294
UNIT 2
Covering, Support, and Movement of the Body
9
Muscle Response to Changes in Stimulus Frequency
Te
nervous system achieves greater muscular force by increasing
the firing rate of motor neurons. For example, if two identical
stimuli (electrical shocks or nerve impulses) are delivered to a
muscle in rapid succession, the second twitch will be stronger
than the first. On a myogram the second twitch will appear to
ride on the shoulders of the first
(Figure 9.15a, b)
.
Tis phenomenon, called
wave
or
temporal summation
, oc-
curs because the second contraction occurs before the muscle
has completely relaxed. Because the muscle is already partially
contracted and more calcium is being squirted into the cytosol
to replace that being reclaimed by the SR, muscle tension pro-
duced during the second contraction causes more shortening
than the first. In other words, the contractions are added to-
gether. (However, the refractory period is always honored. Tus,
if a second stimulus arrives before repolarization is complete,
no wave summation occurs.)
If the stimulus strength is held constant and the muscle is
stimulated at an increasingly faster rate: (1) the relaxation time
between twitches becomes shorter and shorter, (2) the concen-
tration of Ca
2
1
in the cytosol rises higher and higher, and (3) the
degree of wave summation becomes greater and greater, pro-
gressing to a sustained but quivering contraction referred to as
unfused
or
incomplete tetanus
(Figure 9.15b).
the tracing returns to the baseline. If the muscle shortened
during contraction, it now returns to its initial length. No-
tice that a muscle contracts faster than it relaxes, as re-
vealed by the asymmetric nature of the myogram tracing.
As you can see in Figure 9.14b, twitch contractions of some
muscles are rapid and brief, as with the muscles controlling
eye movements. In contrast, the fibers of fleshy calf muscles
(gastrocnemius and soleus) contract more slowly and remain
contracted for much longer periods. Tese differences between
muscles reflect variations in enzymes and metabolic properties
of the myofibrils.
Graded Muscle Responses
Muscle twitches
—like those single, jerky contractions provoked
in a laboratory—may result from certain neuromuscular prob-
lems, but this is
not
the way our muscles normally operate. In-
stead, healthy muscle contractions are relatively smooth and
vary in strength as different demands are placed on them. Tese
variations, needed for proper control of skeletal movement, are
referred to as
graded muscle responses
.
In general, muscle contraction can be graded in two ways:
by changing the frequency of stimulation, and by changing the
strength of stimulation.
Spinal cord
Motor neuron
cell body
Muscle
Branching axon
to motor unit
Nerve
Motor
unit 1
Motor
unit 2
Muscle
fibers
Motor neuron
axon
Axon terminals at
neuromuscular junctions
(a) Axons of motor neurons extend from the spinal cord to the muscle. There each axon divides into
a number of axon terminals that form neuromuscular junctions with muscle fibers scattered
throughout the muscle.
(b) Branching axon terminals form
neuromuscular junctions, one
per muscle fiber (photomicro-
graph 330
m
).
Figure 9.13
A motor unit consists of one motor neuron and all the muscle fibers it
innervates.
(For a related image, see
A Brief Atlas of the Human Body
, Plate 30.)
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