Muscle 375fig. 12.4 ). When the motor axon is activated, all of the muscle
fibers it innervates contract.
So, how can we produce the smooth, sustained contractions
of complete tetanus in vivo? We do this by the asynchronous
activation of motor units. The muscle fibers of some motor
units start to twitch when those of previously activated motor
units begin to relax, producing a continuous contraction of
the whole muscle from the jerky contractions of the separate
motor unit twitches.Treppe
If the voltage of the electrical shocks delivered to an isolated
muscle in vitro is gradually increased from zero, the strength
of the muscle twitches will increase accordingly, up to a maxi-
mal value at which all of the muscle fibers are stimulated. This
demonstrates the graded nature of the muscle contraction. If a
series of electrical shocks at this maximal voltage is given to
a fresh muscle so that each shock produces a separate twitch,
each of the twitches evoked will be successively stronger, up to
a higher maximum. This demonstrates treppe, or the staircase
effect. Treppe may represent a warm-up effect, and is believed
to be due to an increase in intracellular Ca^2 1 , which is needed
for muscle contraction.Types of Muscle Contractions
In order for muscle fibers to shorten when they contract, they
must generate a force that is greater than the opposing forces
that act to prevent movement of the muscle’s insertion. When
you lift a weight by flexing your elbow joint, for example, the
force produced by contraction of your biceps brachii muscle
is greater than the force of gravity on the object being lifted.
The tension produced by the contraction of each muscle fiber
separately is insufficient to overcome the opposing force, but
the combined contractions of numerous muscle fibers may
be sufficient to overcome the opposing force and flex your
forearm. In this case, the muscle and all of its fibers shorten
in length.
This process can be seen by examining the force-velocity
curve. This graph shows the inverse relationship between the
force opposing muscle contraction (the load against which
the muscle must work) and the velocity of muscle shortening
( fig. 12.20 ). The tension produced by the shortening muscle
is just greater than the force (load) at each value, causing the
muscle to shorten. Under these controlled experimental con-
ditions, the contraction strength is constant at each load; this
muscle contraction during shortening is thus called an isotonic
contraction ( iso 5 same; tonic 5 strength).
If the load is zero, a muscle contracts and shortens with its
maximum velocity. As the load increases, the velocity of mus-
cle shortening decreases. When the force opposing contraction
(the load) becomes sufficiently great, the muscle is unable to
shorten when it exerts a given tension. That is, its velocity of
shortening is zero. At this point, where muscle tension does notparticipating in the contraction, skeletal muscles produce
graded contractions.
If the stimulator is set to deliver an increasing frequency
of electric shocks automatically, the relaxation time between
successive twitches will get shorter and shorter as the strength
of contraction increases in amplitude. This effect is known
as incomplete tetanus ( fig. 12.19 ). Finally, at a particular
“fusion frequency” of stimulation, there is no visible relax-
ation between successive twitches. Contraction is smooth and
sustained, as it is during normal muscle contraction in vivo.
This smooth, sustained contraction is called complete tetanus.
(The term tetanus should not be confused with the disease of
the same name, which is accompanied by a painful state of
muscle contracture, or tetany. )
If these procedures were performed on isolated muscle
fibers instead of whole muscles, similar behavior would be
observed. That is, the isolated muscle fiber would twitch to
a single shock and would show summation of twitches if the
shocks occurred quickly one after another. This is because it
only takes about 10 milliseconds for an action potential to be
conducted along the full length of a muscle fiber, whereas the
contraction can last as long as 100 milliseconds. Thus, if action
potentials are produced quickly in succession, Ca^2 1 will remain
in the cytoplasm attached to troponin and the cross-bridge
cycle will continue. In this case the muscle fiber can even be
made to hold a contraction, as in complete tetanus. However,
this behavior of the individual muscle fibers normally does not
occur in vivo. As previously described, a somatic motor axon
innervates a number of muscle fibers to form a motor unit (seeFigure 12.19 Incomplete and complete
tetanus. Stimuli in the form of electric shocks are given to a
muscle and the muscle twitches in response to these stimuli.
When the stimuli are given in rapid succession (for example, 5
to 10 shocks per second), the twitches summate to produce an
incomplete tetanus—a contraction that is sustained but “jerky.”
A faster frequency of stimulation (for example, at 60 shocks per
second) can produce a smooth, sustained contraction known as
complete tetanus. If this frequency of stimuation is maintained,
the muscle gradually loses its ability to maintain the contraction;
it fatigues.Incomplete tetanus Complete tetanusTwitches FatigueMuscle tension
5 shocks
per second60 shocks
per secondTetanus10 shocks
per second