smooth muscle contraction

Question 1

what is a dense body

Answer 1

a cytoskeletal specialization that the thin filaments are anchored to

Question 2

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?

Answer 2

smooth muscle only has:
thin/thick filaments
dense bodies
actin/myosin

Question 3

explain how smooth muscle is activated

Answer 3

spontaneously as in peristalsis or uterus
excited by nerves-ex vascular smooth muscle
hormones
or by stretch

Question 4

describe a slow muscle twitch (in comparison to skeletal muscle)

Answer 4

slow contraction velocity and slow relaxation but muscle shortening is large
duration of tension is usually longer- tonic contractions

Question 5

list 5 things that are contained within a caveoli in a smooth muscle cell

Answer 5

• Muscarinic ACh receptors
• Adrenergic receptors
• L type calcium channels
• ATP sensitive potassium channels
• Calcium sensitive potassium channels

Question 6

when describing skeletal muscle vs smooth muscle action potentials, are both Na+ dependent? If not, what are they?

Answer 6

NO! skeletal muscle is Na+ dependent but smooth muscle is calcium dependent

Question 7

compare the cross bridge cycle timing in smooth muscle vs the cycle in skeletal muscle

Answer 7

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)

Question 8

compare the percentage shortening in skeletal and smooth muscle

Answer 8

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

Question 9

what are the names of the two units that smooth muscle can be divided into?

Answer 9

multiunit

unitary

Question 10

is the statement below describing unitary or multiunit smooth muscle;
“coupled via gap junctions, can be spontaneously activated”

Answer 10

unitary

Question 11

give 3 examples in the human body of multiunit smooth muscle

Answer 11

-smooth muscle fibers in the ciliary and iris of the eye
-piloerector muscles that cause hair erection
-airway smooth muscle
(also some arteriolar muscle)

Question 12

give 4 examples in the human body of unitary smooth muscle

Answer 12

• small blood vessels
• GI tract
• uterus
• most arterial muscle

Question 13

out of smooth muscle, skeletal muscle and cardiac muscle, which utilize 2 sources for Ca?

Answer 13

smooth and cardiac muscle

Question 14

what would be the effect of L-type calcium channel blocking drugs?

Answer 14

decrease influx of calcium, and reduce contractile activity in both smooth and cardiac muscle

Question 15

what are the two ways that calcium is removed from the sarcolemma?

Answer 15

• By Ca++ pumps in the SR membrane and in the sarcolemma

- And by 3Na/ca exchange across sarcolemma

Question 16

what is capactive calcium entry?

Answer 16

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

Question 17

if there is no troponin C in smooth muscle, how is contraction triggered?

Answer 17

by calcium activation of myosin light chain kinase (MLCK)

Question 18

where does calcium bind when initiating an AP in smooth muscle?

Answer 18

to calmodulin which is on the MLCK

Question 19

what is MLCP

Answer 19

a soluble phosphatase in the sarcoplasm that desphosphorylates the regulatory light chain of myosin

Question 20

what are the two categories of smooth muscle contraction?

Answer 20

• electromechanical

- pharmacomechanical

Question 21

list the 4 steps leading to contraction that both the electromechanical and the pharmacomechanical categories of smooth muscle contraction lead to

Answer 21

1. increase in intracelluar ca
2. CICR (ca induced ca release)
3. ca++ calmodulin activation of MLCK
4. activation of smooth muscle myosin
5. contraction

Question 22

what type of contraction pathway does an action potential or stretch stimulate?

Answer 22

electrochemical

Question 23

when is the pharmacomechanical pathway activated?

Answer 23

ligand binding to cell surface receptor

Question 24

what is basal electric rhythm (BER)?

Answer 24

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

Question 25

what can BER control?

Answer 25

BER controls the maximal frequency of contractions, their propagation velocity and the direction in which they propagate

Question 26

can BER initiate action potentials?

Answer 26

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

Question 27

what is the name of the additional step that is seen in the crossbridge cycle of smooth muscle but not that of skeletal muscle?

Answer 27

the “latch” state

Question 28

how is the latch state mechanism theorized?

Answer 28

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

Question 29

what is ET-1

Answer 29

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

Question 30

explain the effects of initial activation of ET-1b

Answer 30

causes formation of NO which then causes vasodilation and hypotension

Question 31

explain the cardiovascular effects of endothelin (ET-1)

Answer 31

initially vasodilation and hypotension (ET-1b) followed by prolonged vasoconstriction and hypertension (smooth muscle ETa and ETb activation)

Question 32

list some diseases or conditions associated with elevated endothelin

Answer 32

hypertension
coronary vasospasm
heart failture

Question 33

which receptor binds epinephrine to stimulate contraction?

Answer 33

alpha 1 receptor

Question 34

which receptor binds epinephrine to stimulate relaxtion?

Answer 34

beta 2 receptor

Question 35

how is local blood flow controlled acutely?

Answer 35

via rapid changes in vasodilation or vasoconstriction

Question 36

what are the two theories that may account for active hyperemia?

Answer 36

• 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

Question 37

what is hyperemia?

Answer 37

when the rate of blood flow through a tissue becomes increased in response to the tissue’s high activity

Question 38

explain the mechanism of A2 adenosine receptors

Answer 38

they couple to Gs leading to adenylate cyclase stimulation and an increase in camp and PKA activity

Question 39

explain the mechanism of A1 adenosine receptors

Answer 39

can couple to ATP sensitive K+ channels causing smooth muscle hyperpolarization and a decrease in calcium influx (shuts calcium channels down)

Question 40

what is 1 difference and 1 similarity between adenosine and adrenergic receptors?

Answer 40

they are DIFFERENT proteins encoded by DIFFERENT genes

but they are both G-coupled receptors that can activate or inhibit similar downstream pathways

Question 41

what is stress relaxation

Answer 41

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

Question 42

name two neurotransmitters that have been identified in the vaso-vagal “receptive response” mechanism

Answer 42

VIP

nitric oxide