Smooth Muscles Flashcards
Define Multi-unit smooth muscle
discrete smooth muscle fibers that contract independently
- Does NOT contain gap junctions
Define Unitary smooth muscle
large number of fibers contract together as a single unit arranged in sheets or bundles via GAP JUNCTIONS.
Describe contractile proteins of Smooth muscle
- Actin - attached to dense bodies
- Myosin - Myosin heads NOT all arranged in same direction - allows for pulling in multiple directions.
- Tropomyosin
- DOES NOT CONTAIN CALCIUM BINDING PROTEINS (toponin)
Define Dense bodies
Similar function to Z disc in skeletal muscle
- Allow for force of contraction to be transmitted from one cell to the next
- dispersed throughout cells
- some attached to cell membrane.
Describe the events of smooth muscle contraction
1) increase in intracellular calcium (extracellular Ca is main source, some from ER)
2) calcium binds to calmodulin
3) calmodulin-Ca++ binds to and activates MYOSIN LIGHT CHAIN KINASE (MLCK)
4) MLCK phosphorylates myosin (regulatory chain) and allows for activation of the myosin ATPase and thus binding of actin and myosin.
5) Attachment-detachment cycle using ATP can continue as long as myosin is phosphorylated.
Define Relaxation of smooth muscle
Decreased MLCK activation and myosin phosphatase activity can lead to relaxation or tonic contraction
Relaxation: (bound the hydrolyzed ATP, but domain is not phosphorylated)
–> intracellular Ca+ conc decreases, which prevents further MLCK activation
–> Dephosphorylation of myosin by myosin phosphatase
Define Latch bridge
Decreased MLCK activation and myosin phosphatase activity can lead to relaxation or tonic contraction.
Tonic contraction/Latch Bridge
–> Low MLCK and myosin phosphatase activity
–> LOW ATPase activity
–> causes you to be stuck in the cross-bridge state. (or very slow power stroke)
Describe differences in Smooth muscle
- Long tonic contractions that can occur for hours or days
- slow cross-bridge cycling
- low energy to produce same tension
- relatively long onset and duration of contraction
- latch bridge can form to produce tonic contraction
- smooth muscle can respond to stretching/filling to maintain constant pressure
Describe some Neural regulation of smooth muscle at NMJ
- No structured NMJ, instead diffuse branches of nerve fibers over sheets of smooth muscle
- Diffuse junctions = nerve fibers do NOT directly contact smooth muscles
- Contact junctions = nerve fibers DO contact smooth muscle (some cases)
Describe neuronal regulation of smooth muscle: NT
Acetylcholine and norepinephrine are primary NT
- acetylcholine excites a tissue then norepinephrine will inhibit the same tissue or vice versa. (depends on organ)
Describe electromechanical stimulation/inhibition to regulate smooth muscle
- Open sodium and or calcium channels leading to a DEPOLARIZATION
- Close sodium and or calcium channels or OPEN POTASSIUM channels to cause HYPERPOLARIZATION
Describe pharmacomechanical stimulation
Activation of signaling molecules or generation of 2nd messengers that ACTIVATE the contractile process
- EX: activation of PLC lead to production IP3, which increases the release of intracellular Ca+
Describe Pharmacomechanical Inhibition
Activation of signaling molecules or the generation of 2nd messenger that INHIBIT the contractile process
- EX: cAMP activation of PKA will result in phosphorylation of MLCK which prevents Ca+-calmodulin from activation MLCK.
- EX: cGMP can lead to activation of the myosin phosphatase.
Describe the two sources of calcium
1) extracellular fluid is the primary source of calcium
2) sarcoplasmic reticulum is not as developed and is next to a less developed transverse tubule or caveolae
What are teh two roles of calcium
1) Membrane depolarization
2) contractile event
