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630: Medical Physiology > Cellular Physiology (Part 3) > Flashcards

Cellular Physiology (Part 3) Flashcards

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USMLE® Step 1 flashcards
1
Q

What are the 3 characteristics of a muscle fiber?

A
  • it functions as a single unit
  • multinucleated
  • contains myofibrils
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2
Q

Myofibrils of a muscle fiber are surround by _____ and are invaginated by _______.

A

endoplasmic reticulum

transverse tubules (T tubules)

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3
Q

What are the thick filaments composed of in a myofibril?

A

myosin

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4
Q

Describe the structure of myosin

A

It is comprised of 6 polypeptide chains (2 heavy and 4 light chains). The heavy chains coil around one another to form the “tail” of the myosin molecule. The 4 light chains and the N terminus of each heavy chain forms 2 globular “heads” which contain an actin-binding site, which is necessary for cross-bridge formation

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5
Q

What are the thin filaments composed of in a myofibril?

A
  • actin
  • tropomyosin
  • troponin
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6
Q

What is the difference between G-actin and F-actin?

A

When actin is in its globular protein form it is called G-actin.
When actin is twisted into an alpha-helix structure to form filamentous actin it is called F-actin

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7
Q

Which form of actin is present in thin filaments?

A

F-actin

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8
Q

Describe the shape actin is in when the muscle is at rest?

A

The myosin-binding sites on the actin are covered by tropomyosin and troponin so that actin and myosin cannot interact

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9
Q

What is tropomyosin’s function?

A

It blocks myosin-binding sites on actin during rest, and when muscle contraction occurs it is moved out of the way so the 2 can interact

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10
Q

What are the 3 globular proteins that form troponin?

A
  • troponin T
  • troponin I
  • troponin C
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11
Q

What is the function of troponin T?

A

It attaches the troponin complex to tropomyosin

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12
Q

What is the function of troponin I?

A

It inhibits the interaction of actin and myosin by covering the myosin-binding sites (with tropomyosin)

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13
Q

What is the function of troponin C?

A

It is a Ca2+ binding protein that plays a central role in the initiation of contraction

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14
Q

What happens at troponin C when Ca2+ concentration is high?

A

Ca2+ binds to troponin C, producing a conformational change in the troponin complex which moves tropomyosin out of the way, permitting the binding of actin to the myosin heads

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15
Q

What is the basic contractile unit of skeletal muscle?

A

sarcomere

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16
Q

Describe the structure of a sarcomere

A

Each sarcomere contains a full A band and one half of 2 I bands on either side of the A band

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17
Q

What structure denotes one sarcomere from the next?

A

Z disks

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18
Q

What are A bands?

A

They are bands located in the middle of the sarcomere that contain the thick (myosin) filaments and also an overlap of thick and thin (actin) filaments which are potential sites of cross-bridge formation

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19
Q

What are I bands?

A

They are bands located on either side of the A band that contain thin (actin) filaments, intermediate filamentous proteins, and Z discs.

*contain no thick filaments

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20
Q

What is the bare zone of a sarcomere?

A

The center of each sarcomere in which there is no thin filaments, thus there can be no overlap of thick and thin filaments or cross-bridge formation

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21
Q

What is the M line?

A

The line in the sarcomere that bisects the bare zone and contains darkly staining proteins that link the central portions of the thick filaments together

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22
Q

What proteins establish the architecture of the myofibrils, ensuring that the thick and thin filaments are aligned correctly and at proper distances with respect to each other

A

cytoskeletal proteins

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23
Q

What do transverse cytoskeletal proteins do?

A

They link thick and thin filaments, forming a “scaffold” for the myofibrils and linking sarcomeres of adjacent myofibrils

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24
Q

What actin-binding protein anchors the entire myofibril arrangement to the cell membrane?

A

dystophin

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25
Q

Patients with a defective or absent dystrophin protein have what pathology?

A

muscular dystophy

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26
Q

Longitudinal cytoskeletal proteins include what 2 proteins?

A

titin and nebulin

27
Q

Describe the structure of titin

A

It is a large molecular weight protein that extends from the M lines to the Z discs through the thick filaments

28
Q

Describe the structure of nebulin

A

This molecule extends from one end of the thin filament to the other serving as a “molecular ruler”, setting the length of the thin filaments during their assembly

29
Q

What anchors the thin filaments to the Z disc?

A

alpha-Actinin

30
Q

What is the function of the T tubules?

A

They are responsible for carrying depolarization from action potentials at the muscle cell surface to the interior of the fiber

31
Q

The sarcoplasmic reticulum is the site or what?

A

storage and release of Ca2+ for excitation-contraction coupling

32
Q

What part of the endoplasmic reticulum comes into contact with the T tubules?

A

the terminal cisternae

33
Q

Describe the action of Ca2+ ATPase in the sarcoplasmic reticulum

A

It pumps Ca2+ from the ICF of the muscle fiber into the interior of the sarcoplasmic reticulum which keeps the intracellular Ca2+ concentration low when the muscle fiber is at rest

34
Q

What binding-protein binds to Ca2+ in the sarcoplasmic reticulum to help keep the concentration of free Ca2+ ions inside the sarcoplasmic reticulum low?

