Digestive System - POSSIBLE QUESTIONS
Describe
the three phases of gastric motility.
Gastric
motility begins as soon as food enters the stomach from the esophagus and is
marked by three independent phases: filling phase, mixing phase and emptying
phase.
The
filling phase begins as soon as food enters the gastric lumen, where the gastric
walls stretch out due to stretching of the smooth muscle walls and smoothing of
the internal folds, called rugae. This is called receptive relaxation and is
caused by two main mechanisms. The first is called stress-relaxation response,
which is a brain stem intrinsic inhibitory response mediated by the vagus nerve.
The second mechanism is called adaptive response and is mediated by stretch
receptors within the muscle cells and these act without any CNS stimulation.
The
second phase is marked by the strong peristaltic contractions occurring within
the gastric wall. These contractions extend from the pyloric region to the
fundic region as waves and they cause complete mixing and churning of the food
(mechanical fragmentation) along with the gastric secretions. The smooth muscle
pacemaker cells set the rhythm of contractions during this phase. These cells
have an unstable membrane potential and they depolarize spontaneously causing
these waves of contractions.
The
last phase is the emptying phase is marked by the ejection of the chyme into the
duodenum of the small intestine. This is largely assisted by the continuation of
the strong peristaltic contractions during the mixing phase, as they provides
the ejection force and also the relaxation of the pyloric sphincter is
important. The stomach empties every 2 to 3 hours. The small intestine is then
responsible for the absorption of nutrients and chemical fragmentation of the
chyme.
Major
Points: What
is gastric motility? -->
Phases involved -->
First Phase -->
food enters, rugae smoothed out stretching of the tunica muscularis -->
what causes this receptive relaxation -->
stress relaxation response -->
adaptive relaxation -->
mixing phase -->
strong wave of peristaltic contractions -->
mix, churning with gastric juice -->
chyme -->
pacemaker smooth muscle cells -->
emptying phase -->
ejection into the small intestine.
Describe the mucosal surface of the stomach, including the types of cells present, their function. Give an account of the protective barriers the stomach has from the corrosive nature of the gastric juice.
Describe
the regulation mechanisms involved when regulating the activity of the liver,
pancreas and gall bladder.
There
are two mechanisms of regulation of the activity of pancreas, liver and gall
bladder. These are neural and hormal mechanisms. Neural mechanisms act via
excitatory parasympathetic fibres (mainly the vagus nerve) which stimulate the
production of bile and pancreatic enzymes. On the other hand, hormones such as
entergastrones mediate the activity of the liver, pancreas and the gall bladder.
Enteroendocrine cells have senses at the tips of their cells to the level of
acidity of the chyme, and also to partially hydrolysed fats and proteins. This
triggers the release of hormones such as secretin and CCK, which have an effect
on the liver, pancreas and gall bladder.
Secretin
has an effect on the liver and pancreas. It has a stimulatory effect on the
composition of the bile, increasing the concentration of bicarbonate ions as
part of bile therefore reducing the acidity of the chyme. CCK also has an effect
on the pancreas. Pancreatic acinar cells have receptors to CCK and when this
binds to them, it stimulates more release of the pancreatic enzymes, which then
act to further break down polypeptides and fats. CCK also has an effect on the
sphincters that control the entry of bile into the small intestine. It relaxes
the pancretic duct sphincter and the common bile duct sphincter and therefore
stimulates the gall bladder to eject some bile into the small intestine.
Major
Points: Neural
regulation -->
parasympathetic fibres such as vagus to stimulate the production of bile and
also pancreatic enzymes -->Hormonal
-->
enterogastrones are responsible for the production of bile and also
enzymes as well -->
have tips of cells which are sensitive to acidic conditions and partially
hydrolysed proteins and fats -->
therefore release cck and secretin -->
Secretin stimulates more bicarbonate composition as part of bile -->
also secretin increases pancreatic enzyme production -->
further breakdown lipids and proteins -->
also has an effect on the gall bladder.
Describe
the structure of the tooth.
The
tooth is the primary structure of the oral cavity and is in motion during
mastication. There are four varieties of tooth in the adult human, these are
incisor, canines, premolars and molars. The incisors help us bite the food, the
canines help us tear the food, the premolars and molars help us crush and grind
our food.
