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Medical Physiology > Exam #1: Smooth Muscle > Flashcards

Exam #1: Smooth Muscle Flashcards

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USMLE® Step 1 flashcards
1
Q

List the characteristics of smooth muscle. How does smooth muscle differ from skeletal muscle?

A
  • Smooth muscle is composed of smaller shorter fibers than skeletal muscle
  • Smooth muscle is non-striated vs. striated skeletal muscle
  • Different types of smooth muscle exist (multi-unit vs. unitary) in different organs
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2
Q

What is the difference between multi-unit & unitary smooth muscle?

A

Multi-unit= discrete smooth muscle fibers that contract independently

  • Each fiber is independently innervated by the ANS
  • Individual fibers do NOT communicate

Unitary= Large number of fibers that contract together as a single unit

  • Neuronal & non-neuronal factors regulate
  • Cells communicate via gap junctions
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3
Q

Locations of multi-unit smooth muscle

A
  • Ciliary & iris muscle of the eye

- Piloerector muscle

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4
Q

Locations of unitary smooth muscle.

A
  • GI Tract
  • Bile Ducts
  • Ureters
  • Uterus
  • Blood vessels
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5
Q

What contractile component is missing from smooth muscle that is present in skeletal muscle?

A

Troponin Complex
- Indicative of a different role of Ca++ in smooth muscle

Smooth muscle does contain actin, myosin, and tropomyosin

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6
Q

How are myosin heads arranged in smooth muscle? How is this different from skeletal muscle?

A

Mysoin heads in smooth muscle are NOT all arranged in the same direction

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7
Q

What takes the place of z-discs in smooth muscle?

A

Dense bodies

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8
Q

What is the function of dense bodies?

A
  • Transmission of force from once cell to another

- Anchor for thin filaments

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9
Q

What attaches to the dense body & what is between them?

A

Thin filaments (actin) are attached to the dense body; thick (myosin) are interspersed between the thin filaments

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10
Q

How are actin & myosin arranged in smooth muscle? What is the functional outcome?

A

Actin & myosin are arranaged in every plane, which allows fro the entire cell to contract i.e. shrink or bulge

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11
Q

Review the contractile events that occur in skeletal muscle, starting with the attached state.

A

1) Myosin attached to actin
2) Myosin binds ATP leading to detachment
3) ATP hydrolysis to ADP & Pi resets the myosin head
4) Cross-bridge forms & myosin binds a new position on actin
5) Pi is released leading to a change in position of myosin–>power stroke
6) ADP released

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12
Q

How does the point of regulation differ from skeletal muscle to smooth muscle?

A
  • Smooth muscle targets myosin

- Skeletal muscle uses Ca++ to regulate actin

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13
Q

Outline the events of contraction in smooth muscle.

A

1) Increased intracellular Ca++ from extracellular space
2) Ca++ binds Calmodulin
3) Calmodulin-Ca++ binds & activates myoskin light chain kinase (MLCK)
4) MLCK phosphorylates myosin regulatory chain & allows for activation of the myosin ATPase
5) Cross bridging occurs when myosin is phosphorylated at the regulatory chain

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14
Q

What is different between contraction of skeletal muscle & smooth muscle?

A

1) In smooth muscle ECF Ca++ is the PRIMARY source of Ca++, NOT SR Ca++ as in skeletal muscle
2) MLCK
3) Myosin ATPase is constitutively active in skeletal muscle; smooth muscle, this is regulated
4) Cross bridging occurs as long as myosin is in the phosphorylated state

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15
Q

Describe the events of actin-myosin cross-bridging in smooth muscle.

A

1) MLCK phosphorylates Myosin ATPase to turn it ON
2) Myosin- ADP+Pi is attached to actin= cross-bridge formed
3) Release Pi + ADP from myosin= power stroke
4) ATP binds myosin= release
5) ATP hydrolysis to ADP+Pi= new cross-bridge formed

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16
Q

What is the result of decreased MLCK activation?

A

Relaxation

17
Q

What causes relaxation in smooth muscle?

A

1) Intracellular Ca++ decreases, preventing MLCK activation & necessary phosphorylation/ activation of Myosin ATPase
2) Dephosphorylation of myosin by myosin phosphatase

18
Q

What leads to tonic contraction in smooth muscle?

