Muscle fiber contraction and relaxation

Question 1

What neurotransmitter signals the contraction of an individual muscle fiber?

Answer 1

ACh

ACh stands for acetylcholine, which is released from motor neurons.

Question 2

What ions enter the muscle fiber, leading to depolarization?

Answer 2

Sodium ions (Na+)

Depolarization is the initial step in generating an action potential in muscle fibers.

Question 3

What triggers the release of calcium ions from the sarcoplasmic reticulum?

Answer 3

Action potential spreading to T-tubules

The action potential causes voltage-gated calcium channels to open.

Question 4

What role do calcium ions (Ca++) play in muscle contraction?

Answer 4

They bind to troponin, keeping actin-binding sites unshielded

This allows for the interaction between actin and myosin.

Question 5

What is necessary for the muscle fiber to continue contracting?

Answer 5

Presence of Ca++ ions and ATP

ATP is required for cross-bridge cycling and muscle contraction.

Question 6

What forms between actin and the myosin heads during contraction?

Answer 6

Cross-bridge

The cross-bridge is essential for the pulling of actin strands by myosin.

Question 7

What happens when signaling from the motor neuron ends?

Answer 7

Repolarization of the sarcolemma and T-tubules occurs

This leads to the closure of voltage-gated calcium channels.

Question 8

What is the result of calcium ions being pumped back into the sarcoplasmic reticulum?

Answer 8

Tropomyosin reshields the binding sites on actin strands

This process stops the muscle contraction.

Question 9

Fill in the blank: A muscle may also stop contracting when it runs out of _______.

Answer 9

ATP

Lack of ATP leads to muscle fatigue.

Question 10

What anatomical limit does the muscle fiber reach during contraction?

Answer 10

The muscle fiber continues to shorten to an anatomical limit

This is the maximum shortening that can occur based on muscle physiology.

Question 12

What is the primary site of muscle fiber contraction?

Answer 12

Sarcomeres

Sarcomeres are the basic contractile units of striated muscle fibers.

Question 13

What occurs to the sarcomeres during muscle contraction?

Answer 13

They shorten as myosin heads pull on actin filaments.

Question 14

What is the significance of the overlap between thick and thin filaments?

Answer 14

It is the site where filament movement starts.

Question 15

What anchors thin filaments at their ends?

Answer 15

Z-discs.

Question 16

Where are the thick filaments anchored?

Answer 16

M-line.

Question 17

What model explains how muscle fibers contract?

Answer 17

The Sliding Filament Model of Contraction.

Question 18

What initiates the sliding of thin filaments past thick filaments?

Answer 18

Exposure of myosin-binding sites on actin filaments.

Question 19

What happens to the Z lines and I band during sarcomere contraction?

Answer 19

Z lines move closer together, and the I band becomes smaller.

Question 20

What remains the same width during muscle contraction?

Answer 20

A band.

Question 21

What is the role of tropomyosin in muscle contraction?

Answer 21

It covers the myosin-binding sites on actin filaments.

Question 22

What complex does tropomyosin form with troponin?

Answer 22

Troponin-tropomyosin complex.

Question 23

What must occur for muscle contraction to initiate?

Answer 23

Ca++ must bind to troponin.

Question 24

What is the first step in the contraction process?

Answer 24

Ca++ binding to troponin.

Question 25

What does ATP provide for muscle contraction?

Answer 25

Energy required for the re-cocking of myosin heads.

Question 26

What is the cross-bridge cycle?

Answer 26

The repeated process of myosin heads pulling, detaching, re-cocking, and attaching to actin.

Question 27

What happens during the power stroke?

Answer 27

The myosin head moves toward the M-line, pulling actin along.

Question 28

What is released when the myosin head goes through the power stroke?

Answer 28

ADP.

Question 29

What occurs to the myosin head after ATP binds?

Answer 29

It detaches from actin.

Question 30

How is the myosin head positioned for further movement?

Answer 30

ATP is converted to ADP and Pi, changing the angle of the myosin head.

Question 31

What happens to the myosin head after the power stroke?

Answer 31

It is in a low-energy position.

Question 32

What causes rigor mortis after death?

Answer 32

Lack of ATP prevents myosin heads from detaching from actin.

Question 33

How many myosin molecules are typically found in one thick filament?

Answer 33

Approximately 300.

Question 34

What is the consequence of losing ATP in skeletal muscles?

Answer 34

Cross-bridges remain in place, causing muscle rigidity.

Question 35

What is the primary role of ATP in muscle contraction?

Answer 35

ATP supplies the energy for muscle contraction and the active-transport Ca++ pumps in the SR.

Question 36

How long can the stored ATP in muscle power contractions?

Answer 36

Only sufficient to power a few seconds worth of contractions.

Question 37

What are the three mechanisms by which ATP can be regenerated in muscle cells?

Answer 37

* Creatine phosphate metabolism * Anaerobic glycolysis * Aerobic respiration

Question 38

What is creatine phosphate and its function in muscle cells?

