Lecture 6 Flashcards
Where does the GI tract start and end ?
The GI tract starts in the mouth and ends in the lower intestine.
How many salivary glands surround the mouth ?
There are three salivary glands - the parotid salivary gland, the sublingual salivary gland, the submandibular salivary gland.
What is the purpose of the salivary glands ?
They provide saliva for lubrication and the initial digestion of food begins.
Where is the pharynx ?
It is found at the back of the mouth and is connected to the oesophagus.
What does the oesophagus connect ?
The oesophagus connects the pharynx to the stomach.
What does the oesophagus run parallel to ?
The windpipe/trachea.
There must be regulatory processes in place ensuring that…
… the food does not enter the trachea instead of the oesophagus.
What is the stomach surrounded by ?
By a number of accessory organs attached to the GI tract.
What does the stomach link to ?
The stomach leads to the small intestine which has a pipe like structure.
Accessory organs
There are important in linking to the GI tract.
They produce secretions that are important for digestion.
The proximity of these organs to the GI tract is important in terms of the speed at which these secretions reach the GI tract.
Types of accessory organs
Liver, gallbladder, pancreas.
What is the end of the GI tract made up of ?
The large intestine, mostly the colon, caecum and rectum.
Where does the GI tract end ?
At the anus.
The main purpose of the GI tract
Allows for food intake - provides nutrients for the body.
Digests the food and absorbs the nutrients - to provide energy.
What is it that regulates the control of food intake ?
A number of different signals.
Types of signals
Levels of glucose circulating the bloodstream.
Hormones - leptin, gherkin, insulin, glucagon.
Stretch receptors in the GI tract (mechanical signals) - these indicate when the stomach is full.
Other components can feed into the control of food intake (neuronal signals) - stress, body temperature, food palatability.
What influences the control of food intake ?
A combination of hormonal, neuronal and mechanical signals.
In a well fed state
There are high levels of glucose, insulin is secreted which enables glucose extraction from the blood.
The glucose is stored in the form of glycogen which can be an energy source when needed - glycogen synthesis.
In a fasting state
There are low levels of glucose.
There are mechanisms in place that provide energy - glucagon triggers glycogen breakdowns, gluconeogenisis is the creation of glucose from other sources like amino acids, glucose sparing processes in some cases e.g. fat utilisation - lipolysis, the breakdown of fat.
When hungry
There is a decrease in glucose, fat and protein and an increase in ghrelin.
These signals trigger signals in the appetite centre in the brain in the lateral hypothalamus telling us to eat.
After a meal
The levels of glucose, fat, protein and leptin are high.
These signal trigger the satiety centre in the medial hypothalamus telling us to stop eating.
There are mechanical signals from the GI tract also telling us we are full.
What can affect the time taken for the food to be processed ?
The composition of food being eaten can affect how fast the food will processed through the GI tract.
The different meals components are digested rates reflecting a difference in the length of the digestive phase.
What can impact the transit through the GI tract ?
Food content can impact this, as larger amounts of food empty slowly begin then more quickly.
The nutrition density (i.e. calories) controls/slows the gastric emptying.
The effects of fat content
Important in delaying factor in gastric emptying.
Its presence in the duodenum will cause the stomach to relax, lowering the intragastric pressure.
Influences food intake at the next meal.
Prolongs the elevation of pH in the stomach.
What would happen if you eat a meal high in fat ?
This would result in a slower transit through the stomach to the small intestine, influencing how long you feel full.
What can impact the ability to absorb drugs ?
Whether the body has eaten or is in a fasting state.
Food can affect drug absorption, different drugs and formulations are affected differently by the presence of food.
What is the structure of the GI tract ?
It is pipe-like structure, it has the same basic structure along the tract.
What are the layers of the GI tract ?
Outer - inner:
Muscular layer made up of longitudinal and circular muscle.
Submucosa.
Mucosa.
Purpose of the submucosa
Where numerous glands are found which are important for GI activity.
Purpose of the mucosa
It is in contact with the lumen of the GI tract - where the food etc. is passed through.
It is important for the absorption of nutrients from end stage product.
