Typically, many action potentials are triggered in a
motor neuron, and so individual twitches do not normally
occur. Tetanic contractions of muscle F
bers are common. On
the whole-muscle level, contractions are smooth rather than
irregular or jerky because the spinal cord stimulates contrac-
tions in different sets of motor units at different moments.
Tetanic contractions occur frequently in skeletal mus-
cles during everyday activities, often in only a portion of a
muscle. ±or example, when a person lifts a weight or walks,
sustained contractions are maintained in the upper limb or
lower limb muscles for varying lengths of time. These con-
tractions are responses to a rapid series of stimuli transmit-
ted from the brain and spinal cord on motor neurons.
Even when a muscle appears to be at rest, its fibers
undergo a certain degree of sustained contraction. This is
muscle tone
(tonus), and it is a response to nerve
impulses originating repeatedly in the spinal cord and travel-
ing to a few muscle F
bers. The result is a continuous state of
partial contraction.
Muscle tone is particularly important in maintaining pos-
ture. Tautness in the muscles of the neck, trunk, and lower
limbs enables a person to hold the head upright, stand, or
sit. If tone is suddenly lost, such as when a person loses con-
sciousness, the body collapses. Muscle tone is maintained in
health but is lost if motor nerve axons are cut or if diseases
interfere with conduction of nerve impulses.
When skeletal muscles contract forcefully, they may generate up
to 50 pounds of pull for each square inch of muscle cross section.
Consequently, large muscles such as those in the thigh can pull with
several hundred pounds of force. Occasionally, this force is so great that
the tendons of muscles tear away from their attachments to the bones.
Types of Contractions
Sometimes muscles shorten when they contract. ±or exam-
ple, if a person lifts an object, the muscles remain taut, their
attached ends pull closer together, and the object is moved.
This type of contraction is termed
(equal force—
change in length), and because shortening occurs, it is called
Another type of isotonic contraction, called a lengthen-
ing or an
eccentric contraction,
occurs when the force a
muscle generates is less than that required to move or lift
an object, as in laying a book down on a table. Even in such
a contraction, cross-bridges are working but not generating
enough force to shorten the muscle.
At other times, a skeletal muscle contracts, but the parts to
which it is attached do not move. This happens, for instance,
when a person pushes against a wall or holds a yoga pose but
does not move. Tension within the muscles increases, but the
wall does not move, and the muscles remain the same length.
Contractions of this type are called
(equal length—
change in force). Isometric contractions occur continuously
fibers per motor unit and can produce very slight move-
ments. Conversely, the motor units of the large muscles
in the back may include a hundred or more muscle F bers.
When these motor units are stimulated, the movements that
result are less gradual compared to those of the eye.
Anatomically, the muscle F
bers of a muscle are orga-
nized into motor units, each of which is controlled by a sin-
gle motor neuron. Each motor unit is also a functional unit,
because a nerve impulse in its motor neuron will contract
all of the F bers in that motor unit at the same time. A whole
muscle is composed of many such motor units controlled by
different motor neurons, which respond to different thresh-
olds of stimulation. If only the more easily stimulated motor
neurons are involved, few motor units contract. At higher
intensities of stimulation, other motor neurons respond,
and more motor units are activated. Such an increase in the
number of activated motor units is called
multiple motor unit
ment). As the inten-
sity of stimulation increases, recruitment of motor units con-
tinues until F
nally all possible motor units are activated in
that muscle.
Sustained Contractions
During sustained contractions, smaller motor units, which
have smaller diameter axons, are recruited earlier. The larger
motor units, which include larger diameter axons, respond
later and more forcefully. The result is a sustained contrac-
tion of increasing strength.
Force of
Force of
Force of
Myograms of (
) a series of twitches, (
) summation,
and (
) a tetanic contraction. Stimulation frequency increases from one
myogram to the next.
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