296
UNIT 2
Covering, Support, and Movement of the Body
9
Increasing the stimulus intensity beyond the maximal stimu-
lus does not produce a stronger contraction. In the body, the
same phenomenon is caused by neural activation of an increas-
ingly large number of motor units serving the muscle.
Te recruitment process is not random. Instead it is dictated
by the
size principle
(Figure 9.17)
. In any muscle:
Te motor units with the smallest muscle fibers are activated
first because they are controlled by the smallest, most highly
excitable motor neurons.
As motor units with larger and larger muscle fibers begin to
be excited, contractile strength increases.
Te largest motor units, containing large, coarse muscle fi-
bers, have as much as 50 times the contractile force of the
smallest ones. Tey are controlled by the largest, least ex-
citable (highest-threshold) neurons and are activated only
when the most powerful contraction is necessary.
Why is the size principle important? It allows the increases
in force during weak contractions (for example, those that
maintain posture or slow movements) to occur in small steps,
whereas gradations in muscle force are progressively greater
when large amounts of force are needed for vigorous activities
such as jumping or running. Te size principle explains how the
same hand that lightly pats your cheek can deliver a stinging
slap at the volleyball during a match.
Although
all
the motor units of a muscle may be recruited
simultaneously to produce an exceptionally strong contraction,
motor units are more commonly activated asynchronously. At
a given instant, some are in tetanus (usually unfused tetanus)
while others are resting and recovering. Tis technique helps
prolong a strong contraction by preventing or delaying fatigue.
It also explains how weak contractions promoted by infrequent
stimuli can remain smooth.
Muscle Response to Changes in Stimulus Strength
Wave
summation contributes to contractile force, but its primary
function is to produce smooth, continuous muscle contractions
by rapidly stimulating a specific number of muscle cells.
Re-
cruitment
, also called
multiple motor unit summation
, con-
trols the force of contraction more precisely. In the laboratory,
recruitment is achieved by delivering shocks of increasing volt-
age to the muscle, calling more and more muscle fibers into play.
Stimuli that produce no observable contractions are
sub-
threshold stimuli
.
Te stimulus at which the first observable contraction occurs
is called the
threshold stimulus
(Figure 9.16)
. Beyond this
point, the muscle contracts more and more vigorously as the
stimulus strength increases.
Te
maximal stimulus
is the strongest stimulus that in-
creases contractile force. It represents the point at which all
the muscle’s motor units are recruited.
Stimulus strength
Proportion of motor units excited
Strength of muscle contraction
Time (ms)
Maximal contraction
Maximal
stimulus
Threshold stimulus
Stimulus voltage
Tension
Stimuli to nerve
1
2
3
4
5
6
7
8
9
10
Figure 9.16
Relationship between stimulus intensity (graph
at top) and muscle tension (tracing below).
Below threshold
voltage, the tracing shows no muscle response (stimuli 1 and 2).
Once threshold (3) is reached, increases in voltage excite (recruit)
more and more motor units until the maximal stimulus is reached
(7). Further increases in stimulus voltage produce no further increase
in contractile strength.
Tension
Motor
unit 1
recruited
(small
fibers)
Motor
unit 2
recruited
(medium
fibers)
Motor
unit 3
recruited
(large
fibers)
Time
Skeletal
muscle
fibers
Figure 9.17
The size principle of recruitment.
Recruitment
of motor neurons controlling skeletal muscle fibers is orderly and
follows the size principle.
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