9 - Smooth Muscle

Question 1

There are two types of smooth muscle. What are they?

Answer 1

1 - Multi-unit

2 - Unitary (syncytial or visceral)

Question 2

What is multi-unit smooth muscle?

Answer 2

Discrete smooth muscle fibers that contract independently of each other

Question 3

Where would you find multi-unit smooth muscle?

Answer 3

Ciliary and iris muscles of the eye as well as piloerector muscles

Question 4

Do multi-unit smooth muscles communicate through gap junctions?

Answer 4

No - they do not communicate with each other

Question 5

What are multi-unit smooth muscle fibers innervated by?

Answer 5

Autonomic nervous system

Question 6

What is unitary smooth muscle?

Answer 6

A group of smooth muscle fibers that contract together as a single unit

Question 7

How are unitary smooth muscle fibers usually arranged?

Answer 7

In sheets or bundles - these arrangements usually work together in concert

Question 8

What are unitary smooth muscle fibers innervated by?

Answer 8

The Autonomic nervous system as well as non-neuronal regulators such as hormones or metabolites

Question 9

Is each fiber of a unitary muscle innervated?

Answer 9

No - this is different from multi-unit smooth muscle.

Each bundle or sheet of unitary smooth muscle is innervated at specific points and the signal is then communicated between fibers via gap jucntions

Question 10

What does this system of innervation and communication allow for?

Answer 10

Unitary smooth muscles are able to exhibit synchronized contractions for the entire group of fibers

Question 11

Where would you find unitary smooth muscle fibers?

Answer 11

• GI tract
• Bile duct
• Ureters
• Uterus
• Blood vessels

Question 12

What contractile elements will you find in smooth muscle?

Answer 12

Actin
Myosin
Tropomyosin

Question 13

What are you missing in smooth muscle contraction that you utilize in skeletal muscle contraction?

Answer 13

Troponin

Question 14

How is myosin different in smooth muscle than it is in skeletal muscle?

Answer 14

In smooth muscle, all the myosin heads are not arranged in the same direction

Question 15

What does the irregular arrangement of myosin in smooth muscle allow for?

Answer 15

Multi-directional contraction or force generation in multiple planes

Question 16

What are dense bodies similar to in skeletal muscle?

Answer 16

Dense bodies are functionally similar to Z discs of skeletal muscle

Question 17

What do dense bodies consist of?

Answer 17

Structural proteins that are disbursed throughout the cell

Question 18

What is the function of dense bodies?

Answer 18

THey serve to anchor adjacent cell to each other

Question 19

What does the cellular connection via dense bodies allow for?

Answer 19

The force of contraction can be transmitted from one cell to the next

Question 20

What else do dense bodies serve to do?

Answer 20

They are the anchoring point for actin thin filaments

Question 21

Do actin and myosin have similar or different functions in smooth muscle and skeletal muscle?

Answer 21

Very similar

Question 22

What does actin attach to in smooth muscle?

Answer 22

Dense bodies

Question 23

Where do you find myosin in smooth muslce?

Answer 23

Interspersed between the actin filaments

Question 24

During smooth muscle contraction, do actin and myosin undergo the same sliding overlap?

Answer 24

It is similar but it has a different contractile structure than skeletal muscle due to the arrangement of mysoin heads

Question 25

What occurs structurally at the molecular level during smooth muscle contraction?

Answer 25

Due to the multidirectional arrangements of myosin heads and the arrangement of structural proteins in all planes of the cell, the contraction allows for SHRINKING and BULGING of the cell

Question 26

How is the shrinking and bulging of the cell in smooth muscle different from the muscle contraction in skeletal muscle?

Answer 26

In skeletal muscle, the muscle fiber simply shortens in length rather than getting pulled in multiple directions

Question 27

How is the regulation of myosin different in smooth muscle than it is in skeletal muscle?

Answer 27

The source of calcium is different

Question 28

What is the primary source of Ca++ in skeletal muscle?

Answer 28

Sarcoplasmic reticulum

Question 29

What is the primary source of Ca++ in smooth muscle?

Answer 29

The extracellular fluid

Question 30

What accounts for this difference?

Answer 30

The sarcoplasmic reticulum in smooth muscle is not well developed and is a weak source of calcium

Question 31

What does the Ca++ of smooth muscles do once it enters the cell?

Answer 31

Binds to a protein called calmodulin

Question 32

What does the calcium-calmodulin complex activate?

Answer 32

Myosin light-chain kinase (MLCK)

Question 33

What does the MLCK enzyme do?

Answer 33

Phosphorylates the regulatory chain of myosin - this ultimately regulates the activity of the myosin ATPase

Question 34

Is the attachment and detachment cycle of myosin and actin using ATP similar or different to the cycle seen with skeletal muscle?

Answer 34

Similar

Question 35

What does a decrease of calcium lead to?

Answer 35

Dephosphorylation of the regulatory domain of myosin

Question 36

What happens when the regulatory domain of myosin is not phosphorylated?

Answer 36

Relaxation

Question 37

What is the latch state?

