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(MUL) Medical Physiology > Chp 6 Contraction of Skeletal Muscle > Flashcards

Chp 6 Contraction of Skeletal Muscle Flashcards

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USMLE® Step 1 flashcards
1
Q

What % of the body is:

  1. Skeletal Muscle
  2. Smooth Muscle
  3. Cardiac Muscle
A
  1. 40%
  2. 10%
  3. 10%
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2
Q

The organisation of skeletal muscle demonstrates that all skeletal muscles are composed of what ?

A

are composed of numerous fibers ranging from 10 to 80 micrometers in diameter…each of the fibers are made up of successively smaller subunits

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3
Q

Define Sarcolemma

A

The Sarcolemma Is a Thin Membrane Enclosing a Skeletal Muscle Fiber

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4
Q

The sarcolemma consists of what type of membrane? What is the outer coat made up of?

A
  1. The sarcolemma consists of a true cell membrane, called the plasma membrane.
  2. the outer coat made up of a thin layer of polysaccharide material that contains numerous thin collagen fibrils
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5
Q

What are Myofibrils composed of?

Muscle is compsed of how many Myofibrils?

A

Myofibrils Are Composed of Actin and Myosin Filaments.

Each muscle fiber contains several hundred to several thousand myofibrils

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6
Q

Define Myofibrils

A

myofibril : basic rod-like unit of a muscle cell. Muscles are composed of tubular cells called myocytes, known as muscle fibers in striated muscle, and these cells in turn contain many chains of myofibrils.

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7
Q

Each myofibril is composed of what type of Filaments and How many Filaments?

A

is composed of about 1500 adjacent myosin filaments and 3000 actin filaments, which are large polymerized protein molecules that are responsible for the actual muscle contraction.

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8
Q
  1. The thick filaments are?
  2. The thin Filaments are?
A
  1. Myosin
  2. Actin

Both filaments partially interdigitate causing the myofibrils to have alternate light and dark bonds

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9
Q

What type of Filaments are present in The light bands ?

A

are actin filaments and are called I bands because they are isotropic to polarized light.

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10
Q

What type of Filaments are present in the dark bands

A

contain myosin filaments, as well as the ends of the actin filaments where they overlap the myosin, and are called A bands because they are anisotropic to polarized light.

- A- for A Bands and Actin filaments, Anisotropic

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11
Q

Define Anisotropic

A

is the property of being directionally dependent, which implies different properties in different directions

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12
Q

Define Isotropic

A
  1. uniformity in all orientations
  2. having a physical property which has the same value when measured in different directions.
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13
Q

What is unique about Myosin filaments?

A

The sides of Myosin Flaments posess small projections kown as Cross-Bridges.

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14
Q

What causes muscular contraction ?

A

the interaction between myosin cross-bridges and the actin filaments causes contraction.

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15
Q

Sarcomere extends from start of what zone to end of which zone?

A

From Z line to Z line.

Remember: The Z line is in the middle of I band.

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16
Q

Myosin Filaments (small projections from the sides of the Myosin) are also called

A

heavy meromyosin

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17
Q

Ends of Actin Filaments extending in both directions to interdigitate with the Myosin Filaments are attached to what ?

A

Z- Disc

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18
Q

How is the Z disk different from the actin and myosin filaments ?

A

The Z disk, which is composed of fila-mentous proteins different from the actin and myosin filaments

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19
Q

The portion of the myofibril (or of the whole muscle fiber) that lies between two successive Z disks is called ?

A

a sarcomere.

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20
Q

The side-by-side relationship between the myosin and actin filaments is maintained by a large number of filamentous molecules of a protein called What?

A

Titin Filamentous Molecules Keep the Myosin and Actin Filaments in Place

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21
Q

Each titin molecule has a molecular weight of ______which makes it one of the largest protein molecules in the body

A

Molecular weight of about 3 million, which makes it one of the largest protein molecules in the body

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22
Q

Draw the approptiate organization of the Skeletal Muscle

A

refer to Fig 6.1

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23
Q

Draw teh appropriate organization of Proteins in a Sarcomere

A

Rerfer to Fig 6.3

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24
Q

What molecules act as a framework that holds the myosin and actin filaments in place so that the contractile machinery of the sarcomere will work.

