M echanism s fo r controlling m uscle tw itch and te ta n u s • Figure 6.9
Motor unit
Neuromuscular
Spinal cord
Muscle fibers (cells)
Increase frequency
of nerve impulses
Increase the number
of motor units involved
Motor unit recruitment
Increased
muscle tension
(twitch
to
tetanus)
fj~ f f Z
L
, ! fl
f|
JLLJLILJIA U A A A A A iL
a.
Single tw itch,
b.
Wave
summation
c. Unfused tetanus
Time (msec)
d. Fused tetanus
Effects of summation on contraction:
a. Single twitch
b.
Wave
summation — when subsequent stimuli arrive before the first wave is finished, the tension is higher
c.
Unfused tetanus — continued wave summations add together, but each stimulus has a partial relaxation (20-30 impulses/s)
d. Fused tetanus — contraction force is steady and sustained (80-100 impulses/s)
As each subsequent stimulus arrives before the previous wave returns to normal, the levels of cytoplasmic Ca2+
accumulate and produce greater contractions up to a maximum when all motor units are recruited
A twitch lasts only a short time and does not produce
much tension. In contrast, tetanus is a longer, sustained
contraction that develops more force than a twitch. There
are two ways to increase the amount of tension and transi-
tion from a twitch to tetanus (Figure 6.9): •
Temporal summation (or wave summation)
increases the frequency of nerve impulses
(that is, the number of impulses per second).
Motor unit
recruitment increases the number
of motor units stimulated at one time.
Most muscles use both methods to control and main-
tain sustained contractions. Sustained contractions are
also maintained by asynchronous (unsynchronized) stimu-
lation of various motor units.
Muscle contraction can be isotonic or isometric. In iso-
tonic contractions, muscle length changes. For example,
164 CHAPTER 6
The Muscular System
when you move your forearm toward your arm, the bi-
ceps muscle shortens, moving the forearm. In contrast,
isometric contraction results in a change of the ten-
sion that is generated but does not cause motion. For ex-
ample, when you stand, your back muscles must contract
continually to maintain your upright posture.
Even when a whole muscle is not con-
tracting, a small number of its motor units
are involuntarily activated to produce a sus-
tained contraction of their muscle fibers.
This process, which results in
muscle tone
,
occurs
in
both
skeletal
and
smooth
muscle.
For
example,
the smooth muscles in your ar-
teries
and
veins
always
have
some degree of tone to maintain
your blood pressure.
motor unit
A motor
neuron together with
the muscle fibers it
stimulates.
muscle tone
A
state of continued
partial contraction.
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