Smooth Muscle Contraction in GI Tract

Smooth Muscle Contraction

• Single-unit smooth muscle is important for peristalsis and segmentation.
• Multi-unit smooth muscle is important for sphincters.
• Smooth muscle can contract due to:
  • Neural innervation
  • Hormonal stimuli
  • Stretch
• Slow wave potentials cause smooth muscle to be partially contracted.

GI Tract Smooth Muscle Control

• In the GI tract, stretch, acetylcholine, and some hormones stimulate smooth muscle contraction by producing slow wave depolarization.
• Norepinephrine and epinephrine inhibit smooth muscle contraction by hyperpolarizing slow waves.
• The autonomic nervous system alters contraction and secretion in the GI tract by modulating enteric nervous system activity.
• Sphincters are primarily contracted until being relaxed.

Peristalsis

• Peristalsis propels food through the GI tract (oral to anal).
• The enteric nervous system coordinates and regulates peristalsis.
• Two things that can disturb peristalsis:
  • Destroying the myenteric plexus
  • Blocking acetylcholine receptors (e.g., by atropine)
• Three things that can trigger peristalsis:
  • Local distension (smooth muscle stretch)
  • Local irritation
  • Parasympathetic discharge
• The myenteric (peristaltic) reflex has 4 steps:
  • Reflex occurs; GI tract contracts.
  • Contraction backtracks towards the mouth.
  • Smooth muscle on the anal side relaxes.
  • Contractile ring moves forward towards the anal direction.

Mixing Movements (Segmentation)

• Mixing movements (e.g., segmentation) produce non-peristaltic rings in different segments.
• In the large intestine, small pouches are called haustra.

Digestive Hormones and Vasodilation

• Digestive hormones (e.g., VIP, CCK, gastrin, secretin) are vasodilators.
• Locally released compounds (e.g., kallidin, bradykinin) are potent vasodilators.
• A drop in oxygen in the GI tract causes vasodilation.
• Parasympathetic innervation causes vasodilation in GI blood vessels.
• Heavy exercise after a meal decreases blood flow to digestive cells, potentially causing cramps or shock.

Mastication (Chewing)

• Mastication is a loop reflex controlled by conscious initiation.
• Benefits of mastication:
  • Increases surface area.
  • Breaks down food structure.
  • Prevents food scrapes.
• Five steps of the chewing reflex:
  • Bolus touches mouth receptors.
  • Masticatory muscles inhibit.
  • Jaw drops.
  • Masticatory muscles stretch.
  • Masticatory muscles reflexively contract.

Swallowing

• Conscious swallowing involves voluntary tongue pushing.
• Involuntary swallowing is triggered by pharyngeal material.
• Six steps after swallowing receptor stimulation:
  • Soft palate moves to protect nasal passages.
  • Palatopharyngeal folds approximate to block large masses.
  • Larynx moves up and anterior; epiglottis covers larynx.
  • Upper esophageal sphincter relaxes.
  • Pharyngeal muscles contract to move food to esophagus.
  • Primary peristaltic waves are triggered by food distension.

Esophagus

• Mucus glands are concentrated at the gastric end of the esophagus to prevent acid reflux.
• Food reaches the stomach faster when standing than sitting.
• A secondary peristaltic wave is generated if food sticks after the primary wave.
• The pharynx and top third of esophagus are skeletal muscle innervated by alpha motor neurons.
• The lower two-thirds of the esophagus up to the external anal sphincter is smooth muscle.
• The enteric and autonomic nervous systems regulate the smooth muscle.
• The lower esophageal sphincter relaxes as each peristaltic wave approaches the stomach.
• Achalasia is poor lower esophageal sphincter relaxation, causing food accumulation.
• Two main muscles protecting against gastric reflux are the lower esophageal sphincter and intra-abdominal pressure.

Stomach

• Three parts of the stomach: cardiac region, fundus (body), antrum.
• The cardiac region has heavy alkaline mucus secretions.
• Weak gastric peristaltic waves are triggered by material entering the fundus; the waves spread to the antrum.
• Peristaltic contractions increase as they approach the pylorus.
• Chyme squirts into the duodenum sparingly, with closed pyloric sphincter.
• Hunger pains result from fundal contractions. Stomach rumbling is from gas movement.
• Important stomach secretions are mainly in the fundus and antrum from oxyntic and pyloric glands.
• The only indispensable function of the stomach is producing intrinsic factor for vitamin B12 absorption.
• Chyme becomes more fluid in the stomach and intense peristaltic waves are triggered closer to the duodenum. This is mildly stimulated by gastrin.

Duodenum and Small Intestine

• The duodenum tends to inhibit gastric emptying.
• The enterogastric reflex inhibits gastric emptying.
• Four stimuli trigger the enterogastric reflex: low pH, amino acids/peptides, hypotonic/hypertonic chyme, fat.
• CCK is released due to fat in the duodenum. CCK (and secretin) block gastrin's effects.
• Peristaltic waves mix and layer chyme in the small intestine for absorption.
• Peristaltic waves increase after a meal due to the gastroenteric reflex.
• Secretin and glucagon can inhibit the gastroenteric reflex and small intestine motility.

Large Intestine

• The myenteric plexus is the nervous supply between the inner and outer muscle layers.
• The peristaltic rush is strong long waves caused by local irritation, quickly moving chyme to the anus.
• The ileocecal valve connects the small and large intestines.
• The colon absorbs water and ions.
• Enteritis is inflammation of the intestine, causing diarrhea.
• Mixing movements in the colon are hausrations, to absorb water and ions and propel chyme.

Defecation

• The defecation reflex is triggered by feces in the rectum.
• The rectum relaxes, internal and external anal sphincters relax.
• Cortical and subcortical control regulates defecation by consciously relaxing the external anal sphincter.

Other Digestive Issues

• Peptic ulcers are caused by H. pylori bacteria.
• Sprue causes malabsorption.
• Constipation is suppression of defecation reflexes, potentially causing megacolon.
• Flatus is gas in the GI tract (swallowed or produced). Primary gases are CO2, methane, and hydrogen gas.
• Nausea is caused by the chemoreceptor trigger zone in the brainstem, triggered by blood contents.
• Vomiting has 10 steps: distension, triggering the vomiting center, antiperistalsis, chyme distension, strong contractions, sphincter relaxation, airway protection, diaphragm and abdominal muscle contractions, and chyme expulsion.