A

Springy Titin Molecules

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25
Q

The many myofibrils of each muscle fiber are sus- pended side by side in the muscle fiber. The spaces between the myofibrils are filled with

Intracellular Fluid known as ?

A

The spaces between the myofibrils are filled with intracellular fluid called sarcoplasm

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26
Q

intracellular fluid called sarcoplasm, containing large quantities of WHAT?

Also present are tremendous numbers of ______that lie parallel to the _____.

A
  1. potassium, magnesium, and phosphate, plus multiple protein enzymes.
  2. mitochondria that lie parallel to the myofibrils
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27
Q

Mitochondria Supplies the contracting Myofibrils with large amount of Energy in the form of What ?

A

Adenosine triphosphate (ATP) formed by the mitochondria.

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28
Q

Define Sarcoplasmic Reticulum

A

the specialized endoplasmic reticulum of cardiac muscle and skeletal striated muscle that functions especially as a storage and release area for calcium.

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29
Q

Define Endoplasmic Reticulm of the Skeletal Muscle

A

The endoplasmic reticulum serves many general functions:

  1. folding of protein molecules in sacs called cisternae
  2. transports synthesized proteins in vesicles to the Golgi apparatus.
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30
Q

The rapidly contracting types of muscle fibers have especially extensive _____ _____.

A

sarcoplasmic reticula.

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31
Q

describe the GENERAL MECHANISM OF MUSCLE CONTRACTION

Step 1.

A
  1. An action potential travels along a motor nerve to its endings on muscle fibers.
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32
Q

describe the GENERAL MECHANISM OF MUSCLE CONTRACTION

Step 2.

A
  1. At each ending, the nerve secretes a small amount of the neurotransmitter substance acetylcholine.
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33
Q

describe the GENERAL MECHANISM OF MUSCLE CONTRACTION

Step 3.

A
  1. The acetylcholine acts on a local area of themuscle fiber membrane to open “acetylcholine- gated” cation channels through protein molecules floating in the membrane.
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34
Q

describe the GENERAL MECHANISM OF MUSCLE CONTRACTION

Step 4.

A

Opening of the acetylcholine-gated channels allows large quantities of sodium ions to diffuse to the interior of the muscle fiber membrane. This action causes a local depolarization that in turn leads to opening of voltage-gated sodium channels, which initiates an action potential at the membrane.

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35
Q

describe the GENERAL MECHANISM OF MUSCLE CONTRACTION

Step 5.

A

The action potential travels along the muscle fiber membrane in the same way that action potentials travel along nerve fiber membranes.

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36
Q

describe the GENERAL MECHANISM OF MUSCLE CONTRACTION

Step 6.

A

The action potential depolarizes the muscle mem-brane, and much of the action potential electricity flows through the center of the muscle fiber. Here it causes the sarcoplasmic reticulum to release large quantities of calcium ions that have been stored within this reticulum.

37
Q

describe the GENERAL MECHANISM OF MUSCLE CONTRACTION

Step 7

A

The calcium ions initiate attractive forces between the actin and myosin filaments, causing them to slide alongside each other, which is the contractile process

38
Q

describe the GENERAL MECHANISM OF MUSCLE CONTRACTION

Step 8

A

After a fraction of a second, the calcium ions are pumped back into the sarcoplasmic reticulum by a Ca++ membrane pump and remain stored in the reticulum until a new muscle action potential comes along; this removal of calcium ions from the myo- fibrils causes the muscle contraction to cease.

39
Q

muscle contraction occurs by a ____ _____ _____

A

sliding filament mechanism.

40
Q

what causes the actin filaments to slide inward among the myosin filaments?

A

This action is caused by forces generated by interaction of the cross-bridges from the myosin filaments with the actin filaments

Note: under resting conditions, these forces are inactive, however when anaction potential travels along the muscle fiber, this causes the sarcoplasmic reticulum to release large quantities of calcium ions that rapidly surround the myofibrils

41
Q

What activates the forces between the myosin and actin filaments for contraction to begin.

A

The calcium ions

42
Q

Energy is needed for the contractile process to proceed. Where does this energy comes from?

How is the energy liberated?

A

This energy comes from high-energy bonds in the ATP molecule, which is degraded to adenosine diphosphate (ADP) to liberate the energy

43
Q

T or F Myosin Filaments Are Composed of Multiple Myosin Molecules

A

True

44
Q

Molecular Wight of the Myosin Molecule ?

