what are the functions of the small intestine?
The functions of the small intestine are digestion and absorption of nutrients
what does motility of the small intestine serve to?
motility of the small intestine serves to:
- mix the chyme with digestive enzymes and pancreatic secretions
- Expose the nutrients to the intestinal mucosa for absorption
- propel the unabsorbed chyme along the small intestine into the large intestine.
what determines the rate of contraction of GIT smooth muscles?
the frequency of slow waves determines the rate at which action potentials and contractions occur.
rates of GIT smooth muscle contractions
Duodenum: 12 waves per minute
Ileum: 9 waves per minute.
Stomach: contractions (called migrating myoelectric complexes) occur every 90 minutes
innervation of small intestine
- There is both parasympathetic and sympathetic innervation of the small intestine.
- Parasympathetic innervation occurs via the vagus nerve, and sympathetic innervation occurs via fibers that originate in the celiac and superior mesenteric ganglia.
- Parasympathetic stimulation increases contraction of intestinal smooth muscle, and sympathetic activity decreases contraction
what do parasympathetic fibers to GIT secrete?
- Although many of the parasympathetic nerves are cholinergic (i.e., they release ACh), some of the parasympathetic nerves release other neurocrines (i.e., they are peptidergic).
- Neurocrines released from parasympathetic peptidergic neurons of the small intestine include VIP, enkephalins, and motilin.
what are the two patterns of contraction in the small intestine?
- segmentation contractions and peristaltic contractions.
- Each pattern is coordinated by the enteric nervous system
what is the importance of segmentation contraction of the small intestine?
Segmentation contractions serve to mix the chyme and expose it to pancreatic enzymes and secretions
mechanism of segmentation contraction of small intestine
- Step 1 shows a bolus of chyme in the intestinal lumen
- A section of small intestine contracts, splitting the chyme and sending it in both orad and caudad directions (Step 2).
- That section of intestine then relaxes, allowing the bolus of chyme that was split to merge back together (Step 3).
- This back-and-forth movement serves to mix the chyme but produces no forward, propulsive movement along the small intestine.
what is the importance of peristalitic contraction of the small intestine?
peristaltic contractions are designed to propel the chyme along the small intestine toward the large intestine
what should happen to accomplish the propulsive movement of food in the small intestine?
- To accomplish such propulsive movements along the small intestine, circular and longitudinal muscles must function oppositely to complement each other’s actions. (Recall that contraction of circular muscle decreases the diameter of that small intestinal segment, whereas contraction of longitudinal muscle decreases the length of that small intestinal segment.)
- Consequently, when the circular muscle of a segment contracts, the longitudinal muscle simultaneously relaxes; when the longitudinal muscle contracts, the circular muscle simultaneously relaxes.
mechanism of peristalitic movement in the intestine
Peristalsis therefore occurs as follows:
- The food bolus in the intestinal lumen is sensed by enterochromaffin-like (ECL) cells of the intestinal mucosa, which release serotonin (5-HT).
- The 5-HT binds to receptors on intrinsic primary afferent neurons (IPANs) that, when activated, initiate the peristaltic reflex in that segment of small intestine, behind the bolus
- excitatory transmitters (e.g., ACh, substance P, neuropeptide Y) are released in circular muscle, while these pathways are simultaneously inhibited in the longitudinal muscle; thus this segment of small intestine narrows and lengthens
- In front of the bolus, inhibitory pathways (e.g., VIP, NO) are activated in circular muscle, while excitatory pathways are activated in longitudinal muscle; thus this segment of small intestine widens and shortens.
