Unit 4.4 - Sliding Filament, Contraction-Relaxation, and Muscle Metabolism

Sliding Filament Contraction

Muscle contraction occurs when the thick and thin filaments slide past each octher.

Thin filaments move towards the M-line.

As sarcomere shortens during contraction, Z-lines move closer together, shortenin the H-zone and I-band.

ATP molecule bounds to myosin head and causes it to detach from the thin filament (the muscle is in relax state).

Myosin head rotates and binds to the binding site.

P dissociated from the myosin head causing to rotate and perform a power stroke, pulling the thin filament (sliding filament theory) and shortens the sarcomere (contraction).

Myosin head remains until a new ATP is bound, then head will detach and go back to initial position (relax).

Repeats if electrical signal sent, Ca released, or ATP is available.

Excitation-Contraction Coupling

Electrical excitation of the surface membrane in the muscle triggering an increase of Ca.

1. Ach is released from the somatic motor neuron.

2. Ach initiates an action potential in the muscle fibre.

3. Action potential triggers Ca release.

4. Ca bind with TnC and initiates muscle contraction.

Short delay between the muscle action potention and the beginning of muscle tension.

Muscle Metabolism

ATP is required to power the muscle contraction cycle.

Muscle fibres extract more ATP from: creatine phosphate, aerobic and anaerobic cellular respiration.

Creatine Phosphate (PCr)

Synthesized from excess ATP when muscle fibres in resting state.

Formation to ATP occurs very rapidly and is the first source.

Aerobic Cellular Respiration

Occurs during rest or light exercise.

With adequate supply of oxygen, pyruvic acid is oxidized to produce 36 molecules of ATP.

Anaerobic Cellular Respiration

Glucose is used to generate 2 molecules of ATP in the absence of oxygen during heavy exercies.

During heavy exercise pyruvic acid gets converted to lactic acid which is converted into glucose.

Oxygen Debt

Extra oxygen acquired after exercise is used to:

1. Convert lactic acid back to glycogen

2. Resynthesize PCr and ATP

3. Replace oxygen removed from myoglobin

4. Sustain the increased rate of chemical reactions due to raised body temperature

5. Sustain extra work performed by organs

6. Allow extra energy consumption for tissue repair