A

molecular weight of about 480,000

45
Q

The myosin molecule is composed of:

A

The myosin molecule is composed of:

  1. (6) polypeptide chains
  2. (2) heavy chains-each with a molecular weight of about 200,000,
  3. (4) light chains with molecular weights of about 20,000 each.
46
Q

What forms the Tail and Head of the Myosin Molecle

A

The two heavy chains wrap spirally around each other to form a double helix called the tail of the myosin molecule.

One end of each of these chains is folded bilaterally into a globular polypeptide structure called a myosin head.

47
Q

What are Actin Filaments composed of

A

Actin Filaments Are Composed of :

Actin (2 helical strands of F-actin molecules)

Tropomyosin Molecules (2 Strands that fit in the grooves betwen Actin Strands)

Troponin (Attached to one end of each tropomyosin molecule is a troponin complex that initiates contraction)

48
Q

In the resting state, WHICH molecules lies on top of the active sites of the actin strands so that attraction cannot occur between the actin and myosin filaments to cause contraction.

A

tropomyosin molecules

49
Q

Define Troponin

What is the role Troponin in Muscle Contraction

A

Troponin (a protein molecule) attached intermittently along the sides of the tropomyosin molecules. (Fig 6.7)

These protein molecules are complexes of 3 loosely bound protein subunits

Each subunit plays a specific role in controlling muscle contraction.

  1. (troponin I) has a strong affinity for actin,
  2. (troponin T) for tropomyosin,
  3. (troponin C) for calcium ions. This complex attaches tropomyosin to the actin. The strong affinity of the troponin for calcium ions to initiate the contraction process,
50
Q

A pure actin filament without the presence of the troponin-tropomyosin complex binds instantly and strongly with what?

What Ion and energy source has to be present for this to take place?

A
  1. Binds instantly and strongly with the heads of the myosin molecules.
  2. in the presence of magnesium ions and ATP
51
Q

What occurs as troponin-tropomyosin complex is added to the actin filament ?

A

The binding between myosin and actin does not take place. Therefore, it is believed that the active sites on the normal actin filament of the relaxed muscle are inhibited or physically covered by the troponin- tropomyosin complex.

52
Q

Describe the “Fenn effect”

A

Large amounts of ATP are cleaved to form ADP during the contraction process, and the greater the amount of work performed by the muscle, the greater the amount of ATP that is cleaved

53
Q

sequence of events during the Phenomenon of “Fenn Effect”

A
  1. Before contraction begins, the heads of the cross- bridges bind with ATP. The ATPase activity of the myosin head immediately cleaves the ATP but leaves the cleavage products, ADP plus phosphate ion, bound to the head
  2. When the troponin-tropomyosin complex binds with calcium ions, active sites on the actin filament are uncovered and the myosin heads then bind with these sites
  3. The bond between the head of the cross-bridge and the active site of the actin filament causes a conformational change in the head, prompting the head to tilt toward the arm of the cross-bridge and providing the power stroke for pulling the actin fila- ment.
  4. Once the head of the cross-bridge tilts, release of the ADP and phosphate ion that were previously attached to the head is allowed. At the site of release of the ADP, a new molecule of ATP binds. This binding of new ATP causes detachment of the head from the actin.
  5. After the head has detached from the actin, the new molecule of ATP is cleaved to begin the next cycle, leading to a new power stroke. That is, the energy again “cocks” the head back to its perpendicular condition, ready to begin the new power stroke cycle.

6.

54
Q

T or F

sarcomeres in different parts of the muscle do not always contract the same amount

A

True

55
Q

when the muscle is at its normal resting length, which is at a sarcomere length of about ?

A

of about 2 micrometers

56
Q

Work output during muscle contraction:

to perform work requires energy which is transferred from the muscle to the external load to lift an object to a greater hight or to overcome resistance to movement.

