Notes
Swallowing & Gastric Motility
Sections
Key concepts: Peristalsis & Slow Waves
Peristalsis is characterized by wave-like smooth muscle contractions that propel food through the GI tract.
Review the muscular layers of the GI tract – we use the stomach as an example.
From outer to inner, the muscularis layer of the stomach comprises a longitudinal, circular, and oblique layer.
As we learn in more detail elsewhere, the longitudinal layer decreases the length of the GI tract; the circular layer decreases the circumference of the tract. Together, these layers help to mechanically propel the food distally and to physically break it into smaller pieces along the way.
The oblique layer, which is unique to the stomach, facilitates a "twisting" action that churns the stomach contents to form chyme.
Slows waves produce the basal electrical rhythm of smooth muscle contraction in the GI tract. They originate from specialized "pacemaker cells" in the interstitial cells of Cajal.
Slow waves are characterized by spontaneous partial waves of depolarization that spread through adjacent smooth muscle cells; they are not action potentials, but action potentials occur in response to the depolarized cell membranes.
Slow waves occur at a much slower frequency than either neuronal or cardiac action potentials, and their frequency differs along the length of the GI tract.
Gastric Motility Regulation
Anatomical Structures
First, the anatomical structures:
The oral cavity is where mastication occurs – the tongue and teeth break down ingested food into smaller particles and mix it with saliva.
The pharynx is a striated muscular tube – we commonly refer to it as the "throat"; it is a shared passageway for air, foods, and liquids.
The esophagus is a posterior continuation of the pharynx; it carries food to the stomach.
The respiratory tract is an anterior continuation of the pharynx; the larynx and trachea deliver air to the lungs.
When we swallow, it's essential that we direct foods and liquids in the pharynx towards the esophagus, and away from the respiratory tract. Show that a cartilaginous leaf-shaped flap, called the epiglottis, covers the opening of the larynx.
Let's see how the epiglottis works to direct the passage of air, food, and liquids in the pharynx.
First, we draw the esophagus and respiratory tract; then, show that the epiglottis remains open to allow air to enter the larynx and trachea.
Then, we re-draw the esophagus and respiratory tract, but show that the epiglottis falls over the opening of the respiratory tract so that foods and liquids are diverted towards the esophagus and blocked from entering the larynx and trachea.
In the stomach, we indicate three anatomical regions: the fundus, body, and pylorus; label the pyloric antrum, specifically.
In addition to anatomical regions, we also need to know the physiological regions; we label the orad region, which comprises the fundus and proximal body (the proximal 1/3 of the stomach), and, the caudad region, which comprises the distal body and the pylorus (the distal 2/3 of the stomach).
Three sphincters regulate the passage of GI contents:
- The Upper Esophageal Sphincter, which regulates the passage of food into the esophagus.
- The Lower Esophageal Sphincter, which regulates the passage of food into the stomach.
- In the stomach, the pyloric sphincter, which regulates the passage of food from the stomach to the small intestine.
One more anatomical note: although most of the GI tract comprises smooth muscle, the pharyngeal wall and upper 1/3 of esophagus, including the muscle that comprise the upper esophageal sphincter, are striated muscle.
Swallowing
Aka, deglutition.
Swallowing is voluntarily initiated in the mouth, then becomes involuntary, aka, under reflex control.
Swallowing occurs in three phases:
First, in the oral phase, the tongue, which is under voluntary control, pushes food towards pharynx, which activates the swallowing reflex.
The swallowing reflex is mediated by the vagus and glossopharyngeal nerves, which carry information to the medullary swallowing center; the medulla then sends motor signals to the striated muscles of the pharynx and upper esophagus.
Then, in the pharyngeal phase, the soft palate pulls upward, blocking off the nasopharynx, and the epiglottis falls over the larynx to larynx to block off the respiratory tract (breathing is inhibited during the pharyngeal phase).
The upper esophageal sphincter relaxes, and peristaltic waves propel the food into the esophagus.
The esophageal phase is an involuntary phase in which a primary peristaltic wave travels down esophagus, pushing the food; If the primary wave isn't sufficient to clear the esophagus, a secondary peristaltic wave is initiated by enteric nervous system.
During this time, the upper esophageal sphincter closes so food can't reflux to pharynx.
When food arrives at the distal esophagus, the lower esophageal sphincter opens and food passes to the stomach; the lower esophageal sphincter opens in response to stimuli from the vagus nerve.
Heartburn:
In its tonically closed state, the lower esophageal sphincter prevents stomach contents, including acid, from refluxing into the esophagus.
However, when intra-abdominal pressure is increased (as in pregnancy or obesity), or when there is an issue with the sphincter itself, stomach contents can be "pushed back" into the esophagus – a condition we call acid reflux, which can cause chest and throat pain known as "heart burn"
Gastric Motility
Motility in the stomach allows for its various functions: the stomach expands to receive and store food, it "churns" to break down food particles and mix them with gastric secretions to form chyme, and, it controls the release of chyme into the duodenum, the first part of the small intestine.
Although we won't show it here, be aware that, even during the fasting state, slow migrating motor complexes form slow peristaltic waves. These waves begin in the fundus and pass through the stomach distally, sweeping food and gastric secretions towards the small intestine.
Filling Phase
The first phase of gastric motility is the filling phase, which relies on the vagovagal reflex (be careful not to confuse this with the vaSovagal reflex!).
When the lower esophageal sphincter opens to release food to the stomach, receptive relaxation promotes reduced pressure and increased volume in the orad region of the stomach. During filling, the stomach can expand several times its empty volume.
Mixing Phase
The next phase is mixing of the stomach contents, which is crucial for digestion and absorption.
Slow tonic contractions of the smooth muscle begin proximally and move distally towards the pylorus; this "churning" action of the stomach breaks food into smaller particles and mixes them with gastric juices to form a semi-liquid called chyme. Recall that the oblique layer adds "twisting" movements to further squeeze the stomach contents.
When food reaches the pylorus, strong contractions in the antrum forcefully push the chyme against the closed pyloric sphincter, which produces retropulsion - food is repelled backwards for increased mixing in the antrum.
Emptying phase
Forceful contractions, sometimes called the "pyloric pump" of the pyloric antrum push small amounts of chyme through the pyloric sphincter and into the duodenum of the small intestine, where further digestion and absorption take place.
Some important notes about the rate of gastric emptying:
Water and other nutrient-poor liquids leave the stomach soon after arriving there; food particles are churned in the antrum until they are about 1mm3 or smaller.
On average, it takes at least a few hours for the stomach to completely empty. As mentioned before, the chemical and physical make-up of the stomach contents determines the emptying rate.
In general, meal ingestion releases hormones that slow gastric emptying, including CCK, leptin, GLP1, glucagon, and insulin. Fatty and/or high protein foods, as well as acidic foods, need longer in the stomach for processing, so they slow the rate of emptying.
During fasting, ghrelin and motilin are released; they increase gastric emptying rate, which keeps the stomach clear of food and digestive juices.