4 – Smooth Muscle Function and Motility

Question 1

Throughout the GI tract there are:

Answer 1

-3 layers of muscle
>submucosa muscularis mucosae
>2 muscle layers

Question 2

Submucosal muscularis mucosae:

Answer 2

-moves villi in the SI

Question 3

Peristalsis that move food down the GI is dependent on:

Answer 3

-smooth muscle
>circular
>longitudinal

Question 4

Circular smooth muscle:

Answer 4

-layer closest to lumen
-fibers running at right angles to the long axis of the intestine

Question 5

Longitudinal smooth muscle:

Answer 5

-outer layer
-fibers running in parallel to the long axis of the intestine

Question 6

What are the 2 types of Gi smooth muscle contractions?

Answer 6

-segmental contraction
-peristalsis

Question 7

Segmental contraction:

Answer 7

-mixing reaction
-carried out by the circular smooth muscle

Question 8

Peristalsis:

Answer 8

-propels a bolus of ingesta down the GI tract aborally
*coordinated contact of longitudinal muscle and circular muscle just behind bolus to push It along

Question 9

Coordinated contact of longitudinal muscle and circular muscle requires:

Answer 9

-entry of calcium from ECF
*process is highly sensitive to hypocalcaemia

Question 10

Smooth muscle contractions:

Answer 10

-as a syncytium (group)
>possible due to gap junctions
>AP in one cell can be propagated and spread through all cells within the group
*nerve fibers do NOT innervate every muscle fiber

Question 11

Smooth vs. striated skeletal muscle:

Answer 11

-contract more slowly
-longer actin filaments (can contract 2-4 times the distance)
-requires entry of calcium from ECF (ex. low calcium can effect this!)
-contract as a syncytium
-individual neurons not required to stimulate each cell

Question 12

Mechanotransduction in the GI tract:

Answer 12

-process where cells convert mechanical forces into electrical and chemical signals that result in cellular responses

Question 13

What do smooth muscle cells in the gut utilize to sense and process mechanical stimuli?

Answer 13

-wide array of membrane associated molecules
>surface receptors
>specialized intracellular cytoskeletal proteins
>extracellular cell-cell connections

Question 14

Examples of surface receptors in the gut:

Answer 14

-ion channels
-GPCR
-kinases

Question 15

Cells can use the mechanical stimuli and convert them into:

Answer 15

-short term effects: changes in ion concentrations and voltage
-long term effects: changes in gene expression

Question 16

GI smooth muscle generally shows:

Answer 16

-spontaneous (myogenic) activity
-conduction of electrical impulses from fiber to fiber
-sensitivity to stretch

Question 17

How is GI smooth muscle activity modulated?

Answer 17

-by autonomic nerves
>but not initiated by them (not required)

Question 18

Interstitial cells of CAJAL:

Answer 18

-some smooth muscles within segments under go rhythmic depolarization
-caused by variations in conductance of Na, Ca, and Cl across the cell membrane at regular intervals

Question 19

What are the interstitial cells of Cajal responsible for?

Answer 19

-basal or basic electrical rhythm (BER)
*resemble pacemaker cells of the heart=pacemakers of the gut

Question 20

Basal or basic electrical rhythm (BER):

Answer 20

-spontaneous depolarization and repolarization of these pacemaker cells in the smooth muscle of stomach, SI and LI

Question 21

What is the resting membrane potential of Cajal cells?

Answer 21

~-50mV

Question 22

What are the 2 ways that depolarization occurs:

Answer 22

1. Slow waves
2. Spikes

Question 23

Slow waves of depolarization:

Answer 23

-spontaneous, slow, transient depolarization
-conducting varying distances along the tract (BER)

Question 24

Spikes of depolarization:

Answer 24

-faster, transient depolarization
-can occur in burst at the periods of maximal depolarization of a slow wave
>proceed and initiated contraction (smooth muscle)

Question 25

Parasympathetic stimulation increases:

Answer 25

-rate at which potentials depolarize the membrane
>increasing rate of spike discharge and contraction

Question 26

Sympathetic stimulation prolongs (slows):

Answer 26

-rate at which generator potential depolarizes the membrane
>slows the rate of spike discharge

Question 27

Slow waves (control of contractions):

Answer 27

-50mV to -60mV
-membrane fluctuates spontaneously (compared to other cells and nerves)
>results in slow waves (5 to 15 mV)
*intrinsic property of smooth muscle (not dependent on nervous stimuli)
*not AP=do not elicit contracts

Question 28

How many slow waves in SI, LI and stomach?

Answer 28

-SI: 10 to 20x per minute
-stomach and LI: 3 to 8x per minute

Question 29

Spikes can occur due to:

Answer 29

-stretched by a bolus of food
-receptors activated by PS stimulation
*rapid entry of Ca ions is important for AP (compared to Na in nerve fibers)
>deficiency in Ca can be a problem for gut health

Question 30

Depolarization causes by AP cause the:

Answer 30

-opening of voltage-dependent calcium channels in smooth muscle
*contractions

Question 31

Tonic smooth muscles:

Answer 31

-are normally contracted and generate a variable steady-state force
Ex. sphincters, blood vessels, airways

Question 32

Phasic smooth muscles:

Answer 32

-commonly exhibit rhythmic contracts (Ex. peristalsis in GI tract)
-may contract intermittently during physiological activities under voluntary control (Ex. voiding of urine from bladder, swallowing)

Question 33

Swallowing in the form of pharyngeal stimulation induces:

Answer 33

-neural reflex relaxation of the LES and proximal part of stomach to allow entry of food

Question 34

Deoxynivalenol (DON)

Answer 34

-vomitoxin
-metabolites produced by fusarium graminearum
-commonly found in corn, barely and wheat
-develops in cool, damp weather
*negative effects begin at ~4ppm but inconsistent

Question 35

What does DON do to the intestine?

Answer 35

-interferes with intestinal motility
>injures the contractility of enteric smooth muscle cells

Question 36

What did 1ppm of DON in piglets (also had ZEN)?

Answer 36

-decrease in growth of SI is greater than reduced feed intake
-greater effects on decreased in smooth muscle layer of jejunum and ilium
>slow down transit time in gut (increase digestibility, but not a positive!)

Question 37

What did purified DON do in mice?

Answer 37

-shorter SI
-motility was decreased
>decreased number and weight of poop pellets
*direct effects on proteins involved in motility of SI

Question 38

What is the take home message on DON?

Answer 38

-may decrease growth and feed intake via actions on intestinal smooth muscle contractility

Question 39

Fusarium Head Blight (FHB):

Answer 39

-disease caused by infection with Fusarium fungal species
-bleached heads and shrivelled chalky/pinkish kernels
-during infection, a variety of mycotoxins can be produced (including DON)

Question 40

What fusarium is most common in western Canadian wheat?

Answer 40

-F. graminearum

Question 41

Fusarium in Canada, 2014:

Answer 41

-black and dark brown soils were contaminated
>had a hard time finding clean wheat for control samples
*not really a problem in dry years