Skeletal Muscle and Muscular Dystrophy | Anatomy Learn

by Robert Tallitsch, PhD | February 24, 2022

Video with Patient Case Example of Muscular Dystrophy and Explanation!

Written by: Robert Tallitsch, PhD

Muscle tissue is one of the four basic tissues. There are three types of muscle tissue: smooth, cardiac, and skeletal muscle. All of these muscle types share four characteristics in common:

Elasticity: The ability of a muscle cell to return to its original length after a contraction.
Excitability: The ability to respond to a stimulus.
Contractility: The ability to actively shorten.
Extensibility: The ability to contract over a range of resting lengths.

This Brain Builder will concentrate on skeletal muscle.

Microscopic Anatomy of Skeletal Muscle

A whole skeletal muscle (such as the biceps brachii) is composed of many individual muscle fibers. Each muscle fiber is actually a single, multinucleated cell. Contraction of these individual muscle fibers ultimately results in contraction of the whole muscle.

When skeletal muscle is viewed with a light microscope or an electron microscope it is easy to see that skeletal muscle is striated, exhibiting regular light and dark bands that are consistently in register from cell to cell. This banding pattern is due to the arrangement of the contractile proteins, actin and myosin, within each muscle fiber. The repeating functional subunits of this banding pattern are termed sarcomeres. The dark band (A band) of the sarcomere is due to the arrangement of myosin, while the light band (I band) is due to the arrangement of actin.

Contraction of Skeletal Muscle

Simply put, understanding the biochemical and mechanical processes that enable the shortening of an individual sarcomere enables one to understand how an entire skeletal muscle contracts. A detailed explanation of the contractile process is beyond the scope of this Brain Builder, but a brief summary of the process follows.

The contraction process of an individual sarcomere and, therefore, an entire muscle, is termed the sliding filament theory of muscle contraction. In this model skeletal muscle contraction is produced by the sliding of actin filaments past the thicker myosin filaments within the skeletal muscle cell. When activated by a series of biochemical events, subunits of myosin attach to an adjacent actin filament, forming what is termed a cross bridge. Cross bridges facilitate the sliding of the actin filaments past the myosin filaments. This sliding action results in a progressive overlap of the actin and myosin filaments, reducing the length of the sarcomere. However, the length of the individual actin and myosin filaments does not change. The force generated within each sarcomere is determined by the number of cross bridges formed within that sarcomere. The greater the number of cross bridges formed and activated the greater the force generated by each sarcomere.

Gross Anatomy of Skeletal Muscle

Each skeletal muscle has three concentric layers of connective tissue. The epimysium surrounds the entire muscle. The perimysium divides the muscle into smaller compartments termed fascicles. The endomysium surrounds each individual skeletal muscle fiber.

At each end of the skeletal muscle the connective tissue fibers of the epimysium, perimysium and endomysium taper to form a tendon, which attaches the muscle to another muscle or to skin, cartilage, or bone. This arrangement of connective tissue layers enables each individual skeletal muscle fiber to contribute to the force placed on the tendon.

Origins and Insertions

Each skeletal muscle has at least one origin, which is the less movable attachment, and at least one insertion, which is the more movable attachment. Current anatomical terminology is reducing the usage of “origin” and “insertion”, and is replacing these terms with modifiers such as proximal, distal, medial, or lateral attachment. This new terminology enables a better visualization of the location of the muscle and the movement it causes during contraction.

Muscle Shape

Skeletal muscles are classified according to the shape and arrangement of the individual fibers relative to the direction of muscle pull. These classifications, and an example of each, are

Circular muscles (orbicularis oris)
Parallel muscles (biceps brachii)
Convergent muscles (pectoralis major)
Unipennate muscles (extensor digitorum)
Bipennate muscles (rectus femoris)
Multipennate muscles (deltoid)

Roles of Skeletal Muscles

Skeletal muscles are grouped into four categories, depending on the type of action they perform.

Agonist, or prime mover: Contraction of this type of muscle produces the desired movement.
Antagonist: Contraction opposes the action of the agonist.
Synergist: Contraction of this type of muscle assists the agonist in its production of the desired movement.
Fixator: Simultaneous contraction of an agonist and an antagonist stabilizes a joint of the body and produces a stable or immovable base.

Names of Skeletal Muscles

Gross anatomy is often thought of as a subject requiring a lot of memorization. However, if one pays close attention, the names of skeletal muscles are often based on the shape, location, and/or action of the muscle.

Palmaris longus: A long muscle that terminates in the palm of the hand.
Extensor digiti minimi: The muscle that extends the little finger.
Deltoid: A muscle shaped like a triangle (Greek letter delta)
Biceps: A muscle with two heads.
Tibialis anterior: A muscle found on the anterior surface of the tibia.

Muscular Dystrophy

Muscular dystrophy is a group of diseases caused by one or more genetic mutations that prevent the normal formation and development of one or more proteins needed to make skeletal muscle function properly. The main symptom of muscular dystrophy is progressive skeletal muscle weakness. Specific signs and symptoms develop at different ages, depending on the type of muscular dystrophy.

There are approximately thirty forms of muscular dystrophy. Duchenne muscular dystrophy is the most common form. Some of the symptoms, which begin in early childhood, include frequent falls, difficulty in rising from a lying or sitting position, difficulty running or jumping, walking on one’s toes, muscle pain and stiffness, and delayed growth.

Becker muscular dystrophy has symptoms very similar to those of Duchenne muscular dystrophy. However, this form develops later in life and progresses more slowly.

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Key Terms:

Epimysium - Connective tissue sheath surrounding an entire muscle.

Myosin - One of the main contractile proteins within skeletal muscle. The dark portion of the banding pattern of a skeletal muscle cell is due to the arrangement of myosin.

Actin - One of the main contractile proteins within skeletal muscle. The light portion of the banding pattern of a skeletal muscle cell is due to the arrangement of myosin.

Sliding filament theory - The theory that explains the contractile mechanism of skeletal muscle.

Agonist - Muscle that produces the desired action.

Fixator - The simultaneous contraction of two skeletal muscles (agonist and antagonist) that results in the stabilization a joint of the body and produces a stable or immovable base.

Origin - The least movable attachment of a skeletal muscle.

Tendon - Attaches the muscle to another muscle or to skin, cartilage, or bone.

Duchenne muscular dystrophy - The most common form of muscular dystrophy. This form of muscular dystrophy typically begins in early childhood.

Questions:

What are the four characteristics of skeletal muscle?
A: Elasticity, excitability, contractility, and extensibility
What are the two contractile proteins within each muscle fiber?
A: Actin and myosin
What is the name for the category (or role) of a skeletal muscle contraction that assists the prime move in the production of a particular contraction?
A: Synergist
What is the name of the connective tissue layer that divides a skeletal muscle into smaller compartments termed fascicles?
A: Perimysium
What is the term that describes a skeletal muscle cell’s ability to return to its original length after a contraction?
A: Elasticity
What is the term that describes a skeletal muscle cell’s ability to respond to a stimulus?
A: Excitability
What is the name of the more movable attachment of a skeletal muscle?
A: Insertion
Yes or no: During the contraction process of a skeletal muscle, does the length of the individual sarcomeres change?
A: No
What is the most common form of muscular dystrophy?
A: Duchenne muscular dystrophy
What is the cause of muscular dystrophy?
A: Defective genes involved in the making of protein fibers
What is the main symptom of muscular dystrophy?
A: Progressive skeletal muscle weakness

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