L20. Neural and Hormonal Control of Gastrointestinal Function 1 Flashcards
What are the basic functions of the GIT? [5]
- Digestion
- Absorption
- Excretion of waste
- Prevention of invasion by pathogens
- Contains the microbiome
The gut coordinates widely separated regions in a complex manner. What are the key regulatory requirements/functions of the gut? [3]
- Controls intestinal smooth muscle contractions for peristalsis
- Regulation of the secretion of digestive enzymes
- Control of re-absorption of water and electrolytes
There are many interacting control systems with some redundancies in function. Why is this so?
There needs to be failsafes built in to ensure that this system doesn’t succumb to damage that it is so susceptible to (in direct contact with the outside environment)
What are the two main interacting control systems of the gut?
Local (enteric) and endocrine signals
Describe the local control of the gut
- The enteric nervous system: controls secretion of salt and water and is pre-programmed (the brain has no impact on what, only how much)
- Interstitial cells of Cajal: modified excitable cells that produce pacemaker, rhythmic changes in membrane potential of smooth muscles (motor activity/motility)
Describe the endocrine control of the gut
Signals from the intestinal mucosa to and from the ANCILLARY ORGANS like the brain, pancreas and gall bladder:
- secretion of enzymes
- food seeking behaviour and appetite
What is the difference between the mucosa of the small intestine vs. the colon?
Small intestine has villi in the mucosa extending out and pits going in while the Colon has a relatively flat mucosal surface with deep pits/crypts
Describe the basic structure of the enteric nervous system in the gut wall (in relation to the muscles)
Longitudinal muscle and circular muscle layers of the gut wall.
Between the two muscle layers is the MYENTERIC PLEXUS
And under the circular muscle (between it and the submucosa) is the SUBMUCOSAL PLEXUS
What is the myenteric plexus responsible for?
Stimulation leads to:
- increased tone of the gut wall
- increased intensity of the rhythmical contractions
- a slight increase in rate of the rhythm of contraction
- an increased velocity of peristalsis
- inhibition of contraction of the pyloric and the ileocecal valves
What is the submucosal plexus primarily responsible for?
concerned with the control of functions within the inner walls of each gut segment i.e. local absorption, secretion, contraction
Describe the Interstitial Cells of Cajal (ICC)
Specialised cells in both plexuses
Form a 2D sheet of cells interconnected by gap junctions forming an ELECTRICAL SYNCYTIUM
- they generate large depolarising potentials with rhythms
- electrically coupled to the muscle layers
What are the mucosal Enteroendocrine (EE) cells?
Several subtypes exist
Endocrine cells localised and specialised to the gut
Contain and secrete many mediators including serotonin, CCK, somatostatin, Glucagon-like peptides, pancreating polypeptide
What are some examples of enteroendocrine cells?
Enterochromaffin cells: secrete serotonin
G cells: secrete gastrin
D cells: secrete somatostatin
Enterochromaffin-like cells: histamine
Where to the enteroendocrine cells lie in the mucosa?
Span the apical surface of the epithlium at the level of lumen. Activation at one end (lumen) leads to release of their contents at the basolateral surface to act in an endocrine or paracrine manner
What are the 3 major interacting control reflexes acting on the gut? (one is a subset of the other)
- Vago-vago reflex
- Intestino-intestinal reflex
Vago-vago is a subtype of the intestino-intestinal reflex
- CNS control
Describe the vago-vago reflex
Pathway that coordinates the movement in the upper GI tract through the vagus nerve
Activation at the peripheral level by sensory stimuli
= primary afferent neurons via vagus to the brain to activate neurons
= descending signals via the dorsal motor vagal pathway produces efferent control of the gut
What kind of the reflexes are controlled by the vago-vagal pathway?
Swallowing
Acid secretion in the stomach
Peristalsis of the oesophagus
Contraction of the stomach and duodenum
What happens to vagal influence moving down the gut tube?
It wanes progressively
It only controls about 0.5 of the 6m long gut tube
Describe the intestino-intestinal reflexes
They run out of the gut into the CNS and then back down into the intestine via vagal or other means
eg. Dorsal root ganglia and spinal cord
What are the major differences between the vagal pathway vs. the spinal nerve pathway?
VAGUS:
excitatory
tends to deal with day to day monitoring
SPINAL:
inhibitory
deal with nociception (irritation/pain)
A class of intestino-intestinal reflexes involves cell bodies in the enteric nervous system (embedded in the gut wall) projecting out into what pre-vertebral sympathetic ganglia?
Celiac
Inferior mesenteric ganglia
What is the general trend of the intestino-intestinal reflex pathways in terms of synaptic connections and sympathetic outflow?
Outflow goes BACK to inhibit various parts of the intestine
The signalling cells and controlling cells lie distally to the effector cells and the neurons they innervate
Give a basic example of an intestino-intestinal reflex pathway
Distention or strong stimuli sensed by the colon leads to inhibition of motor activity in the small intestine (distal to proximal direction)
Tells the earlier parts to slow down as the later parts are not ‘ready’ for more.
What are the CNS control pathways involved in?
Anticipation, mood, activity (mediated through cognitive function)
How do vagal and sympathetic input modulate the enteric neural circuits of the enteric nervous system?
The produce a complex motor output INDIRECTLY (don’t act on the muscle but rather controls the amount of activity of the intrinsically programmed system)
To be full of flavour and taste, what must the food have?
Significant amounts of fat
Free amino acids to excite umami receptors
sugar or sweeteners to be aromatic
What is the cephalic phase?
A reflex triggered by sight, smell and taste (and thought) of food
Causes salivation, gastric acid and pepsin secretion and relaxation IN ANTICIPATION of food.
Through what does the cephalic phase work through?
The vagus nerve
What is the major function of the cephalic phase?
To set up the stomach to store food and begin the barrier function of the GIT
If this didn’t occur, there is less acid secretion and the stomach isn’t ready to cope with the inflow of food.
The acid is also very important to killing bacteria in food
How is primary peristalsis of the oesphagus controlled?
It is entirely vagal (neural)
What are the four main mediators that interact in regulating acid secretion?
- ACh from enteric neurons excited by vagus
- Gastrin from G cells in antrum and duodenum
- Histamine from ECL cells
- Somatostatin inhibition from D cells
What happens when food enters the stomach?
Distention caused by contents is sensed and activates ENTERIC and VAGO-VAGAL REFLEXES that secrete more acid and pepsin
How do the Interstitial cells of cajal act in this stomach phase of digestion?
They’re activity propagates from the corpus (body) to the antrum causing ripples of constriction to propagate food towards the pylorus
What happens to food in the antrum?
Triggers a reflex INHIBITION of acid secretion in the body (triggers D cells to release somatostatin)
Strong constrictions from the ICC prortentials drive cells to the CLOSED sphinchter.
What happens to the content of the food in the stomach?
Acid, protease and mechanical activity causes components of food to separate fat out
The fat floats to the top of the stomach in the fundus