A

Calsequestrian

35
Q

What is the significance of keeping the concentration of Ca2+ concentration low in the sarcoplasmic reticulum?

A

It reduces the work of the Ca2+ ATPase pump

36
Q

What is excitation-contraction coupling?

A

The mechanism that translates the muscle action potential into the production of tension

37
Q

Describe the 3 temporal sequence of events in excitation-contraction coupling in skeletal muscle

A

1) muscle action potential
2) rise in intracellular Ca2+ concentration
3) muscle contraction

38
Q

What are the 7 steps involved in excitation-contraction coupling?

A

1) action potentials in the muscle cell membrane are propagated to the T tubules by the spread of local currents
2) depolarization of the T tubules causes a critical conformational change in its voltage-sensitive dihydropyridine receptor which opens the Ca2+ release channels on the sarcoplasmic reticulum
3) Ca2+ is released from its storage site in the SR into the ICF of the muscle fiber, resulting in an increase in intracellular Ca2+ concentration
4) Ca2+ binds to troponin C on the thin filaments, causing a conformational change in the troponin complex
5) the conformational change in troponin causes tropomyosin to be moved out of the way so that cross-bridging cycling can begin
6) myosin heads bind to actin and form cross-bridges
7) Relaxation occurs when Ca2+ is reaccumulated in the SR by the Ca2+ ATPase

39
Q

Describe the sequence of events in the cross-bridge cycle

A

1) No ATP is bound to myosin, and myosin is tightly attached to actin in a “rigor” position
2) ATP binds to the cleft on the back of the myosin head which produces a conformational change in myosin that decreases its affinity for actin, thus myosin is released from the original actin-binding site
3) The cleft closes around the bound ATP molecule, producing a further conformational change in myosin that causes myosin to be displaced toward the plus end of actin and ATP is hydrolyzed to ADP and Pi
4) Myosin binds to a new site on actin (toward the plus end) constituting the power stroke
5) ADP is released and myosin is returned to its original state with no nucleotides bound (rigor state)

40
Q

How far does the myosin head “walk” along the actin filament during each power stroke?

A

10 nm

41
Q

What happens if there is insufficient time for the SR to reaccumulate the Ca2+ and the intracellular Ca2+ concentration never returns to the low levels that exist during relaxation?

A

There is continued binding of Ca2+ to troponin C and continued cross-bridge cycling. In this state there is a sustained contraction called tetanus, rather than a single twitch

42
Q

The _____-_____ relationship in muscle refers to the effect of muscle fiber length on the amount of tension the fiber can develop

A

Length-Tension Relationship

43
Q

What are the 3 measurements of tension?

A
  • passive
  • total
  • active
44
Q

What is passive tension?

A

The tension developed by simply stretching a muscle to different lengths

45
Q

What is total tension?

A

The tension developed when a muscle is stimulated to contract at different preloads

46
Q

Total tension is the sum of what 2 things?

A

The active tension developed by the cross-bridge cycling in the sarcomere and the passive tension caused by stretching the muscle

47
Q

How does active tension represent?

A

The active force developed by cross-bridge cycling

48
Q

How can you determine active tension?

A

Subtract passive tension from the total tension

49
Q

Active tension is ________ to the number of cross-bridges. What does this mean?

A

proportional

When there is maximal active tension, there is maximal overlap of thick and thin filaments and maximal possible cross-bridges

50
Q

The _____-_____ relationship demonstrates the relationship between the velocity of shortening when the force against which the muscle contracts (afterload) is varied

A

force-velocity

51
Q

In a force-velocity relation what is fixed?

A

The force, thereby producing an isotonic contraction

52
Q

During a isotonic contraction what does the velocity of shortening reflect?

A

The speed of cross-bridge cycling

53
Q

The velocity of shortening will be maximal when the afterload on the muscle is ____.

A

zero

54
Q

What distinguishes smooth muscle from skeletal and cardiac muscle?

A

it lacks striations due to the unorganized nature of the sarcomeres

55
Q

What are the 2 functions of smooth muscle?

A
  • To produce motility

- To maintain tension

56
Q

What are the 2 types of smooth muscle?

A
  • Unitary

- Multiunit

57
Q

What type of smooth muscle has gap junctions between cells, which allows for the fast spread of electrical activity throughout the organ, followed by a coordinated contraction?

A

unitary

58
Q

Where can unitary smooth muscle be found?

A

In the GI tract, bladder, uterus, and ureter

59
Q

What determines the frequency of contractions in unitary smooth muscle?

A

Spontaneous pacemaker activity called slow waves

60
Q

What type of smooth muscle has little or no coupling between cells?

A

multiunit

61
Q

Where can multiunit smooth muscle be found?

A

in the iris, ciliary muscles of the lens, and in the vas deferens

62
Q

What innervates multiunit smooth muscle?

A

Postganglionic fibers of the parasympathetic and sympathetic nervous systems

63
Q

What are the 3 mechanisms that increase intracellular Ca2+ concentration in smooth muscle?

A
  • voltage-gated Ca2+ channels
  • ligand-gated Ca2+ channels
  • IP3-gated sarcoplasmic reticulum Ca2+ channels

630: Medical Physiology (24 decks)

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