Muscle Contraction and Excitation-Contraction Coupling: Key Concepts and Processes

Sliding Filament Theory

Process where myosin heads pull actin filaments toward the center of the sarcomere causing contraction without changing filament length.

Myosin

Motor protein that binds to actin and performs the power stroke during muscle contraction.

Actin

Thin filament that interacts with myosin for contraction.

Membrane Potential

Voltage difference across a cell membrane due to ion distribution.

Depolarization

A reduction in membrane potential making the inside of the cell less negative.

Resting Membrane Potential

The stable negative charge of a cell when it is not active (~ -70 mV).

Action Potential

A rapid change in membrane potential that travels along the sarcolemma to trigger contraction.

Excitation-Contraction Coupling

Sequence linking muscle fiber excitation to contraction via Ca2+ release.

Neuromuscular Junction (NMJ)

Synapse between a motor neuron and a muscle fiber where ACh is released.

Acetylcholine (ACh)

Neurotransmitter that initiates muscle fiber depolarization by opening Na+ channels.

Motor Unit

A motor neuron and all the muscle fibers it innervates.

Sarcolemma

The plasma membrane of a muscle cell.

T-tubules

Extensions of the sarcolemma that help propagate action potentials into the muscle fiber.

Sarcoplasmic Reticulum (SR)

Organelle that stores and releases calcium ions in muscle cells.

Calcium (Ca2+)

Ion released from SR that binds to troponin, enabling actin-myosin interaction.

Troponin

Protein that binds Ca2+ and moves tropomyosin to expose actin binding sites.

Tropomyosin

Protein that blocks actin's binding sites until moved by troponin.

Cross Bridge

Connection formed when myosin binds to actin during contraction.

Power Stroke

The action of myosin pulling actin toward the center of the sarcomere.

Steps of Excitation-Contraction Coupling

1) Action potential (AP) starts in brain or spinal cord,

2) AP arrives at axon terminal, releases acetylcholine (ACh), binds to receptors on sarcolemma,

3) Binding opens chemically gated ion channels; Na+ flows into muscle fiber,

4) Na+ influx causes voltage gated ion channels to open = wave of depolarization, 5) AP propagated down sarcolemma and into T-tubules,

6) Triggers Ca2+ release from sarcoplasmic reticulum (SR),

7) Ca2+ enables actin-myosin contraction, 8) Sliding Filament Theory.