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Introduction

The respiratory muscles constitute a complex pump system. Several muscles comprise this system, represented schematically in Figure 3-1. Breathing under all circumstances requires a coordinated contraction of different respiratory muscles. The most important inspiratory muscle is the diaphragm. The conditions under which this respiratory muscle system weakens and eventually will fail are addressed in other chapters (see Chapters 83, 85, and 104). This chapter focuses on structural and functional properties of the respiratory muscles, respiratory muscle action, and respiratory muscle interaction.

Figure 3-1

Idealized drawing of the respiratory muscles.

Structural and Functional Properties of Respiratory Muscles

The respiratory muscles are skeletal muscles, and, in essence, their structural and functional properties are within the range of other skeletal muscles located in the limbs. Adaptations to their specific function, however, make them distinctly different from other skeletal muscles in a number of respects. First, limb muscles are essentially designed to produce movements, and hence, primarily work against inertial loads. Respiratory muscles mainly have to overcome resistive and elastic loads.

Second, peripheral muscles contract rhythmically during movements, whereas respiratory muscles contract rhythmically and continuously, and they are the only skeletal muscles on which life depends. These vital muscles thus have to be well equipped to sustain continuous rhythmic contraction. These adaptations include high fatigue resistance, high oxidative capacity, greater capillary density, and greater maximal blood flow, and they depend upon structural and functional properties of the muscles.

Structural Properties

Structural properties of muscles in general, and respiratory muscles in particular, depend upon fiber types present in the muscle, morphological characteristics of the fibers, and motor unit organization.

Fiber Types

Four types of muscle fibers are usually present in skeletal muscles. They are distinguished on the basis of the myofibrillar myosin adenosine triphosphatase (ATPase) activity and its pH dependence and pretreatment with paraformaldehyde.1,2 Thus, after acid preincubations at pH 4.5, type I fibers are stained dark, type IIa fibers are stained lighter than type IIb and type IIx fibers. In addition, pretreatment with paraformaldehyde after alkaline preincubation at pH 10.4 further allows the distinction between type IIb fibers staining lighter than type IIx fibers. Alternatively, muscle fibers may be distinguished through myosin heavy chain gene expression using electrophoresis and western blotting or via immunostaining.3 This latter technique has the advantage of revealing the presence of coexpression of different myosin heavy chain isoforms within the same muscle fiber. It has also been revealed that myosin heavy chain 2b is not expressed in human muscle.4 Type I fibers, or slow oxidative fibers, have a slow contraction profile but are high in endurance and rich in oxidative enzymes.5 Type II fibers are fast-twitch fibers that develop tension rapidly. They either ...

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