Muscle
fibres differ from species to species of animal and between
parts of the same animal. Apart from the distinction between
voluntary and involuntary muscles, muscles differ in structure
and activity.
Muscles differ in the arrangement of their myofilaments.
The principal typesof muscle are striated muscle, in which
the filaments are organized in transverse bands as in Figure
2; obliquely striated muscle, in which the filaments are
staggered, making the bands oblique (Figure 3); and smooth
muscle, in which the filaments are arranged irregularly.
In vertebrates, all voluntary muscles are striated and all
involuntary muscles are smooth, except for cardiac muscle,
which is involuntary but striated. Obliquely striated muscle
is found only in some invertebrate groups (the nematodes,
annelids, and mollusks) and has the protein paramyosin in
the thick filamentsas well as myosin.
Muscles
differ in the stimuli required to activate them. In vertebrates,
voluntary muscles require action potentials (electrical
signals) in their nerves to initiate every contraction.
Some involuntary muscles are spontaneously active, and the
action potentials in their nerves only modify the natural
rhythm of contraction. The leg muscles of all insects, and
the wing muscles of many, require action potentials to initiate
every contraction; however, the wing muscles of other insects
consist of fi
lar muscle, which requires only occasional
action potentials to maintain its rapid rhythmic contractions.
The wings of these insects are attached to the body in such
a way as to have a resonant frequency of vibration (like
a guitar string that vibrates, when plucked, at its resonant
frequency). When fi
lar muscles are active, they contract
so as to maintain the vibrations of the resonant system.
Muscles
differ in the ability to exert stress. Muscles that exert
large stresses have long, thick filaments that carry larger
numbers of cross bridges.The result is more cross bridges
than in other muscles. This means that more force can be
transmitted from each thick filament to the adjacent thin
filaments, and larger stresses can be exerted. Less stress
can be exerted when the fibres are shortening than when
they are maintaining constant length, and more when they
are being forcibly stretched.
Muscles
differ in the manner in which their forces are controlled.
Most of the fibres in the voluntary muscles of mammals can
only be switched on or off, and different degrees of force
are obtained by activating different numbers of fibres.
In many other muscles, however, the force exerted by each
fibre can be varied. In these muscles, force is not controlled
by activating different numbers of fibres but by changing
the intensity of muscle activation as a whole.
Muscles
differ in the ranges of length over which they can operate.
Smooth muscles generallywork over wider ranges of length
than striated ones, but there are a few exceptional striated
muscles. One such muscle in the tongue of chameleons can
shorten to one-sixth of its fully extended length.
Muscles
also differ in their speed of action, including the rates
at which they develop force andshorten. If a muscle shortens
by one-tenth of its length in one-tenth of a second, its
rate of shortening is one length per second. Maximum rates
of shortening vary between species and between muscle fibres
in a single animal. For example, two muscles in the limbs
of mice have maximum shortening speeds (at 37° C) of 24
and 13 lengths per second.
Finally,
muscles differ in their metabolism. The adenosine triphosphate
(ATP) that they use as their immediate energy source may
be produced either by oxidative reactions, in which food
is oxidized to carbon dioxide and water, or by processes
that do not require oxygen (anaerobic). Vertebrates and
crabs use the anaerobic process of glycolysis, converting
the carbohydrate glycogen to lactic acid, for short bursts
of vigorous activity such as sprinting. The burst of activity
is followed by a recovery period in which oxygen is used
to oxidize some of the lactic acid, releasing the energy
needed to convert the rest back to glycogen. The advantage
of using anaerobic metabolism in this way is that the intensity
of activity during the burst is not limited by the rate
at which the blood can bring oxygen to the muscles.
In
vertebrates, many muscle fibres perform only oxidative metabolism
or only glycolysis, though some perform both. Oxidative
fibres are commonly red, owing to the presence of the pigment
myoglobin. Most fishes show an obvious distinction between
the main bulk of white swimming muscle and a narrow strip
of red muscle along the side of the body. Slow swimming
is powered by the red (oxidative) muscle and bursts of fast
swimming by the white (glycolytic) muscle. Red and white
muscles are also easy to distinguish in the domestic chicken,
in which the pale meat of the breast consists mainly of
white fibres and the dark meat of the legs the red fibres.
The breast muscles are the main muscles of the wings, which
are used by chickens only for occasional short bursts of
flight. Other birds that practice sustained flight (e.g.,
hummingbirds) mainly have red breast muscles.
