Chapter 6 - Guyton Flashcards

1
Q

General skeletal muscle fiber characteristics.

A

numerous fibers ranging from 10-80 micrometers, most fibers extend the length of entire muscle, each fiber is usually innervated by one nerve ending near the middle of each fiber

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2
Q

The membrane of a skeletal muscle fiber?

A

sarcolemma (fuses with tendon fiber at the end)

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3
Q

General characteristics of myofibrils.

A

each muscle fiber consists of several thousand, composed of myosin and actin interdigitated causing alternating light and dark bands (2:1 actin to myosin)

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4
Q

I bands

A

light bands that contain only actin, isotropic to polarized light (appear the same from all directions)

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5
Q

A bands

A

dark bands that contain only myosin filaments at the ends of actin filaments where they overlap myosin, anisotropic to polarized light (appear different in different directions)

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6
Q

Cross-bridges

A

small projections from the sides of the myosin filaments that connect the actin and myosin allowing for skeletal muscle contraction

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7
Q

M-line

A

thick, mid region of myosin

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8
Q

Z-disc

A

where the ends of actin filaments attach, extend in both directions to connect myosin (creating striations)

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9
Q

Sarcomere

A

portion that lies between two z-discs

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10
Q

When is the muscle fiber capable of producing its strongest contraction force?

A

actin filaments completely overlap myosin filaments and actin filaments are beginning to overlap one another

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11
Q

What is the filamentous molecule of protein that keeps the actin and myosin in place?

A

titin (attaches Z-discs to M-line)

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12
Q

What is one of the largest protein molecules in the body (molecular weight of ~3 million)?

A

titin

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13
Q

What is the sarcoplasm?

A

intracellular fluid between the myofibrils, contains large amounts of potassium, magnesium, phosphate, and some protein enzymes

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14
Q

What lies parallel to the myofibrils in the sarcoplasm?

A

mitochondria to supply the ATP to the contracting myofibrils

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15
Q

Explain the general mechanism of muscle contraction (deep breath here we go…).

A
  1. AP travels along motor neuron to its ending on muscle fibers 2. Ach released at ending 3. ligand-gated Ach channels are opened through protein molecules floating in the membrane 4. Na diffuses into muscle fiber leading to depolarization and opening of voltage-gated Na channels initiating AP at the membrane 5. AP depolarizes muscle membrane, SR releases stored Ca 6. calcium ions cause actin and myosin to slide alongside in contractile process 7. Ca pumped back into SR by pump until new AP
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16
Q

What is responsible for the muscle contraction stopping?

A

removal of calcium ions from the myofibrils

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17
Q

Describe the positioning of actin filaments in a relaxed state.

A

the ends of actin filaments extend from two successive z-discs and are barely beginning to overlap the A-band

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18
Q

Describe the positioning of actin filaments in a contracted state.

A

the actin filaments have pulled inward over the myosin heads, Z-discs have been pulled by actin to the ends of the myosin filaments

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19
Q

Molecular Characteristics of Myosin

A

total molecular weight ~480,000, composed of 6 polypeptide chains, golf club

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20
Q

What is the significance of the heavy and light chains of myosin?

A

2 heavy chains form the head (site of ATPase activity) and tail, 4 light chains also form the head and help control the movement

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21
Q

Molecular Characteristics of Actin

A

F-actin: double-stranded protein molecule that acts as backbone to actin filament, each strand is composed of polymerized G-actin molecules, 1 ADP is attached to each G-actin, which is the active site of actin filaments with which the cross-bridges of myosin interact

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22
Q

What is tropomyosin?

A

protein component of actin filament; wrapped spirally around the sides of the F-actin helix, in resting state lies on top of actin so that actin and myosin cannot produce contraction

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23
Q

What is troponin?

A

protein molecule attached to the sides of tropomyosin, believed to initiate contraction process

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24
Q

What are the three protein subunits of troponin?

A

Troponin I - strong affinity for actin; Troponin T - strong affinity for tropomyosin; Troponin C - strong affinity for calcium

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25
Q

What causes the conformational change in the T-T complex to allow for contraction to occur?

A

calcium binds to Troponin C, uncovering the active site and allowing actin and myosin to bind

26
Q

Explain the “walk-along” theory of contraction.

A

when a myosin head attaches to an active site, it simultaneously causes changes in the intramolecular forces between the head and arm of the cross-bridge, the myosin heads of the cross-bridges move back-and-forth “walking-along” the actin filament, pulling the ends of the actin towards the center of the myosin

27
Q

What is the power stroke?

A

tilting of myosin head toward the arm to “drag” actin filament along

28
Q

What is the Fenn effect?

A

the greater the amount of work performed by the muscle, the greater the amount of ATP cleaved

29
Q

Explain the chemical events in the motion of the myosin heads.

A
  1. myosin heads binds with ATP before a contraction occurs and ATPase enzyme cleaves the ATP, leaving ADP + a phosphate ion bound to the head 2. T-T complex binds with Ca, uncovering the active sites 3. binding between the myosin head and actin filament cause the head to tilt toward the arm providing the power stroke event to pull the actin filament. The energy
    required for the powerstroke is energy that has been stored in the myosin head from
    when ATP was cleaved 4. when the myosin head of the cross-bridge tilts, ADP and the phosphate ion are released and a new molecule of ATP binds, causes detachment of the head from
    actin 5. after detachment, a new molecule of ATP is cleaved to begin a new cycle, leading to a
    new power stroke. (the head is placed back to its perpendicular positioning to begin the power stroke) 6. “cocked” head binds with a new active site on the actin filament and provides a
    new power stroke
30
Q

When does the contraction-coupling cycle cease?