Work is defined by following equation:

A

W=L×D

W= Work output

L= Load

D= distance of movement against the load

57
Q

3 Sources of Energy are required for muscle Contraction:

name each:

Source 1:

A
  1. first source of energy that is used to reconstitute the ATP is the substance phosphocreatine, which carries a high-energy phosphate bond similar to the bonds of ATP. phosphocreatine is instantly cleaved, and its released energy causes bonding of a new phosphate ion to ADP to reconstitute the ATP
58
Q

3 Sources of Energy are required for muscle Contraction:

name each:

Source 2:

A
  1. The second important source of energy, which is used to reconstitute both ATP and phosphocreatine, is “glycolysis” of glycogen previously stored in the muscle cells.Rapid enzymatic breakdown of the glycogen to pyruvic acid and lactic acid liberates energy that is used to convert ADP to ATP; the ATP can then be used directly to energize additional muscle contraction.
59
Q

3 Sources of Energy are required for muscle Contraction:

name each:

Source 3:

A
  1. The third and final source of energy is oxidative metab- olism, which means combining oxygen with the end prod- ucts of glycolysis and with various other cellular foodstuffs to liberate ATP. More than 95 percent of all energy used by the muscles for sustained, long-term contraction is derived from oxidative metabolism
60
Q

Define Isotonic contraction

A

Isotonic contraction occurs when the force of the muscle contraction is greater than the load and the tension on the muscle remains constant during the contraction; when the muscle contracts, it shortens and moves the load

Shorten muscle at Constant Tension

61
Q

Define Isometric contraction

A

Isometric contraction occurs when the load is greater than the force of the muscle contrac- tion; the muscle creates tension when it contracts, but the overall length of the muscle does not change.

Do not shorten muscle

62
Q

sports physiology, every muscle of the body is composed of a mixture of so-called :

A

fast and slow muscle fibers

63
Q

Type 1 Red Muscle is defined by what Fibers:

A

Slow Fibers

64
Q

Slow Fibers (Type 1, Red Muscle).

Name the (5) characteristics of slow fibers:

A

Slow Fibers (Type 1, Red Muscle). The following are characteristics of slow fibers:

  1. Slow fibers are smaller than fast fibers.
  2. Slow fibers are also innervated by smaller nerve fibers.
  3. Compared with fast fibers, slow fibers have a more extensive blood vessel system and more capillaries to supply extra amounts of oxygen.
  4. Slow fibers have greatly increased numbers of mitochondria to support high levels of oxidative metabolism
  5. Slow fibers contain large amounts of myoglobin, an iron-containing protein similar to hemoglobin in red blood cells. Myoglobin combines with oxygen and stores it until needed, which also greatly speeds oxygen transport to the mitochondria. The myoglo- bin gives the slow muscle a reddish appearance and hence the name red muscle.
65
Q

Type II White Muscle is defined by what Fibers:

A

Fast Fibers

66
Q

FastFibers (Type II, WhiteMuscle).

The following are characteristics of fast fibers:

A
  1. Fast fibers are large for great strength of contraction.
  2. An extensive sarcoplasmic reticulum is present for rapid release of calcium ions to initiate contraction.
  3. Large amounts of glycolytic enzymes are present for rapid release of energy by the glycolytic process.
  4. Fast fibers have a less extensive blood supply than do slow fibers because oxidative metabolism is of secondary importance.
  5. Fast fibers have fewer mitochondria than do slow fibers, also because oxidative metabolism is secondary. A deficit of red myoglobin in fast muscle gives it the name white muscle.
67
Q

All the muscle fibers innervated by a single nerve fiber are called?

A

a motor unit

68
Q

Define Summation:

Summation occurs in two ways:

A

1. Summation means the adding together of individual twitch contractions to increase the intensity of overall muscle contraction.

  1. Summation occurs in two ways:
    (1) by increasing the number of motor units con- tracting simultaneously, which is called multiple fiber summation
    (2) by increasing the frequency of contraction, which is called frequency summation and can lead to tetanization.
69
Q

A motor unit consists of what:

review Fig 6.14

A

A motor unit consists of a motor neuron and the group of skeletal muscle fibers it innervates.

A single motor axon may branch to innervate several muscle fibers that function together as a group. Although each muscle fiber is innervated by a single motor neuron, an entire muscle may receive input from hundreds of differ- ent motor neurons.

70
Q

Define Tetanization:

A

When the frequency reaches a criti- cal level, the successive contractions eventually become so rapid that they fuse together and the whole muscle contraction appears to be completely smooth and con- tinuous, as shown in the figure

71
Q

Why does Tetany occur in a muscle?

