regulation of GI function

Which stimuli do sensory cells in the GI tract detect?

Wall stretch, nutrient and metabolite concentration, osmolarity, pH and mucosal irritation

Stimulation of these sensory cells initiates reflexes that coordinate smooth muscle activity and hormone release.

What type of feedback mainly regulates digestion?

Negative feedback

Digestion is mainly regulated by negative feedback, coordinated by neural and hormonal regulation.

what role does the CNS play in regulating the GI function?

sight + smell of food and regulation of appetite

Short reflex arcs correspond to which nervous system?

Enteric nervous system - uses sensory and motor nerve cells within the wall of the GI tract.

Long reflex arcs correspond to which nervous system?

autonomic nervous system with connection with the central nervous system

In long reflex arcs, what do the two ANS divisions do to digestion?

Parasympathetic = rest and digest (promotes)

Sympathetic = halts digestion

Can the enteric nervous system operate independently of the rest of the body?

Yes - the ENS consists of short reflex arcs in the GI wall and can operate independently.

How many nerve plexuses make up the ENS?

2 nerve plexuses that synapse with each other.

What do ENS sensory cells respond to?

Content of the lumen and degree of wall stretch

What do ENS motor cells stimulate?

drive motility by stimulating smooth muscle (motility) and epithelial cells (secretion of digestive juices or hormones)

Simple vs complex ENS reflex arcs: what is the difference?

Simple: one sensory + one motor cell

Complex: simple arcs linked by interneurons, spreading the impulse wider

why are short reflex arcs important in the GI tract?

enable GI tract to have extensive control of its activities

What is the main transmitter of ENS reflexes, and are most reflexes stimulatory or inhibitory?

Most ENS reflexes are stimulatory with acetylcholine as main transmitter. Other inhibitory transmitters also exist.

Where do inhibitory ENS transmitters usually act, and with what effect?

Inhibitory transmitters usually relax sphincters such as the gastroduodenal and ileocolic junctions.

what are pre-ganglionic fibres?

nerve fibres that originate in the central nervous system (brain and spinal cord) and run out toward the organs.

what are post-ganglionic fibres?

These are the subsequent nerve fibres that emerge from a ganglion (where the pre-ganglionic fibre has synapsed) and connect directly to the target tissue or the Enteric Nervous System (ENS) within the wall of the GI tract.

what is a ganglion?

a collection or cluster of nerve cell bodies located outside of the central nervous system (brain and spinal cord).

In the autonomic nervous system pathways:

• It acts as a relay station or junction point where a pre-ganglionic nerve fibre synapses (connects) with a post-ganglionic nerve fibre.

• the ganglion is typically located embedded right within the wall of the target organ (like the GI tract).

Parasympathetic supply to the GI tract: which nerve carries the pre-ganglionic fibres and what is the transmitter?

Pre-ganglionic fibres run in the vagus nerve with acetylcholine neurotransmitter. Post-ganglionic fibres sit in the wall of the GI tract, connect to the ENS, and are also cholinergic (acetylcholine transmitter), stimulating secretion and motility.

Sympathetic supply to the GI tract: where do pre-ganglionic and post-ganglionic fibres run?

Pre-ganglionic fibres run in the splanchnic nerves and are cholinergic.

Post-ganglionic fibres run along arteries to organ of innervation or in hypogastric nerves and synapse with ENS or reduce acetylcholine release at parasympathetic pre-synapses

What do sympathetic post-ganglionic fibres release and what are the effects on the GI tract?

secrete noradrenaline (adrenergic) and inhibit secretion and motility and decrease GI blood supply

What are entero-enteric reflexes?

• Reflexes coordinating activity between different parts of the GI tract

• Example: mastication stimulates release of saliva, gastric juice, pancreatic juice and bile.

• Example: gastro-colic reflex

What is the gastro-colic reflex?

Stomach stretch relaxes the ileocolic sphincter so small intestine contents move on, making room for food from the stomach (chyme)

Gastrin hormone: production site, stimulus and effect?

• Caudal stomach

• Peptides, amino acids and ACh

• Stimulates HCl and growth of gastric/intestinal mucosa

Gastrin from the caudal stomach drives acid secretion and mucosal growth.

Secretin hormone: production site, stimulus and effect?

• Mainly duodenum

• HCl

• Stimulates pancreatic bicarbonate (HCO3−)

Acid entering the duodenum triggers secretin, which brings bicarbonate to neutralise it.

Cholecystokinin (CCK) hormone: production site, stimulus and effects?

• Mainly duodenum

• fatty acids, monoglycerides, amino acids, peptides

• stimulates pancreatic enzymes and contracts gall bladder

CCK responds to fat and protein products, releasing enzymes and bile.

Gastric inhibitory peptide (GIP) hormone: production site, stimulus and effects?

• Cranial small intestine

• fat, glucose, amino acids

• inhibits HCl and stimulates insulin production

GIP inhibits gastric acid and promotes insulin release.

what are the three phases of regulation?

cephalic, gastric, and intestinal phase

Cephalic phase: what triggers it and what coordinates it?

• pertains to head

• Anticipation of food (Pavlov's dog) and emotion

• ANS (stimulation via parasympathetic, inhibition via sympathetic)

Phases relate to the site of the stimulus; the cephalic phase pertains to the head.

Gastric phase: what triggers it and what coordinates it?

• pertains to stomach

• Stomach distension and presence of peptides

• ANS, ENS and hormones (mainly gastrin)

The gastric phase pertains to the stomach; gastrin is the main hormone.

Intestinal phase: what triggers it and what coordinates it?

• pertains to intestines

• Intestinal distension and lumen contents

• ANS, ENS and hormones (mainly secretin, CCK and GIP)

The intestinal phase pertains to the intestines.

where is the appetite regulated?

controlled by the hypothalamus

Where are the appetite and satiety centres, and what does each do?

Appetite centre: ventrolateral hypothalamus (food searching, voracious eating - direct effect on animal behaviour)

Satiety centre: ventromedial hypothalamus (food refusal, inhibits appetite centre)

Memory aid: Lateral = Look for food; Medial = Me full.

What are the 3 theories for how appetite is regulated?

• Glucostat (glucose levels)

• CCK theory (CCK levels)

• lipostat (fat levels, possibly leptin)

The mechanism is still unknown

what is motility?

co-ordinated contraction of smooth muscle in GI tract

what are the 4 different types of contraction?

• segmental

• peristalsis

• anti-peristalsis

• mass movement

What are the GI pacemaker cells, where are they, and how often do they oscillate?

• Interstitial cells of Cajal

• Between circular and longitudinal smooth muscle

• About 5 oscillations per minute

They produce repetitive, spontaneous membrane potential oscillations.

How are pacemaker oscillations passed on to synchronise smooth muscle contraction?

Via gap junctions

When does a pacemaker oscillation cause smooth muscle contraction?

Only when a neural or hormonal stimulus (e.g. ACh, gastrin after a meal) lifts depolarisation past threshold, firing action potentials, causing smooth muscle contraction

Without a stimulus, depolarisation is too weak to reach threshold, so no contraction occurs.

What causes the smooth muscle action potential, and what determines contraction strength?

• Ca2+ channel opening stimulates action potential

• frequency of action potentials determines contraction strength - higher frequency = stronger action potentials