7.4 Control of the Gut Flashcards
What are some functions of the gut that need to be regulated
-Motility
-Secretions
-Water and electrolyte transport
-Regeneration of epithelial cells
-Blood flow to digestive tract
-Mediation of immune system
-Accessory organs
What are the 2 predominant mechanisms for gut control
hormonal control and nervous control
What are the subdivisions of nervous control of the gut
Nervous control can be via extrinsic innervation (nerves not coming from the gut itself) where it is either sympathetic or parasympathetic. Or, it can be via intrinsic innervation coming from the enteric nervous system in the gut.
Describe sympathetic innervation of the gut
Sympathetic activity generally decreases GI activity by directing blood vessels, directing blood away from digestive system. There is also a sensory component, detecting pain in the digestive tract.
Describe parasympathetic innervation of the gut
The vagus nerve innervates most of the digestive tract, nerves from the sacral spinal cord innervate the last 1/3 of the colon.
There are sensory components, responding to changes in stretch, temperature and osmolarity, these take part in vagovagal reflexes (controlling gastrointestinal muscle in response to distension of the tract by food).
The motor components are generally stimulatory, promoting secretion and motility.
Instead of post ganglionic parasympathetic neurons, preganglionic parasympathetic neurons usually synapse with enteric neurons, these can either be inhibitory or excitatory.
Describe enteric innervation of the gut.
The enteric system is known as the mini brain and can operate independently of the central nervous system. It integrates commands from the parasympathetic and sympathetic fibres with sensory information from the gut.
The system has a complex 3D structure extending from the oesophagus to the anus. The neurons form a plexus. Plexus are networks of neurons with individual ganglia. There are 2 plexus in the enteric system
What are the 2 plexus in the enteric system
Submucosal plexus and myenteric plexus
Describe the myenteric plexus in the enteric system
It is located in between the circular and longitudinal smooth muscle layers. As the muscle is dense, there is not much room for the plexus so it has to be regularly arranged and densely packed to form a polygonal network. It is mostly composed of motor neurons which mainly supply the muscularis. It is also known as Auerbach’s plexus.
What are the functions of the myenteric plexus
Muscle tone of gut wall, increased intensity of rhythmical contractions, slightly increased rate of contractions, increased conduction velocity of the gut wall and inhibiting sphincters
Describe the submucosal plexus
It is scattered throughout the submucosa, as the submucosa is connective tissue and can be less dense. Hence it can be loosely packed ganglia and more spread out than the myenteric system. Projections are less organised and are mostly associated with the mucosa and the lumen. It is also known as Meissner’s plexus
What are the functions of the submucosal plexus
Local secretion, local absorption and local contraction of the muscularis mucosae
Describe some gut-brain peptides
motilin - Stimulates migrating motor complex, acts on ENS neurones to begin these contractions
cholecystokinin (CCK) - Increases pancreatic secretion, contraction of the gallbladder, intestinal motility, Inhibits gastric emptying.
5-HT - Released by interneurons. SSRIs (drugs) may cause decreased GI motility.
gastric inhibitory polypeptide (GIP) - Promotes secretion of insulin, Inhibits motor and secretory activity in the stomach at pharmacological doses.
Secretin - Increase the bicarbonate secretion by the pancreas and biliary system. Inhibits gastric acid production
Describe an additional control mechanism of the gut that is neither hormonal or neuronal
Myogenic control of the smooth muscle via the interstitial cells of cajal. The interstitial cells of cajal sit in the smooth muscle of the digestive tract and regulate the intrinsic rhythmicity of the muscle. They can spontaneously depolarise causing contraction of the smooth muscle. The contraction is sporadic and can be overridden or controlled by hormonal or neuronal mechanisms