A

Once the cross-bridge is formed, dephosphorylation of the Myosin ATPase regulatory site

  • Reduced ATPase activity follows
  • Very slow power stroke occurs
  • Maintenance of the cross-bridge for a long time= tonic contraction
19
Q

List the differences between smooth muscle & skeletal muscle in terms of contraction. Specifically address: 1) General Characteristics, 2) Cross-Bridge Cycling, 3) Energy
4) Contraction/Relaxation, 5) Maximum Force, 6) Latch Mechanism, & 7) Stress-relaxation.

A
  • Smooth muscle can produce long tonic contractions that occurs for hours or days vs. rapid contraction & relaxation in skeletal muscle
  • Slow cross-bridge cycle in smooth muscle vs. fast cycle in skeletal
  • Low energy demand in smooth muscle vs. high energy demand in skeletal muscle
  • Relatively long onset & duration of contraction in smooth muscle vs. fast onset, duration, & relaxation in skeletal muscle
  • Higher maximum force of contraction in smooth muscle vs. skeletal muscle
  • Latch can produce tonic contraction in smooth, which is not present in skeletal muscle
  • Relaxation & contraction can occur in response to stretching & filling to maintain constant pressure in smooth muscle, which does not occur in skeletal muscle
20
Q

How does the neural regulation of smooth muscle differ from skeletal muscle?

A
  • No structural neuromuscular junction like in skeletal muscle
  • Diffuse branches of nerves overlie smooth muscle
  • Multiple varicosities along the nerve fiber instead of end feet
  • Increased space between varicosity & muscle fibers
21
Q

What is the difference between diffuse junctions & contact junctions in smooth muscle?

A
  • Diffiuse junction= seen in unitary smooth muscle; nerve fiber does NOT contact the sheet of muscle
  • Contact junction= seen in multi-unit smooth muscle; nerver fiber does contact the muscle fibers

Note that contact junctions resembe neuromuscular end feet seen in skeletal muscle

22
Q

What are the primary neurotransmitters that regulate smooth muscle?

A

Acetylcholine & NE

23
Q

In general, what effect will acetylcholine & NE have?

A

Opposite; it is difficult to tell if it will cause contraction of relaxation (tissue-dependent)

24
Q

What is the effect of NO on smooth muscle?

A

Relaxation

25
Q

Define electromechanical stimulation.

A
  • Change in membrane permeability resulting in depolarization of smooth muscle

I.e. opening Na+ &/or Ca++ membrane channels leading to a depolarization

26
Q

Define pharmacomechanical stimulation.

A
  • NOT CHANGING MEMBRANE POTENTIAL
  • Activation of signaling molecules that through the generation of second messengers activate the contractile process.

E.g. Hormone activates PLC, increasing IP3 & Ca++; increase of intracellular Ca++ causes contraction of smooth muscle

27
Q

Define electromechanical inhibition.

A
  • Change in membrane permeability that results in hyperpolzarization of smooth muscle

E.g. close Na+ &/or Ca++ channels or open K+ membrane channels.

28
Q

Define pharmacomechanical inhibition.

A
  • NOT CHANGING MEMBRANE POTENTIAL
  • Activation of signaling molecules that through the generation of second messengers inhibit the contractile process.

E.g. Hormone activates PKA that phosphorylates MLCK, preventing Ca++-Calmodulin from binding & activating MLCK

29
Q

What are the two sources of Ca++ in smooth muscle? Which is the primary source?

A 
ECF= primary 
SR= secondary
30
Q

What are the two roles of Ca++ in smooth muscle?

A

1) Membrane depolarization i.e. influx of Ca++ & Na+ are important for action potential
2) Contraction via the activation of MLCK

31
Q

What are the two types of action potentials that occur in smooth muscle?

A

1) Spike potentials

2) Action potentials with plateaus

32
Q

Draw a spike potential & List the different ions that important for the phases the action potential.

A

N/A

33
Q

Draw an action potential with plateau & list the different ions that important for the phases of the potential.

A

N/A

34
Q

What are slow wave potentials?

A

A continuous cycling of depolarization & repolarization without eliciting a spike potential that leads to a contraction.

  • Seen in some types of smooth muscle
  • Serve as pacemakers for some types of smooth muscle
  • An action potential could occur at the peak of the slow wave

*THIS IS NOT AN ACTION POTENTIAL

35
Q

What causes slow-wave potentials?

A

Na+ pumping or rhythmic changes in ion conductance

36
Q

What ion is primarily responsible for depolarization during action potentials in smooth muscle?

A

Ca++

*Note that Na+ does play a role, but Ca++ is primary, like cardiac action potentials.

37
Q

What ion is responsible for the prolonged state of depolarization seen in action potentials with plateaus?

A

Ca++

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