Types of muscle fibers

Two major differences between the types of muscle fibers are:

• Speed of contraction

• Metabolic pathway used to form ATP

As such, three types of muscle fibers (see Table 11.1) exist:

• Slow-twitch oxidative

• Fast-twitch oxidative

• Fast-twitch glycolytic

Table 11.1 Features of Skeletal Muscle Fiber Types

Slow-twitch

Fast-twitch

Fast-twitch

Feature

oxidative

oxidative

glycolytic

Myosin ATPase

Slow

Fast

Fast

activity

Speed of contraction

Slow

Fast

Fast

Removal of calcium

Slow

Fast

Fast

Duration of

Long

Short

Short

contraction

Mitochondria

Many

Many

Few

Capillaries

Many

Many

Few

Myoglobin content

High

High

Low

Color of fiber

Dark red

Red

Pale

Diameter of fiber

Small

Intermediate

Large

Source of ATP

Oxidative

Oxidative

Glycolysis

phosphorylation

phosphorylation;

glycolysis

Glycogen content

Low

Intermediate

High

Onset of fatigue

Delayed

Intermediate

Rapid

Fast-twitch muscle fibers develop tension two to three times faster than slow-twitch muscle fibers because of more rapid splitting of ATP by myosin ATPase. This enables the myosin crossbridges to cycle more rapidly. Another factor influencing the speed of contraction involves the rate of removal of calcium from the cytoplasm. Muscle fibers remove Ca++ ions by pumping them back into the sarcoplasmic reticulum. Fast-twitch muscle fibers remove Ca++ ions more rapidly than slow-twitch muscle fibers, resulting in quicker twitches that are useful in fast precise movements. The contractions generated in slow-twitch muscle fibers may last up to 10 times longer than those of fast-twitch muscle fibers; therefore, these twitches are useful in sustained, more powerful movements.

Muscle fibers also differ in their ability to resist fatigue. Slow-twitch muscle fibers rely primarily on oxidative phosphorylation for the production of ATP. Accordingly, these muscle fibers have a greater number of mitochondria, the organelles in which these metabolic processes are carried out. These fibers also have an extensive capillary network for delivery of oxygen. Furthermore, the high myoglobin content within slow-twitch muscle fibers facilitates diffusion of oxygen into the cells from the extracellular fluid. Myoglobin imparts the characteristic red color to these muscle fibers; thus, they are referred to as "red muscle."

Finally, slow-twitch muscle fibers have a small diameter. This facilitates the diffusion of oxygen through the fiber to the mitochondria where it is utilized. Taken together, each of these characteristics enhances the ability of these fibers to utilize oxygen. Therefore, in slow-twitch oxidative muscle fibers oxidative phosphorylation predominates and fatigue is delayed.

Fast-twitch muscle fibers fall into two categories. Fast-twitch glycolytic muscle fibers have fewer mitochondria, fewer capillaries, less myoglobin, and large diameters. As a result, these fibers rely primarily on glycolysis for the production of ATP. The resulting accumulation of lactic acid and decrease in pH hastens the onset of fatigue. Because this type of muscle fiber has less myoglobin, it has a much paler appearance than the slow-twitch oxidative muscle fibers. Therefore, it is referred to as "white muscle." Fast-twitch oxi-dative muscle fibers have many mitochondria and capillaries, a significant amount of myoglobin, and intermediate-sized diameters. These muscle fibers utilize a combination of oxidative and glycolytic metabolism to produce ATP. As a result, fast-twitch oxidative muscle fibers are more resistant to fatigue than fast-twitch glycolytic muscle fibers.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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