Smooth Muscle

Question 1

Innervation of smooth muscle

Answer 1

-ANS

Question 2

Smooth muscle differences:

Answer 2

• slower movements
• monomnucleated
• actin/myosin not organized into sarcomeres, instead they run parellel to the long axis of the cell (contractile filament domains)
• much more MYOSIN than actin (allows for more shortening, less volume but generates greater force due to the length)
• are some of the smallest cells in the body
• NO TROPONIN, caldesmon or calponin instead
• develop LATER, from non-muscle cells

Question 3

Actin and Myosin organization in smooth muscle

Answer 3

-into groups of overlapping filaments within the cells

Question 4

Cross bridge cycling in smooth muscle

Answer 4

• myosin is morphologically same as skeletal but biochemically different
• ATPase normally OFF and must be turned on, activation due to phosphorylation of myosin light chain
• calcium influx activates calmodulin which activates the MLCK which phosphorylates ATP and activates myosin
• only need about 50% of the sites activated for the system to be maximum

Question 5

Junctions between smooth muscle cells:

Answer 5

simple opposition

intermediate contact

desmosome

gap junction

Question 6

Calcium sources for smooth muscle contraction

Answer 6

1) electromechanical coupling: entry of calcium via voltage gated calcium channels (L type) *action potential
2) pharmacological coupling: receptor-operated calcium channels or SR release

Question 7

SR release of calcium in smooth muscle

Answer 7

• not due to depolarization but triggered by PLC - PIP2 — IP3 second messengers
• smooth muscle can contract without a change in membrane potential
• ACh is one type of agonist/receptor for this
• SR amount varies

Question 8

How does smooth muscle force vary with MLC phorphorylation?

Answer 8

• increased phorphorylation = increased tension

* smooth muscle is never totally relaxed

Question 9

Relationship between action potential and the number of SR

Answer 9

• vascular smooth muscle has no action potential and therefore an increased number of SR
• GI has less SR because action potential used

Question 10

Regulation of smooth muscle contraction:

Answer 10

• is accomplished via inhibition of MLCP

- via activation of MLCK via calcium

Question 11

Regulation of smooth muscle relaxation:

Answer 11

• accomplished via activation of phosphatase

- via inactivation of MLCK activity (decrease calcium)

Question 12

Agonist activation of smooth muscle:

Answer 12

relies on MLCP inactivation

• GP activation of rho activates rho kinase which phosphorylates the myosin binding site of MLC phosphatase = INACTIVATION
• Protein kinase C can also inactivate via CPI-17

Question 13

How to activate MLCP

Answer 13

• subset of smooth muscle used NO which increased cGMP which activates PKG which bind/activates MLCP
• desensitized to calcium levels, mechanism of Viagra

Question 14

Beta-receptor stimulated decrease of MLCK

Answer 14

-increased cAMP leads to phosphorylation of MLCK which DECREASES activity

• asthma treatment shuts down MLC and relaxes
• via beta agonist

Question 15

How to decrease calcium during relaxation

Answer 15

• take away agonist

- stop membrane depolarization

Question 16

Location of SR in smooth muscle

Answer 16

-close to the membrane for refilling

Question 17

The impact of force (tone) on BP

Answer 17

• as tone falls BP falls and flow to organ increase

- as tone increases, BP increases and flow decreases (contraction)

Question 18

Force maintenance in smooth muscle:

Answer 18

calcium and MLC phosphorylation only peak for a short time however force is maintained

muscle resistance to fatigue, ATP hydrolysis drops

Question 19

Phasic contraction:

Answer 19

what you expect to see in skeletal muscle

-calcium increase leads to cross bridge phosphorylation, twitch contraction

Question 20

Tonic contraction

Answer 20

• calcium and velocity of phosphorylation are the same but there is a SUSTAINED stimulation
• force is maintained even through calcium/phos are not
• force maintained with lower cross bridge cycling rates, lower shortening velocity and lower ATP consumption rate

Question 21

Force length relationship of smooth muscle:

Answer 21

• stretching increases force generated by isometric contraction until muscle reaches Lo (optimal length for force generation)
• after Lo isometric force begins to decline

Question 22

Force velocity relationship of smooth muscle:

Answer 22

• hyperbolic

- has an internal load so the force the muscle generates must be greater than the internal load for shortening to occur

Question 23

Classification of smooth muscle:

Answer 23

• neuronal regulation

- functional classification

Question 24

Neuronal regulation classification:

Answer 24

• single unit (primarily GI, bladder, uterine)

- mutli unit (primarily vacular)

Question 25

Function classification:

Answer 25

• tonic: resting tone and slow contraction (arterial)

- phasic: more rapid contraction and relaxation (GI tract)