Muscle Contraction

Excitation-Contraction Coupling

Excitation-contraction coupling is the physiological process by which an electrical stimulus (action potential) is converted into a mechanical response (muscle contraction). This process involves the transmission of the action potential from the motor neuron to the muscle fiber, leading to the release of calcium ions and subsequent contraction.

• Action Potential Initiation: The motor neuron releases acetylcholine at the neuromuscular junction, triggering an action potential in the muscle fiber.

• Propagation: The action potential travels along the sarcolemma and down the T-tubules.

• Calcium Release: The action potential stimulates the sarcoplasmic reticulum to release Ca2+ into the cytosol.

• Contraction: Elevated cytosolic Ca2+ initiates muscle contraction by enabling crossbridge cycling.

• Latent Period: The brief delay between the action potential and the onset of contraction is called the latent period.

Example: The diagram shows the sequence from acetylcholine release to muscle fiber depolarization and calcium release.

Calcium Regulation of Contraction

Calcium ions play a central role in regulating muscle contraction by removing the inhibition of crossbridge cycling.

• Troponin Binding: Ca2+ binds to troponin, causing a conformational change in the troponin-tropomyosin complex.

• Exposure of Binding Sites: This change moves tropomyosin away from actin's myosin-binding sites, allowing myosin heads to bind to actin.

• Initiation of Crossbridge Cycling: The exposure of binding sites enables the crossbridge cycle to begin.

Example: When Ca2+ is present, muscle contraction occurs; when Ca2+ is removed, contraction ceases.

Crossbridge Cycling

The crossbridge cycle is the sequence of events that leads to muscle contraction at the molecular level, involving interactions between actin and myosin filaments.

1. ATP Binding: ATP binds to the myosin head, reducing its affinity for actin and causing detachment.

2. ATP Hydrolysis: ATP is hydrolyzed to ADP and inorganic phosphate (Pi), cocking the myosin head.

3. Crossbridge Formation: The cocked myosin head binds to a new position on actin.

4. Power Stroke: Release of Pi triggers the power stroke, moving actin and generating force.

5. ADP Release: ADP is released, and myosin remains attached until another ATP binds.

Equation:

Example: The sliding filament theory describes how actin and myosin filaments slide past each other during contraction.

Removal of Calcium Ends a Contraction

Muscle relaxation occurs when Ca2+ is removed from the cytosol, allowing tropomyosin to cover the myosin-binding sites on actin.

• SERCA Pump: The sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA) pump actively transports Ca2+ back into the sarcoplasmic reticulum using ATP.

• Other Mechanisms: Ca2+ can also be removed via Na+/Ca2+ exchangers and Ca2+ pumps in the plasma membrane, but these play a minor role.

• Relaxation: As Ca2+ levels fall, the troponin-tropomyosin complex returns to its resting state, blocking myosin binding sites and ending contraction.

Example: Rigor mortis occurs after death when ATP is depleted and Ca2+ cannot be removed, resulting in sustained muscle contraction.

ATP Supply in Skeletal Muscle

ATP Utilization in Muscle

Muscle cells require a continuous supply of ATP for contraction, relaxation, and maintenance of ion gradients.

• Myosin ATPase: Powers contraction by hydrolyzing ATP during crossbridge cycling.

• SERCA ATPase: Uses ATP to pump Ca2+ back into the sarcoplasmic reticulum for relaxation.

• Na+/K+ ATPase: Maintains membrane potential after action potentials.

Sources of ATP in Muscle

Muscle fibers utilize several mechanisms to generate ATP, depending on activity duration and intensity.

• Phosphocreatine System: Stores high-energy phosphate bonds; rapidly regenerates ATP during short, intense activity.

• Anaerobic Glycolysis: Converts glycogen to glucose, then to pyruvate, producing ATP without oxygen. Pyruvate is converted to lactate under anaerobic conditions.

• Aerobic Metabolism: Occurs in mitochondria; pyruvate enters the citric acid cycle and electron transport chain, producing large amounts of ATP in the presence of oxygen.

Equations:

Anaerobic glycolysis:

Aerobic metabolism:

Energy Supply Over Time

The contribution of each ATP source varies with exercise intensity and duration.

Example: During sprinting, phosphocreatine and anaerobic glycolysis supply most ATP; during marathon running, aerobic metabolism predominates.

Additional info:

• Glycogen Storage: Muscles store glucose as glycogen, which can be rapidly mobilized for ATP production.

• Fat Metabolism: During prolonged exercise, beta-oxidation of fatty acids contributes to ATP production.