L39. Organization and Regulation Flashcards
(17 cards)
What is the GI tract?
The gastrointestinal system is:
- A long epithelium lined tube that originates during embryonic folding
- Functional sections are separated by sphincters
- Connected to accessory exocrine glands
What are the functions of the GI system?
The GI tract takes in food, digests it to extract and absorb metabolites for the growth and energy needs of the body plus fluid and electrolytes to replace losses, and expel the remaining waste; to do this the GI tract must:
- Reduce the size of food to allow absorption
- Deliver material to site of absorption
- Absorb necessary material and excrete the rest
While also being a barrier to:
- Pathogens and disease
- To digestive enzymes and acid
Mechanisms:
- Motility
- Digestion and secretion
- Absorption
- Excretion of wastes
What are the layers of the gut wall?
Mucosal layer
- Epithelium
- Lamina propria
- Muscularis mucosae
Sub-mucosal layer
- Submucosal plexus
- Connective tissue blood vessels
Muscularis layer
- Inner circular muscle
- Outer longitudinal muscle
- Myenteric plexus
Serosal layer
- Outer sheath
What is special about epithelial structure?
The structure of epithelial lining is specialised for each functional section of the GI tract
E.g. the epithelia lining the small intestine has absorptive cells near the surface and secretory cells located in the crypts
What are the specialised structures of the mucosa and submucosa?
The structure of the mucosa is different for each functional section
Small intestine:
- Plicae circulares: folds containing a core of submucosa
- Villi: Folds containing a core of lamina propria
- Microvilli: Finger-like projections on the surface of epithelial cells
Tell me about blood supply in the GI system?
Microscopically
- Blood and lymphatic vessels: carry nutrients to and from the submucosa
Dense capillary beds penetrate:
Mucosa of stomach - secretion
Villi of small intestine - absorption
Crypts of small intestine - secretion
Mucosa of large intestine - secretion and absorption
Macroscopically
~25% cardiac output at rest
Posrprandial hyperemia (this means increased blood flow after eating)
Venous return - via liver (hepatic portal vein) allowing for removal of toxins
What is the enteric nervous system ENS?
Neuron types
Enteric NS neurons:
- Afferent sensory
- Mechanoreceptors
- Chemoreceptors (inputs from enteroendocrine cells)
- Interneurons
- Efferent neurons (motor and secretory)
ENS strucutres:
- Submucosal plexus which controls the mucosa (secretion, absorption, movement of the villi)
- Myenteric plexus which controls the muscularis (motility)
What are the general principles of regulation?
Regulation is by homeostasis (trying to get back to resting)
Controlled variables: conditions in the lumen
Receptors: chemoreceptors, osmoreceptors, and mechanoreceptors
Signals: both neural and hormonal
Effectors: GI tract smooth muscle and GI tract epithelial cells
What is phased regulation?
The GI tract has different functions at different phases of digestion
The regulation of the stomach changes to facilitate these different functions
Cephalic phase (preparative):
- Detect: food - sight, smell, taste
- Prepare: the GIT lumen (CNS via ENS)
Gastric phase (digestive):
- Detect: distension, pH, nutrients
- Prepare: send signals via ENS, CNS, hormones (e.g. gastrin)
Intestinal phase (controlled release):
- Detect: stretch, acid, osmolarity, nutrients
- Prepare: ENS, CNS, hormones (e.g. CCK and secretin)
What is neural regulation like in the GIT?
Neural signaling is used when rapid and precise regulation is required
Extrinsic reflexes involve the CNS
–> Regulation and coordination of GI function over long distances e.g. complex behaviours such as swallowing
Intrinsic (local) reflexes of enteric nervous system (ENS)
–> Regulation over centimeters (local reflexes)
E.g. relaxation of the internal anal sphincter in response to rectal distension
How does regulation happen by the ENS?
ENS regulation provides:
- Rapid precise regulation of smooth muscle and secretory and absorptive epithelial cells to coincide with digestive events and phases (e.g. secretion of gastric acid when peptides are detected in the stomach)
Involves:
- Regulation over short distances (centimeters)
- Intrinsic (local) regulation within the ENS
- No input from the CNS is needed
What are the receptors and neurotransmitters involved in regulation by the ENS?
Receptors:
Located in the GI tract epithelium and wall, respond to various stimuli:
- Stretch, pH, osmolarity, products of digestion
Neurotransmitters:
–> Stimulation = acetylcholine and tachykinins
–> Inhibitory = mainly nitric oxide (NO), also VIP, ATP
How does regulation happen by the CNS?
Extrinsic external regulation where the CNS acts on the ENS
- Coordination of GI function over long distances
- Coordination of complex behaviours such as swallowing
- Regulation based on external factors such as stress and anticipation of food
Two sensory pathways
1. Long reflex loop (gut to brain and back again)
- Receptors in the GI tract send messages to the brain via the ENS and afferent autonomic pathways. The brain then sends coordinating messages back to the ENS via efferent autonomic neurons
- External receptors
- Somatic and special senses (such as sight, taste and smell) detect conditions that might impact the GI tract and the CNS sends messages to the ENS via the autonomic NS
Go into more detail on regulation by the CNS?
- Acts via the peripheral autonomic pathways modulating the activity of the ENS
- Both sympathetic and parasympathetic interact with:
- Afferent fibres of sensory ENS neurons which detect information regarding state of gut and relay it to the CNS
- Efferent fibres of the ENS which signal a response in smooth muscle cells or epithelial cells
- As a rule, no direct innervation of GI tract - acts via the ENS
Exceptions to this rule:
- Upper structures involved in swallowing and external anal sphincter
- Internal stimuli - long reflex (gut to brain and back)
- External stimuli - mood, thought, smell, sight
Sympathetic nervous system:
- Usually inhibitory
- Noradrenaline, detected by alpha adrenergic receptors
- Short preganglionic and long post ganglionic
Parasympathetic nervous system:
- Usually stimulates activity
- Acetylcholine, detected by muscarinic receptors
- Long preganglionic and short post ganglionic
What is endocrine regulation in the GI tract?
- Hormones have a slow but sustained action, which is important as it can take many hours to digest and absorb a meal
- GI hormones are secreted by enteroendocrine cells throughout the intestinal tract
- Largest endocrine organ in body
- Hormones can be regulated by both intrinsic and extrinsic stimuli
- GI hormones have multiple effector sites - coordinate phases of digestion
Go into more detail on endocrine regulation?
- GI hormones have mechanisms that include endocrine, paracrine, and neurocrine actions (some do more than one of these)
- Sensors for endocrine responses are enteroendocrine cells
- Exposed to the lumen of the GI tract
- Respond to changes in luminal contents by releasing (via exocytosis) granules containing hormones from their basolateral pole in the blood
- These same cells can also be neural receptors
What are the components of the immune system in the GI tract?
- Epithelial barrier
- Mesenteric lymph nodes
- Peyers patches
- A range of immune cells
- Intraepithelial lymphocytes, B cells, T cells, mast cells, macrophages, eosinophils
