Skeletal, or voluntary, muscle constitutes the principal organ
of locomotion, as well as a vast metabolic reservoir. Disposed in
more than 600 separate muscles, this tissue makes up as much as
40 percent of the weight of adult human beings. An intricacy of
structure and function undoubtedly accounts for its diverse susceptibility
to disease, for which reason the main anatomic and clinical facts
are provided as an introduction to the following several chapters
on muscle disease. The essential physiologic facts concerning the
function of muscle are in Chap. 45 and those
of the neuromuscular junction, in Chap. 53.
A single muscle is composed of thousands of muscle fibers that
extend for variable distances along its longitudinal axis. Each
fiber is a relatively large and complex multinucleated cell varying
in length from a few millimeters to several centimeters (34 cm in
the human sartorius muscle) and in diameter from 10 to 100 μm.
Some fibers span the entire length of the muscle; others are joined
end to end by connective tissue. Each muscle fiber is enveloped
by an inner plasma membrane (the sarcolemma) and an outer basement
membrane. The multiple nuclei of each fiber, which are oriented
parallel to its longitudinal axis and may number in the thousands,
lie beneath the plasma membrane (sarcolemma); hence they are termed subsarcolemmal, or sarcolemmal
The cytoplasm (sarcoplasm) of the cell is abundant and contains
myofibrils and various organelles such as mitochondria and ribosomes.
Each myofibril is enveloped in a membranous net, the sarcoplasmic
reticulum (SR; see Fig. 45-2). Extensions
of the plasma membrane into the fiber form the transverse tubular
system (T tubules), which are extracellular channels of communication
with the intracellular sarcoplasmic reticulum. The SR and T tubules
are anatomically independent but functionally related membrane systems.
The junctional gap between the T tubules and SR is occupied by protein
formations that are attached to the SR and are referred to as junctional
feet; the latter have been identified as ryanodine receptors
and are responsible for the release of calcium from the SR, which
is a critical step in exciting the muscle (see Franzini-Armstrong).
The myofibrils themselves are composed of longitudinally oriented
interdigitating filaments (myofilaments) of contractile proteins
(actin and myosin), additional structural proteins (titin and nebulin),
and regulatory proteins (tropomyosin and troponin). The series of
biochemical events by which these proteins, under the influence
of calcium ions, accomplish the contraction and relaxation of muscle
is described in Chap. 45. Droplets of stored
fat, glycogen, various proteins, many enzymes, and myoglobin, the
latter imparting the red color to muscle, are contained within the
sarcoplasm or its organelles.
Although the muscle fiber represents an indivisible anatomic
and physiologic unit, disease may affect only one part of it, leaving
only that part, or sometimes the remainder, to become dysfunctional
or to atrophy, degenerate, or regenerate, depending on the nature
and severity of the disease ...