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Twitch
and tetanus responses
Skeletal
muscles respond to a single electric shock of sufficient
magnitude by rapid, intense contractions called phasic contractions.
If the ends of a frog sartorius muscle (at 0° C) are fixedto
prevent shortening, the tension increases for about 200
milliseconds and then begins to decrease, at first rather
rapidly and then more slowly. More happens during this mechanical
response to a single stimulation, called a twitch, than
the tension record suggests.
The
mechanical response to repeated stimulation depends on the
rate of the stimulation. Muscle, like other excitable tissues,
has a period following its action potential during which
themembrane will not respond to stimulation regardless of
the strength. This absolute refractory period in the frog
sartorius at 0° C lasts about 10 milliseconds after stimulation.
Therefore, a second pulse coming within that time span will
not elicit any response. If, however, the pulses are 300
milliseconds apart, the muscle will be relaxing when the
second pulse is given, and the tension will appear in waves
in phase with the stimulation, causing an unfused tetanus.
It is possible to stimulate the muscle at a frequency between
these extremes so that the tension developed by the muscle
remains constant. This latter type of contraction is called
a fused tetanus, and the rate of stimulation that produces
it is called the fusion frequency. The exact rate depends
upon the particular muscle and the temperature.
Usually,
the maximum tetanus tension is from 1.2 to 1.8 times greater
than the maximum tension during a twitch. Within the muscle
many elastic structures, connected in series with thecontractile
elements, are stretched during contraction. The attachment
of the muscle fibres to the tendons at the end of the muscle
and the attachment of the thin filaments to the Z line contribute
to this elastic component. In single fibres, however, most
of the elasticity of the series elastic elements is contributed
by the actin-myosin cross bridges themselves. Full maximum
tension is not apparent at the end of the muscle until the
contractile elements have shortened enough to stretch the
elastic elements—somewhat like taking up the slack in rope
before a pull on one end can be felt at the other end. In
a twitch, the activity of the muscle is so brief that the
contractile elements cannot extend the elastic elements
completely before relaxation begins; as a result, the tension
at the ends of the muscle does not reach the maximum possible
level. During a tetanus, on the other hand, the activity
of the contractile elements is maintained, and they can
eventually shorten enough to extend fully the series elastic
elements. When this has been accomplished, the maximum tension
is apparent at the ends of the muscle.
