Chapter 6 - Guyton

Question 1

General skeletal muscle fiber characteristics.

Answer 1

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

Question 2

The membrane of a skeletal muscle fiber?

Answer 2

sarcolemma (fuses with tendon fiber at the end)

Question 3

General characteristics of myofibrils.

Answer 3

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

Question 4

I bands

Answer 4

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

Question 5

A bands

Answer 5

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)

Question 6

Cross-bridges

Answer 6

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

Question 7

M-line

Answer 7

thick, mid region of myosin

Question 8

Z-disc

Answer 8

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

Question 9

Sarcomere

Answer 9

portion that lies between two z-discs

Question 10

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

Answer 10

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

Question 11

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

Answer 11

titin (attaches Z-discs to M-line)

Question 12

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

Answer 12

titin

Question 13

What is the sarcoplasm?

Answer 13

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

Question 14

What lies parallel to the myofibrils in the sarcoplasm?

Answer 14

mitochondria to supply the ATP to the contracting myofibrils

Question 15

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

Answer 15

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

Question 16

What is responsible for the muscle contraction stopping?

Answer 16

removal of calcium ions from the myofibrils

Question 17

Describe the positioning of actin filaments in a relaxed state.

Answer 17

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

Question 18

Describe the positioning of actin filaments in a contracted state.

Answer 18

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

Question 19

Molecular Characteristics of Myosin

Answer 19

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

Question 20

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

Answer 20

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

Question 21

Molecular Characteristics of Actin

Answer 21

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

Question 22

What is tropomyosin?

Answer 22

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

Question 23

What is troponin?

Answer 23

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

Question 24

What are the three protein subunits of troponin?

Answer 24

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

Question 25

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

Answer 25

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

Question 26

Explain the “walk-along” theory of contraction.

Answer 26

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

Question 27

What is the power stroke?

Answer 27

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

Question 28

What is the Fenn effect?

Answer 28

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

Question 29

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

Answer 29

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

Question 30

When does the contraction-coupling cycle cease?

Answer 30

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

Question 31

Explain the sarcomere length-tension relationship.

Answer 31

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

Question 32

Total tension of a muscle is equal to?

Answer 32

active and passive tension

Question 33

What is the contraction velocity-load relationship?

Answer 33

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

Question 34

Work=

Answer 34

Load x Distance

Question 35

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

Answer 35

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

Question 36

Sources of energy to replenish ATP in the muscle.

Answer 36

phosphocreatine, glycogen, and oxidative metabolism

Question 37

Is there a lot or a little phosphocreatine in muscle?

Answer 37

only a little

Question 38

Can glycolysis occur in the absence of oxygen?

Answer 38

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

Question 39

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

Answer 39

oxidative metabolism

Question 40

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

Answer 40

Carbohydrates yum

Question 41

Efficiency of Muscle Contractions

Answer 41

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

Question 42

What occurs if muscle is contracted too rapidly?

Answer 42

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

Question 43

Isotonic Contraction

Answer 43

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

Question 44

Isotonic contraction is dependent on?

Answer 44

load the muscle is contracting against and inertia against the load

Question 45

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

Answer 45

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

Question 46

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

Answer 46

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

Question 47

What gives slow-twitch fibers their red appearance?

Answer 47

myoglobin

Question 48

What is force summation?

Answer 48

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

Question 49

What is the size principle?

Answer 49

smaller motor units will be recruited first over larger ones

Question 50

What is tetanization?

Answer 50

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)

Question 51

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

Answer 51

calcium

Question 52

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

Answer 52

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

Question 53

What is the Staircase Effect (Treppe)?

Answer 53

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

Question 54

What results in skeletal muscle tone?

Answer 54

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

Question 55

General characteristics of muscle fatigue.

Answer 55

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

Question 56

The lever systems of the body depend on:

Answer 56

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

Question 57

Muscle atrophy occurs as a result of what pathway?

Answer 57

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

Question 58

What is contracture?

Answer 58

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

Question 59

How does muscle function recover during poliomyelitis?

Answer 59

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

Question 60

What causes rigor mortis?

Answer 60

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