Ah ha, found this
A twitch is a muscle contraction that occurs in
response to a single, rapid stimulus that evokes a
single, isolated action potential in a muscle fiber.
Although single, isolated twitches are not in and
of themselves very useful for generating
controlled, coordinated movements needed for
maintaining homeostasis, observations of twitch
contractions present invaluable insights into the
basic physiology by which muscle fibers
generate tension.
Because the action potential is an “all or
none” response, the contraction of a muscle fiber
in response to a single action potential is
likewise an all or none response. Therefore,
there is a minimum stimulus strength that must
be applied to the muscle fiber in order to reach
threshold, evoke the action potential and, in turn,
induce the contraction.
Once the action
potential occurs, though, no further increase in
stimulus strength will increase the strength of
contraction, as the Ca
2+
gates in the sarcoplasmic
reticulum are open for a fixed amount of time
once opened.
Individual muscle fibers respond to isolated
stimuli in an all or none fashion. However, a
muscle organ, such as the gastrocnemius muscle,
is composed of many individual muscle fibers.
By varying the number
motor units
(groups of
muscle fibers innervated by a singe somatic
motor neuron) contracting at a given time, the
amount of tension gene
rated by the whole
muscle can vary. In one of the experiments we
are performing today, you will note that the
strength of the contraction varies with the
strength of the stimulus applied (Fig 9.7). This
does not violate the all or none principle.
Rather, as stimulus strength is being increased,
progressively more muscle fibers reach their
thresholds and contract. Thus, the change in
tension is due to the number of contracting
muscle fibers,
not a change in how much tension
the individual fibers are generating.
Most sustained
contractions are generated by a combination of
twitches and partial-tetanic contractions by
different motor units whose motor neurons are
stimulating the fibers at different intervals and at
different frequencies.
Interestingly, the amount of tension
generated during a tetani
c contraction is often
substantially higher than that of a maximal
twitch. There are several reasons for this. First,
when a muscle begins to contract, some of the
tension generated by the muscle is absorbed by
stretching elastic elements within the muscle’s
attachments.
This can reduce the total tension
generated on the attachments in a twitch
contraction whereas tetany, these elastic
elements are fully stretched and more tension is
exerted directly on the attachments.
So, inside the fiber, every twitch is full force, but the force transmitted to the end of the fiber, (the connective tissue) can vary with twitch speed (rate coding) due to elasticity soaking up some of the force so to speak. So WHERE are the points of elasticitiy?
Apparantly the crossbridge force is fixed, but what about the ends of the sarcomeres?
Where the fibrils attach?
Is there a 'shock absorber' attribute in those areas?
If so, I have to rethink all of this!
Bryan, where are you? !!!!!!!!!