Answer 38

A molecule that stores energy in its phosphate bonds and quickly regenerates ATP during muscle contraction.

Question 39

How long can creatine phosphate provide energy for muscle contraction?

Answer 39

Approximately 15 seconds.

Question 40

What process do muscles turn to for ATP production after creatine phosphate is depleted?

Answer 40

Glycolysis.

Question 41

What is glycolysis and how does it produce ATP?

Answer 41

An anaerobic process that breaks down glucose to produce ATP, yielding two ATP and two molecules of pyruvic acid.

Question 42

What happens to pyruvic acid if oxygen is not available?

Answer 42

It is converted to lactic acid.

Question 43

What is the role of lactic acid in muscle metabolism during strenuous exercise?

Answer 43

It recycles NAD+ from NADH, allowing glycolysis to continue.

Question 44

What is aerobic respiration?

Answer 44

The breakdown of glucose or other nutrients in the presence of oxygen to produce carbon dioxide, water, and ATP.

Question 45

What percentage of ATP for resting or moderately active muscles is provided by aerobic respiration?

Answer 45

Approximately 95 percent.

Question 46

How many ATPs are produced by aerobic respiration per molecule of glucose?

Answer 46

Approximately 36 ATPs.

Question 47

What is myoglobin and its function in muscle cells?

Answer 47

A protein that stores excess oxygen for more efficient muscle contractions.

Question 48

What is muscle fatigue?

Answer 48

When a muscle can no longer contract in response to signals from the nervous system.

Question 49

What factors are correlated with decreased muscle contraction during fatigue?

Answer 49

* Reduced ATP reserves * Lactic acid buildup * Imbalances in Na+ and K+ levels * Damage to the SR and sarcolemma

Question 50

What is oxygen debt?

Answer 50

The amount of oxygen needed to compensate for ATP produced without oxygen during muscle contraction.

Question 51

What processes require oxygen to restore normal muscle function after intense activity?

Answer 51

* Restoring ATP and creatine phosphate levels * Converting lactic acid to pyruvic acid * Converting lactic acid into glucose or glycogen in the liver

Question 52

What initiates the relaxation of skeletal muscle fibers?

Answer 52

The motor neuron stops releasing ACh into the synapse at the NMJ.

Question 53

What happens during the repolarization of a muscle fiber?

Answer 53

Gates in the SR close, and ATP-driven pumps move Ca++ back into the SR.

Question 54

What is the result of calcium being moved back into the SR?

Answer 54

The actin-binding sites on the thin filaments are reshielded, leading to muscle relaxation.

Question 55

What determines the number of skeletal muscle fibers in a given muscle?

Answer 55

Genetically determined and does not change ## Footnote The number of muscle fibers is fixed and influenced by genetics.

Question 56

Muscle strength is directly related to the amount of what within each muscle fiber?

Answer 56

Myofibrils and sarcomeres ## Footnote These components are crucial for muscle contraction and strength.

Question 57

What is the term for the increase in muscle mass and bulk due to increased production of myofibrils and sarcomeres?

Answer 57

Hypertrophy ## Footnote This can occur due to hormones, stress, and anabolic steroids.

Question 58

What happens to skeletal muscle when it experiences decreased use?

Answer 58

Atrophy ## Footnote Atrophy results in the loss of sarcomeres and myofibrils but not muscle fibers.

Question 59

What is a common consequence of a limb being in a cast?

Answer 59

Atrophied muscles ## Footnote Muscle atrophy can be observed when the cast is removed.

Question 60

What is Duchenne muscular dystrophy (DMD)?

Answer 60

A progressive weakening of the skeletal muscles ## Footnote DMD is one type of muscular dystrophy.

Question 61

What causes Duchenne muscular dystrophy?

Answer 61

Lack of the protein dystrophin ## Footnote Dystrophin is essential for muscle fiber integrity.

Question 62

What happens to the sarcolemma in DMD due to insufficient dystrophin?

Answer 62

It tears during muscle contractions ## Footnote This leads to cellular damage and muscle fiber degradation.

Question 63

How is Duchenne muscular dystrophy inherited?

Answer 63

Through an abnormal X chromosome ## Footnote DMD primarily affects males due to X-linked inheritance.

Question 64

At what age is Duchenne muscular dystrophy usually diagnosed?

Answer 64

Early childhood ## Footnote Symptoms often appear before the age of 5.

Question 65

What is one of the first symptoms of DMD?

Answer 65

Difficulty with balance and motion ## Footnote This symptom progresses to more severe mobility issues.

Question 66

What is the ultimate cause of death in individuals with Duchenne muscular dystrophy?

Answer 66

Respiratory failure ## Footnote Affected individuals typically do not live past their 20s.

Question 67

What type of cells were considered for treating DMD by introducing healthy genes?

Answer 67

Myoblasts ## Footnote Myoblasts are embryonic cells that develop into muscle tissue.

Question 68

What recent approach has been explored as a potential treatment for DMD?

Answer 68

Boosting the production of utrophin ## Footnote Utrophin may substitute for dystrophin and protect muscle cells.