the frequency of motor neuron
stimulation a single action potential
from a motor neuron will produce a
single contraction in muscle fibers of
its motor unit this isolated contraction
is called a twitch the tension produced
by a single twitch can be measured by a
Maia Graham which is an instrument that
measures the amount of tension produced
over time each twitch undergoes three
phases a latent period a contraction
phase and a relaxation phase during the
latent period the action potential is
being propagated along the sarcolemma
and calcium ions are released from the
sarcoplasmic reticulum
however contraction has not yet occurred
in the contraction phase calcium ions in
the sarcoplasm have bound to troponin
tropomyosin has shifted away from active
binding sites and crossbridges have
formed here the circum years are
actively shortening to the point of peak
tension in the relaxation phase tension
decreases as contraction stops calcium
ions are pumped out of the sarcoplasm
back into the sarcoplasmic reticulum and
crossbridge cycling stops this returns
the muscle fiber to its resting state
although a person can experience a
muscle twitch a single twitch does not
produce any significant muscle activity
in a living body a series of action
potentials to the muscle fibers is
necessary to produce a muscle
contraction that can produce work normal
muscle contraction is more sustained and
it can be modified by input from the
nervous system to produce varying
amounts of force this is called a graded
muscle response the frequency of action
potentials or nerve impulses from a
motor neuron as well as the number of
motor neurons transmitting action
potentials both affect the tension
produced in skeletal muscle
if the fibers are stimulated while a
previous twitch is still occurring the
second twitch will be stronger this
response is called wave summation we
have summation occurs because the
excitation contraction coupling effects
of successive motor neuron signaling is
summed or added together at the
molecular level summation occurs because
the second stimulus triggers the release
of more calcium ions which become
available to activate additional Cir
commuters while the muscle is still
contracting from the first stimuli
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summation results in greater contraction
of the motor unit if the frequency of
motor neuron signaling increases
summation and subsequent muscle tension
in the motor unit continues to rise
until it reaches a peak point the
tension at this point is about three to
four times greater than the tension of a
single twitch this state is referred to
as incomplete tetanus during incomplete
tetanus the muscle goes through quick
cycles of contraction with a short
relaxation phase for each if the
stimulus frequency is so high that the
relaxation phase disappears completely
the contractions become continuous in a
process called complete tetanus during
complete tetanus the concentration of
calcium ions in the sarcoplasm allows
virtually all of the sarcomeres to form
cross bridges and to shorten this allows
the contraction to continue
uninterrupted tetanus is the fusion of
contractions to produce a continuous
contraction
1 окт 2024
