Digestive System Motility Notes
Stomach Functions and Structure
The stomach serves two main functions:
Storage vat to control the rate of food delivery to the small intestine.
Grinder and sieve that reduces the size of food particles, releasing them only when they are appropriately broken down for small-intestinal digestion.
The stomach can be divided into two physiological regions:
Proximal region:
Located at the esophagus end of the stomach.
Functions primarily as a storage area.
Characterized by weak continuous contractions that help shape the gastric wall per its content and provide gentle propulsion of contents into the distal stomach.
The major muscular reflex is adaptive relaxation, where muscles relax as food enters the stomach.
Little mixing occurs due to passive muscular activity in this region.
As the stomach empties, wall tension in the proximal region slightly increases, pushing food towards the distal stomach for processing.
Distal region:
Serves the grinding and sieving function, breaking solid food into smaller particles.
Features intense slow-wave activity and frequent muscular contractions.
Peristaltic waves begin at the junction of the proximal and distal areas, moving toward the pylorus.
Pylorus constricts during these waves, causing some food to be crushed and propelled back into the proximal stomach, while some finely ground material passes into the duodenum.
Gastric Motility Control
The motility of the stomach and other parts of the GI tract is controlled by secretory substances from the Enteric Nervous System (ENS) and the endocrine and paracrine systems.
Fibers of the vagus nerve synapse with the cell bodies of the gastric myenteric plexus, exerting high control over gastric motility.
In the proximal region, vagal activity suppresses muscular contraction, permitting adaptive relaxation.
In the distal stomach, vagal stimulation results in intense peristaltic activity.
Events within the Central Nervous System (CNS), as well as stimuli from the stomach and intestines, stimulate vagal action.
For example, the anticipation of food increases vagal stimulation and prepares the stomach for incoming food, which intensifies once food enters the stomach.
Regulation of Gastric Emptying
Rate of gastric emptying must align with the rate of digestion and absorption in the small intestine.
Afferent sensor neurons in the duodenum detect content characteristics:
Low pH
High osmolality
Presence of fat
Activation of these sensors triggers reflexes that regulate gastric emptying.
Between meals, specialized motility known as interdigestive motility complex is required to clear indigestible materials.
Peristaltic waves of the interdigestive motility complex occur at approximately 1-hour intervals when the stomach is relatively empty.
Vomiting Reflex
Vomiting is a complex reflex coordinated by the brainstem involving multiple muscle groups and structures outside of the GI tract.
Associated actions of vomiting include:
Relaxation of stomach and lower esophageal sphincter muscles and closure of the pylorus.
Contraction of abdominal muscles, increasing intra-abdominal pressure.
Expansion of the chest cavity while maintaining a closed glottis.
Opening of the upper esophageal sphincter.
Antiperistaltic motility in the duodenum.
Afferent stimulation for the vomiting reflex comes from numerous receptors, particularly mechanoreceptors in the pharynx and tension & chemoreceptors in gastric and duodenal mucosa.
These receptors send signals to the vomit center in the brainstem.
Notably, vomiting isn't always a primary GI issue but can relate to stimuli from other organs.
Small Intestine Motility
Motility of the small intestine occurs in two distinct phases:
Digestive period after food intake.
Inter-digestive period when little food is present.
Digestive phase: Two primary motility patterns:
Non-propulsive (segmentation):
Localized contractions of circular muscles in 3 to 4 cm segments.
Creates portions of constricted and dilated lumen.
This action mixes digestive contents and circulates them over absorptive surfaces, effectively “milking” them back and forth.
Slows down material movement due to lumen closure in constricted segments.
Propulsive movements:
Peristaltic contractions push ingesta down the gut in sync with slow waves.
Short-distance propulsion intercalates with segmentation to effectively mix and move ingesta.
Inter-digestive phase:
Characterized by migrating motility complexes (MMC), which consist of powerful peristaltic waves sweeping over large lengths of the small intestine.
MMC serves housekeeping functions, pushing undigested materials out and controlling bacterial populations in the upper gut.
The duodenum generally maintains low bacterial populations, while higher concentrations are found in the ileum and colon, with MMC aiding in impeding bacteria migration to the duodenum.
Movement to the Colon
Ingesta moves from the small intestine to the large intestine at the ileocecal junction.
Ileocecal sphincter:
Prevents retrograde movement of colonic contents into the ileum.
Typically tightly closed but relaxes during ileal peristaltic activity to permit movement into the colon.
Increased colonic pressure causes tighter constriction of the ileocecal sphincter.
Functions of the Colon
The colon has three primary functions:
Absorption of water and electrolytes.
Storage of feces.
Fermentation of organic matter that escapes digestion in the small intestine.
Function relative importance varies among species, leading to differences in colon size and shape.
Colon size is chiefly determined by the significance of colonic fermentation for energy needs.
Despite anatomical differences, motility patterns remain similar across species: mixing activity is key to both absorption and fermentation functions.
Colonic Motility
A distinct characteristic of colonic motility is retropulsion (antiperistalsis):
This movement migrates orally, opposing normal peristalsis.
It hinders ingesta movement, enhances mixing, and causes material accumulation in proximal colon regions.
The colon serves as a critical site for storage and absorption.
Material entering a carnivore's colon is often fluid; through mixing in the ascending and transverse colon, significant water and electrolytes are absorbed.
By the time contents reach the descending colon, they become semisolid and form feces.
Anal Sphincters and Defecation
The descending colon connects with the anal sphincter, composed of two layers with differing innervation:
Internal sphincter:
Comprised of smooth muscle, an extension of the circular muscle layer of the rectum.
Maintains tonic contraction for anal continence.
Parasympathetic innervation relaxes it; sympathetic stimulation causes constriction.
External sphincter:
Made of striated muscle, maintaining some tonic contraction.
Innervated by general somatic efferent fibers with cell bodies in cranial-sacral spinal segments.
Rectosphincteric Reflex and Defecation
Entry of feces into the rectum triggers a rectosphincteric reflex involving:
Relaxation of the internal anal sphincter.
Peristaltic contractions of the rectum to aid defecation.
In certain animals like humans, defecation can be voluntarily suppressed, leading to relaxation of the rectum and regained tone of the internal anal sphincter.
Uninhibited animals respond to fecal presence in the rectum with voluntary actions linked to defecation.
In carnivores, contraction of diaphragm and abdominal muscles raises intra-abdominal pressure while striated muscles of the anal canal relax during the defecation posture.
Differences Between Avian and Mammalian Digestive Systems
Major differences include:
Birds lack teeth and have distinct anatomical regions for gastric functions.
Pharynx in birds is simpler than in mammals (lacking a soft palate).
The beak in carnivorous birds is modified for tearing food into small pieces that are swallower-friendly.
The esophagus features a large diameter to accommodate un-masticated food and includes an out-pouch called the crop.
The proventriculus is the glandular stomach portion, separate from the muscular stomach called the ventriculus or gizzard.
The rectum leads to the cloaca, a common passage for digestive, urinary, and reproductive discharges.