smooth muscle contraction Flashcards
what is a dense body
a cytoskeletal specialization that the thin filaments are anchored to
out of the following list, which are components of smooth muscle; thin filaments, thick filaments, sarcomeres, dense bodies, A bands, I bands, striations, actin, myosin, T tubules, troponin C?
smooth muscle only has:
thin/thick filaments
dense bodies
actin/myosin
explain how smooth muscle is activated
spontaneously as in peristalsis or uterus
excited by nerves-ex vascular smooth muscle
hormones
or by stretch
describe a slow muscle twitch (in comparison to skeletal muscle)
slow contraction velocity and slow relaxation but muscle shortening is large
duration of tension is usually longer- tonic contractions
list 5 things that are contained within a caveoli in a smooth muscle cell
- Muscarinic ACh receptors
- Adrenergic receptors
- L type calcium channels
- ATP sensitive potassium channels
- Calcium sensitive potassium channels
when describing skeletal muscle vs smooth muscle action potentials, are both Na+ dependent? If not, what are they?
NO! skeletal muscle is Na+ dependent but smooth muscle is calcium dependent
compare the cross bridge cycle timing in smooth muscle vs the cycle in skeletal muscle
The proportion of time spent in the tension generating phase of the cross bridge cycle is longer for smooth muscle than that of skeletal muscle thus resulting in greater force of smooth muscle contraction with less energy expenditure (ATP hydolysis)
compare the percentage shortening in skeletal and smooth muscle
Smooth muscle can contract to less than 1/3 of its stretched length where as skeletal muscle only contracts about 1/4-1/3 of its stretched length
what are the names of the two units that smooth muscle can be divided into?
multiunit
unitary
is the statement below describing unitary or multiunit smooth muscle;
“coupled via gap junctions, can be spontaneously activated”
unitary
give 3 examples in the human body of multiunit smooth muscle
-smooth muscle fibers in the ciliary and iris of the eye
-piloerector muscles that cause hair erection
-airway smooth muscle
(also some arteriolar muscle)
give 4 examples in the human body of unitary smooth muscle
- small blood vessels
- GI tract
- uterus
- most arterial muscle
out of smooth muscle, skeletal muscle and cardiac muscle, which utilize 2 sources for Ca?
smooth and cardiac muscle
what would be the effect of L-type calcium channel blocking drugs?
decrease influx of calcium, and reduce contractile activity in both smooth and cardiac muscle
what are the two ways that calcium is removed from the sarcolemma?
- By Ca++ pumps in the SR membrane and in the sarcolemma
- And by 3Na/ca exchange across sarcolemma
what is capactive calcium entry?
the SR is refilled by calcium from outside the cell- this seems to occur in a way that entering calcium does not trigger a contraction
this can occur when the SR calcium stores become depleted
if there is no troponin C in smooth muscle, how is contraction triggered?
by calcium activation of myosin light chain kinase (MLCK)
where does calcium bind when initiating an AP in smooth muscle?
to calmodulin which is on the MLCK
what is MLCP
a soluble phosphatase in the sarcoplasm that desphosphorylates the regulatory light chain of myosin
what are the two categories of smooth muscle contraction?
- electromechanical
- pharmacomechanical
list the 4 steps leading to contraction that both the electromechanical and the pharmacomechanical categories of smooth muscle contraction lead to
- increase in intracelluar ca
- CICR (ca induced ca release)
- ca++ calmodulin activation of MLCK
- activation of smooth muscle myosin
- contraction
what type of contraction pathway does an action potential or stretch stimulate?
electrochemical
when is the pharmacomechanical pathway activated?
ligand binding to cell surface receptor
what is basal electric rhythm (BER)?
refers to waves of rhythmic depolarization of intestinal smooth muscle cells, which originate at a specific point and then are propagated along the length of the GI track
what can BER control?
BER controls the maximal frequency of contractions, their propagation velocity and the direction in which they propagate
can BER initiate action potentials?
no!
it is only when there is a release of neurotransmitters from enteric nerve endings that are superimposed on the BER waves that an AP can occur
what is the name of the additional step that is seen in the crossbridge cycle of smooth muscle but not that of skeletal muscle?
the “latch” state
how is the latch state mechanism theorized?
It is thought that this state occurs because: the MLCP dephosphorylation of the myosin light chain occurs while the myosin head IS bound to actin
what is ET-1
amino acid peptide produced by vascular endothelium
its formation and release are stimulated by angiotension II, ADH, thrombin, cytokines, reactive oxygen species and shearing forces acting on vascular endothelium
explain the effects of initial activation of ET-1b
causes formation of NO which then causes vasodilation and hypotension
explain the cardiovascular effects of endothelin (ET-1)
initially vasodilation and hypotension (ET-1b) followed by prolonged vasoconstriction and hypertension (smooth muscle ETa and ETb activation)
list some diseases or conditions associated with elevated endothelin
hypertension
coronary vasospasm
heart failture
which receptor binds epinephrine to stimulate contraction?
alpha 1 receptor
which receptor binds epinephrine to stimulate relaxtion?
beta 2 receptor
how is local blood flow controlled acutely?
via rapid changes in vasodilation or vasoconstriction
what are the two theories that may account for active hyperemia?
- oxygen lack theory: low O2 causes smooth muscle relaxation of the sphincter
- vasodilatory theory: substances (adenosine) are released by active muscles which cause relaxation of the sphincter
what is hyperemia?
when the rate of blood flow through a tissue becomes increased in response to the tissue’s high activity
explain the mechanism of A2 adenosine receptors
they couple to Gs leading to adenylate cyclase stimulation and an increase in camp and PKA activity
explain the mechanism of A1 adenosine receptors
can couple to ATP sensitive K+ channels causing smooth muscle hyperpolarization and a decrease in calcium influx (shuts calcium channels down)
what is 1 difference and 1 similarity between adenosine and adrenergic receptors?
they are DIFFERENT proteins encoded by DIFFERENT genes
but they are both G-coupled receptors that can activate or inhibit similar downstream pathways
what is stress relaxation
certain areas of smooth muscle (ie bladder) can have a decline in pressure over time at a constant volume strain
the amount of stress relaxation a smooth muscle can undergo is related to its amount of elastin
name two neurotransmitters that have been identified in the vaso-vagal “receptive response” mechanism
VIP
nitric oxide
