What is the muscle of the GI tract?
except the muscle of the pharynx, the upper 1/3 of the esophagus and the external anal sphincter, all of the GI muscle is smooth muscle
What is the ratio of actin to myosin in smooth muscle? striated?
What is nexus?
smooth muscle is organized in bundles that are connected to each other via nexus connections, which are clusters of gap junctions
How is smooth muscle innervated?
only some of the smooth muscle fibers are innervated by neurotransmitters, but all fibers contract together via nexus connections
Note that in smooth muscle there is no discrete neuromusculae junction and that transmitters are released from swelling of nerve called varicosities that act upon receptors
What is the function of Interstitial cells of Cajal?
these are found in intestinal smooth muscle (myenteric and intramuscular) and function as pacemakers for slow-wave production (dont cause muscle contraction) smooth muscle contraction by receiving the neurotransmitter signal from varicosities before they are relayed to smooth muscle
What causes Gi smooth muscle contraction?
there must be spike potential superimposed on slow-wave propagation to induce smooth muscle contraction
What is very important in mediating smooth muscle contraction?
Calcium. This can be induced by changing the electrochemical potential of the cell
Ca binds to calmodulin and together they bind to MLCK (myosin light chain kinase) to phosphorylate myosin which can then participate in contraction with actin
T or F. A spike potential without a resting wave can be enough to induce contraction of smooth muscle
What are the main events of the pharyngeal phase of swallowing?
A. Propulsion into oropharynx
B. Closure of nasopharynx (caused by action of the upper constrictor and epiglottis action)
C. Propulsion through upper esophagus (via the middle and lower constrictors)
D. Propulsion of bolus
How does swallowing occur?
When food reaches the upper pharynx, affarent fibers transmit to the swallowing center in the brainstem which activates the nucleus abiguus to transmit via the vagus nerve to promote contraction
Describe the muscle of the esophagus
The muscle in the upper 1/3 is striated, then mixed in the middle, and then smooth muscle in the lower 1/3
A= during fasting
B= following swallowing
What is secondary peristalsis?
this occurs from distension in the esophagus if the primary wave of peristalsis isnt able to fully evacuate the esophagus or during reflux
T or F. Peristalsis is required for fluid movement
What are the functional aprts of the stomach based on motility?
the upper orad area which is concerned with accomodating food entry and the
lower caudad area near the pylorus which is concerned with mixing and propelling the food into the duodenum
What happens in the orad area as food passes through the lower esophageal sphincter?
Notice that both the velocity and the amplitude of contractions increase as they approach the pylorus
T or F. In the stomach, slow waves can induce contraction alone
T. Unlike the intestines, where a spike potential is needed
Sympathetic inenrvation inhibits gastric emptying (in situations like stress). Note that motilin can induce very rapid bursts of peristalsis via MMCs to cleanse the stomach about every 90 minutes even during fasting
Motility of Small Intestine
It is a local (3-4cm) phenomenon (entire SI doesnt contract at the same time) that occur about every 5 seconds (or multiples of 5 seconds) that cause intraluminal pressure waves
T or F. During fed conditions, MMCs completely dissappear to allow more time for absorption to occur
T. And phasic contractions become more uniform
MMCs occur about every 90 minutes due to motilin
The purpose of intestinal contractions not being completed coordinated is to promote mixing
NOTE: in the fed state, MMCs are not occurring and there is more coorinated movement, whereas in the fasting state MMCs do occur and segmentation doesnt occur
In contrast, coordinate small intestine contractions promote movement