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Smooth Muscle

From MedPhys notes (some from Guyton)
smooth muscle is not attached to _____.
what are the 2 types of smooth muscle contraction? which type is rhythmic?
1) tonic (such as sphincters)
2) phasic (such as GI tract)
phasic contractions are rhythmic
what is the significance of gap junctions in smooth muscle?
- allow for coordinated contractions
- allow ion to flow from one cell to another (esp since most smooth muscles tend to be connected to each other)
* this is typical of single unit smooth muscle"
smooth muscle has __1__ and __2__ similar to skeletal muscle but no ___3___.
1) actin
2) myosin
3) troponin
Smooth muscles have _______ which act like "z disks in skeletal"
dense bodies
where are two places dense bodies can be found?
1) attached to the cell membrane
1) anchored to cytoskeletal components of the cytosol
how far can smooth muscle be stretched and still contract?
2.5 times resting length
compare the difference in lengths of the resting length and the length of myosin filaments for smooth vs. skeletal muscle.
- resting length of smooth muscle is much shorter than skeletal
- smooth myosin filaments are much longer compared to skeletal
what is the Stretch-Relaxation Response? how does it work?
it is the ability of smooth muscle to be stretched then relax maintaining constant pressure
1) If the smooth muscle is quickly stretched, it contracts.
2) over time, it gradually relaxes adjusting to new length
note the Pressure does change transiently but then it returns to relatively normal
what is a major factor that determines F of contraction in smooth muscle?
the fraction of time the crossbridges are attached to actin
what are 3 dihydropyridine antagonists? and what do they do? and they commonly used to treat what?
- verapamil, nifedipine, norvasc
- they block Ca2+ binding to receptors causing muscle relaxation
- used to treat hypertension
smooth muscle myosin has more/less ATPase activity compared to skeletal muscle so that even a relatively fast contraction of ______ muscle is many time slower than ______ muscle.
smooth muscle uses what fraction of E to sustain the less/more/same tension of contraction when compared to skeletal muscle? how is this possible?
1/10 to 1/300
same tension of contraction
this is because of slow cycling of ATP
what stores of E are lacking in smooth muscle compared to skeletal muscle?
creatine phosphate and glycogen
how does the slow cycling of myosin crossbridges affect the F of contraction?
it increases the time myosin is attached to actin thus increasing the F of contraction
compared to skeletal muscle, how is the actin filament different in smooth muscle?
1) does NOT contain troponin
2) tropomyosin does NOT block the binding sites of myosin
generally speaking, in what part of the skeletal and smooth muscle does the change occur to affect binding?
in skeletal muscle: actin
in smooth muscle: myosin
in smooth muscle, myosin cannot bind to actin unless what happens?
unless myosin is phosphorylated
what triggers the chain of biochemical events that results in the phosphorylation of myosin?
during excitation, an increase in cytosolic Ca2+ which acts as an intracellular messenger initiates the chain of events
name 3 innervations that can cause in increase of cytosolic Ca2+ in smooth muscle
nerve stimulation of the smooth muscle, hormonal stimulation, or stretch of the fibers
what is the regulatory protein in smooth muscle that is similar to troponin in skeletal muscle?
what is the pathway resulting in the phosphorylated myosin head?
calmodulin + Ca2+ --> Calcium-Calmodulin
inactive myosin kinase (MK) + calcium-calmodulin -->act. MK
this activated myosin kinasse = myosin LC kinase
inactive myosin + active MK --> phosphorylated myosin
phosphorylated myosin binds actin = CONTRACTION
what happens when Ca2+ is removed?
myosin is dephosphorylated (via myosin phosphatase in cytosol which dephosphorylates the regulatory LC) and detaches from actin = the End of Contraction
how does calcium enter the cell?
in contrast to skeletal muscle, smooth muscle does NOT have T tubules and only has very small SR. Calcium enters the cell from the ECF via Ca2+ channels
when calcium enters the cell in smooth muscle, it goes through channels called what?
Calcium goes through dihydropyridine receptors.
what is a unique feature of the cross bridges in smooth muscle myosin?
- they are "side polar" cross bridges
- this set up allows myosin to pull actin in one direction on one side and simultaneously pulls another actin filament in the opposite direction on the other side
unitary smooth muscle (single unit smooth muscle) has more of what particular type of channel compared to skeletal/cardiac muscle?
voltage sensitive calcium channels
skeletal/cardiac muscle have more of what particular type of channel compared to smooth muscle?
voltage sensitive sodium channels
what is the major ion involved in the depolarization of smooth muscle?
how do calcium channels behave differently than sodium channels and how does this affect APs?
- calcium channels open/close more slowly than sodium channels
- this is why smooth muscle has a slower depolarization/repolarization compared to skeletal/cardiac muscle
what are the 2 groups into which smooth muscle can be classified?