Answer 37

A state that forms which allows persistent contraction with low energy requirements

Question 38

What occurs that allows the latch state to be maintained

Answer 38

Once the actin-myosin bridge forms, the regulatory site can be dephosphorylated There is reduced ATPase activity and therefore a "slow" power stroke This causes the actin-myosin bridge to be maintained for a relatively long period of time in a tonic contraction

Question 39

Are neuromuscular junctions found in smooth muscle like they are in skeletal muscle?

Answer 39

No

Question 40

Instead of neuromuscular junctions, what type of innervation do we see from neurons?

Answer 40

DIffuse branches of nerve fibers that lay over sheets of smooth muscle

Question 41

Is there an end foot at the end of a nerve terminal in smooth muscle?

Answer 41

No - instead there are varicosities, which release neurotransmitters

Question 42

What types of neurotransmitters regulate smooth muscle contraction?

Answer 42

Norepinephrine | Acetylcholine

Question 43

What chemicals regulate smooth muscle contraction?

Answer 43

Metabolic products, ions, gases

Question 44

What hormones regulate smooth muscle contraction?

Answer 44

Endothelin Angiotensin Vasopressin

Question 45

What is electromechanical stimulation?

Answer 45

A change in membrane permeability that results in a depolarization of smooth muscle

Question 46

When would electromechanical stimulation occur?

Answer 46

If sodium or calcium channels were opened

Question 47

What is electromechanical inhibition?

Answer 47

A change in membrane permeatiblity that results in the hyperpolarization of smooth muscle

Question 48

When would electromechanical inhibition occur?

Answer 48

If membrane sodium or calcium channels were closed or if potassium channels were opened

Question 49

What is pharmacomechanical stimualtion?

Answer 49

The stimulation of smooth muscle contraction by signaling molecules

Question 50

When would pharmacomechanical stimulation occur?

Answer 50

Some hormones will activate the phospholipase C (PLC) pathway through their receptor, which then increases the intracellular concentration of the second messenger (IP3) which then allows intracellular stores of calcium to be released and smooth muscle to contract

Question 51

What is pharmacomechanical inhibition?

Answer 51

The inhibition of smooth muscle contraction which is mediated by signaling molecules

Question 52

When would pharmacomechanical inhibition occur?

Answer 52

Some hormones will activate the Protein Kinase A (PKA) pathway, which then phosphorylates MLCK, whcih prevents the calcium-calmodulin complex from activating MLCK, which blocks muscle contraction Some hormones activate myosin phosphatase, which leads to an inhibition of the contractile process as well

Question 53

How does calcium participate in electrical events in smooth muscles?

Answer 53

Depolarization due to movement of Ca++ into the cell

Question 54

How does calcium participate in mechanical events in smooth muscle?

Answer 54

Calcium inside the cell binds to calmodulin, which is required to activate MLCK and contract the muscle

Question 55

What is a spike potential of smooth muscle?

Answer 55

A type of smooth muscle action potential that resembles a normal action potential

Question 56

What causes the depolarization (incline) of a spike potential?

Answer 56

Depolarization due to Ca++ and Na+ movement into the cell

Question 57

What causes the hyperpolarization (decline) of a spike potential?

Answer 57

Repolarization due to K+ movement out of the cell

Question 58

What is an action potential with a plateau in smooth muscle?

Answer 58

A type of smooth muscle action potential that depolarizes until it reaches its peak, begins to hyperpolarize and decline, but experiences a prolonged plateau before eventually completing the hyperpolarization back to resting potential

Question 59

What causes the plateau in an action potential in smooth muscle?

Answer 59

Prolonged and slow opening of Ca++ channels

Question 60

What is a resting slow wave of a smooth muscle membrane?

Answer 60

A continuous cycling of depolarization and repolarization without eliciting a spike potential that would lead to a contraction

Question 61

Is the membrane of a resting slow wave ever at rest?

Answer 61

No - it continues to depolarize and repolarize

Question 62

What occurs at the peak of depolarization of a slow wave?

Answer 62

Rapid depolarization or a spike potential can occur at this peak of depolarization - this can lead to contraction

Question 63

Where would you find smooth muscle that exhibits a resting slow wave?

Answer 63

Smooth muscle of the gastrointestinal tract

Question 64

What type of smooth muscle contains cells that are connected via gap junctions?

Answer 64

Unitary smooth muscle

Question 65

What type of smooth muscle would you find in the small intestine?

Answer 65

Unitary smooth muscle

Question 66

What is a contractile protein that is present in skeletal muscle but not smooth muscle?

Answer 66

Troponin

Question 67

What protein can bind to calcium in smooth muscle?

Answer 67

Calmodulin

Question 68

What protein can bind to calcium in skeletal muscle?

Answer 68

Troponin

Question 69

Which type of regulator will stimulate smooth muscle contraction, acetylcholine or epinephrine?

Answer 69

CAN'T TELL - it is tissue specific

Question 70

What is the primary source of calcium for smooth muscle contraction?

Answer 70

Extracellular fluid

Question 71

What does the calcium from the extracellular fluid allow for?

Answer 71

Change in membrane potential and a contractile event

Question 72

What ions are responsible for the depolarization of smooth muscle?

Answer 72

Sodium and calcium

Question 73

True or false: a change in membrane potential is required to induce smooth muscle contraction

Answer 73

No - it CAN, but it is not required