297
CHAPTER NINE
Muscular System
simultaneously. To record how a whole muscle responds to
stimulation, a skeletal muscle can be removed from a frog or
other small animal and mounted on a special device. The mus-
cle is then electrically stimulated, and when it contracts, it pulls
on a lever. The lever’s movement is recorded as a myogram.
The myogram results from the combined twitches of muscle
F bers taking part in the contraction, so it looks essentially the
same as the twitch contraction depicted in F gure 9.15.
Sustained contractions of whole muscles enable us to
perform everyday activities, but the force generated by those
contractions must be controlled. ±or example, holding a styro-
foam cup of coffee F rmly enough that it does not slip through
our F ngers, but not so forcefully as to crush it, requires pre-
cise control of contractile force. In the whole muscle, the force
developed refl ects (1) the frequency at which individual mus-
cle F bers are stimulated and (2) how many F bers take part in
the overall contraction of the muscle.
Summation
The force that a muscle F ber can generate is not limited to
the maximum force of a single twitch
(f g. 9.17
a
)
. A muscle
F
ber exposed to a series of stimuli of increasing frequency
reaches a point when it is unable to completely relax before
the next stimulus in the series arrives. When this happens,
the individual twitches begin to combine, and the contrac-
tion becomes sustained. In such a
sustained contraction,
the
force of individual twitches combines by the process of
sum-
mation
(F
g. 9.17
b
). When the resulting forceful, sustained
contraction lacks even partial relaxation, it is called a
tetanic
(te
˘-tan
ik)
contraction
(tetanus) (F
g. 9.17
c
).
Recruitment of Motor Units
The number of muscle F bers in a motor unit varies consider-
ably. The fewer muscle F bers in the motor units, however,
the more precise the movements that can be produced in a
particular muscle. ±or example, the motor units of the mus-
cles that move the eyes may include fewer than ten muscle
contract at their optimal lengths. Some activities, such as
walking up stairs two at a time or lifting something from an
awkward position, put F
bers at a disadvantageous length
and compromise muscle performance.
A muscle f
ber brought to threshold under a given set oF conditions
contracts completely, and each twitch generates equal Force. This
is an
all-or-none
response. However, “all-or-none” is misleading,
because in normal use oF muscles, the Force generated by muscle
f
bers and by whole muscles must vary.
Understanding the contraction of individual muscle
F bers is important for understanding how muscles work, but
such contractions by themselves are of little signiF cance in
day-to-day activities. Rather, the actions we need to perform
usually require the contribution of multiple muscle F
bers
(b)
Overly shortened
(c)
Overly stretched
(a)
Optimal length
Muscle fiber length
Force
Force of contraction
Time
Time of
stimulation
Period of
contraction
Period of
relaxation
Latent
period
FIGURE 9.15
A myogram oF a single muscle twitch.
FIGURE 9.16
The Force a muscle f
ber
can generate depends on the length to
which it is stretched when stimulated.
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