Chapter 9
Muscles and Muscle Tissue
295
9
Finally, as the stimulation frequency continues to increase,
muscle tension increases until it reaches maximal tension. At
this point all evidence of muscle relaxation disappears and the
contractions fuse into a smooth, sustained contraction plateau
called
fused
or
complete
tetanus
(tet
9
ah-nus;
tetan
5
rigid,
tense) (Figure 9.15c). (Note that this term is o±en confused
with the bacterial disease called tetanus that causes severe invol-
untary contractions.) In the real world, fused tetanus happens
infrequently, for example, when someone shows superhuman
strength by li±ing a fallen tree limb off a companion.
Vigorous muscle activity cannot continue indefinitely. Pro-
longed tetanus inevitably leads to muscle fatigue, a situation in
which the muscle cannot contract and its tension drops to zero.
Latent
period
Latent period
Single
stimulus
Extraocular muscle (lateral rectus)
Gastrocnemius
Soleus
0
40
20
60
80
100
120
140
Time (ms)
(a) Myogram showing the three phases of an isometric twitch
Period of
contraction
Period of
relaxation
Percentage of
maximum tension
Single
stimulus
0
40
80
120
160
200
Time (ms)
(b) Comparison of the relative duration of twitch responses of
three muscles
Percentage of
maximum tension
Figure 9.14
The muscle twitch.
Tension
Time (ms)
(a) A single stimulus is delivered. The muscle contracts and
relaxes.
100
200
Contraction
Relaxation
Maximal tension of a single twitch
Stimulus
Stimuli
Partial relaxation
Stimuli
300
Tension
Time (ms)
(b) If another stimulus is applied before the muscle relaxes
completely, then more tension results. This is wave (or
temporal) summation and results in unfused (or incomplete)
tetanus.
100
200
300
Tension
Time (ms)
(c) At higher stimulus frequencies, there is no relaxation at all
between stimuli. This is fused (complete) tetanus.
100
200
300
Single stimulus
single twitch
Low stimulation frequency
unfused (incomplete) tetanus
High stimulation frequency
fused (complete) tetanus
0
0
0
Figure 9.15
A muscle’s response to changes in stimulation
frequency.
(Note that tension is measured in grams.)
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