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Actin and Myosin in Muscle Contraction

Actin and Myosin in Muscle Contraction

Actin and myosin are contractile proteins that form the sarcomere found in skeletal muscle tissues for regulating muscle contraction. Actin, a globular contractile protein, interacts with myosin for muscle contraction. The skeletal tissue appears striped or striated under a microscope due to the repeated arrangement of contractile proteins actin and myosin along the length of myofibrils. Dark A bands and light I bands repeat along myofibrils, and the alignment of myofibrils in the cell causes...

The Sarcomere

The Sarcomere

A sarcomere is a microscopic segment repeating in a myofibril. The sarcomere fundamentally consists of two main myofilaments: thick filaments called myosin and thin filaments called actin. These filaments interact by sliding past each other in response to stimulus. In addition to myosin and actin, several other proteins, such as tropomyosin, troponin, titin, nebulin, myomesin, α-actinin, and dystrophin, play crucial roles in regulating, structuring, and functioning of the sarcomere.
Each myosin...

Excitation-Contraction Coupling in Skeletal Muscles

Excitation-Contraction Coupling in Skeletal Muscles

Excitation-contraction coupling is a series of events that occur between generating an action potential and initiating a muscle contraction. It occurs at the triad, a structure found in skeletal muscle fibers that comprise a T-tubule and terminal cisternae of the sarcoplasmic reticulum on each side. These triads are visible in longitudinally sectioned muscle fibers. They are typically located at the A-I junction — the junction between the A and I bands of the sarcomere.
When an action potential...

Cross-bridge Cycle

Cross-bridge Cycle

As muscle contracts, the overlap between the thin and thick filaments increases, decreasing the length of the sarcomere—the contractile unit of the muscle—using energy in the form of ATP. At the molecular level, this is a cyclic, multistep process that involves binding and hydrolysis of ATP, and movement of actin by myosin.

Generation of Straight or Branched Actin Filaments

Generation of Straight or Branched Actin Filaments

The straight or branched structure formation of actin filaments is controlled by nucleating proteins such as the formins and Arp2/3 complex. Formin-mediated assembly results in straight filaments, whereas Arp2/3 protein complex-mediated assembly results in branched actin filaments.
Arp2/3 Complex
Arp2/3 complex is a seven-subunit complex consisting of two proteins similar to actin- Arp2 and Arp3, and five other subunits that help keep Arp2 and Arp3 inactive. When required, the complex is...

Relaxation of Skeletal Muscles

Relaxation of Skeletal Muscles

The period of muscle contraction primarily influences the duration of stimulation at the neuromuscular junction (NMJ), the presence of free calcium ions in the sarcoplasm, and the availability of energy or ATP to support contractions.
When an action potential reaches the axon terminal, it depolarizes the membrane and opens voltage-gated sodium channels. Sodium ions enter the cell, further depolarizing the presynaptic membrane. This depolarization causes voltage-gated calcium channels to open.

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Related Experiment Video

Updated: Jun 17, 2026

"Avatar", a Modified Ex vivo Work Loop Experiments Using In vivo Strain and Activation

Published on: August 18, 2023

Myofilament length dependent activation.

Pieter P de Tombe¹, Ryan D Mateja, Kittipong Tachampa

¹Department of Cell and Molecular Physiology, Stritch School of Medicine, Loyola University Chicago, Chicago, IL 60153, USA. pdetombe@lumc.edu

Journal of Molecular and Cellular Cardiology

|January 8, 2010

Summary

This summary is machine-generated.

The Frank-Starling law explains how heart muscle adjusts its pumping force based on stretch. This review explores the molecular basis of myofilament length-dependent activation, crucial for cardiac function.

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Last Updated: Jun 17, 2026

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Procedures for Rat in situ Skeletal Muscle Contractile Properties

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Assessment of Myofilament Ca2+ Sensitivity Underlying Cardiac Excitation-contraction Coupling

Assessment of Myofilament Ca²⁺ Sensitivity Underlying Cardiac Excitation-contraction Coupling

Published on: August 1, 2016

Area of Science:

Cardiovascular Physiology
Muscle Biology
Molecular Cardiology

Background:

The Frank-Starling law describes the heart's intrinsic ability to match cardiac output to venous return.
Myofilament length-dependent activation, a cellular mechanism, enhances myocyte responsiveness to calcium at longer sarcomere lengths.
Despite its long-standing recognition, the precise molecular mechanisms linking sarcomere length to ventricular pressure remain unclear.

Purpose of the Study:

To review and discuss the molecular mechanisms underlying myofilament length dependency in cardiac muscle.
To explore the roles of inter-filament spacing, thick and thin filament regulation, and sarcomeric proteins in this process.

Main Methods:

This is a review article, synthesizing existing research.
Focuses on theoretical and experimental evidence regarding molecular interactions within the sarcomere.
Discusses regulatory proteins and structural components of cardiac muscle.

Main Results:

Myofilament length-dependent activation is a key cellular process for the Frank-Starling mechanism.
Inter-filament spacing and the regulation of thick and thin filaments are implicated in length adaptation.
Sarcomeric regulatory proteins play a role in modulating myofilament responsiveness to calcium.

Conclusions:

Understanding the molecular basis of myofilament length dependency is critical for comprehending cardiac adaptation.
Further research is needed to elucidate how the contractile apparatus senses and transduces sarcomere length information.
This knowledge is fundamental for understanding cardiac function and dysfunction.