Muscle
is composed of many long, cylindrical-shaped fibres from
0.02 to 0.08 millimetre in diameter. In some muscles the
fibres run the entire length of the muscle (parallel fibres),
up toseveral tens of centimetres long. In others a tendon
extends along each edge, and the fibres rundiagonally across
the muscle between the tendons (pennate fibres). Considerable
variation can be found among the different skeletal muscles,
the actual arrangement of the fibres depending on the function
of the muscle.
There
is a high degree of organization within the living fibre,
a series of alternately dark and bright bands. Each band
extends perpendicular to the length of the fibre. Each fibre
is surrounded by a complex, multilayered structure called
the sarcolemma. The outermost layer is a fine network of
fi
s, which, at the ends of the muscle, extend into the
tendons and form the structural link with them. The next
layer of the sarcolemma is a foundation, or basement, membrane.
The innermost layer is a plasma membrane similar to the
ones that surround most cells. The plasma membrane consists
of a lipid bilayer with proteins embedded in it. Some of
theproteins are embedded entirely within the lipid layer;
others extend to one or the other surface; still others
span the whole width of the two layers. These proteins represent
enzymes, receptors, and various channels (such as those
involved in the movement of ions between the exterior and
interior of the cell). The plasma membrane maintains the
electrical potential, whichplays a major role in stimulating
muscle contraction.
Sarcoplasm
is the cytoplasm of a muscle fibre. It is a water solution
containing ATP and phosphagens, as well as the enzymes and
intermediate and product molecules involved in many metabolic
reactions. The most abundant metal in the sarcoplasm is
potassium. Sodium and magnesium are present in lower concentrations.
Most of the calcium of muscle is bound to proteins or stored
in the sarcoplasmic reticulum. Contraction is initiated
by the release of calcium ions (Ca2+) upon the depolarization
of the membrane, which is induced by nerve impulses.
Each
striated muscle cell, or fibre, contains many nuclei. This
is the result of the fusion of singly nucleated cells that
occurs during the embryological development of striated
muscle. After fusion, the cells never again divide.
Mitochondria
in the sarcoplasm of the muscle fibre contain the enzymes
involved in the Krebs cycle and in oxidative phosphorylation.
Granules in the sarcoplasm of muscle cells contain glycogen,
the storage form of carbohydrate. The breakdown of glycogen
and the metabolism of the individual units of the resulting
carbohydrate through glycolysis, the Krebs cycle, and oxidative
phosphorylation are important sources of ATP, the immediate
source of energy for muscle contraction. Muscles that contain
many fibres that operate at a steady, low level of activity
are red, owing to the presence of cytochromes (molecules
involved in oxidative phosphorylation) and myoglobin (an
oxygen-carrying molecule in the sarcoplasm). Muscles that
work in bursts of activity contain fibres that have fewer
mitochondria and fewer molecules of cytochromes or myoglobin,
are white, and depend more heavily on reactions that do
not require oxygen to make ATP.