single unit smooth muscle & multi unit smooth muscle
describe multiunit smooth muscle.
(how is it innervated?)
(how would you describe the contraction?)
(how are AP affected?)
-has multiple discrete units (cells) that function independently from each other
- each cell has to be innervated individually
- contraction is "neurogenic"
- there is NO action potential
where are 4 places multiunit smooth muscle can be found?
1) wall of large b.v.
2) large airways to the lung (i.e. trachea, bronchi)
3) muscle of the eyes (lens and iris)
4) erector pili muscles attached to hair
describe single unit smooth muscle.
(what unique structure can be found here that allows the contractions to be coordinated?)
(what is another name for single unit sm. musc?)
- it is the most Abundant of the smooth muscle in the body
- also called "visceral smooth muscle"
- when excited, it contracts as a single unit (forms a functional syncytium)
- electrically linked by gap junctions
where is single unit smooth muscle found in general? name 4 examples.
in the walls of hollow organs of viscera
i.e. digestive tract, reproduction (uterus), urinary tracts (ureters), and small b.v.
Explain how the innervation to single unit smooth muscle affects contractions.
- has innervation but the innervation does NOT determine if contraction occurs
- Parasymp/Sympathetic innervation influences the rate & strength of contraction by inhibiting/stimulating pacemaker cells
so in other words, single unit smooth muscle is self-excitable and does not require nervous stimulation for contraction
what are pacemaker cells in smooth muscle?
- specialized, non-contractile, smooth muscle cells that have spontaneous electrical activity and can depolarize in their own rhythmic way without stimulation from outside sources.
- they do NOT have a resting membrane potential.
what is the nerve-independent contraction of single unit smooth muscle called?
myogenic activity
what are the 2 types of spontaneous depolarization?
1) pacemaker potential (found in reproduction and urinary tracts)
2) slow wave potential (Only in the gut)
what is the nervous system of the gut called?
enteric system
what is Peristalsis? how does it come about in the gut (i.e. what causes the AP to occur)?
Peristalsis = rhythmic waves of contraction in the gut
1) slow wave depolarization cycle hits about -35mV
2) then a series of APs occur
peristalsis is what type of contraction?
1) what type of receptors are the NT receptors on smooth muscle? 2) how do NT affect the calcium channels?
1) metabotropic (receptors evenly distributed throughout the cell membrane)
2) NT cause the calcium channels to open via second messenger systems
how do NE and ACh generally affect smooth muscle (excit/inhib....) and Ca2+ influx?
NE (sympathetics) and ACh (parasympathetics) can be excitatory or inhibitory but are usually ANTAgonists to each other.
-they bind receptors and either allow/inhibit Ca@+ going into the cell
how is NT release different in smooth muscle compared to skeletal muscle?
in smooth muscles, there are regions of the axon that release NT called varicosities.
name 4 hormones (in addition to NE and ACh) that can bind receptors and alter cytosolic Ca2+ levels?
oxytocin, SER, angiotensin and histamine
what are the 2 major categories of Ca2+ channels in smooth muscle?
1) voltage sensitive Ca2+ channels
2) receptor-mediated channels (via G proteins)
Multiunit smooth muscle does not have __1__; it cannot have a contraction without __2__. Single unit smooth muscle cells are linked by __3__; can respond to __4__ factors i.e. hormones that affect smooth muscle contraction.
1) AP
2) NT
3) gap junctions
4) humoral
in Multiunit smooth muscle, __1__ have NT vesicles associated with __2__ or __3__ nerves, so that the NT is released and __4__ to the cell membrane. There are also receptors for __5__ which are circulating in the blood which can __6__ to the cell membrane. Both ways cause an increase in [Ca2+] triggering a response.
in Single unit smooth muscle, are receptors for __7__ which are circulating in the blood which can __8__ to the cell membrane and receptors for __9__. Both ways cause an increase in [Ca2+] triggering propagation from cell to cell via __10__ (2 words).
1) varicosities
2) autonomic
3) intrinsic
4) diffuses
5) hormones
6) diffuse
7) hormones
8) diffuse
9) NT
10) gap junctions
Stimulation of skeletal muscle is via the ___ ___.
Stimulation of smooth muscle is via what 3 things?

what is found in NT vesicles associated with skeletal muscle? what is found in NT vesicles associated with smooth muscle?
nervous system
nervous system (nerve signals), hormones, stretch

NT with skeletal: ACh
NT with smooth: ACh and NE
name 7 hormones that are associated with muscle contraction.
NE, epi, ACh, oxytocin, SER, histamine, and angiotensin
what are 3 responses to local tissue factors for smooth muscle?
1) lack of oxygen in local tissues causes sm. musc relaxation and thus vasoD
2) excess carbon dioxide causes vasoD
3) increased [H+] also causes vasoD
T/F Smooth muscle depends on an AP to contract. (and explain why this is true/false)
False! Smooth muscle can contract without an AP (does Req a depolarization!!) because contraction can be affected by a variety of factors that do not necessarily cause APs.