52. Motoric activity and electrophysiology of the gastrointestinal tract Flashcards
Slow wave and spikes
The resting potential of each smooth muscle cell of the gastrointestinal canal is different. These cells show a permanent, and small activity at which two types can be differentiated: Slow wave and spikes.
slow wave
- BER = Basic Electrical Rhythm = electric activity generated by the slow waves.
- Slow wave potential changes between 5-15 mV.
- Slow wave frequency: In stomach 3/min, in duodenum 12/min.
- Slow waves do not have contractile force, so they do not cause muscle contraction on their own, but their rhythm defines the frequency of the contraction. They are also called pacemaker cells.
- Resting membrane potential: 50-60 mV.
Spike potential
- It is the action potential of the GI smooth muscle cells.
- Slow waves triggers spike potential when threshold level is reaches (about 40 mV).
- Depolarization can be triggered in different ways:
- By stretching the muscle cell
- Acethylcholine from parasympathetic nerve endings
- Parasympathetic mimics.
- Specific GI hormones.
- Hyperpolarization (less excitable muscle cell) can be caused by:
- Adrenaline in blood
- Noradrenaline, released from sympathetic nerve endings.
- Specific GI hormones
Neurohormonal regulation
- Central neural regulation: chewing, swallowing, defecation
- Peripheral neural regulation: gastric, pancreatic, bile secretion, gastrointestinal motility, intestinal juice production
Extrinsic neuronal regulation
- Sympathetic: postganglionic
- Noradrenaline causes hyperpolarization, and decreases resting membrane potential
- Leads to decreased function of intestinal tract
- Parasympathetic: preganglionic
- Cause hypopolarization
- Leads to increased motility and secretion
Intrinsic neuronal regulation
- Plexus myentericus: efferent fibers of the intrinsic regulation
- Plexus submucosae: afferent fibers and receptors of the intrinsic regulation
- Effect is mediated through neurotransmitters:
- Activation: acetylcholine
- Inhibition: VIP and opoid peptides
- Controls: GI secretion, peristalsis, mingling, local inhibition, absorption, intensity and frequency of contraction.
Gastrointestinal reflexes
- Intestines → prevertebral ganglion → intestines
- Gastic-colon reflex: colon empties by the effect of the fullness of the stomach
- Colon-gastric reflex: dilation of the colon terminates gastric secretion and movement
- Colon-ileum reflex: the fullness of the colon inhibits emptying of the ileum into the colon
- Intestines →spinal chord, brain stem → intestines
- Reflex influencing gastric motor function and secretion
- Noiciceptive reflex: cause paralysis of the GI tract
- Defecation reflex: contraction of the colon, rectum and abdominal muscles
Movements of the GI tract
Passing movement:
- Stretch receptors in the intestines cause the oral part to narrow while the aboral part dilates.
- The circular muscle layer behind the bolus is inhibited for further movement, while the longitudinal layer in front is dilated.
Mixing movement
Mingling bolus
Villi movement:
- By mucosal muscle layer
- Secures contact between epithelium and intestinal content.
Motor movements of stomach
Filling: As stomach fills up, smooth muscles become longer, but intragastric pressure hardly changes (LaPlace law)
Step 1:
A circular retraction which is a rolling-mixing tonic contraction.
Step 2:
A circular contraction can be observed in the middle area of the corpus, in a fulfilled stomach.
Step 3:
The contraction rings become more and more tight.
Step 4:
Through the gap in the contraction ring, chyme whirls back towards the oral parts of the stomach, the pressure increases (mixing).
Only the distal part of the stomach takes part in peristalsis, and the increase in the tone of the proximal part presses the gastric content in an aboral direction.
(Peristalsis is a series of wave-like muscle contractions that moves food)
Step 5:
The more diluted the gastric content is - the more is pressed through into the small intestines, depending on the pressure conditions of stomach-pylorus-duodenum.
This process is strengthened by the activity of the pre-pyloric region of the antrum and it somehow “helps on” the emptying of the chyme. Later the emptying of the chyme becomes continuous.
Regulation
Duodenal inhibiting effects:
- Neuronal reflexes: dilation, irritation, osmolarity, degradation of products
- Hormonal: secretin, GIP, CCK
Gastric facilitating effects:
- Volume of foodstuff (stretch gastric wall)
- Gastrine (facilitates pyloric pump)
Motor movements of ileum
Mingling: Increases the breakdown of fat and degree of contact with the intestinal wall, increases absorption. -Segmental movement -Villi movement -Slow peristalsis
Ileocecal valve prevents backflow of colon content.
Ileocecal sphincter is normally light contracted, this slows down the process of passing.
-Chyme remains longer in ileum, which results in a greater degree of absorption.
Regulation:
Influenced by n.splanchnicus: sympathetic inhibition and n.vagus: parasympathetic excitation
Motor movements of colon
- Colon is responsible for microbial synthesis
- Reabsorption of water and electrolytes
Ruminants:
(ileum – colon – caecum – colon)
This is to slow down the flow of the intestinal content
Horses:
1st type contraction:
-Content pressed into basal part and copula, segredation of copula leads content into colon
2nd type contraction:
-Gas pressed into copula, liquid sink, gas into colon
3rd type contraction:
-Constant contraction appears in the haustrum which creates continuous mixing in a way that contracts during peristalsis in oral and aboral direction.
-Peristalsis + antiperistalsis mixing in colon
Carnivores:
- Small microbial synthesis
- Fast movement caused by an aboral-mass peristalsis.
- Small peristalsis can pass the intestinal content for a large distance.
Motor movements of rectum
Main role: evacuation. (Feces are stored in colon)
- Defecation is under hypothalamic and cortical control.
- During feces evacuation reflex stretch receptors in the wall sends afferent neurons to S1-S4.
- Efferentation: Pelvic nerves increase contraction and peristalsis, relaxes sphincters.