Biomedical Engineering Reference
In-Depth Information
applied to the muscle after the initial stimulus, other responses may be noted. For
example if two stimuli are applied one immediately after the other, the muscle will
respond to the first stimulus but not to the second. When a muscle fiber receives
enough stimulation to contract, it temporarily loses its excitability and cannot
respond again until its responsiveness is regained.
This period of lost excitability is called the refractory period and is a character-
istic of nerves and muscle cells. The duration of the refractory period varies with
the muscle involved. Skeletal muscle has a short refractory period of about 5 msec.
Cardiac muscle has a long refractory period of about 300 msec.
When two stimuli are applied and the second is delayed until the refractory
period is over, the skeletal muscle will respond to both stimuli. In fact, if the second
stimulus is applied after the refractory period but before the muscle fiber has finished
relaxing, the second contraction will be stronger than the first. This phenomenon in
which stimuli arrive at different times and cause larger contractions is called wave
summation or temporal summation.
If a muscle is stimulated at a rate of 20 to 30 times per second, it can only partly
relax between stimuli. The result is a sustained contraction called incomplete
(unfused) tetanus. Stimulation at an increased rate (80-100 stimuli per second)
results in complete (fused) tetanus, a sustained contraction that lacks even partial
relaxation between stimuli. Both kinds of tetanus result from the addition of Ca
2+
released from the SR by the second and subsequent stimuli to the Ca
still in the
sarcoplasm from the first stimulus. Relaxation is partial or does not occur at all.
Most voluntary muscular contractions involve short-term tetanic contractions and
are thus smooth, sustained contractions.
When a muscle has been relaxed for some time and then is stimulated to contract
by several identical stimuli that are too far apart for wave summation to occur, each
of the first few contractions is a little stronger than the last. This phenomenon is
known as the staircase effect or treppe. After the first few contractions, the muscle
reaches its peak of performance and can undergo its strongest contractions. The
explanation for the staircase effect may be the same as for tetanus, which is a
progressive buildup of Ca
2+
in the sarcoplasm. Successive stimuli cause calcium ions
to flow out of the SR faster than the active transport pumps take them back in. Up
to a certain point, as Ca
2+
builds up and binds to troponin, more power strokes can
occur and filament sliding intensifies.
In addition, other internal conditions in the muscle such as temperature, pH, and
viscosity have changed. A rise in temperature, for example, could provoke stronger
contractions. One advantage of warming up for athletes may be taking advantage of
the staircase effect to improve performance.
A skeletal muscle fiber contracts when myosin cross-bridges of thick filaments
connect with actin on the thin filaments. A muscle fiber develops its greatest tension
when there is optimal overlap between thick and thin filaments (fig. A.8). At the
optimum sarcomere length, the number of myosin cross-bridges making contact with
thin filaments brings about a maximal force of contraction. If the sarcomeres of a
muscle fiber are stretched to a longer length, fewer myosin cross-bridges can make
contact with thin filaments and the force of contraction decreases. If a skeletal muscle
2+
