Week 3 - PHYS 20008

What is the hierarchical structure of muscle? (largest to smallest structure)

1) Muscle (whole muscle belly)

2) Fascicle (bundles of muscle fibres)

3) Myofibrils (each muscle fibre is made of this)

4) Sarcomeres (the basic contractile unit of muscles)

What defines a sarcomere and why is it important?

It’s the region between 2 Z-lines. In between the Z lines are thick filaments (myosin) and thin filaments (actin). It is responsible for muscle contraction. It is also the basic functional unit of muscles.

What causes the striped (striated) appearance of muscle?

The regular arrangement of: Myosin (thick filaments and Actin (thin filaments). Their overlapping pattern creates visible stripes.

What are the key features of myosin (thick filaments)?

It’s shaped like a 2 headed golf club. It has a head which contains binding sites for actin (thin filaments) and ATP. It has a tail. It also has a hinge region which allows movement (Hinge movement is crucial for contraction).

How is actin structured and regulated?

Actin is arranged in helical strands and contains binding sites for myosin. It’s regulated by Tropomyosin (which blocks binding sites) & Troponin (controls tropomyosin). At rest, binding sites are blocked by Tropomyosin leading to no unnecessary muscle contraction.

How does calcium enable muscle contraction?

1) Calcium released from sarcoplasmic reticulum (SR)

2) Binds to troponin

3) Moves tropomyosin away

4) Exposes actin binding sites

5) Myosin can now attach

Why is ATP essential for muscle contraction?

Myosin detaches from actin due to ATP binding. Myosin head gets energised due to ATP splitting. It provides energy for cross-bridge cycling. Without ATP, rigour mortis will be caused (permanent attachment causing stress on muscles seen during death)

What is the role of cytoskeletal proteins like titin and nebulin?

They provide structural stability, keep filaments aligned and prevent damage during contraction. Loss of these proteins will lead to muscle diseases (due to fragility & damage).

What is the function of the tendon’s elastic component?

It acts as a spring. It stores and releases elastic energy. It must be stretched before force is transmitted. The component is important for force production and injury risks like avulsions.

What does the sarcoplasmic reticulum do?

It stores calcium and releases it during contraction. It also reabsorbs calcium via ATP dependent pumps. It keeps calcium levels tightly regulated.

What is excitation–contraction (EC) coupling?

It’s a process linking electrical signal to muscle contractions. (elec to chem to mechanical)

Steps:

1) Action potential generated from nerve

2) It travels along membrane + T Tubules

3) Triggers calcium release from sarcoplasmic reticulum

4) Calcium enables contraction

What is the role of T-tubules?

Carry action potentials deep into muscle fibre and Trigger calcium release from SR. Overall ensures rapid, uniform contraction.

How does cross-bridge cycling produce force?

1. Myosin binds actin

2. ATP binds → detachment

3. ATP splits → energises head

4. Myosin reattaches

5. Power stroke (hinge movement)

6. Repeats while ATP + calcium present

👉 Thousands occur simultaneously → muscle shortens.

How do muscles shorten (sliding filament theory)?

• Filaments do NOT shorten (Actin and myosin simply slide past each other)

• Sarcomeres shorten causing whole muscle to shorten

What determines how much force a muscle produces?

The degree of actin–myosin (filament) overlap (More optimal overlap → more force)

What are the 2 main types and causes of muscle fatigue?

Two main types:

• Central fatigue → brain/spinal cord

• Peripheral fatigue → muscle-level issues

Causes include:

• ATP depletion

• Calcium handling problems

• Metabolic stress

How does muscle structure/shape affect function?

• Fusiform (parallel) → long fibres, more movement

• Pennate (angled) → more fibres packed → more force

👉 Structure = function specialisation.

What determines how strong a muscle is?

The cross sectional area. A larger cross sectional area = more fibres = greater force

Why do different muscles have different functions?

• Small muscles → fine control (e.g., eye, hand)

• Large muscles → power and force (e.g., legs)

What are the three types of muscle actions?

• Concentric (shortening)

  • Force > load

  • ex// pumping with dumbbels

• Isometric (same length)

  • Force = load

  • ex// pulling at something impossible to lift like strong barbed wire causing straight arm

• Eccentric (lengthening)

  • Load > force

  • ex// like holding heavy grocery bags

How do muscles coordinate movement?

• Agonist → produces movement

• Antagonist → opposes movement

👉 One shortens while the other lengthens.

How does muscle length affect force?

F∝ overlap between actin and myosin (filaments)

• Ascending limb →sarcomere is overly compressed, so actin filaments overlap and interfere. Myosin won’t have proper space to bind = fewer effective cross-bridges → low force

• Plateau → optimal overlap of actin and myosin→ max force

• Descending limb → sarcomere is too stretched. actin and myosin barely overlap → low force

What is the difference between active and passive force?

• Active force (contractile) → from contraction (cross-bridges)

• Passive force (elastic) → from stretching elastic tissues (tendons)

Why are eccentric contractions significant?

• Produce higher force

• Common in injuries

• Important in:

  • Sport

  • Rehabilitation

What happens if calcium regulation fails?

• Excess calcium → muscle damage

• Poor reuptake → continuous contraction

Thus its linked to muscle diseases.

Why do we need to understand basic muscle structure?

Because it explains:

• Movement

• Fatigue

• Injury

• Disease

• Adaptation to exercise