Smooth Muscle Flashcards
Where is smooth muscle located?
in the walls of holloow organs (gallbladder, uterus, bladder)
Tubes (GI tract, blood vessels)
What is a major purpose of smooth muscle?
maintain organ shape and push contents along
Describe smooth muscle contraction
slower and longer than skeletal muscle
more efficient
responds to variety of stimuli
latch state possible
Why is smooth muscle contraction more efficient than skeletal muscle contraction?
Generates comparable force using 300X less ATP
gives up speed for ability to adapt and adjust
What does the latch state do?
provides prolonged contraction w/o input of ATP
What is the structure of smooth muscle?
dense bodies (instead of z lines)
tropomyosin
caveolae (instead of t tubules)
underdeveloped SR
What do dense bodies do?
anchor actin
What holds dense bodies in place?
intermediate filaments
What does smooth muscle lack that skeletal muscle has?
sarcomeres (no striations)
troponin
t-tubules
Why is the SR not well developed in smooth muscle?
intracellular Ca2+ is less important
How is actin/myosin arranged in smooth muscle?
oriented diagonally
diamond-shaped lattice (not parallel with long axis)
What does sliding cause in smooth muscle?
causes cells to shorten and expand
What do long thin filaments allow in smooth muscle?
a large range of shortening (to fully expel bladder)
What are the two types of smooth muscle?
Single Unit
Multiunit
Define: Single Unit Smooth Muscle
only a few muscle fibers innervated in each group
stimulatd together, contract together
Single Unit smooth muscle can be stimulated by…
innervation, stretch, and hormonally
Explain Single Unit smooth muscle
Few muscles innervated in each group
impulse spreads through gap junctions
whole sheet contracts as unit
stimulated together, contract together
What is the structure of neuromuscular synapse in smooth muscle?
neurons have multiple varicosities (swellings of axons)
No complex structure at synapse (no MEP)
one smooth muscle can get input from both PNS and SNS
What do pacemaker cells do in single unit smooth muscle?
initiate spontanoeous electrical activity that spreads throughout the muscle
What are the two types of fluctuations in electrical activity in single unit smooth muscle?
Slow Wave Potentials
Action (spike) Potentials
Define: Slow Wave Potentials
coordinate muscle contractions in the gut by controlling the appearance of a second type of depolarizing event
Where do action potentials occur in single unit smooth muscle?
only occur at the crests of slow waves
Excitation-Contraction Coupling in smooth muscle
RMP is relatively low (-50 to -60 mV)
Long AP = 10-50 ms
No voltage gated Na+ channels at “motor end plate”
Voltage gated Ca2+ channels (dihydropyridine)
What is the dihydropyridine receptor in skeletal muscle?
voltage sensor
What is the dihydropyridine receptor in cardiac muscle?
voltage-dependent Ca2+ channel
only lets a little Ca2+ in
What is the dihydropyridine receptor in smooth muscle?
voltage-dependent Ca2+ channel lets in enough Ca2+ for AP
What does Ca2+ effect in smooth muscles?
the state of thick filaments
Where does Ca2+ come from in smooth muscles?
Mostly outside the cell, a little is released by the SR
How does Ca2+ enter the smooth muscle cells from the outside?
Depolarization -> voltage-gated Ca2+ channels open
NTs, hormones, etc. open Ca2+ channels
Smooth muscle contraction
Extracellular Ca2+ enters cell
Ca2+ binds calmodulin
activates myosin light chain kinase (MLCK)
MLCK phosphoylates light chain of myosin
phosphorylated myosin allows crossbridges to form and break repeatedly
In smooth muscle, myosin is….
“off”
In skeletal muscle, myosin is….
always “on”
Smooth muscle relaxation mechanism
Contractile stimulus is removed -> Ca2+ concentration decreases
or direct actionh of a substiture that inhibits the contractile mechanis, (increase myosin phosphatase activity)
Pharmacological Relaxation of smooth muscle
L-type Ca2+ channel blocker
increase in cAMP or cGMP
What does cAMP do in smooth muscle?
(EPI via β2) inhibits MLCK
What does cGMP do in smooth muscles?
(nitroglycerin) stimulates myosin phosphatase
What is EPI used or in relation to smooth muscle?
used in treatment of bronchospasm of asthma
What is nitroglycerin used for in terms of smooth muscles?
used to relax cornary arteries
converted to nitric oxide in the body
What is Sildenafil used for?
cGMP phophodiesterases for ED
Sexual stimulation -> production and release of __ -> activates __ -> prodices __-> results in __-> increase in BF
NO
guanylate cyclase
cGMP
smooth muscle relaxation
How can vascular smooth muscle (aorta) endure 60 “insults” per minute and sustain BP w/o expending a lot of ATP?
Develop force
smooth muscle has a way for cross bridges to remain attached, cycle slowly, and consume less ATP
partial constriction
Latch State
Mechanism of Tonic Contraction
Dephosphorylation of myosin while still attached to actin -> ATPases activity decreases -> more difficult to release myosin heads from actin; slow cross-bride cycling (low ATP use)
What does the mechanism of tonic contraction result in?
more difficult to release myosin heads from actin; slow cross-bride cycling (low ATP use)
Increased intracellular Ca2+ in smooth muscle…
regulates myosin
Ca2+ is increased in smooth muscle through:
Mechanically gated Ca2+ channels
Ligand gated Ca2+ channels (ANS, hormones, paracrine)
Voltage gated Ca2+ channels
Smooth muscle is capable of which types of contraction?
phasic (short term) and tonic
Overlapping myofilaments in smooth muscle
slide and generate force