What are the two nerve plexuses found between the muscularis and submucosal layer ?
Myenteric plexus (Auerback's plexus). Submucosal plexus (Meissner's plexus).
What is the purpose of the nerve plexuses ?
They modulate GI activity.
Myenteric plexus + submucosal plexus =
Enteric nervous system.
What is the structure of the ENS like ?
It is pretty much the same from the stomach to the latter part of the large intestine, the rectum.
What is the central nervous system made up of ?
The brain and the spinal cord.
What is the purpose of the peripheral nervous system ?
It connects the CNS to the limbs and organs.
The subdivisions the PNS is split into
Autonomic nervous system - influences organ function.
Enteric nervous system - intrinsic to the GI tract.
What is the purpose of the vagus nerve ?
It connects the CNS and ENS, they are connected in both directions i.e. the brain influences the gut and the gut can influence/send signals to the brain.
What is the vagus nerve ?
The primary route the gut bacteria use to transmit information to the brain.
Why is there a strong link between the gut and the brain ?
This is because the neuronal input between the brain and the gut are very similar if not the same.
What is the purpose of afferent neurons (sensory receptor neurons) ?
They carry nerve impulses away from the receptors of seconds organs and to the CNS.
What is the purpose of efferent neurons (motor/effector neurons) ?
They carry nerve impulses away from the CNS to effectors like muscles or glands.
Where is the ENS found ?
Within the wall of the GI tract from the oesophagus to the anus.
What is the role of the myenteric plexus ?
Largely motor in function and influences muscle activity in the tract.
What is the role of the submucosal plexus ?
Largely sensory, receives signals from the epithelium and stretch receptors and influences secretory activity.
What is the ENS subject to ?
The ENS is subject to modulation by the ANS since fibres from both the SNS and PNS can synapse with the neurons in the ENS (cross talk).
The CNS can influence GI tract activity.
How are nerve plexuses linked to the CNS ?
Via different fibres activated by stretch (mechanical) stimulation and chemical stimulation.
Types of efferent innervation the nerve plexuses receive from the ANS
Parasympathetic input.
Sympathetic input.
Parasympathetic input
Rest and digest.
Stimulate motility and secretory activity.
Sympathetic nerves
Fight and flight.
Causes presynaptic inhibition of parasympathetic-induced contraction, i.e. inhibits digestion.
What are short reflexes ?
Reflexes which occur within the GI tract.
ENS only.
What are long reflexes ?
Reflexes which depend on the CNS input and so the CNS sends signals to the nerve plexus (ENS) resulting in a response, the CNS is impacting on the ENS and triggering responses in the GI tract.
What cane used to affect gut transit/motility ?
Certain drugs.
Longitudinal muscle
This type of muscle runs along the length of the tract.
Circular muscle
This type of muscle runs along the circumference of the tract.
What is the importance of the longitudinal and circular muscle ?
They are important for mixing and propulsion of food along the tract.
What is segmentation ?
It facilitates the mixing of food.
What is peristalsis ?
It is concerned mainly with the propulsion of food along the tract.
How do drugs affect the gut transit/motility ?
They do so by affecting the muscle function and segmentation and peristalsis - involved in movement of the gut and food along the tract.
How long does gut transit take ?
Anything between 12-48 hours, depends on age, what time of day, gender.
What happens if gut transit time is too fast ?
The absorption of nutrients from the food is affected because the food is not staying long enough in the tract to be absorbed and so the result is diarrhoea.
What happens if gut transit time is too slow ?
This results in poor gut health because if there are substances staying in the gut for too long, this can influence the gut bacteria and this results in constipation.
What can affect gut transit time ?
Drugs/classes of drugs can affect the gut transit time through side effect because the nature of the receptors that they act on.
The drug prescribed could gave been prescribed for something else but also affects the GI tract because of the non-specific nature of the the drug.
Drugs acting on specific receptors…
… may be acting in different areas of the body also because the same receptors may be expressed in different parts of the body.
Other drugs can be used to…
… affect gut transit time as that is the desired end effect.
Various drugs will also target…
… different receptors in the GI tract but can also target those same receptors elsewhere in the body and can result indifferent effects.
