Regulation of GI function: Nerves and Smooth Muscle

Question 1

syncytium

Answer 1

muscle fibers of gut are electrically connected through large numbers of gap junctions
- thus AP spreads throughout all neighboring fibers

Question 2

Slow waves

Answer 2

GI contractions occur rhythmically, this rhythm is determined by frequency of “slow waves”
- slow undulating changes int eh resting membrane potential

Question 3

interstitial cells of Cajal

Answer 3

slow wave cells that act as electrical pacemakers for smooth muscle cells
- undergo cyclic changes in membrane potential due to unique ion channels

Question 4

spike potentials

Answer 4

true action potentials

occur on top of slow waves, when Vm is more positive than -40 mV

Question 5

channels responsible for APs?

Answer 5

calcium-sodium channels

- much slower to open and close than rapid Na+ channels

Question 6

Factors that depolarize the membrane and make it more excitable?

Answer 6

1. stretching of muscle
2. stimulation by ACh released from PS nerves
3. stimulation by several GI hormones

Question 7

important factors that hyperpolarize membrane? make it less excitable?

Answer 7

1. NE/ Epi

2. stimulation of sympathetic nn.

Question 8

how does smooth muscle contracts

Answer 8

entry of Ca2+ ions
- slow waves only cause influx of Na+, thus don’t cause muscle contraction. The spike potentials result in the Ca2+ entering fibers

Question 9

tonic contraction of smooth muscle

Answer 9

continuous, not associated with slow waves
caused by continuous repetitive spike peotentials or by hormones that bring about partial depolarization
can also be caused by continuous entry of Ca2+

Question 10

enteric nervous system

Answer 10

GI tract Nervous system

-lies entirely in wall of gut from esophagus to anus

Question 11

two plexuses of ENS?

Answer 11

myenteric/auerbach’s plexus: outer plexus b/w longitudinal and circular muscle layers. controls GI mvmts

inner/Meissner’s plexus: lies in submucosa. controls Gi secretion and bloodflow

Question 12

Myenteric plexus

Answer 12

controls GI mvmt: lies within the muscle

1. increased tonic contraction of gut wall
2. increased intensity of contractions
3. increased rate of rhythm of contraction
4. increased velocity of peristaltic waves

VIP is inhibitory to this plexus

Question 13

Submucosal plexus

Answer 13

controls GI secretion and bloodflow

Question 14

ACh

Answer 14

excites GI activity
secreted via vagus nn.

cranial and sacral plexuses

Question 15

postganglionic neurons

Answer 15

of GI PS are located in myenteric and submucosal plexuses

Question 16

Sympathetic fibers

Answer 16

inhibit GI tract activity

originate at T5-L2

Question 17

Multi-unit SM

Answer 17

one fiber –> one SM
for “fine tuning”
little or no gap junctions

Question 18

Single-unit SM

Answer 18

one fiber/nerve –> multiple cells
coord. via gap jns.
visceral unitary smooth m.
myogenic “pace maker activity” –. slow wave

Question 19

depolarization APs

Answer 19

spike potentials: opening of slow Ca2+ channels, results in Ca2+ influx

Question 20

repolarizing

Answer 20

delayed opening of voltage gated K+ channels

K+ efflux

Question 21

sources of Ca2+ in smooth m?

Answer 21

1. voltage gated Ca2+ channels (from outside)
2. Sarcoplasmic reticulum (Ca2+-induced Ca2+ release and IP3 Ca2+ channel)
3. store operated Ca2+ channel (from outside)

Question 22

Ca2+ sequestering?

Answer 22

1. SERCA of SR
2. Ca2+ ATPase pump (sarcolemma)
3. 3Na+/Ca2+ antiporter (sarcolemma)

Question 23

role of Ca2+ in excitation contraction coupling?

Answer 23

in Smooth m. Ca2+ turns on X bridge by inducing chemical change in myosin thick filament via phosphorylation so that it can bind with actin

• skeletal m. induces a PHYSICAL change in thin filament

actin = thin 
myosin = thick

Question 24

chemical excitation contraction coupling

Answer 24

CA2+ binds with calmodulin (CaM) which activates myosin light chain complex (MLCK)
- results in conformational change of myosin head and powerstroke via increased ATPase activity

Question 25

latch state

Answer 25

maintenance of tension in tonic smooth m. without fatigue