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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...

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...

Smooth Muscle Contraction

Smooth Muscle Contraction

Smooth muscle contraction is a complex process vital for various bodily functions, from maintaining blood vessel tension to facilitating the movement of food through the digestive tract. Unlike striated muscles, smooth muscle contraction begins more slowly and lasts longer.
The onset of contraction is triggered by an increase in calcium ions within the sarcoplasm, similar to the process in striated muscle. However, smooth muscles have a relatively smaller reservoir of the sarcoplasmic...

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.

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.

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...

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Related Experiment Video

Updated: Jun 17, 2026

Isolating Myofibrils from Skeletal Muscle Biopsies and Determining Contractile Function with a Nano-Newton Resolution Force Transducer

Isolating Myofibrils from Skeletal Muscle Biopsies and Determining Contractile Function with a Nano-Newton Resolution Force Transducer

Published on: May 7, 2020

Force depression in single myofibrils.

V Joumaa¹, W Herzog

¹Human Performance Laboratory, University of Calgary, Alberta, Canada.

Journal of Applied Physiology (Bethesda, Md. : 1985)

|December 17, 2009

Summary

Muscle force depression after shortening is not caused by sarcomere length nonuniformities. Instead, this study shows force depression is an intrinsic property of individual sarcomeres during muscle contraction.

Area of Science:

Muscle physiology
Cellular biomechanics

Background:

Force depression is a known phenomenon in muscle preparations after active shortening.
Previous hypotheses attributed force depression to sarcomere length nonuniformities.

Purpose of the Study:

To investigate the role of sarcomere length nonuniformities in force depression.
To directly measure individual sarcomere lengths during and after muscle shortening.

Main Methods:

Studied force depression in single myofibrils (n=11).
Simultaneously tracked individual sarcomere lengths (n=60) before, during, and after shortening.
Compared measurements with a purely isometric contraction at the final length.

Main Results:

Shortening caused significant force depression (30.9 +/- 3.9%).

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Dissecting Mechanoenzymatic Properties of Processive Myosins with Ultrafast Force-Clamp Spectroscopy

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Last Updated: Jun 17, 2026

Isolating Myofibrils from Skeletal Muscle Biopsies and Determining Contractile Function with a Nano-Newton Resolution Force Transducer

Isolating Myofibrils from Skeletal Muscle Biopsies and Determining Contractile Function with a Nano-Newton Resolution Force Transducer

Published on: May 7, 2020

Probing Myosin Ensemble Mechanics in Actin Filament Bundles Using Optical Tweezers

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Published on: May 4, 2022

Dissecting Mechanoenzymatic Properties of Processive Myosins with Ultrafast Force-Clamp Spectroscopy

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Published on: July 1, 2021

Sarcomere lengths became nonuniform during shortening, but sarcomere length nonuniformity (SD) remained consistent before and after shortening.

Nonuniformity of half-sarcomere lengths also showed no significant change.

Conclusions:

Development of sarcomere or half-sarcomere length nonuniformities does not significantly contribute to force depression.
Force depression appears to be an intrinsic property of individual sarcomeres and the muscle contraction process.