What is motility?
The action of the muscles of the GI tract that mix and propel its contents from mouth to anus
What is peristalsis?
The involuntary constriction and relaxation of the muscles of the intestine or another canal, creating wavelike movements that push the contents of the canal forward
What is segmentation?
Contractions are a common type of mixing motility seen especially in the small intestine-segmental rings of contraction chop and mix the ingests. Alternating contraction and relaxation of the longitudinal muscle in the wall of the gut also provides effective mixing of its contents
Describe the big picture of enteric motility
Gastrointestinal (GI) motility focuses on digestive motor function and the transit of ingested material within the GI tract
- Motility involves the coordination of smooth muscle and nerve function to mix, titrate, and propel products of digestion
- While the esophagus, stomach, small bowel, and large intestine has its own specialized functions, all work collaboratively to aid in digestion and motility
What are the types of motility patterns?
Tonic contractions
Phasic contractions
Depolarization of circular or longitudinal muscle leads to a contraction that alters diameter or length
Define What are tonic contractions?
Maintain constant level of contraction without regular relaxation(sustained)
-Lower esophageal, pyloric, Ileocecal, internal anal sphincters
What are phasic contractions?
- Periodic contractions followed by relaxation; seconds
- Esophagus, gastric antrum, small intestine- colicky contractions
Describe in depth what are tonic and phasic contractions
The time course of contractions among smooth muscles in the GI tract varies. Some muscles such as those found in the esophagus, small intestines and gastric antrum contract and relax in a matter of seconds (phasic contractions) other smooth muscles as those found in the lower esophageal sphincter, oral stomach, ileocecal and internal anal spchincters show slow sustained contractions that last from minutes to hours. These are called tonic contractions. The type of contraction is governed by the smooth muscle itself or the interstitial cells of Cajal located in them
What are tonic contractions governed by?
By smooth muscle itself or the interstitial cells of Cajal located in them
Generate the slow waves, act as intrinsic pacemaker cells for enteric motility
Located mostly between circular and longitudinal muscles, but also within each muscle layer (different subtypes of ICC)
ICCs form and electrical synctium
-electrically coupled by gap junctions to each other, the ENS and to neighboring SM cells, enabling motor coordination
Integrate gut mechanics (stretch) into motility pattern
Bolus/chyme distends the gut, stretching the walls
ICCs are stretch sensitive and their membrane potential in the stretched section becomes more depolarized
when a slow wave passes over this area of sensitized smooth muscle a contraction is more likely, aiding peristalsis and segmental contractions
Do phasic and tonic contractions depend on neural or hormonal input?
They don’t. Neurocrines, endocrine and paracrine are important because they modulate them basic contractile activity, so that the amplitude of the contractions of phasic muscle varies and the tone of the tonic muscle increases or decreases
What are some uses of phasic contractions?
Peristalsis, segmentation, MMCs
Peristaltic contractions create forward movement
Propulsive segment- relaxation of longitudinal muscle; contraction of circular muscle
Receiving segment- Contraction of longitudinal muscle; inhibition of circular muscle
There are also Segmental contractions: which result in mixing with no net forward movement
What helps the basic processes to occur?
Slow waves
Slow waves are oscillating membrane potentials
- occur spontaneously
- originate in the interstitialcells of Cajal,(pacemaker for GI smooth muscle)
- not action potentials
- Determine pattern of contraction
What helps the basic processes to occur?
ICCs cause cyclic opening of Ca2+ channels(depolarization) opening of K+ (repolarization)
- Depolarization during each slow wave brings the membrane potential of smooth muscle to threshold
- therefore, increases the probability that action potentials will occur
What helps the basic processes to occur?
