Muscle Fiber Contraction

Overview

• Muscle fiber contraction involves a series of coordinated steps beginning with nerve stimulation.
• Nerve stimulation leads to an action potential in the sarcolemma, causing an increase in intracellular calcium levels from the sarcoplasmic reticulum.
• Key components include the neuromuscular junction, excitation-contraction coupling, and the sarcoplasmic reticulum.

Neuromuscular Junction

• Somatic Motor Neurons: Nerve cells whose cell bodies are in the brain or spinal cord with axons extending towards skeletal muscles.
• Axon Terminal: End of the axon; forms the neuromuscular junction or motor endplate.
• Synaptic Cleft: Narrow space separating the axon terminal from the muscle cell.
• Acetylcholine (ACh): Neurotransmitter stored in synaptic vesicles released into the synaptic cleft to communicate with the muscle cell.

Process of Muscle Contraction

1. Action Potential Initiation: Propagated along the neuron's axon, reaching the axon terminal.
2. Calcium Channels: Voltage-gated calcium channels in the axon terminal open, allowing calcium influx.
3. Exocytosis: Calcium triggers exocytosis of ACh-containing vesicles into the synaptic cleft.
4. ACh Binding: ACh diffuses across the synaptic cleft, binding to nicotinic receptors on the muscle cell's junctional folds.
5. Ion Channel Activation: Ligand-gated ion channels open, sodium influx occurs, leading to depolarization (end-plate potential).
6. Acetylcholinesterase Action: Enzyme breaks down ACh to terminate the signal, closing ion channels.

Action Potential in Muscle Cells

• End Plate Potential (EPP): Local depolarization at the neuromuscular junction due to sodium influx.
• Depolarization & Repolarization:
  • If EPP is strong enough, it triggers depolarization (action potential) by opening voltage-gated sodium channels.
  • Repolarization follows, restoring resting conditions via potassium efflux.

Excitation-Contraction Coupling

• Propagation: Action potential spreads across the sarcolemma and down T-tubules.
• Calcium Release: Voltage-sensitive proteins trigger calcium release from the sarcoplasmic reticulum.
• Contraction Initiation: Calcium binds to troponin, causing tropomyosin to uncover actin binding sites, allowing myosin to form cross-bridges.

Cross-Bridge Cycle

1. Formation: Myosin head binds to actin.
2. Power Stroke: Myosin head pivots, pulling actin filament towards the M-line.
3. Detachment: ATP binds, causing myosin to detach from actin.
4. Recocking: Myosin returns to a high-energy state, ready to bind again.

ATP Role

• ATP is essential for myosin head detachment and recocking.
• In the absence of ATP, such as in rigor mortis, myosin heads remain bound to actin, leading to muscle stiffness.