exercise phys chp 1 processes

Step 1 of Excitation-Contraction Coupling

An action potential originates in the central nervous system and travels down an alpha-motor neuron to the axon terminal.

Step 2 of Excitation-Contraction Coupling

Acetylcholine (ACh) is released into the synaptic cleft at the neuromuscular junction (NMJ) and binds to receptors on the plasmalemma.

Step 3 of Excitation-Contraction Coupling

An action potential travels along the plasmalemma and penetrates deep into the muscle fiber via Transverse Tubules (T-Tubules).

Step 4 of Excitation-Contraction Coupling

The electrical signal reaching the T-tubules triggers the Sarcoplasmic Reticulum (SR) to release stored calcium ions (Ca2+) into the sarcoplasm.

Step 5 of Excitation-Contraction Coupling

Calcium ions (Ca2+) bind to troponin on the thin filament, causing a structural change in the troponin complex.

Step 6 of Excitation-Contraction Coupling

Troponin pulls tropomyosin away from the active sites on actin, exposing the myosin-binding sites.

Step 7 of Excitation-Contraction Coupling

The myosin head binds to the exposed active site on actin, forming a cross-bridge.

Step 8 of Excitation-Contraction Coupling

The power stroke occurs: the myosin head tilts and pulls the thin filament past the thick filament toward the M-line (releasing ADP and Pi).

Step 9 of Excitation-Contraction Coupling

A new ATP molecule binds to the myosin head, causing the cross-bridge to detach from actin.

Step 10 of Excitation-Contraction Coupling

Myosin ATPase hydrolyzes ATP into ADP + Pi, resetting ("cocking") the myosin head back to its energized resting state.

Step 11 of Excitation-Contraction Coupling (Relaxation)

Nerve stimulation stops, and calcium (Ca2+) is actively pumped back into the sarcoplasmic reticulum using an ATP-driven pump.

Step 12 of Excitation-Contraction Coupling (Relaxation)

Without calcium bound to troponin, tropomyosin moves back over actin's active sites, blocking cross-bridge formation and relaxing the muscle.

Process of ATP Hydrolysis in Muscle Contraction

Myosin ATPase breaks down ATP into ADP, inorganic phosphate (Pi), and energy, which powers the repositioning of the myosin head for the next stroke.

Mechanism of Muscle Fiber Length Change in Sarcomeres

During contraction, thin filaments slide over thick filaments, pulling Z-discs closer together; the H-zone and I-band shorten, while the A-band length stays constant.

Process of Motor Unit Recruitment (Size Principle)

Motor units are recruited in order of increasing size based on force demands: smaller Type I motor units first, followed by Type IIa, and finally Type IIx for maximal efforts.

Process of Rate Coding (Frequency Summation)

Increasing the frequency of neural action potentials sent to a motor unit causes individual twitches to summate, eventually producing a smooth, maximal sustained contraction called tetanus.

Process of Concentric Muscle Action

Muscle tension overcomes external resistance, causing the muscle to shorten while generating force as cross-bridges pull filaments toward the M-line.

Process of Eccentric Muscle Action

External resistance exceeds muscle tension, causing the muscle to lengthen while producing force as cross-bridges pull in opposition to stretching forces.

Process of Isometric Muscle Action

Muscle force equals external resistance, resulting in cross-bridge engagement and tension development without any overall change in muscle length.

Mechanism of the Length-Tension Relationship

Force generation is maximized at optimal sarcomere length because maximum cross-bridge overlap is possible; over-stretching reduces cross-bridge attachment, while over-shortening causes filament interference.

Mechanism of the Force-Velocity Relationship (Concentric)

At high contraction velocities, force capability decreases because myosin cross-bridges cannot form and detach fast enough to generate high force.

Mechanism of the Force-Velocity Relationship (Eccentric)

At higher elongation velocities, force capability increases because stretching active cross-bridges creates greater resistance and mechanical energy storage.