A

actin filaments pull the z-membrane up against the myosin filaments OR until the load on the muscle becomes too great for further pulling to occur

31
Q

Explain the sarcomere length-tension relationship.

A

tension increases as length decreases until all actin and myosin is overlapped, zero tension at full length and at 1.65 micrometers, at 2.0 micrometers full tension is reached

32
Q

Total tension of a muscle is equal to?

A

active and passive tension

33
Q

What is the contraction velocity-load relationship?

A

when loads are applied, the contraction velocity becomes progressively less as the load increases, when the load has been increased to equal the max. force that the muscle can exert, the velocity of contraction becomes zero and no contraction results, low Vmax means slow shortening, cycling
ex. soleus has a low Vmax, 200 ms

34
Q

Work=

A

Load x Distance

35
Q

Name the three ways ATP is used during a muscle contraction.

A

walk-along mechanism by which the cross-bridges pull the actin filaments (most of the ATP), pumping Ca from sarcoplasm into SR after contraction, pumping NA and K ions through muscle fiber membrane

36
Q

Sources of energy to replenish ATP in the muscle.

A

phosphocreatine, glycogen, and oxidative metabolism

37
Q

Is there a lot or a little phosphocreatine in muscle?

A

only a little

38
Q

Can glycolysis occur in the absence of oxygen?

A

Certainly! Good thing too because the rate of making ATP by glycolysis is ~2.5 x faster than cellular foodstuffs reacting with oxygen

39
Q

Over 95% of all all energy used for sustained muscle contractions use this source.

A

oxidative metabolism

40
Q

The greatest proportion of energy comes from fats but for period of 2-4 hours energy comes from this source.

A

Carbohydrates yum

41
Q

Efficiency of Muscle Contractions

A

the percentage of input energy into a muscle that can be converted to work is less than 25%, the rest becoming heat; low efficiency is because about half of the energy in foodstuffs is lost when making ATP; max efficiency occurs when a muscle is contracted at moderate velocity

42
Q

What occurs if muscle is contracted too rapidly?

A

large quantities of energy are used to overcome the friction within the muscle, reducing contraction efficiency

43
Q

Isotonic Contraction

A

muscle does shorten but the tension on the muscle remains constant throughout the contraction

44
Q

Isotonic contraction is dependent on?

A

load the muscle is contracting against and inertia against the load

45
Q

General characteristics of fast-twitch Type II white muscle fibers.

A

large fibers for greater contraction strength, extensive SR for increased release of Ca, large quantities of glycolytic enzymes for rapid release of ATP, less extensive blood supply, fewer mitochondria, less quantity of myoglobin

46
Q

General characteristics of slow-twitch Type I red muscle fibers.

A

small fibers, more extensive blood supply, more mitochondria to support more oxidative metabolism, large amounts of myoglobin

47
Q

What gives slow-twitch fibers their red appearance?

A

myoglobin

48
Q

What is force summation?

A

adding together of individual twitch contractions to increase the intensity of overall muscle contraction (occurs through multiple fiber or frequency)

49
Q

What is the size principle?

A

smaller motor units will be recruited first over larger ones

50
Q

What is tetanization?

A

when the frequency reaches a critical point and the successive contractions fuse together to form one smooth contraction (slightly past tetanization, the muscle reaches its max. strength of contraction and cannot increase any further in contractile force)

51
Q

What limits tetanization and thus the strength of the muscle contraction?

A

calcium

52
Q

What is the maximum strength of tetanic contraction of a muscle at normal resting length?

A

averages 3-4 kg/cm2 of muscle or 50 lbs/in2

53
Q

What is the Staircase Effect (Treppe)?

A

when a muscle begins to contract after a long period of rest, the initial strength of contraction maybe be as little as ½ of its strength, 10-50 twitches later the strength of the contractions increases to a plateau caused by the increasing Ca ions in the cytosol because the ions are being released rapidly from the SR and the sarcoplasm cannot recapture them immediately

54
Q

What results in skeletal muscle tone?

A

low rate of nerve impulses constantly coming from the spinal cord (controlled by signals transmitted to the brain and signals from muscle spindles)

55
Q

General characteristics of muscle fatigue.

A

results from prolonged and strong muscle contraction, increases to the rate of depletion of muscle glycogen, interruption of blood flow in a contracting muscle will lead to immediate muscle fatigue

56
Q

The lever systems of the body depend on:

A

point of muscle insertion, its distance from the fulcrum of the lever, length of the lever arm, position of the lever

57
Q

Muscle atrophy occurs as a result of what pathway?

A

ATP-dependent ubiquitin-proteasome pathway - accounts for much of protein degradation

58
Q

What is contracture?

A

when the fibrous tissue that replaces the atrophied muscle fibers is in a continued shortened state

59
Q

How does muscle function recover during poliomyelitis?

A

nerve fibers that are not destroyed branch off forming new axons to innervate paralyzed muscle fibers causing macromotor units which decrease the motor control of a muscle but can allow one to regain strength

60
Q

What causes rigor mortis?

A

lack of ATP to undo cross bridges, lasts until proteins deteriorate about 15-25 hours later (autolysis by enzymes released from lysosomes)