particlescan slide along the epithelium with great ease.
  • Fourth,mucus causes fecal particles to adhere to one anotherto form the feces that are expelled during a bowel movement.
  • Fifth, mucus is strongly resistant to digestion bythe gastrointestinal enzymes.
  • sixth, the glycoproteinsof mucus have amphoteric properties, whichmeans that they are capable of buffering small amountsof either acids or alkalies;
  • seventh, mucus often containsmoderate quantities of bicarbonate ions which specificallyneutralize acids.

Secretion of Saliva

The principalglands of salivation are:

(1)parotid,

(2)submandibular,

(3)sublingual glands;

(4)many verysmall buccalglands.

Daily secretion of saliva normallyranges between 800 and 1500 milliliters, as shown bythe average value of 1000 milliliters in Table 64–1.

Saliva contains two major types of protein secretion:

(1)a serous secretion that contains ptyalin (alpha-amylase), which is an enzyme for digesting starches,

(2)mucus secretion that contains mucin

The parotid glands secrete almost entirely theserous type of secretion,

The submandibular andsublingual glands secrete both serous secretionandmucus.

The buccal glands secrete only mucus.

Salivahas a pH between 6.0 and 7.0, a favorable range for the digestive action of ptyalin.

Secretion of Ions in Saliva.

Saliva contains especiallylarge quantities of potassium and bicarbonate ions.

Conversely, the concentrations of both sodium andchloride ions are several times less in saliva than inplasma.

One can understand these special concentrationsof ions in the saliva from the following descriptionof the mechanism for secretion of saliva.

Salivary secretion is a two-stageoperation:

The first stage involves the acini.

The acini secrete a primarysecretion that contains ptyalin and/or mucin in a solutionof ions in concentrations not greatly differentfrom those of typical extracellular fluid.

The second stage in the salivary ducts.

As theprimary secretion flows through the ducts, two majoractive transport processes take place that markedlymodify the ionic composition of the fluid in the saliva:

First, sodium ions are actively reabsorbed from allthe salivary ducts and potassium ions are secreted in exchange for the sodium. Therefore, thesodium ion concentration of the saliva becomesgreatly reduced, whereas the potassium ion concentrationbecomes increased. However, there is excesssodium reabsorption over potassium secretion, andthis creates electrical negativity of about -70 millivoltsin the salivary ducts; this in turn causes chloride ionsto be reabsorbed passively.Therefore, the chloride ionconcentration in the salivary fluid falls to a very lowlevel, matching the ductal decrease in sodium ionconcentration.

Second, bicarbonate ions are secreted by the ductalepithelium into the lumen of the duct.

  • This is at leastpartly caused by passive exchange of bicarbonate forchloride ions,
  • but it may also resultpartly from anactive secretory process.

The net result of these transport processes is that:

Under resting conditions, the concentrations of:

sodiumand chloride ions in the saliva are only about 15 mEq/L each, about one seventh to one tenth their concentrationsin plasma.

potassium ions is about 30 mEq/L, seven times as greatas in plasma

bicarbonateions is 50 to 70 mEq/L, about two tothree times thatof plasma.

During maximal salivation,the salivary ionic concentrationschange considerably because the rate offormation of primary secretion by the acini canincrease as much as 20-fold. This acinar secretionthen flows through the ducts so rapidly that the ductalreconditioning of the secretion is considerablyreduced. Therefore, when copious quantities of salivaare being secreted,

sodium chloride concentrationdrops to one half or two thirds that of plasma,

potassium concentration rises to only four timesthat of plasma.

Function of Saliva for Oral Hygiene.

Under basal awake conditions,about 0.5 milliliter of saliva, almost entirely ofthe mucous type, is secreted each minute;

but duringsleep, secretion becomes very little.

This secretion playsan exceedingly important role for maintaining healthyoral tissues. The mouth is loaded with pathogenic bacteriathat can easily destroy tissues and cause dentalcaries. Saliva helps prevent the deteriorative processesin several ways.

First,the flow of saliva itself helps wash away pathogenicbacteria as well as food particles that provide theirmetabolic support.

Second, saliva contains several factors that destroybacteria.

Thiocyanate ionsand

Several proteolytic enzymes(most important,lysozyme)which

  • attack the bacteria,
  • aid the thiocyanateions in entering the bacteria where these ionsin turn become bactericidal, and
  • digest food particles,thus helping further to remove the bacterial metabolicsupport.

Third,saliva often contains significant amounts of antibodies that can destroy oral bacteria, includingsome that cause dental caries.

In the absence ofsalivation, oral tissues often become ulcerated and otherwiseinfected, and caries of the teeth can become out of control.

Nervous Regulation of Salivary

Secretion

Thesalivary glands are controlled mainly byparasympatheticnervous signals all the way from thesuperior andinferior salivatory nucleiin the brain stem.

The salivatory nuclei are located approximately at the junction of the medulla and pons and are excited by both taste and tactile stimuli from the tongue and other areas of the mouth and pharynx.

The Many tastestimuli, especially the sour taste (caused by acids),elicit copious secretion of saliva—often 8 to 20 timesthe basal rate of secretion. Also, certain tactile stimuli,such as the presence of smooth objects in the mouth(e.g., a pebble), cause marked salivation, whereasrough objects cause less salivation and occasionallyeven inhibit salivation.

Salivation can also be stimulated or inhibited bynervous signals arriving in the salivatory nuclei fromhigher centers of the central nervous system. Forinstance, when a person smells or eats favorite foods,salivation is greater than when disliked food is smelledor eaten.The appetite areaof the brain, which partiallyregulates these effects, is located in proximity to theparasympathetic centers of the anterior hypothalamus,and it functions to a great extent in response to signalsfrom the taste and smell areas of the cerebral cortexor amygdala.

Salivation also occurs in response to reflexes originatingin the stomach and upper small intestines; particularlywhen irritating foods are swallowed or whena person is nauseated because of some gastrointestinalabnormality. The saliva, when swallowed, helps toremove the irritating factor in the gastrointestinal tractby diluting or neutralizing the irritant substances.

Sympathetic stimulation can also increase salivationa slight amount, much less than does para-sympatheticstimulation. The sympathetic nerves originatefrom the superior cervical ganglia and travel along thesurfaces of the blood vessel walls to the salivaryglands.

A secondary factor that also affects salivary secretionis the blood supply to the glands because secretionalways requires adequate nutrients from the blood.

  • The parasympathetic nerve signals that induce copioussalivation also moderately dilate the blood vessels.Inaddition,
  • salivation itself directly dilates the bloodvessels, thus providing increased salivatory glandnutrition as needed by the secreting cells.Part of thisadditional vasodilator effect is caused by kallikreinsecreted by the activated salivary cells, which in turnacts as an enzyme to split one of the blood proteins,an alpha2-globulin, to form bradykinin, a strongvasodilator.

Esophageal Secretion

The esophageal secretions are entirely mucous in characterand principally provide lubrication for swallowing.

The main body of the esophagus is lined with manysimple mucous glands.

At the gastric end and to a lesserextent in the initial portion of the esophagus, thereare also many compound mucous glands.The mucussecreted by the compound glands in the upper esophagusprevents mucosal excoriation by newly enteringfood, whereas the compound glands located near theesophago-gastric junction protect the esophageal wallfrom digestion by acidic gastric juices that often refluxfrom the stomach back into the lower esophagus.Despite this protection, a peptic ulcer at times can stilloccur at the gastric end of the esophagus.