Biomedical Engineering Reference
In-Depth Information
The inability of a muscle to maintain its strength of contraction or tension is called
muscle fatigue. It occurs when a muscle cannot produce enough ATP to meet its
needs. Several factors appear to contribute to muscle fatigue, including insufficient
oxygen, depletion of glycogen, buildup of lactic acid, failure of action potentials in
the motor neuron to release enough ACh, and unexplained fatigue mechanisms in
the central nervous system.
A single action potential in a motor neuron elicits a single contraction in all the
muscle fibers of its motor unit. The contraction is said to be “all or none” because
individual muscle fibers will contract to their fullest extent. In other words, muscle
fibers do not partially contract. The force of their contraction can vary only slightly,
depending on local chemical conditions and whether or not the motor unit has just
contracted.
However, we know that muscle as a whole can have graded contractions to
perform different tasks. The amount of tension that a skeletal muscle can develop
depends on the frequency of stimulation of muscle fibers by motor neurons, the
length of muscle fibers just before they contract, the number of muscle fibers
contracting (number of motor units recruited and size of individual motor units),
and structural components of the muscle.
A twitch contraction is a brief contraction of all the muscle fibers in a motor
unit in response to a single action potential in its motor neuron. In the laboratory, a
twitch also can be produced by direct electrical stimulation of a motor neuron or its
muscle fibers.
Much of our knowledge of muscle contraction comes from experiments per-
formed on isolated, excised muscle. The record of muscle contraction is called a
myogram (fig. A.7). A latent period is a brief period between application of the
stimulus and the beginning of the contraction. During this time, Ca
is being released
from the SR; the filaments start to exert force (usually taking some slack out of the
system) and, finally, shortening starts. The latent period lasts about 2 msec. The
second phase, the contraction period, lasts from 10 to 100 msec.
The third phase, the relaxation period, also lasts about 10-100 msec. It is caused
by the active transport of the Ca
2+
back into the SR, which results in relaxation. The
duration of these periods varies with the muscle involved. If additional stimuli are
2+
Contraction
period
Relaxation
period
Latent
period
0
10
30
Time in msec
20
40
50
FIGURE A.7
Myogram of twitch contraction.
