biol 3410 10/22

Skeletal Muscle Contraction

  • Overview of Topics

    • Discussion on muscle contraction and neuromuscular junction.

    • Focus on action potentials and their role in muscle contraction.

    • Connection between electrical signals and chemical signals (excitation-contraction coupling).

Sliding Filament Theory

  • Concept

    • Refers to the sliding of thin filaments past thick filaments during muscle contraction.

    • The thin filaments (actin) slide towards the center of the sarcomere.

  • Changes in Sarcomere

    • H-zone becomes smaller, while the zone of overlap increases; A band remains the same size.

Neuromuscular Junction

  • Definition

    • Synapse between a motor neuron and a skeletal muscle fiber.

  • Function

    • Site of electrical to chemical to electrical signal transduction.

  • Mechanism of Action

    • Action potentials trigger the release of acetylcholine (neurotransmitter) from the synaptic terminal.

    • Acetylcholine diffuses across the synaptic cleft and binds to receptors on muscle fibers, initiating muscle contraction.

Action Potentials

  • Resting Membrane Potential

    • Defined as the charge difference across the plasma membrane when the cell is not active, approximately -70 mV in neurons and about -90 mV in skeletal muscle.

    • Results from differing concentrations of sodium (higher outside) and potassium (higher inside).

    • Sodium-potassium pump maintains this gradient by expelling 3 sodium ions for every 2 potassium ions imported.

  • Generation of Action Potential

    • Triggered when the neuron reaches a threshold potential (-55 mV).

    • Rapid depolarization occurs as sodium channels open, followed by repolarization as potassium channels open.

    • Phases:

      • Depolarization: Inside becomes more positive.

      • Repolarization: Restores resting state as potassium moves out of the cell.

      • Hyperpolarization: Membrane potential dips below resting potential due to slow closing of potassium channels.

Voltage-Gated Ion Channels

  • Function

    • Allow ions to cross the membrane in response to membrane potential changes.

    • Sodium channels: Open during depolarization; close during repolarization.

    • Potassium channels: Open after sodium channels, assisting in returning to resting potential.

  • Refractory Period

    • Absolute Refractory Period: No action potential can be initiated during this time due to inactivation of sodium channels.

    • Relative Refractory Period: A stronger stimulus could generate an action potential during this time, though it is less likely.

Excitation-Contraction Coupling

  • Steps in Muscle Contraction

    • Action potential from the motor neuron reaches the synaptic terminal.

    • Voltage-gated calcium channels open, allowing calcium influx and triggering the release of acetylcholine.

    • Acetylcholine binds to receptors on muscle fibers, depolarizing the membrane and generating a muscle action potential.

    • Muscle action potential travels down T-tubules and triggers the release of calcium from the sarcoplasmic reticulum.

    • Calcium enables the sliding filament mechanism (interaction of actin and myosin), facilitating contraction.