Skeletal Muscle: Functional Anatomy and Pathophysiology

ABSTRACT

Muscle is generally divided into three subtypes—skeletal, cardiac, and smooth—but because this edition focuses on the musculoskeletal system, this article concentrates on skeletal muscle. We review ultrastructure and function and then look at the latest scientific ideas concerning the physiological basis of muscle contraction. It is important to appreciate the different muscle types and how they act with respect to muscle growth and adaptation. Finally, what happens to muscle cells when they are damaged and the reparative response is considered.

KEYWORDS

Myocyte - myofibril - myofilaments - sarcomere - action potential - contraction - muscle type - damage - repair

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1 Some texts use sarcolemma and plasmalemma interchangeably, but in this article it can be assumed that they will be used as defined earlier.

2 ATP binding to release myosin from actin is needed to allow the myosin head to move to the next binding site as contraction continues.

3 The exact timing of ATP hydrolysis and phosphate release with respect to the power stroke is still a matter of debate.

4 It is accepted that resistance training does cause muscle injury at a microscopic level. The pathophysiology of delayed onset muscle soreness, a condition where muscles are sore a day or two after heavy exercise, highlights this.

5 Given that we are discussing skeletal muscle, the focus is on physiologic hypertrophy, rather than pathological hypertrophy, such as that seen in cardiac failure.

6 It may be that muscle fiber numbers do increase with resistance training but that some of the methods used to detect these changes are not entirely accurate.

7 Hyperplasia is defined as an “increase in the number of cells in an organ or tissue, which may then have increased volume” (Kumar V, Fausto N, Abbas A. Robbins & Cotran Pathologic Basis of Disease. 7th ed. Philadelphia, PA: Saunders; 2004:272).