Overview of muscle types.
The three major types of contractile tissues in the body,
, have many properties
in common, but differ in many other ways.
is usually, but not always, attached to the bones of the skeleton and
is specialized to execute rapid voluntary movements of the limbs, digits, head, etc., in response to signals from the central nervous sys-
tem (CNS). Skeletal muscle cells are long, thin, tubular structures with multiple nuclei clustered just under the cell membrane. Motor
neuron axons form synapses (motor endplates) on each skeletal muscle F
ber. Contraction in skeletal muscle is produced by a calcium-
mediated interaction between myoF laments that are composed primarily of the proteins actin and myosin. The actin and myosin
F laments are arranged into highly organized, repeating units called sarcomeres, which give skeletal muscle a striped (“striated”)
appearance when viewed at higher magniF
cations in light microscopy. The calcium necessary to initiate the actin-myosin reaction
is stored in modiF ed endoplasmic reticulum structures called the sarcoplasmic reticulum. Calcium is released when electrical charges
ﬂ ow down the transverse tubule system, which is formed by invaginations of the cell membrane, and is located adjacent to parts
of the sarcoplasmic reticulum within the muscle cells.
in contrast, is specialized for repeated, rhythmic, automatic
contractions over many years without ceasing. The contractile mechanism is similar to that of skeletal muscle: actin and myosin
myoF laments are arranged in sarcomeres and their interaction is mediated by calcium release. However, cardiac muscle cells are
short and split into two or three branches; these branching cells are joined end to end by intercalated disks. The overall structure of
cardiac muscle is, therefore, one of a meshwork of contractile tissues, instead of being a collection of independent, parallel units such
as is found in skeletal muscle. The autonomic axons that innervate cardiac muscle release their neurotransmitters into the intracel-
lular space rather than onto individual cells at motor endplates as in skeletal muscle. The nervous system, therefore, modulates the
rhythm of contraction of cardiac muscle, but does not command individual contractions.
is found in many organ
systems including the circulatory, respiratory, gastrointestinal, reproductive, and urinary systems. ±or the most part, smooth muscle
is specialized for automatic, slow, rhythmic contraction, although a few muscles such as the ciliary muscle of the eye are exceptions.
Like skeletal and cardiac muscles, smooth muscle uses actin and myosin F
laments to produce contraction, but the myoF
are not organized into sarcomeres. Instead, actin F
laments are anchored to dense plaques in the smooth muscle sarcolemma, and
a myosin F lament contacts several individual actin F
laments at both of its ends. These actin-myosin combinations are arranged in
a random, crisscross pattern in some muscles and in parallel patterns in other muscles. As in skeletal and cardiac muscles, calcium
is a critical factor in initiating a contraction, but in smooth muscle the calcium is stored in the intercellular space rather than in a
sarcoplasmic reticulum. As in cardiac muscle, autonomic motor nerves release neurotransmitters into the intercellular space rather
than into motor endplates. The nervous system, therefore, modulates the inherent contractile rhythm of smooth muscle. This rhythm
can also be inﬂ
uenced by hormones in the bloodstream and by mechanical stretching of the muscle. Electrical excitation and, hence,
muscle contraction, can also spread directly from cell to cell via gap junctions between the membranes of adjacent cells.