Muscle Structure, Fiber Types, and Mitochondria/Energy

Myofiber

Muscle cell; functions: force and heat production

Mitochondria

Organelles that produce ATP

SR

Stores/releases Ca2+ for contraction

Diameter of myofiber

50-70 μm; affects force production

Length of myofiber

2-45 cm; affects speed and range of contraction

Sarcolemma

Basement membrane + plasmalemma

Plasmalemma

Excitable membrane; conducts AP

T-tubules

Invaginations of plasmalemma; conduct AP into fiber

Triad

T-tubule + 2 terminal cisternae of SR

Satellite cells

Muscle stem cells for repair and growth

Myonuclei

Control gene expression; maintain nuclear domain

Nuclear domain

Cytoplasmic volume maintained by one nucleus

Myofibrils

Cylindrical structures within myofiber; contain sarcomeres

Sarcomere

Z-line to Z-line; basic contractile unit

Thick filament

Myosin

Thin filament

Actin, troponin, tropomyosin

A-band

Entire length of myosin

I-band

Actin only

H-zone

Central region with only myosin

M-line

Center proteins

Z-line

Anchors actin filaments; defines sarcomere boundaries

Cross-bridge cycle

Myosin binds actin, power stroke, ATP detaches

Structural proteins

Titin (elasticity), nebulin (actin length), dystrophin

Dystrophin

Links cytoskeleton to ECM; mutations → muscular dystrophy

Protein synthesis

Transcription (nucleus) → translation (ribosomes)

Protein turnover

Synthesis - degradation

Epigenetics

Regulates gene expression without changing DNA

Mutations in nuclear genes

Can impair muscle function

What are the primary classification schemes for muscle fiber types?

Based on color, primary energy metabolism, myosin heavy chain (MHC) isoforms, and speed of contraction.

What characterizes Type I muscle fibers?

They contain MHC Type I, are slow contracting, fatigue-resistant, and have high mitochondrial content.

What are Type IIA muscle fibers known for?

They contain MHC Type IIa, are fast-twitch, fatigue-resistant, and have a mix of oxidative and glycolytic properties.

What distinguishes Type IIX muscle fibers?

They contain MHC Type IIx, are fast-twitch, and are more fatigable with lower mitochondrial content.

How does the color of muscle fibers relate to their properties?

Dark fibers have high mitochondria, myoglobin, and capillaries, while light fibers have low amounts of these components.

What is the primary energy metabolism of oxidative muscle fibers?

They rely on aerobic metabolism and have high numbers of mitochondria.

What is the primary energy metabolism of glycolytic muscle fibers?

They rely on anaerobic metabolism and have high content of glycolytic enzymes.

What factors affect the development of muscle fiber types?

Genetics, exercise training, hormones, and potentially gut bacteria.

What is the significance of SERCA pumps in muscle fibers?

They affect the speed of relaxation; Type II fibers have more SERCA pumps for faster Ca2+ uptake.

How does muscle fiber type influence contraction speed?

Type I fibers contract slowly, while Type II fibers contract quickly.

What is the relationship between muscle fiber type and power output?

Type I fibers have the lowest peak power, Type IIA have intermediate, and Type IIX have the highest peak power.

What does Vmax represent in muscle fibers?

Vmax is the maximal shortening velocity of a myofiber when no load is applied.

How do fast-twitch fibers differ from slow-twitch fibers in terms of ATP use?

Fast-twitch fibers use more ATP due to higher rates of contraction and relaxation.

What is the effect of endurance training on muscle fiber types?

It increases mitochondrial volume and may increase MHC IIa isoform while decreasing MHC IIx isoform.

What is the role of thyroid hormones in muscle fiber development?

Increases in thyroid hormones (T3 and T4) can enhance MHC II isoforms and decrease MHC I isoforms.

What is the significance of the myosin heavy chain (MHC) isoform?

MHC isoforms determine the speed of contraction and the metabolic properties of muscle fibers.

What does the term 'fiber switching' refer to?

The ability of muscle fibers to change from one type to another based on training and physiological conditions.

What is the peak power difference between Type IIx and Type I fibers?

Type IIx fibers have peak power 10 times greater than Type I fibers.

How does muscle fiber type affect muscle function?

Different fiber types influence contraction speed, endurance, force output, and fatigue resistance.

What are the characteristics of slow-twitch oxidative fibers?

They are fatigue-resistant, have high mitochondrial content, and are suited for endurance activities.

What are fast-twitch glycolytic fibers known for?

They are designed for quick bursts of power but fatigue quickly due to lower mitochondrial content.

What is the role of gut bacteria in muscle fiber development?

Emerging research suggests gut bacteria may influence the development of muscle fiber types.

What is the relationship between muscle fiber type and fatigue?

Type I fibers are more fatigue-resistant, while Type II fibers are more fatigable.

What is the impact of exercise on muscle fiber composition?

Exercise can lead to adaptations in muscle fiber types, enhancing performance and endurance.

What is the primary function of mitochondria?

To produce ATP through aerobic metabolism.

What type of DNA do mitochondria contain?

Mitochondrial DNA that expresses critical enzymes for mitochondrial function.

What are the three energy systems used to produce ATP?

Phosphagen system (Creatine Kinase), Glycolysis, and Oxidative phosphorylation.

What is apoptosis?

Programmed cell death, a normal process for removing excess tissue.

What are the two pathways of apoptosis?

Intrinsic pathway (cell stress signals) and Extrinsic pathway (signals from other cells).

How does excessive apoptosis affect the body?

It causes atrophy.

What is the result of insufficient apoptosis?

Uncontrolled cell proliferation, such as cancer.

What is mitochondrial plasticity?

The ability of mitochondria to adapt through biogenesis or mitophagy.

What are the main components of mitochondria?

Outer membrane, inner membrane, cristae, and matrix.

What metabolic processes occur in mitochondria?

Krebs cycle (TCA), electron transport chain, and aerobic metabolism.

What is the role of the Na+-K+ ATP pump?

To maintain cellular ion balance and contribute to ATP consumption.

What happens to mitochondrial content with physical activity?

It increases, enhancing mitochondrial biogenesis.

What is the significance of mitochondrial DNA?

It contains 13 critical genes necessary for oxidative phosphorylation.

What is the relationship between apoptosis and mitochondria?

Mitochondria release proteins that trigger the intrinsic pathway of apoptosis.

What is the role of creatine kinase in energy production?

It helps regenerate ATP from creatine phosphate in the phosphagen system.

What is the effect of inactivity on mitochondria?

It leads to a decrease in mitochondrial content and function.

What is the function of cristae in mitochondria?

To increase the surface area for ATP production during oxidative phosphorylation.

How is the Krebs cycle related to mitochondrial function?

It is a key metabolic pathway that occurs in the mitochondrial matrix, producing ATP and CO2.

What is the outcome of mitochondrial biogenesis?

Increased number and function of mitochondria in response to physical activity.

What is mitophagy?

The process of degrading damaged mitochondria to maintain cellular health.