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Major
types of vertebrate muscle
In
terms of its microscopic structure, the musculature of vertebrates
is usually divided into three types: striated, cardiac,
and smooth muscle. Smooth and cardiac muscle are under the
control of the involuntary, or autonomic, nervous system.
Striated muscle, on the other hand, ismainly under the control
of the voluntary, or central, nervous system. Smooth and
cardiac muscle are also similar in their development, being
generally associated with the yolk sac. Striated muscle
develops directly from the middle of the three embryonic
layers, arising largely from the mesodermal somites (see
below). In the adult, smooth and cardiac muscle are associated
with organs or tubes (viscera), and striated (skeletal)
muscle with the bony or cartilaginous skeleton.
Although
some evidence suggests that a twofold division of the musculature
into visceral and skeletal muscle might be appropriate,
one set of striated muscles is distinct in development and
in innervation from all other striated muscle. In humans
these include both the muscles ofthe jaw and some of the
muscles of the shoulder and are called the branchiomeric,
or branchial, muscles. Branchiomeric muscles, even in humans,
are innervated by visceral nerve fibres as is the case with
smooth and cardiac muscle, not the somatic nerve fibres
(thosethat supply the outer body wall or soma), and thus
are usually grouped with the smooth and cardiac muscles
despite their striated nature.
The
two major divisions of the vertebrate musculature are thus
the visceral musculature (smooth, cardiac, and branchiomeric)
and the somatic musculature (the striated muscles of thebody
wall). Somatic musculature may be divided into appendicular,
or limb, muscles and axial muscles. The axial muscles include
the muscles of the tail, trunk, and eyeballs as well as
a group of muscles called hypobranchial muscles, which separate
and migrate from the others during development.
