Digestive System - Paper 1999

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1999_2^nd semester_Q6  

There are four main cell types that are found in the gastric glands of the stomach in the fundic region. In addition, the mucosal lining contains an abundance of goblet cells which are responsible for secreting mucous which forms a protective layer on topic of the gastric wall, protecting against the acidity of the gastric juice produced. The glands themselves contain four different types of cells.  

At the apex of the gastric pits, there are mucous neck cells and the function of these is not yet identified. The parietal cells are located dispersed around the chief cells within the gland. These cells are responsible for the secretion of hydrochloric acid, which provides the acidity of the gastric juice. It is also responsible for the production intrinsic factor which is used for the absorption of vitamine B12 in the small intestine. The third type of cell is the chief cell, and this is mainly found at the bottom of the gastric glands. These cells produce pepsinogen which is the inactive form of pepsin. The pepsinogen is stimulated to convert to pepsin by hydrocholoric acid secreted by the parietal cells. The presence of pepsin causes self activation of this process, called a positive feedback response. The last type of cells present are the enteroendocrine cells which are present at the base of the gastric glands and these are responsible for the secretion of enterogastrones, which are hormones that have regulatory effects on the gastric secretions during the different phases of control. Hormones releases include, secretin, gastrin and cck.  

There are two mechanisms which control the secretion of gastric juice and all these mechanisms occur during the different phases of the gastric secretion control, namely: cephalic, gastric and intestinal. The two mechanisms are: neural and hormonal. The neural mechanism involves long and short reflexes. Long reflexes involved an afferent limb (for example: distension of the smooth muscle around gastric wall), an integration center (CNS) and an efferent limb (parasympathetic vagal influence). Short reflex involved intramural neurons synapsing between themselves without any outside influence. This leads to the concept of the Enteric Nervous System. Hormonal influences are also present. Enterogastrones like gastrin are secreted during the gastric phase of regulation and this has a stimulatory effect on gastric motility and secretion. This is produced by G cells in response to the properties of the chyme (i.e: pH etc). Also other hormones have excitatory effects such as intestinal hormones, which are released during the intestinal phase. Also, the enterogastric reflex is a neural response during the intestinal phase and has the effect of tighten the pyloric sphincter and causes release of several inhibitory hormones such as: secretin, cck and histamine.

1999_2^nd semester_Q7_PartA  

The two main functions of the small intestine is to chemically break down the chyme into its functional components and to absorb the nutrients from this substance. It has many structural modifications which enable the efficiency of the breaking down and absorbing process.  

Structurally, the small intestine is divided into three components. The duodenum, the jejunem and the ileum. The duodenum is responsible for receiving chyme from stomach, bile from gall bladder (via common bile duct), pancreatic enzymes from pancreas. It also contains enteroendocrine cells in its mucose which secrete secretin and CCK which induce production of more bile, contract the ducts of the gall bladder and stimulation secretion of pancreatic enzymes.  

The Jejunem is meant for mostly chemically digesting the chyme into its functional components. It break downs the polypeptides and polysaccarides into amino acid and monosaccarides. Lastly the ileum is mainly responsible for the absorption of the nutrients from the broken down chyme.  

The small intestine is made up of a simple columanar epithelium which contains many microvilli and also some goblet cells. The small intestine has a huge surface area to enable the efficiency of its functions. This is due to mucosal folds called plicae circilaris, the huge length of the small intestine, the villi projections which contain the core of connective tissue, and the microvilli projections on the luminal aspect of individual epithelial cells. All of these account for an increase in surface area, something in the order of 600 times more than a normal flat surface. Also between the villi, there are small intestinal crypts which are responsible for the intestinal secretions and hormones produced (namely: enterogastrones).  

The tunica muscularis also contains two layers of smooth muscle which enable it to perform segmentation and peristaltic contractions. Segmentation is due to the out of phase contractions of the circular layer of smooth muscle in neighbouring sections of the small intestine. This results in complete mixing of the chyme, in the duodenum, with the pancreatic enzymes, bile salts and intestinal secretions. Segmentation is caused by pacemaker cells of the smooth muscle, but each set of cells has its own rate of firing. Peristaltis is the contraction of the longitudinal layer of muscle a little before the circular layer, and this has the effect of pushing chyme forward along the length of the small intestine.  

The small intestine is equipped for its functions thanks to the structurally modifications to the its mucosal surface and muscularis surface.  

Major Points: Main functions --> which sections perform what function -->  structurally modifications --  increase in surface area --> epithelium type --> goblet cells -->  plicae circularis, length, villi, microvilli --> segmentation -->  peristaltis --> two layers of muscle evident --> pacemaker cells --> different firing rate between adjacent sections.