Muscle Contraction: Sequence and Energy Systems

Events at the Neuromuscular Junction (NMJ)

The neuromuscular junction is the site where a motor neuron communicates with a muscle fiber, initiating muscle contraction. This process involves the conversion of a neural signal into a chemical stimulus that excites the muscle cell.

• Stimulus: A somatic motor neuron transmits an action potential to its axon terminal.

• Calcium Influx: Voltage-gated calcium channels open, allowing to enter the synaptic knob.

• Exocytosis: Calcium triggers synaptic vesicles to release Acetylcholine (ACh) into the synaptic cleft.

• Receptor Binding: Two ACh molecules bind to each receptor on the motor end plate, opening chemically-gated sodium-potassium channels.

• End-Plate Potential (EPP): Rapid sodium influx depolarizes the sarcolemma from a resting state of to a peak of .

Muscle Fiber Excitation & Propagation

Once the muscle fiber is excited, the electrical signal spreads across the sarcolemma, leading to further activation of the muscle cell.

• Depolarization: Sodium ions diffuse to adjacent regions, shifting the membrane potential to the threshold of .

• Action Potential: Voltage-gated sodium channels open, causing a rapid rise to .

• Repolarization: Sodium channels inactivate; voltage-gated potassium channels open, allowing to exit and return the cell to .

• Propagation: This cycle repeats along the sarcolemma and down the Transverse (T) tubules, ensuring the entire muscle fiber is activated.

Excitation-Contraction Coupling

This phase links the electrical excitation of the muscle fiber to the mechanical events of contraction, primarily through the release of calcium ions.

• Triad Activation: The action potential traveling down T-tubules opens voltage-gated calcium channels in the terminal cisternae of the sarcoplasmic reticulum (SR).

• Calcium Release: floods the cytosol.

• Regulatory Shift: Calcium binds to troponin, causing tropomyosin to rotate and expose myosin-binding sites on the actin filament.

The Cross-Bridge Cycle

The cross-bridge cycle describes the mechanical process by which muscle fibers contract, resulting in the shortening of the sarcomere.

1. Cross-Bridge Formation: Energized myosin heads bind to exposed actin sites.

2. Power Stroke: ADP and are released; the myosin head pivots, pulling the actin filament toward the M-line.

3. Detachment: A new ATP molecule binds to the myosin head, causing it to release actin.

4. Re-energization: ATP undergoes hydrolysis (ADP + ), cocking the myosin head back into its high-energy position.

Example: During muscle contraction, repeated cross-bridge cycles result in the sliding of actin filaments past myosin, shortening the muscle.

Energy Systems for Contraction

Muscle contraction requires energy, which is supplied by several metabolic pathways. Each pathway differs in duration, efficiency, and mechanism.

Example: Sprinting relies on direct phosphorylation and anaerobic respiration, while endurance activities depend on aerobic respiration.

Contraction Modalities

Muscle contractions can be classified based on changes in muscle length and tension.

• Isometric: Muscle length remains constant while tension increases (e.g., attempting to lift an immovable object).

• Isotonic: Muscle tension remains constant while length changes.

  • Concentric: Muscle shortens (e.g., upward phase of a bicep curl).

  • Eccentric: Muscle lengthens (e.g., controlled lowering of a weight).

Muscle Contraction Sequence Flowchart

The following sequence summarizes the steps of muscle contraction:

• ACh Release → Motor End Plate Activation

• Motor End Plate Activation → Sarcolemma Depolarization

• Sarcolemma Depolarization → T-Tubule Propagation

• T-Tubule Propagation → SR Calcium Release

• SR Calcium Release → Troponin/Tropomyosin Shift

• Troponin/Tropomyosin Shift → Cross-Bridge Cycle

Rigor Mortis

Rigor mortis is a post-mortem phenomenon where muscles remain contracted due to the absence of ATP, preventing myosin heads from detaching from actin. It typically begins 3–4 hours after death and persists until protein decomposition occurs at 48–60 hours.

Additional info: The notes cover the physiology of muscle contraction, excitation-contraction coupling, and energy systems, corresponding to Anatomy & Physiology chapters on muscle tissue and the muscular system.