Introduction to Skeletal Muscles
Skeletal muscle is the type of muscle
that we can see and feel. When a body builder works
out to increase muscle mass, skeletal muscle is what
is being exercised. Skeletal muscles attach to the
skeleton and come in pairs -- one muscle to move the
bone in one direction and another to move it back
the other way. These muscles usually contract voluntarily,
meaning that you think about contracting them and
your nervous system tells them to do so. They can
do a short, single contraction (twitch) or a long,
sustained contraction (tetanus).
Parts of a Skeletal Muscle
The basic action of any muscle is contraction.
For example, when you think about moving your arm
using your biceps muscle, your brain sends a signal
down a nerve cell telling your biceps muscle to contract.
The amount of force that the muscle creates varies
-- the muscle can contract a little or a lot depending
on the signal that the nerve sends. All that any muscle
can do is create contraction force.
A muscle is a bundle of many cells called
fibers. You can think of muscle fibers as long cylinders,
and compared to other cells in your body, muscle fibers
are quite big. They are from about 1 to 40 microns
long and 10 to 100 microns in diameter. For comparison,
a strand of hair is about 100 microns in diameter,
and a typical cell in your body is about 10 microns
in diameter.
A muscle fiber contains many myofibrils,
which are cylinders of muscle proteins. These proteins
allow a muscle cell to contract. Myofibrils contain
two types of filaments that run along the long axis
of the fiber, and these filaments are arranged in
hexagonal patterns. There are thick and thin filaments.
Each thick filament is surrounded by six thin filaments.
Thick and thin filaments are attached
to another structure called the Z-disk or Z-line,
which runs perpendicular to the long axis of the fiber
(the myofibril that runs from one Z-line to another
is called a sarcomere). Running vertically down the
Z-line is a small tube called the transverse or T-tubule,
which is actually part of the cell membrane that extends
deep inside the fiber. Inside the fiber, stretching
along the long axis between T-tubules, is a membrane
system called the sarcoplasmic reticulum, which stores
and releases the calcium ions that trigger muscle
contraction.
Energy for Muscle Contraction
Muscles use energy in the form of ATP.
The energy from ATP is used to reset the myosin crossbridge
head and release the actin filament. To make ATP,
the muscle does the following:
1 Breaks down creatine phosphate, adding
the phosphate to ADP to create ATP
2 Carries out anaerobic respiration,
by which glucose is broken down to lactic acid and
ATP is formed
3 Carries
out aerobic respiration, by which glucose, glycogen,
fats and amino acids are broken down in the presence
of oxygen to produce ATP (see How Exercise Works for
details).
Muscles have a mixture of two basic types of fibers:
fast twitch and slow twitch. Fast-twitch fibers are
capable of developing greater forces, contracting faster
and have greater anaerobic capacity. In contrast, slow-twitch
fibers develop force slowly, can maintain contractions
longer and have higher aerobic capacity. Training can
increase muscle mass, probably by changing the size
and number of muscle fibers rather than the types of
fibers.
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