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Related Concept Videos

Actin and Myosin in Muscle Contraction01:16

Actin and Myosin in Muscle Contraction

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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...
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Excitation-Contraction Coupling in Skeletal Muscles01:20

Excitation-Contraction Coupling in Skeletal Muscles

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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...
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Cross-bridge Cycle01:26

Cross-bridge Cycle

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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.
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Smooth Muscle Contraction01:25

Smooth Muscle Contraction

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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...
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The Sarcomere01:08

The Sarcomere

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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...
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Relaxation of Skeletal Muscles01:29

Relaxation of Skeletal Muscles

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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: Aug 17, 2025

Ex Vivo Assessment of Contractility, Fatigability and Alternans in Isolated Skeletal Muscles
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Calcium activation through thick and thin?

Michael J Previs1

  • 1Molecular Physiology and Biophysics Department, Larner College of Medicine, University of Vermont, Burlington, VT, USA.

The Journal of General Physiology
|December 16, 2022
PubMed
Summary

This study reviews the super-relaxed (SRX) state, interacting heads motif (IHM), and calcium's role in muscle contraction. It provides historical context for these key concepts in muscle physiology.

Area of Science:

  • Muscle physiology
  • Biophysics
  • Biochemistry

Background:

  • The super-relaxed (SRX) state is a key regulatory mechanism in muscle.
  • The interacting heads motif (IHM) influences myosin motor function.
  • Calcium ions are critical for initiating muscle contraction.

Purpose of the Study:

  • To provide a historical overview of the SRX state.
  • To explore the functional significance of the IHM.
  • To examine the impact of calcium on muscle contractility.

Main Methods:

  • Literature review
  • Historical analysis
  • Conceptual synthesis

Main Results:

  • The SRX state's discovery and characterization.

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  • Evolution of understanding the IHM.
  • Elucidation of calcium's role in excitation-contraction coupling.

    • Conclusions:

    • The SRX state, IHM, and calcium are fundamental to understanding muscle function.
    • Historical perspectives offer insights into current research.
    • Further research continues to refine our understanding of muscle mechanics.