A

occurs because enough calcium ions are maintained in the muscle sarcoplasm, even between action potentials, so that full contractile state is sustained without allowing any relaxation between the action potentials.

72
Q

define Muscle Hypertrophy ;

A

The increase of the total mass of a muscle

73
Q

define Muscle Atrophy:

A

When the total muscle mass decreases

74
Q

Virtually all muscle hypertrophy results from an increase in the number fo what in each muscle fiber?

causing enlargement of the individual muscle fibers; this condition is called simply ?

A

increase in the number of actin and myosin filaments in each muscle fiber (myogibrils)

fiber hypertrophy

75
Q

muscle remains unused for many weeks, the rate of____ _____ ____ ___ is more rapid than the rate of replacement. Therefore, muscle ____occurs

A
  1. degradation of the contractile proteins
  2. atrophy
76
Q

What Pathway appears to account for much of the protein degradation in a muscle undergoing atrophy?

A

ATP-dependent ubiquitin-proteasome pathway.

77
Q

ATP-dependent ubiquitin-proteasome pathway

Break down the pathway:

A

Proteasomes are large protein complexes that degrade damaged or unneeded proteins by proteolysis.

Proteolysis a chemical reaction that breaks peptide bonds.

Ubiquitin is a regulatory protein that basically labels which cells will be targeted for proteosomal degradation.

78
Q

Define Fiber hyperplasia

A

the increase in fiber number due to extreme muscle force generation

Very Rare!!

79
Q

define Contracture

A

When the fibrous tissue replaces the muscle fibers during denervation atrophy also has a tendency to continue shortening for many months

80
Q

Define Poliomyelitis

A

When some but not all nerve fibers to a muscle are destroyed, the remaining nerve fibers branch off to form new axons that then innervate many of the paralyzed muscle fibers.

81
Q

In the Case of Poliomyelitis, as remaining nerve fibers branch off to form new axons that then innervate many of the paralyzed muscle fibers:

this process results in what ?

A

This process results in large motor units called macromotor units, which can contain as many as five times the normal number of muscle fibers for each motoneuron coming from the spinal cord.

82
Q

In case of Poliomyelitis, as the remainng nerve fibes branch off to form new axons which innervate many of the poralyzed muscel fibers results in formation of Macromotor Units:

What happens with “recovery of Muscle contraction in Poliomyelitis”

A

The formation of large motor units decreases the fineness of control one has over the muscles but does allow the muscles to regain varying degrees of strength.

83
Q

Rigor Mortis

A
  1. all the muscles of the body go into a state of contracture called “rigor mortis” ( the muscles contract and become rigid, even without action potentials)
  2. rigidity results from loss of all the ATP, which is required to cause separation of the cross-bridges from the actin filaments during the relaxation process.
  3. proteins deteriorate about 15 to 25 hours later, resulting from autolysis caused by enzymes released from lysosomes.
84
Q

Muscular Dystrophy

A
  1. includes several inherited disorders that cause progressive weakness and degeneration of muscle fibers, which are replaced by fatty tissue and collagen.
  2. Most common: Duchenne muscular dystrophy (DMD). This disease affects only males because it is transmitted as an X-linked recessive trait and is caused by a mutation of the gene that encodes for a protein called dystrophin, which links actins to proteins in the muscle cell membrane
85
Q

in the Case of “ Duchenne muscular dystrophy (DMD)”

Name one important effect of abnormal Dystrophin:

A

One important effect of abnormal dystrophin is an increase in membrane permeability to calcium, thus allowing extracellular calcium ions to enter the muscle fiber and to initiate changes in intracellular enzymes that ultimately lead to proteolysis and muscle fiber breakdown.

86
Q

What are symptoms of DMD

A

muscle weakness that begins in early childhood and rapidly progresses, so that the patients are usually in wheelchairs by age 12 years and often die of respiratory failure before age 30 years.

87
Q

What is a milder form of Muscular dystrophy disease called?

A

Becker muscular dystrophy (BMD), is also caused by mutations of the gene that encodes for dystrophin but has a later onset and longer survival. It is estimated that DMD and BMD affect 1 of every 5,600 to 7,700 males between the ages of 5 through 24 years.

no TX.

88
Q
A

(MUL) Medical Physiology (11 decks)

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