Physio of Smooth muscle Flashcards
How is the sarcoplasmic reticulum different in smooth muscle than in skeletal muscle?
The sarcoplasmic reticulum in smooth muscle is very underdeveloped and thus cannot release very much calcium
What is the main source of calcium for smooth muscle?
Extracellular sources, especially for sustained contraction
Does smooth muscle actin have tropomyosin on it?
NO; It has troponin but instead has 2 light chains on the myosin head
Describe how the myosin head binds to the actin filament in smooth muscle
Calcium comes in and binds to calmodulin
Calmodulin then activates MLCK (myosin light chain kinase)
MLCK phosphorylates the light chain on myosin
What does the MLCK do for the affinity of myosin for actin?
INCREASES
How do you relax smooth muscle?
Pump calcium out, calmodulin no longer activates MCLK, and a myosin light chain phosphorlyase dephosphorylates the light chain on the myosin head to decrease its affinity for actin
How many ATP does smooth muscle use during one contraction cycle?
2 - 1 to hydrolyze in order to attach a phosphate to the light chain on myosin and 1 to bind to myosin head and dissociate it from actin (recocks once hydrolyzed)
Does smooth muscle use more or less ATP than skeletal muscle?
LESS still!!
What are the mechanisms by which smooth muscle can use less ATP overall than skeletal muscle?
Its myosin isoform is slower
Latch mechanism
What is the latch mechanism?
The phosphate on the myosin light chain can be removed at any time, so if it gets removed after the myosin head has bound to actin, the myosin will continue to do the powerstroke, but will then be stuck there!
It will now have a very low affinity for ATP and will not dissociate from actin resulting in continuation of tension without using ATP
What neurotransmitters does smooth muscle use compared to skeletal muscle?
ACH, norepinephrine, Nitric oxide
ACH only thing used in skeletal muscle
What supplies the NTs for smooth muscle and functions as the presynaptic terminal?
Varicosities - swelling in the axon
What hormones have control over smooth muscle?
Epinephrine, CCK, oxytocin
Skeletal muscle cannot be excited by hormones
Smooth muscle can be excited and can be inhibited, can skeletal muscle be inhibited?
Nope
Describe smooth muscle activation by paracrine agents
Nitric oxide made by surrounding endothelial cells releases it directly to smooth muscle cells and relaxes smooth muscle
