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smooth muscle


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Where is smooth muscle located?
the majority is in walls of hollow organs and tubes
What are 2 unique problems smooth muscle is responsible for?
1. Being able to contract over a wide range of lengths.

2. Being able to contract for sustained periods of time.
what innervates smooth muscle?
autonomic nerves
what's unique about autonomic innervation?
-the axon lays across the muscle cells.
-no real terminal.
-receptors are farther away so NT has to diffuse further.
what's the name for the point analogous to a nerve terminal?
of smooth muscle, what is the characteristic
shape = spindle
nucleus = one
size = smaller than skeletal
compared to skeletal muscle is smooth more or less developed?
-doesn't extend length of muscle
-has SR, but no t-tubules.
are smooth muscles coupled
Yes - both.
Mechanical: conected so all contract together.
Electrical: most coupled through gap junctions to behave as a single unit.
what are 5 characteristics of smooth myofilaments?
-smooth, not striated
-longer thick filaments
-thin filaments have no troponin
-Intermediate filaments
-Dense bodies
what's different about
-thick filaments
-thin filaments
and why?
thick: longer to allow more tension to develop over longer length.
thin: contain no troponin because tropomysin doesn't cover the binding site for myosin.
what are
-intermediate filaments?
-dense bodies?
int. fils = structural proteins that support the cell's shape.

dense bodies = analag. to Z lines - hold Actins together
what are the two types of smooth muscle units?
-single unit
-multi unit
what is multiunit smooth muscle?
-multiple, discrete units that function independently.
-stim by nerves separately.
what are some sites of multi-unit smooth muscle?
lens adjusters, iris, base of hair follicles, large airways, large blood vessels.
what is single-unit smooth muscle?
-muscle that acts as one single unit; contracts together.
-gap junctions couple together.
what does myogenic mean?
refers to muscle that needs no nervous stimulation; self-excitable - smooth musc is.
where is single-unit smooth muscle found?
-GI tract
-small blood vessels
-reproductive/urinary tract walls
what are the steps in smooth muscle cross-bridge cycling?
1. Ca2+ binds calmodulin
2. Calmodulin/Ca binds myosin lightchain kinase (MLCK)
3. MLCK phosphorylates myosin crossbridges
4. Cross-bridges bind actin
5. Crossbridge shortens/tense
how does the cross-bridge cycling get stopped?
when calcium is taken back up into both sources.
what is the result of calcium re-uptake?
MLCP - phosphatase - takes the phosphate off myosin crossbridge.
what are the two sources of cytosolic calcium for crossbridge cycling?
-sarcosplasmic reticulum
-exctracellular calcium
what are 3 things that can cause smooth muscle membrane activation?
-spontaneous electrical activity
-nerves and hormones
-local factors
does smooth muscle have a resting membrane potential?
no; it depends on the site of the membrane you're looking at.
what are 2 characteristics of smooth muscle electrical activity?
-Pacemaker potentials
-Slow waves - the mp oscillates, and can be at different levels depending on time/factors.
what do nerves and hormones to to smooth muscle?
-change the baseline resting potential by changing calcium levels.
what 3 ways do nerves and hormones alter calcium levels?
1. depolarizing and causing an action potential.
2. opening voltage-gated Ca2+ channels.
3. Activating G-proteins to cause IP3 generation
what does IP3 do to smooth muscle?
nerves/hormones activate G proteins to generate IP3, it then makes the sarcopl. retic release calcium.
Are action potentials necessary for smooth muscle contraction?
-can use voltg-gtd Ca2+ channels
-can use Gproteins/IP3
what 3 local factors can change to activate smooth muscle?
1. acid - pH decrease
2. O2 levels
3. Stretching
-can constrict or dilate bronchial muscle; cause intestinal constractions
is there recruitment in smooth muscle contraction? why?
b/c they are already all contracting together as one unit.
does the cytosolic calcium level alter?
yes - by the control mechanisms noted before; allows for greater/lesser strengths of contractions.
how is the length-tension relationship in smooth muscle compared to skeletal?
tension increases over much broader lengths.
define the pacemaker potential
after an action potential, the resting membrane continues to depolarize until threshold, then into action potential again.
what type of smooth muscle units are electrically coupled?
single unit
how are cells in smooth muscle arranged?
NOT striated; no sarcomere pattern!
why are thick filaments in sm. muscle LONGER than skeletal?
so they can develop tension over a longer range; actin and myosin filaments don't collide as soon.
is myosin light chain phosphatase always active or no?
always - just when calmodulin binds calcium is MLCkinase active enough to phosphorylate the myosin crossbridge.
what allows calcium reuptake into the SR and extracellular fluid?
Calcium ATPase
what is the major difference between single and multi-unit smooth muscle?
multi = needs nerve stimulation

single= self-excitable, doesn't require nerve stimulation!
what is neurogenic muscle?
muscle requiring nerve stimulation

-skeletal, multi-unit smooth
what is myogenic muscle?
muscle that is self-excitable

does smooth muscle have troponin?
no troponin

yes, tropomyosin, but it has no role.
in smooth muscle, what is the molecule analagous to troponin?
calmodulin kind've.. at least it's what is active first.
how is electrical activity spontaneously generated in smooth muscle cells?
by pacemaker potentials.
what is the resting membrane potential of smooth muscle?
constantly fluctuating.
About -50 mV at baseline.
When very negative, causes a pacemaker potential to depolarize the membrane.
does smooth muscle have T tublules?
does smooth muscle have troponin?
what is in place of troponin?
where does calcium come from in smooth muscle?
-Sarcoplasmic reticulum
-extracellular fluid
what couples smooth muscle cells electrically?
what will increase excitation of smooth muscle cells?
anything causing an increase in intracellular calcium
in smooth muscle, how many actin thin filaments are there for every thick filament?
eleven - 11
what allows for calcium reuptake?
Calcium ATPase - works to reuptake into both SR and extracellular
how do nerves and hormones alter contractile activity of smooth muscle?
by releasing (at varicosities for nerves) near muscle cells, and causing inhibition or excitation of the cells.

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