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Muscle Contractions: Micro Level

Muscle Contractions: Micro Level

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This lesson will go into more detail describing how a muscle contracts.

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Tutorial

What's Covered

This lesson will cover micro level view of muscle contractions, looking specifically at sarcomeres and how they act in order to produce a muscle contraction.

  1. Structure of a Sarcomere
  2. How Contractions Happen
  3. Types of Contractions

1. Structure of a Sarcomere

Muscle contractions basically are just the shortening of fibers within muscles which acts to generate force, and the sarcomere is the basic unit of contraction. Muscle contractions rely on calcium, and can occur when there's an increase in calcium ions. Calcium is either stored or released by the sarcoplasmic reticulum. The sarcoplasmic reticulum is like the endoplasmic reticulum of muscle fibers. It stores and releases calcium necessary for these muscle contractions.

Terms to Know

    • Contraction
    • A muscle fiber generates tension, causing the muscle to shorten.
    • Sarcomere
    • The functional and contractile units of skeletal and cardiac muscles; created by a specific arrangement of myofilaments called actin and myosin; each sarcomere is bordered by a z-line.
    • Calcium
    • A mineral/ion that has many physiologic uses to humans; calcium is stored in the sarcoplasmic reticulum (SR) of skeletal muscles and when released it prepares actin and myosin for interaction.
    • Sarcoplasmic reticulum (SR)
    • A specialized form of smooth endoplasmic reticulum (SER) found within skeletal muscles used for calcium storage.

Use this diagram below, just to give you a general idea, so you have a picture of what we're talking about when we talk about a sarcomere, or a muscle fiber. Muscles contain different bundles of fibers, and if we break this down, we can take a look at one fiber. That fiber is actually made of a myofibril. These myofibrils have this banded appearance, and when bundled together give muscles their striated, or striped look. Sarcomeres, the basic unit of contraction, are portions of myofibrils.

musclecontractionsmicro (2).png

Regions of the sarcomere:

  • Z-bands- mark the ends of sarcomeres. When a sarcomere shortens, or when a contraction happens, it's because Z-bands are moving closer together.
  • I-bands- a band on either side of the Z-band where actin is not overlapped by myosin
  • A-band- when a contraction happens the thick and thin filaments are overlapping each other within our A-band region.

musclecontractionsmicro (3).png

Within our sarcomere, we have something called thick and thin filaments. And those thick and thin filaments work together in order to produce a contraction. Actin is a protein that's referred to as the thin filament in a contraction, whereas myosin is the protein referred to as a thick filament.

musclecontractionsmicro2 (2).png

Terms to Know

    • Actin
    • Referred to as the thin filament of a sarcomere, creates the lighter color within a sarcomere and interacts with myosin to create movement.
    • Myosin
    • Referred to as the thick filament of a sarcomere, creates the darker colors within a sarcomere and contains various heads that pull on actin filaments to create movements.

Actin looks like a strand of beads, and the above image shows several molecules of myosin together. Just one molecule of myosin is composed of a head and a tail. The head plays an important role in contraction.


2. How Contractions Happen

musclecontractionsmicro2 (4).png

Use this diagram to visualize what's happening during a contraction; imagine this is one sarcomere. When a contraction happens actin and myosin overlap with each other, and the Z-bands will move closer together. The top image is a relaxed muscle fiber, and the bottom would be one that has contracted.

This contraction happens when myosin heads bind to actin, and this occurs when the sarcoplasmic reticulum releases calcium. Calcium prepares actin and myosin for interaction. Once myosin has bound to actin, the myosin heads will pull the actin filaments toward the center of the sarcomere.musclecontractionsmicro2 (3).png

ATP energy is also required for this to happen. When ATP is released it will then bind to myosin, and myosin will detach from actin. The sarcomere will return to its resting state. This whole process is known as the Sliding filament mechanism.

Terms to Know

    • Sliding filament mechanism
    • A theory used to describe how the microfilaments within the sarcomere interact with one another; actin and myosin cling together and slide past one another.
    • ATP
    • The primary energy molecule that cells use for energy; composed of adenosine and three phosphate groups; when the bonds that hold the phosphates together are broken energy is released for work.

3. Types of Contractions

So there are kind of two types of contractions that can occur:

  • Twitch-a type of contraction where just one contraction will happen in response to the firing of a motor neuron.
  • Tetanus- a sustained contraction that's caused by repeated muscle twitches.

In other words, if a muscle twitches over and over and over and over again, it can sustain that contraction. Contractions are caused by impulses from the nervous system. An action potential is the technical term for these impulses and a motor unit describes one motor neuron and all of the muscle fibers it innervates at once.

Terms to Know

    • Action potential
    • The technical term for a nervous impulse; when a wave of depolarized electrical energy travels down the length of a cell/tissue.
    • Motor unit
    • The term used to describe one motor neuron and all of the muscle fibers it innervates at once.
    • Twitch
    • The way a skeletal muscle contracts, sarcomeres quickly pull inward and create a quick, jerky twitch.
    • Tetanus
    • A term used to described a prolonged muscle contraction; often used to describe a person who has been exposed to Clostridium tetani toxin.

Summary

The sarcomere is the basic unit of contraction within muscle fibers and make up myofibrils. The structure of the sarcomere include 3 regions: Z-bands, I-bands, and A-bands; they also have thin and thick filaments. Contractions happen when calcium is released, and myosin, the thick filament, binds to actin, the thin filament. They move to overlap each other in the A-band region, pulling the Z-bands closer together. There are two types of muscle contractions: twitch and tetanus. Tetanus is a prolonged contraction made up of multiple twitches.

Keep up the learning and have a great day!

Source: Source: THIS WORK IS ADAPTED FROM SOPHIA AUTHOR AMANDA SODERLIND

TERMS TO KNOW
  • Sarcomere

    The functional and contractile units of skeletal and cardiac muscles; created by a specific arrangement of myofilaments called actin and myosin; each sarcomere is bordered by a z-line.

  • Actin

    Referred to as the thin filament of a sarcomere, creates the lighter color within a sarcomere and interacts with myosin to create movement.

  • Myosin

    Referred to as the thick filament of a sarcomere, creates the darker colors within a sarcomere and contains various heads that pull on actin filaments to create movements.

  • ATP

    The primary energy molecule that cells use for energy; composed of adenosine and three phosphate groups; when the bonds that hold the phosphates together are broken energy is released for work.

  • Action potential

    The technical term for a nervous impulse; when a wave of depolarized electrical energy travels down the length of a cell/tissue.

  • Calcium

    A mineral/ion that has many physiologic uses to humans; calcium is stored in the sarcoplasmic reticulum (SR) of skeletal muscles and when released it prepares actin and myosin for interaction.

  • Sarcoplasmic reticulum (SR)

    A specialized form of smooth endoplasmic reticulum (SER) found within skeletal muscles used for calcium storage.

  • Contraction

    A muscle fiber generates tension, causing the muscle to shorten.

  • Twitch

    The way a skeletal muscle contracts, sarcomeres quickly pull inward and create a quick, jerky twitch.

  • Tetanus

    A term used to described a prolonged muscle contraction; often used to describe a person who has been exposed to Clostridium tetani toxin.

  • Sliding filament mechanism

    A theory used to describe how the microfilaments within the sarcomere interact with one another; actin and myosin cling together and slide past one another.

  • Motor unit

    The term used to describe one motor neuron and all of the muscle fibers it innervates at once.