270
Chapter 9
one ATP splits during each cross-bridge cycle, the rate of ATP
splitting determines the shortening velocity. Increasing the
load on a cross-bridge slows its forward movement during the
power stroke. This reduces the overall rate of ATP hydrolysis,
and thus decreases the velocity of shortening.
Frequency-Tension Relation
Because a single action potential in a skeletal muscle fi ber lasts
only 1 to 2 ms but the twitch may last for 100 ms, it is possible
for a second action potential to be initiated during the period
of mechanical activity.
Figure 9–19
illustrates the tension
generated during isometric contractions of a muscle fi ber in
response to multiple stimuli. The isometric twitch following
the fi rst stimulus, S
1
, lasts 150 ms. The second stimulus, S
2
,
applied to the muscle fi ber 200 ms after S
1
, when the fi ber has
completely relaxed, causes a second identical twitch. When a
stimulus is applied before a fi ber has completely relaxed from
a twitch, it induces a contractile response with a peak tension
greater than that produced in a single twitch (S
3
and S
4
). If
the interval between stimuli is reduced further, the resulting
peak tension is even greater (S
5
and S
6
). Indeed, the mechani-
cal response to S
6
is a smooth continuation of the mechanical
response already induced by S
5
.
The increase in muscle tension from successive action
potentials occurring during the phase of mechanical activ-
ity is known as
summation.
Do not confuse this with the
summation of neuronal postsynaptic potentials described in
Chapter 6. Postsynaptic potential summation involves additive
voltage effects on the membrane whereas here we are observing
the effect of additional attached
cross-bridges. A maintained
contraction in response to repetitive stimulation is known as a
tetanus
(tetanic contraction). At low stimulation frequencies,
the tension may oscillate as the muscle fi ber partially relaxes
between stimuli, producing an
unfused tetanus.
A
fused
tetanus,
with no oscillations, is produced at higher stimula-
tion frequencies (
Figure 9–20
).
As the frequency of action potentials increases, the level
of tension increases by summation until a maximal fused
tetanic tension is reached, beyond which tension no longer
increases even with further increases in stimulation frequency.
This maximal tetanic tension is about three to fi ve times greater
than the isometric twitch tension. Different muscle fi bers have
different contraction times, so the stimulus frequency that will
produce a maximal tetanic tension differs from fi ber to fi ber.
Why is tetanic tension so much greater than twitch
tension? We can explain summation of tension in part by
considering the relative timing of calcium availability and
cross-bridge binding. The isometric tension produced by a
muscle fi ber at any instant depends mainly on the total num-
ber of cross-bridges bound to actin and undergoing the power
stroke of the cross-bridge cycle. Recall that a single action
potential in a skeletal muscle fi ber briefl y releases enough cal-
cium to saturate troponin, and all the myosin-binding sites
on the thin fi
laments are therefore
initially
available. But the
binding of energized cross-bridges to these sites (step 1 of
the cross-bridge cycle) takes time, while the calcium released
into the cytosol begins to be pumped back into the sarcoplas-
mic reticulum almost immediately. Thus, after a single action
potential, the calcium concentration begins to fall and the
troponin/tropomyosin complex reblocks many binding sites
before cross-bridges have had time to attach to them.
In contrast, during a tetanic contraction, the successive
action potentials each release calcium from the sarcoplasmic
reticulum before all the calcium from the previous action poten-
tial has been pumped back into the reticulum. This results in a
persistent elevation of cytosolic calcium concentration, which
prevents a decline in the number of available binding sites on
the thin fi laments. Under these conditions, more binding sites
remain available, and many more cross-bridges become bound
to the thin fi
laments.
Other causes of the lower tension seen in a single twitch
are elastic structures, such as muscle tendons and the protein
titin, which delay the transmission of cross-bridge force to the
ends of a fi ber. Because a single twitch is so brief, cross-bridge
activity is already declining before force has been fully trans-
mitted through these structures. This is less of a factor dur-
ing tetanic stimulation because of the much longer duration
of cross-bridge activity and force generation.
Length-Tension Relation
The spring-like characteristic of the protein titin (see Figure
9–3), which is attached to the Z line at one end and the thick
fi laments at the other, is responsible for most of the
passive
elastic properties of relaxed muscles. With increased stretch,
the passive tension in a relaxed fi ber increases, not from active
cross-bridge movements but from elongation of the titin fi la-
ments. If the stretched fi
ber is released, it will return to an
equilibrium length, much like what occurs when releasing a
Tension
100
200
300
400
0
S
1
S
2
500
600
700
S
3
S
4
800
900
1000
S
5
S
6
Time (ms)
Figure 9–19
Summation of isometric contractions produced by shortening the time between stimuli.
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