CPPS 302 Muscular System "Determinants of muscle fiber phenotype for skeletal muscle" Sept. 4, 2026

How do muscle fiber phenotypes differ between endurance runners vs sprinters

Endurance runners: mainly slow-twitch, oxidative fibers (Type I/IIa) emphasizing endurance

Sprinters: mainly fast-twitc, glycolytic fibers (Type IIb), emphasizing force and speed

How does training influence skeletal muscle fiber phenotype

Training can change muscle fiber characteristics, including contraction speed, force generation, endurance, and metabolism (muscle plasticity)

What are the determinants of muscle fiber phenotype

Cell lineage

Nerve-evoked electrical activity

Mechanical conditions

Paracrine, autocrine, and hormones

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During embryonic development, where do all 3 types of muscles originate from

Somatic Mesoderm

What is cell lineage, and how does it influence muscle fiber phenotype?

Cell lineage refers to a cell’s developmental origin and ancestry, which can influence whether a myoblast develops into a slow or a fast twitch muscle fibre

What are myoblasts, adn what is thier role in muscle development?

Myoblasts are muscle precursor cells that migrate and differentiate into muscle fibers with specific phenotypes.

What was the main purpose of the ceolomic graft experiment?

To determine whtehr muscle fiber phenotype is determined by myoblast cell lineage independently or external influences, particularly nerve input

Explain the ceolomic graft experiment and its main findings regarding muscle fiber phenotype

Experiment: Chick hindlimb buds (stages 16-18) were transplanted inot the ceolomic cavity, isolating them from migrating myoblasts and normal nerve input

Finding 1: Older grafts developed more muscle tissue

Finding 2: Muscle fibers developed a near-normal distirbution of fiber types even WITHOUT nerve input

Conclusion: Cell lineage plays an important role in determining muscle fiber phenotype, even wihtout external nerve influences during development

What did the muscle transplantation studies reveal about cell lineage and muscle fiber phenotype?

Even when the uscles are moved to a different location, they retain some characteristics of their original muscle. This hsows that muscle fiber phenotype is partly determined by cell lineage and where it comes form, not just its location

Compare and contrast the differentiation between stages 16 to 17 vs a 17 to 18 limb tissue

Stages 16 - 17: Mostly Slow myotubes

Stages 17 - 18: Mostly fast myotubues

What are satellite cells, and where are they loacted

Satellite cells are normally inactive muscle stem cells located between the basal lamina and plasma membrane of skeletal muscle fibres

What activates satellite cels, and what are their funcitons

Acitvated by: muscle injury or intensive exercise

Functions: fuse damaged muscle fibers to repair them or contribute to the formation of new muscle fibers

True or false: Satellite cells can fuse among themselves to make new muscle fibres or fuse with damaged muscle fibers to repair damage

TRUE

How do human satellite cells differ from avian and rodent satellite cells

Avian / rodent (heterogeneous): Each satellite cell is restricted to producing either fast or slow muscle fibers

Human (homogeneous): A satellite cell can potentially produce either fast or slow muscle fibre

How does nerve-evoked electrical activity influence fiber phenotype?

Nerve signals can change muscle fiber characteristics, including contraciton speed and endurance.

A muscle can adapt its phenotype depending on the nerve activity it recieves

What did nerve transplantation experiments demonstrate about muscle fiber phenotype?

When a nerve from a fast muscle is transplanted to a slow muscle, the moved muscle changed its phenotype to the new nerve supply and vise versa

• Conclusion: nerve input plays an important role in determining muscle fiber phenotype, even in adults

Compare the patterns of electrical activity in Type 1 and Type 2 muscle fibers

Type 1 (slow-twitch): Long periods of low-frequency stimulation, with many impulses throughout the day

Type 2 (fast twitch): short bursts of high frequency stimulaiton, with fewer impulses throughout the day

Why are type 1 muscle fibers acitvated more frequently throughout the day than Type IIb fibers?

Type 1 fibers are used for continious, low intensity activities such as maintaining posture and standing. Therefore, they recieve more total nerve impulses throughout the day.

Type IIb fibers are mainly recruited for short, powerful movements like sprinting, so they are activated less often

How can electrical stimulation cause fast-twitch muscle fibers to develop slow-twitch characteristics

Adding prolonged low-frequency electrical stimulation can cause fast-twitch muscle fibers to develop slow-twitch characterstics (fast-to-slow transformation)

• and vise versa

Why is the slow to fast muscle fiber transfomration difficult to study using external nerve stimulaiton

Slow muscles already recieve high amounts of nerve activity from the CNS

Because external stimulation only ADDS impulses rather than reducing existing acitivty, researchers cannot easily reproduce the low activity patterns associated with fast muscles

Therefore, the existing nerve activity interferes with studying slow-to-fast transformation

Why do type 1 muscle fibers recieve more total nerve impulses per day than Type II fibers, despite having a lower firing frequency?

Type i fibers are activated for long periods to maintain posture and support endurance activites, resulting in more total daily impulses. Type II fibers fire at higher frequencies but are acitvated for shorter periods.

What does depolairzation lead to in terms of length of muscle

Depolarization leads to shortening and/or mechanical tension in muscle

How does mechanical stress influence muscle size and phenotype

Contracting against greater resistance produces greater mechanical tension, which can stimulate muscle hypertrophy

Explain the hindlimb suspension experiment and its main conclusion about mechanical stress

Experiment: rats were suspended by their tails to remove weight stress form their hindlimbs

Findings: electrical activity initially decreased but returned to normal after a few days. However, thie rmuscles continued to shrink (atrophy) because they were not experiencing the normal mechanical loading

Conclusion: mechanical tension is important for maintaining muscle size. Even with normal electrical activity, muscles can shirnk if mechanical loading is reduced

What does the hindlimb suspension experiment suggest about the importance of electrical activity vs mechanical tension

electrical activity alone is not enough to maintain muscle size. Mechanical tension plays an important role in preventing muscle atrophy

How can changes in muscle usage lead to changes in muscle fiber phenotype?

Changes in muscle activity acitvate signaling pathways that cause muscle fibers to adapt to their chharacteristics. When you use your muscles, you create changes inside the muscle cells. These cahnges act as signals that tell the muscle how to adapt.

Potential signals include:

• Intracellular Ca

• Metabolites

• Hypoxia

• Mechanical tension

What three major factors regulate muscle fiber size

Number of nuclei

Rate of protein synthesis

Rate of protein degregation

What is myotatin, and what is its role in muscle development

Myotatin = a member of the TGF-B superfamily that acts as an inhibitor of muscle growth

What happens when the myostatin gene is disrupted

Muscles become abnormally enlarged due to:

• hypertrophy: increased muscle fiber size

• Hyperplasia: increased number of muscle fibers

What is IGF-1, and what is its main rolein skeletal muscle

IGF-1 is a growth factor that promotes muscle hypertrophy by stimulating muscle growth and protein synthesis

How does iGF-1 promote muscle hypertrophy

IGF-1 promotes growth by:

• increasing protein synthesis

• decreasing protein breakdown

• increasing myotube diameter

• Increasing the number of nuclei in developing muscle fibers

What studies did studies involving increased or decreased IGF-1 demonstrate about muscle growth

Increaased IGF-1: greater muscle mass

IGF-1 deficiency: Severe muscular abnormalities, inluding muscle dystorphy in mice