9.02 Skeletal Muscle Fibers and Contraction

Skeletal Muscle Structure

Understanding muscle structure is important to the understanding of muscle contraction. Image 1 illustrates the structures making up skeletal muscle. Take a moment to look at the image. We will learn about each structure.

Image 1: Structure of Skeletal Muscle
Open larger version of Image 1 here.

There are 3 major types of connective tissue found within the skeletal muscle:

1. Epimysium – is a connective tissue covering the entire muscle.
2. Perimysium – is a connective tissue covering a group of muscle fibers.
3. Endomysium – is a connective tissue covering individual muscle fibers.

Find these on the image above.

1. Fascicle – is another name for a group of muscle fibers.
2. Sarcolemma – is the cell membrane of a muscle fiber.
3. Fiber – is a single muscle cell.
4. Myofibrils – are the contractile structures of the cells made up of repeating protein units called sarcomeres.
5. Myofilaments – are the filaments of myofibrils constructed from the proteins actin and myosin. (These myofilaments are illustrated in the image protruding from the myofibril and not labeled.)

This is a concept map of the skeletal muscle organization from the macroscopic level to the microscopic level:

Image 2: Skeletal Muscle Organization
Open larger version of Image 2 here.

Go to Get Body Smart: Skeletal Muscle Fiber Location and Composition for an interactive diagram of these structures. Be sure to click the highlighted words on that page.

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How a Muscle Contracts

To understand how a muscle contracts, let's look at the structure of a sarcomere below. Sarcomeres are the functional contractile units of muscle fibers that are made up of thick (myosin) and thin (actin) filaments.

Image 3: Sarcomere Structure
Open larger version of Image 3: Sarcomere Structure.

• Notice these filaments illustrated in red and blue on the bottom part of the Image 3: Sarcomere Structure. The top part of the image is a stained microscopic view of these filaments.
• The H Zone is the area between the actin filaments. This zone will narrow as the muscle contracts due to myosin arms pulling the actin closer together.

On Image 4 below, notice the narrow H Zoneof the sarcomere in the image on the right. This happens when the muscle contracts after receiving a signal from a neuron. The neuron releases acetylcholine a neurotransmitter stimulating muscle contractions which is taken up by the muscle resulting in a cascade of events. These events culminate in contraction and relaxation of the muscle.

Image 4: H Zone diagrams
Open larger version of Image 4: H Zone diagrams.

Summary

Summary of the normal events in muscle contraction:

1. A neuron sends a signal (action potential) to the muscle fibers.
2. The neuron releases acetylcholine.
3. Acetylcholine causes an action potential along the surface of the muscle.
4. The action potentially causes calcium ions to be released from the sarcoplasmic reticulum (an organelle in the muscle).
5. Calcium binds to a protein causing myosin and actin to connect (cross ridge formation).
6. Cross bridge cycling results in muscle shortening as long as ATP is available.

Summary of the normal events in muscle relaxation:

1. Calcium ions are taken back to the sarcoplasmic reticulum.
2. Calsequestrin binds and stores calcium in the sarcoplasmic reticulum.
3. Actin and myosin cannot bind anymore without calcium.
4. Sodium (Na) is pumped out of the cell in exchange for potassium (K).

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