Skeletal Muscle Contraction

Introduction

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Video Focus

• Skeletal muscle contraction

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Skeletal Muscle Contraction

Neurons and Muscle Fiber

• Begins with a neuron (somatic neuron)
• Skeletal muscle contracts when stimulated by a somatic motor neuron
• Somatic motor neuron: Innervates 3 to 1,000 muscle fibers
• Motor unit: Complex of neuron’s nerve fiber and muscle fiber

Muscle Fiber Structure

• Muscle fiber attaches to bone via tendon
• Structures within muscle: fascicles, which contain muscle fibers
• Muscle fiber contains many myofibrils, mitochondria (providing ATP), and multiple nuclei
• Membrane: Sarcolemma with many receptors and ion channels

Myofibril Details

• Myofibrils contain thin (blue) and thick (red) filaments
• Sarcomere: Unit from one Z line to another Z line
• T-tubules: Surrounds myofibrils; important for contraction
• Sarcoplasmic reticulum (SR): Surrounds T-tubules, contains calcium ions
• Terminal cisternae: Part of SR, contains calcium ions
• Calcium ion channels: Initially blocked, need to be opened

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Mechanism of Muscle Contraction

Action Potential

• Begins at the neuron’s axon terminal (end bulb)
• Action potential causes vesicles with acetylcholine to release it

Acetylcholine and Sarcolemma

• Acetylcholine binds to receptors in sarcolemma
• Opens receptors: Sodium ions enter, leading to action potential

Propagation & Calcium Release

• Action potential travels across sarcolemma
• Travels down T-tubule
• Voltage change opens calcium ion channels
• Calcium from terminal cisternae enters T-tubule

Interaction with Myofibrils

• Calcium binds to thin filaments
• Along with ATP, calcium aids in muscle contraction

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Additional Information

• Sliding filament theory: Learn about muscle contraction at a molecular level
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