When the bolus distends the gut, stretching stimulates nerves in the wall of the gut to release neurotransmitters into smooth muscle at the site of distension
Neurotransmitters increase the intracellular Ca++ via voltage gated channels
- When these channels are activated, rapid transients in membrane potential causing the membrane potential of the section smooth muscle becomes “more depolarized”
- Each time the peak of the slow waves temporarily become more positive than -40mv, spike potentials appear on those peaks
- The higher the slow wave potential rises, the greater the frequency of the spike potentials, ranges between 1-10 spikes/seconds
- Action potentials, produced on top of the background of slow waves, then initiate phasic
- Results in contractions of the smooth muscle
When a slow wave passes over this area of sensitized smooth muscle, spike potentials form and contraction results
- The contraction moves around and along the gut in the coordinated manner because the muscle cells are electrically coupled through gap junctions
- However, sympathetics cause hyperpolarization (down-regulation)
Describe the frequency of slow waves
- Varies along the GI tract but is constant and characteristic for each part of the GI tract
- is not influenced by neural or hormonal output. In contrast, the frequency of the action
- Potentials that occur on top of the slow waves is modified by neural and hormonal influences
- sets the maximum frequency of contractions for each part of the GIbtract.
- is lowest in the stomach (3 slow waves/min) and highest in the duodenum (12 slow waves/min)
What type of motility pattern is expected to be seen in oral cavity and esophagus?
Swallowing, chewing
-primary and secondary peristalsis
- Role of Sphincters
- Clinical correlation - GERD(heart burn), Achalsia
What types of motility pattern do you expect to see in the stomach?
Peristaltic mixing and propulsion
Fed state:
- Receptive relaxation
- Mixing and digestion
- gastric emptying
Fasting state: MMCs
What types of motility pattern do you expect to see in the small intestine?
Mixing and propulsion primarily by segmentation:
- Segmentation contractions
- Peristaltic contractions
-Gastroileal reflex
What types of motility pattern do you expect to see in the large intestine?
Segmental Mixing ; mass movement for propulsion
- Role of sphincters
- Segmentation, mass movements
- Defecation
- Reflexes
Clinical correlation
- Constipation/Diarrhea
- Hirschsprung disease
Describe the motility of oral cavity
The mouth serves as the first site of mechanical and chemical digestion of food
Mastication/chewing:
- Mixing, reducing particle size and aiding in digestion
- Controlled by voluntary and involuntary nerves
- pressure of food in the mouth triggers the chewing
- Size of the swallowed particle has no effect on the digestive process
- Mixes the food with saliva
- Pattern of chewing is based on input from the medulla
- BUT increases taste pleasure
Describe the stages of esophagus motility
Swallowing reflex- coordinated by CN X and X; to/from the medulla
The act of swallowing is coordinated largely initiated voluntarily and becomes involuntary initiated
- Voluntary stage- Tongue pushes bolus against soft palate and back of mouth, triggering swallowing reflex
- Pharyngeal stage- upper esophageal sphincter relaxes while epiglottis closes to keep swallowed material out of the airways
- Esophageal stage- Food moves downward into the esophagus, propelled by peristaltic waves and aided by gravity
What is the effect of the nervous system on esophagus motility?
A parasympathetic response involves afferent feedback to the swallowing center followed by efferent response through other nuclei.
- This coordination allows muscle to be contracted in a proximal-distal manner down the esophagus. (Primary peristaltic wave)
- The pressure in the thoracic esophagus is below atmospheric and stomach pressure. Therefore, must have sphincters.
- The upper 1/3 is skeletal muscle, the lower 2/3 is smooth muscle
- Non-vagal- coordinates the activity of CN V, VII, XII
- Nucleus Ambiguus- somatic nerves regulate the pharynx and striated areas of the esophagus
- Dorsal Motor Nucleus- Autonomic nerves, visceral motor type to the smooth muscle of esophagus via ENS
Differentiate primary and secondary peristalsis of the esophagus
Primary peristalsis
-Once the bolus passes the UES, series of coordinated movement sends a positive pressure wave to propel the bolus into the stomach
- Contraction occurs proximal to bolus, relaxation caudal to bolus.
- This propels the bolus to the stomach and opens the LES as it approaches. Gravity accelerates the movement
- this takes about 6-1 0 seconds
- Relaxation of LES- mediated by Vagus, nitric oxide and Vasoactive intestinal peptide
Secondary Peristaltic valve
- IF bolus not propelled to stomach in first wave:
- Stretch of esophagus by bolus
- Activated pressure receptors
- Second peristaltic wave
What is GERD?
- Reflux of gastric contents into the lower esophagus
- Decreased lower esophageal sphincter tone
- Barium swallow or upper endoscopy with biopsy
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Motor Systems 112
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Endocrine System 421
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Acid base physiology39
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Respiratory Tract 1: Gas Transport30
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