Smooth Muscle

Question 1

Airway and blood vessel smooth muscle are examples of __ contractions.

Urinary bladder and intestinal smooth muscle are examples of __contractions.

Answer 1

Tonic contraction

Phasic contraction

Question 2

Structural differences between smooth and skeletal muscle

Answer 2

Smooth muscle are single-nucleated and have a centrally placed nuceus.

Question 3

How to skeletal muscles contract together​? How do smooth muscles do it?

Answer 3

Skeletal muscle is organized into motor untis

Smooth muscle have gap junctions that span the membranes of 2 cells; also have structural juncitons between the cells that link them into a contractil eunit

Question 4

While skeletal muscle is organized into striations aligned with the long axis of the cell, the contractile filaments of smooth muscle are arranged …

Answer 4

Diagnoally across the cell

Thinf ialments are connected to dense bodies (like the Z disc) which are linked by intermediate filaments

Question 5

The process of crossbridge cycling is the same in skeletal and smooth muscles, but the mechanism by which calcium controls smooth muscle contraction is different:

Answer 5

Smooth muscle has thin filament regulation (troponin & actin), but ALSO has thick filament regulation:

1. Rise in intracellular Ca2+
2. Four calciums bind calmodulin
3. Ca-calmodulin binds and activates MLCK
4. MLCK phosphorylates myosin’s actin binding site on the light chain so it can interact with actin

Question 6

Some smooth muscle organs have to perform tonic contraction, so they have to save their ATP. How do they do it?

Answer 6

Latched state: maintain contact between myosin and actin without ATP hydrolysis

Question 7

During phasic vs tonic contraction, what’s the relationship between calcium and force?

Answer 7

Phasic contraction: force follows calcium conc (just like skeletal)

Tonic contraction: force increases with calcium, but doesn’t fall when calcium does; it’s maintained as long as Ca+ is above resting levels

Question 8

How do you generate the latched state?

Answer 8

Myosin phosphatase dephosphorylates the myosin while its heads are still in contact with actin; possible in smooth muscle because binding is slower than it is in skeletal

To sustain force: at low [calcium], the # of actin-myosin crossbridges entering the latched state = # leaving the state

Question 9

Myosin light chain phosphatase (MLCP) has what effect on smooth muscle?

Answer 9

Relax

Question 10

Calcium sensitization

Answer 10

RhoA-activated ROK or PKC-potentiated CPI-17 inhibit MLCP

–> potentiates and prolongs the effect of IP3/Ca mediated muscle contraction

Question 11

Vasopressin, endothelin, angiotensin, muscarinic agonists

Answer 11

activates the GPCRs that trigger calcium sensitization (block MLCP to sustain contraction)

Question 12

In sekeltal muscle, calcium is released from

Answer 12

the SR and the SR only!

Question 13

3 mechanisms to raise intracellular calcium conc in smooth muscle

Answer 13

1. Release from plasma membrane
   
   1. Receptor-activated Ca+ channels (e.g. neurotransmitter)
   2. Potential dependent Ca+ channels
2. Release from SR via GPCR pathway that induces PLC to metabolize PIP2 into DAG and IP3 –> IP3 binds/opens SR receptors

Question 14

SR release of calcium in skeletal muscle is ___, but in smooth muscle it’s ___

Answer 14

mechanical

chemical

Question 15

Different types of smooth muscle cells show different electrical activities that induce contraction

Answer 15

1. AP: Rapid depolarization & repolarization
2. Pacemaker potential: Brief action potentials “riding” on a slow wave
   
   1. Depolarization by Na+ & Ca2+
   2. Repolarization by K+
3. Subthreshold potential change: No action potentials - resting membrane potential is sufficiently depolarized to activate a few voltage-dependent Ca+ channels -> constant influx of Ca maintains constant tone
   
   1. Slow changes in membrane potential induced by neurotransmitters and hormones
4. Pharmaco-cehmical: Certain agents can alter smooth muscle force without altering membrane potential
   
   1. Involves release Ca+ from SR via IP3

Question 16

All hormones and signal molecules that induce muscle CONTRACTION are mediated by__

The ones that induces RELAXATION are done by __

Answer 16

contract: IP3
relax: cAMP

Question 17

Multiunit smooth muscle

Answer 17

Has few gap junctions

Each smooth muscle cell acts individually, with little-no connections between them; mostly innervated by autonomic nervous system

Ex) Ciliary muscle of the eye, iris, and piloerector muscles

Question 18

Single unit smooth muscle

Answer 18

Many gap junctions, so electrical signals are rapidly passed between cells.

Poorly innervated by the autonomic nervous system

Ex) GI taract, uterus, ureters, blood vessels

Question 19

Why does total stress continue to increase with length even when active stress is reduced?

Answer 19

Passive stress increases from elasticity of smooth muscle cells

Question 20

Increasing the level of myosin phosphorylation …

Answer 20

Increases the contraction velocity and max force

Question 22

The decision of the skeletal muscle contraction is ultimately made by the ____. Whereas, smooth muscle..

Answer 22

For skeletal muscle, the motor neuron decides on contraction.

For smooth muscle, the muscle cell receives and integrates many signals that impact the degree of contraction - so the muscle cell makes the decision.

Question 23

The nervous innervation of __ muscle is much more diffuse.

Answer 23

smooth.

Smooth muscle, unlike skeletal muscle, can survive without nervous input.

Question 24