Biomedical Engineering Reference
In-Depth Information
swivels toward the center of the sarcomere like
the oars of a boat. This action draws the thin fila-
ments past the thick filaments toward the H-zone.
As the myosin heads swivel, they release ADP.
Once the power stoke is complete, ATP again
combines with the ATP-binding sites on the
myosin cross-bridges. As ATP binds, the myosin
head detaches from actin. Again, ATP is split,
imparting its energy to the myosin head, which
returns to its original upright position. It is then
ready to combine with another myosin-binding
site further along the thin filament. The cycle
repeats over and over.
The myosin cross-bridges keep moving back
and forth like the cogs of a ratchet, with each
power stroke moving the thin filaments toward
the H-zone. At any instant, about half of the
myosin cross-bridges are bound to actin and are
swiveling. The other half are detached and
preparing to swivel again.
Contraction is analogous to running on a
nonmotorized treadmill. One foot (myosin
head) strikes the belt (thin filament) and pushes
it backward (toward the H-zone). Then the other
foot comes down and imparts a second push.
The belt soon moves smoothly while the runner
(thin filament) remains stationary. And, like the
legs of the runner, the myosin heads need a
constant supply of energy to keep going. The
power stoke repeats as long as ATP is available
and the Ca
2
+
level near the thin filament is high.
This continual movement applies the force
draws the Z-discs toward each other and the
sarcomere shortens. The myofibrils thus con-
tract and the whole muscle fiber shortens. Dur-
ing a maximal muscle contraction, the distance
between Z-discs can decrease to half the resting
length. However, the power stroke does not
always result in shortening of the muscle fibers
and the whole muscle. Isometric contraction or
contraction without shortening occurs when the
cross-bridges generate force but the filaments do
not slide past one another.
Sustained small contractions give firmness to
a relaxed skeletal muscle, known as
muscle tone
.
At any instant, a few muscle fibers are con-
tracted while most are relaxed. This small
amount of contraction firms up a muscle with-
out producing movement and is essential for
maintaining posture. Asynchronous firing of
motor units allows muscle tone to be sustained
continuously.
A single action potential in a motor neuron
elicits a single contraction in all the muscle fibers
of its motor unit. The contraction is said to be
all-or-none,
because individual muscle fibers will
contract to their fullest extent. In other words,
muscle fibers do not partially contract. The force
of their contraction can vary only slightly,
depending on local chemical conditions and
whether or not a motor unit has just contracted
previously.
In addition, other internal conditions in the
muscle, such as temperature, pH, and viscosity
change. A rise in temperature, for example,
could provoke stronger contractions.
6.1.3 Electromyography
The electrical signal associated with the contrac-
tion of a muscle is called an
electromyogram
or
EMG.
Electromyography
, which is the study of
EMG, has revealed some basic information. Vol-
untary muscular activity results in an EMG that
increases in magnitude with tension. However,
other variables influencing the signal at any
given time are velocity of shortening or length-
ening of the muscle, rate of tension buildup,
fatigue, and reflex activity.
Muscle tissue conducts electrical potentials
somewhat similarly to axons of the nervous sys-
tem.
Motor unit action potential
(m.u.a.p.) is an
electrical signal generated in the muscle fibers
because of the recruitment of fibers as the motor
unit. Electrodes placed on the surface of a muscle
or inside the muscle tissue will record the alge-
braic sum of all m.u.a.p.'s being transmitted
along the muscle fibers at that point in time.
Those motor units away from the electrode site
