CoursesPhysiologyMuscular systemSkeletal muscleSkeletal muscle contraction

Skeletal muscle contraction

Learning objectives

After completing this study unit, you will be able to:

Describe muscle fiber excitation at the neuromuscular junction.
Explain excitation-contraction coupling and crossbridge cycling.
Contrast the mechanisms of muscle contraction and relaxation.

Introduction

When we need to move, our muscles contract to generate a pulling force called tension. Relaxation is the opposite process, where the muscle returns to its resting state and tension is released.

Skeletal muscle fiber contraction is coordinated by the somatic efferent nervous system. Every skeletal muscle fiber is innervated by a single somatic motor neuron at the neuromuscular junction. Each motor neuron action potential triggers a single action potential in all the muscle fibers it innervates, thereby evoking a discrete contraction-relaxation event known as a muscle twitch.

Skeletal muscle contraction can be divided into three phases:

Muscle excitation: a neuronal action potential triggers a muscle fiber action potential
Excitation-contraction coupling: the muscle fiber action potential triggers the release of Ca²⁺ inside the muscle cell, allowing crossbridge formation
Crossbridge cycling: myosin heads make thin filaments slide inwards, shortening the sarcomere to generate muscle tension

Muscle relaxation includes active processes like restoration of the ion gradient of the sarcolemma and Ca²⁺ reuptake.

Explore concepts

Skeletal muscle contraction and relaxation

Muscle contraction (muscle excitation, excitation-contraction coupling, crossbridge cycling) and relaxation regulate muscle tension generation.

Muscle excitation

During muscle excitation, a motor neuron action potential causes acetylcholine release at the neuromuscular junction, triggering a muscle fiber action potential.

Excitation-contraction coupling

During excitation-contraction coupling, the muscle fiber action potential propagates into the triad and triggers Ca²⁺ release into the sarcoplasm, allowing crossbridge formation.

Crossbridge cycling

During crossbridge cycling, myosin heads slide thin filaments inwards, shortening the sarcomere to generate muscle tension.

Muscle relaxation

During relaxation, tension is released as the sarcomeres return to their resting length.

Take a quiz

Test your knowledge about muscle contraction with this quiz!

Summary

Key points about skeletal muscle contraction
Innervation	Each skeletal muscle fiber is innervated by a single somatic motor neuron. One motor neuron action potential > one muscle fiber action potential > one muscle twitch (contraction-relaxation event)
Muscle contraction	Muscle excitation Excitation-contraction coupling Crossbridge cycling
Muscle excitation	Location: Neuromuscular junction (motor neuron terminal, synaptic cleft, motor end-plate) Action potential reaches the axon terminal Neuron releases acetylcholine into the synaptic cleft Acetylcholine opens nicotinic channels on the motor end-plate Net influx of positive charges generates end-plate potential End-plate potential starts muscle fiber action potential propagation
Excitation-contraction coupling	Locations: Triad (T-tubule, sarcoplasmic reticulum), sarcomere Action potential propagates along the sarcolemma into T-tubules Depolarization releases stored Ca²⁺ into the sarcoplasm Ca²⁺ binds to troponin, moving tropomyosin to expose binding site
Crossbridge cycling	Location: Sarcomere ATP hydrolysis 'cocks' myosin heads into a high-energy position Myosin heads bind actin and form a crossbridge Myosin heads slide actin toward the M-line (power stroke) ATP binding detaches myosin head
Muscle relaxation	Locations: Neuromuscular junction, triad, sarcomere Acetylcholinesterase degrades acetylcholine SERCA reuptakes Ca²⁺ into the sarcoplasmic reticulum Ca²⁺ dissociates from troponin Myosin heads detach from actin Titin helps restore the sarcomere resting length