The
primary division of the tooth is its crown and root. The crown component is the
bit that is visible, and the root is the bit embedded into the gigiva (gum). The
outer most surface of the tooth contains a very hard substance called enamel.
This is the hardest substance in our body and is non-living. Any damage will
mean that the underlying structures are at risk. Enamel can be eroded or
demineralised by acids. The internal structure of the tooth comprises of the
dentin, which is living connective tissue. The dentin surrounds the dental pulp
which contains nerves, vessels and lymphatics of the tooth. These enter via the
root canal. The dental pulp is contained within the pulp cavity.
The
outer surface of the root is covered with living tisse called cementum. From
these peridontal ligaments anchor the tooth to the surround bone, and this
provides the structural integrity of the tooth.
Draw
a diagram if asked this question in the exam, which there is a good possibility
because it has not been in the exam for the past 5 years.
Describe
the tongue. Includes its features
and functions.
The
tongue is the main organ which initiates swallowing. It also helps form food
into a bolus, a combination of food and saliva secreted by the salivary glands.
The tongues contains a stratified sqamous epithelium and this aids in protect against abrasion from the constant eating of food. The tongue is largely made up of skeletal muscles fibres which are interlaced three dimensionally. These fibres can be split up into two groups. Extrinsic and Intrinsic fibres. Intrinsic muscles are ones which have no attachment to the surrounding bone, therfore contract can change the shape of the tongue. The extrinsic muscles are attached to the surrounding bone, therefore contraction can change the position of the tongue such as: protrusion, retraction and folding. The tongue also house specialised structures on its dorsal surface. These are filiform, funiform and circumvalate papillae. The filiform papillae give the tongue its roughness which aids in gripping of the food and also swallowing the bolus. The fungiform and circumvalate papillae house taste buds, which assisting in tasting the different flavours. Posteriorly there is the sulcus terminalis which is a structural division between the anterior 2/3’s and the posterior 1/3. The posterior aspect contains the lingual tonsil.
Describe
the break down of proteins, carbohydrates, nucleic acids, and lipids by the
jejunem of the small intestine. Absorption is also another function of the small
intestine. Describe the process of absorption in the intestine.
The
small intestine breaks down the chyme into its functional components, and this
is assisted by the pancretic juice.
Proteins
Proteins
are broken down by pancreatic proteases and peptidases. Trypsinogen is the
inactive form of trypsin, which is secreted by the pancreas. It is converted to
its active form by brush border enzymes present on the microvilli. Once
converted, they convert other inactive enzymes to their active forms. These
enzymes break down the proteins into their dipeptide and amino acid form. Brush
border enzymes further break down these substances.
Carbohydrates
Carbohydrates
are first broken down to dissacarides by the pancreatic amylase. Then they are
further broken down into monosaccarides by brush border enzymes
Nucleic
acids
Nucleic
acids are broken down to nucleotides by pancreatic nucleases. Then these are
further broken down into phosphates, sugars and nitrogenous bases by brush
border enzymes.
Lipids
Lipids
are large molecules so they need to be first emulsified by the bile salts.
Emulsification is the process of breaking down the lipids into smaller molecules
so that the surface area for contact is increased dramatically. Then pancreatic
lipase hydrolyses the bonds between the lipid molecules to form monoglycerides.
Absorption
is another function of the small intestine. It occurs in the ileum, the final
anatomical structure of the small intestine. Absorption by the small intestine
involves passage into the cell through the luminal surface and then passage into
the underlying connective tissue through the basal surface. The process involved
here are:
v
Osmosis
(the passage of water along its concentration gradient)
v
Active
transport (ATP is required for this process)
v
Secondary
active transport (the transport of glucose follows movement of sodium ions for
example)
v
Facilitated
diffusion (eg. Transport of fructose)
v
Simple
diffusion (transport of lipid soluble substance like triglycerides)
The
second part of absorption of fats and lipids is slightly different to the
others. Once monogylcerides enter the cell, they transformed into triglycerides
and bonded to cholesterol, phospholipids to form cholymicrons. Cholymicrons are
then exocytosed out of the cell into the connective tissue, and then into the
lymphatics and capillary blood. The hepatic portal vein connects the small
intestine to the liver, and nutrient rich blood is sent to the liver where the
essential nutrients is absorbed. Then the remaining blood is sent to the venous
return of the heart.