GI Smooth Muscle

Question 0

What’s in sm. m

Answer 0

Thick filaments,
Thin filaments,
SR (less developed than sk. m) that contains Ca2+

Question 1

smooth muscle must be able to

Answer 1

contract and maintain that contraction for a long period of time
must be ENERGY EFFICIENT - use little ATP
maintain shape of organ, but allow for distension
continue to generate active tension even when stretched

Question 2

Thin filaments in sm. m

Answer 2

actin
anchored to dense bodies
2X as in sk.m
tropomyosin - but doesnt cover active site
NO TROPONIN

Question 3

Thick filaments in sm. m

Answer 3

1/4 of that found in sk. m
different isoform
contains kinase, and phosphatase
slower contraction than sk. m.

Question 4

can sm m be directly inhibited

Answer 4

yes

Question 5

Intrinsic innervation of sm. m

Answer 5

gut, trachea, (enteric plexus)
neurons (sensory and motor)
independent of CNS and PNS

Question 6

Extrinsic innervation of sm. m

Answer 6

ANS, allows CNS to control viscera

Question 7

Ach

Answer 7

excites some smooth m (gut)

may inhibit other (cause relaxation)

Question 8

Norepi or epi

Answer 8

causes contraction of vascular sm. m

inhibits gut sm. m

Question 9

NO

Answer 9

vasodilator and relaxor
major inhibitory influence on sm. m
acts via cGMP mechanism

Question 10

Sm. m neuromuscular neuromuscular contact

Answer 10

no motor end plate, synapses en passante

varicosities: swellings in axon. NT is released from the varicosity, then diffuses to find receptors on sm. m

Question 11

Sm. m. neurotransmitter receptors

Answer 11

muscarinic cholinergic
adrenergic alpha and beta

NO is extremely lipid soluble, doesnt require receptor, diffuses through and has action on cGMP system

Question 12

Hormones that can elicit sm. m contraction

Answer 12

epi
CCK
gastrin, motilin

Question 13

Activation of GI sm.m by paracrine agents

Answer 13

EDRF= endothelial derived relaxinf factor, now known as NO

inhibits smooth muscle

Question 14

other forms of sm. m activation

Answer 14

can be activated by stretch. some of the sm. m in the vasculature can be activated this way

Question 15

Calcium regulation in Sm. m contraction

Answer 15

VERY dependent on Ca2+, even more so than cardiac m
comes from extracellular stores or SR
regulated seperately and play different roles in contraction

Question 16

AP always necessary for sm. m contraction

Answer 16

NO

Question 17

Release of Ca2+ from SR

Answer 17

Epi or n-epi binds –> Gqprotein –>increase IP3 and DAG= Ca released from Sr
PKC is activated as part of this process
BECAUSE OF THIS, SM. M CAN BE ACTIVATED TO CONTRACT IN ABSENCE OF AP

Question 18

Extracellular Ca2+

Answer 18

particular importantance in sm. m. does somewhat rely on AP
voltage gated Ca-ch opens with depol
ligang gated Ca-ch opens with binding of ligand
IMPORTANT sustained contraction of sm. m require Ca2+ from extracellular stores,
longer contration, more it relies on extracelluar Ca

Question 19

Sm. m at rest

Answer 19

tropomyosin not covering active site. we have no troponin
addition of 2 myosin light chains, = decreased affinity of myosin head for actin sites

at rest, the MYOSIN LIGHT CHAIN is dephosphorylated and myosin isnt particularly interested in binding to actin active site. no need to cover with tropomyosin.
still have MYOSIN HEAD bound to ATP and Pi

Question 20

Smooth Muscle Contraction

Answer 20

Step 1= increase in intracellular calcium
Step 2= calmodulin binds with Ca. = activates the calmodulin
Step 3= calmodulin activates MLCK, which takes an ATP, hydrolyzes it to make ADP and Pi
Step 4= MLCK phosphorylates the light chain
Step 5= myosin cross bridge can begin cycle

now we get myosin binding actin, Pi dissociates and then ADP dissociates= power stroke. these 2 filaments are stuck until ATP comes in and binds the myosin head. when it does that, we see the release of the myosin head from the active site as we hydrolyze atp to adp and pi.

Question 21

Where do the two distinct Pi work in sm. m contraction?

Answer 21

one at they myosin head, the other at the myosin light chain

Question 22

Sm. m. Relaxation

Answer 22

occurs when we remove Ca from the calmodulin, inactivate the MLCK, and activate the phosphatase.
inactivating MLCK will activate a phophatase, which removes the Pi from the myosin light chain and allow relaxation

Question 23

The Latch mechanism

Answer 23

the Pi can be removed from the myosin light chain at any point in the cycle, the cycle will continue but VERY slowly, and a new cycle can not be started
any activated cross bridges are still generating tension. but not using energy.
WITHOUT ATP, AND WITH A DEPHOS MYOSIN LIGHT CHAIN
INCREASES TENSION AND DECREASES ATP USAGE

Question 24

Length- Tension relationship in sm. m

Answer 24

stretch muscle, passive tension increases a little, but actin and myosin rearrange and passive tension dissapates

the random arrangement of actin - allows us to disappate passive tension and to maintain active tension over a wider range of lengths.

Question 25

Sm. m plasticity

Answer 25

capable of dedifferentiation and differentiation
in damage, can become a fibroblast to secrete collagen for repairs
once damage is repaired, differentiates back into sm. m of blood vessel