Week 4- Muscular System

Explain how muscles contract use terms (ACh, Na+, Ca2+)

• Begins with a signal from a motor neuron.

• Acetylcholine (ACh) is released and binds to receptors on the muscle fibre.

• Local membrane of the muscle fibre depolarises.

• Sodium ions (Na⁺) enter, triggering an action potential.

• Action potential spreads along the fibre.

• Calcium ions (Ca²⁺) are released.

• Ca²⁺ triggers muscle contraction.

• ATP is required to sustain contraction.

Explain how muscles relax, use terms (Ca2+, tropomyosin)

• Contraction stops when motor neuron signalling ends.

• Sarcolemma and T-tubules repolarise.

• Ca²⁺ is pumped back into the sarcoplasmic reticulum.

• Tropomyosin covers actin binding sites, preventing cross-bridge formation.

What is the Sliding Filament model of contraction

• When a muscle contracts, contraction occurs within sarcomeres.

• Z lines move closer together.

• I band becomes smaller.

• A band remains the same width.

• At full contraction, actin (thin) and myosin (thick) filaments fully overlap.

Describe ATP and muscle metabolism (How ATP is used)

• Actin slides past myosin during contraction.

• Myosin heads bind to actin at binding sites.

• Myosin heads pull, detach, and reattach repeatedly.

• This cross-bridge cycle requires ATP for:

  • Myosin head movement.

  • Detachment from actin.

  • Re-cocking of the myosin head.

Mysia

The 3 connective tissue layers (epimysium, perimysium, endomysium)

How does skeletal muscle work with tendons to move body?

• Collagen in mysia connects to tendon which is fused with bone’s periosteum OR aponeurosis

Aponeurosis

Sheath/flat sheet of connective tissue that connects muscle to bones

intercollated disc

Allows cardiac muscle to contract in wave pattern

Gap junction

forms channels between adjacent cardiac muscle fibres that allow action potential to flow from one cell to another

Isotonic contraction

• Muscle changes length while producing force

2 types: concentric and eccentric

Concentric contraction

Muscle shorten while holding tension

Lifting dumbbell in Bicep curl

Eccentric contraction

lengthening muscle while holding tension

-Lowering dumbbell after a curl

Length-tension relationship

Actin filaments overlap too much if too shortened (contracted), fewer cross-bridges can form

Actin filaments don’t overlap enough if muscle stretches too far, fewer myosin heads to bind to actin

80-120% is optimal

Latent period

Period of muscle twitch, action potential moves along sarcolemma and Ca2+ are released from sarcoplasmic reticulum

Contraction period

Period of muscle twitch, Ca2+ have bound to troponin, tropomyosin has shifted away from actin-binding sites, cross-bridges have formed, and sarcomere are shortening to point of tension

Relaxation period

Period of muscle twitch, tension decreases and contraction stops, muscle fibres return to resting state (Ca2+ pumped out, cross-bridge cycling stops)

Wave summation

Wave summation occurs when multiple stimuli are delivered to a muscle fibre in rapid succession, before it has time to fully relax between contractions. This results in stronger and more sustained contractions.

Treppe

When a skeletal muscle has been dormant for an extended period and then activated to contract, initial contraction is about one half of later contractions

Tetanus

High Ca2+ concentration allows all sarcomeres to shorten so muscle contracts until fatigue

What source of ATP for muscles is used for 95% of resting and moderately active muscles?

Creatine phosphate, anaerobic or aerobic respiration?

Aerobic respiration

fascia

connective tissue between skin and bones

Cardiac muscle fibre ends are connected to each other by what?

intercalated discs