OP
T

thebillsfan

Unseasoned Veteran
10+ Year Member
5+ Year Member
Dec 22, 2008
778
0
Status
Pre-Medical
for constriction, sympathetic stim. releases norepinephrine which causes the smooth muscle to contract. my question is now...how does it cause it to contract? (i think this may be beyond the scope of the mcat)
 

boaz

shanah alef
10+ Year Member
7+ Year Member
Dec 31, 2007
1,373
25
bachelor pad
Status
Medical Student
The mechanism is beyond the scope of the MCAT. But what IS important to know is that sympathetic system cause constriction in some vessels (gastro) and dilation in others (e.g. arteries leading to leg muscles). Reverse goes for PS system.
 
May 8, 2009
289
1
Status
Pre-Medical
Smooth muscles use a different system that still involves Ca+ and actin a myosin filaments to contract. Their stimulation is due to the influx of extracellular Ca+ which bind calmodulin and activates myosin light chain kinase. The activates the myosin to contract.

Thats how they contract.

Sympathetic will vasodilate skeletal muscle and vasoconstrict GI smooth muscle blood vessels upon activation. The sympathetic is always responsible for some peripheral resistance (vasoconstriction) all over the body. I think upon activation if it let up its NT release then you would get vasodilation.
 

Buding

10+ Year Member
5+ Year Member
2+ Year Member
7+ Year Member
Apr 2, 2009
14
0
Status
Pre-Medical
for constriction, sympathetic stim. releases norepinephrine which causes the smooth muscle to contract. my question is now...how does it cause it to contract? (i think this may be beyond the scope of the mcat)
I believe that norepi works through a GPCR pathway in which phospholipase c cleaves PIP2 into IP3 and DAG. IP3 binds onto the ER which subsequently releases Ca2+ into the cell->bind onto troponin->etc, etc... . I guess it would be assuming that the ER in this case would be interchangable with the SR though.