Effect of Stimulus Intensity on Skeletal Muscles

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Does stimulus intensity (electric shocks) affect on skeletal
muscles respond differentially , interns of developing different
frequency tension !
Introduction:
Muscles are one of the most important parts in our body. Help in
movement, holding bones together and responding to different
homeostasis. Muscles vary in size, shape, length dependents on it’s
function and where it can found in the body . Skeletal muscles are a
part of the peripheral nerve system, a cylinder shape structure. Is
composed of Muscles fascicle, which composed of a number of muscle
cells forms the smallest unit in the muscles called Sarcomeres ,each
muscles fiber or muscle cell is made up with myofibrils (actin and
myosin)that collaborate together to give a particular respond to a
specific stimulate(Figure1).
A number of motor axons from the CNS are innervating to each
individual fibers. Transferring an action potential that travels all the
way down from the CNS to the peripheral nerve system throughout
the axon to the sarcomere on the skeletal muscles forming a motor
neuron unit. This conduction of the nerve impulses to the skeletal
muscle cells causes movements of the skeletal muscles. As a single
nerve innervation in the skeletal cell muscles is able to contract
multiple muscle fibers, so with a single contraction multiple fibers can
contract at same time. Once, a single fiber is contracted, the protein
filament in the muscle fiber transmits the contraction through sliding
filament mechanism. At the end of the each motor neuron unit, there
Figure1: A basic anatomy of the skeletal muscles.
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is an axonal branch that form a neuromuscular junction in between
skeletal muscle fibers ( Sarcomere) and nerve endings ( Hall 2015).
Each of Neuromuscular junction is connected to one sarcomere only
(refer to Figure 2)
Body( review the knowledge ):
Action potential is created by a stimulus (whether is was voluntarily
or electrical) throughout the motor unit .It conducted along the nerve
fiber ,until it reached the end of the motor unit where specific
chemicals are found within vesicles that contains
Neurotransmitters. Neurotransmitters (In this case acetylcholine)
is diffused via exocytosis to the area of Neuromuscular junction or
synaptic cleft. Refer to Figure 3.
Figure2: The makeup of motor units. Which shows the transformation of information from the
CNS through to the peripheral nerve system Neuromuscular junction to a particular
sarcomere.
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As the releasing of neurotransmitter is taking place; caused by the
opening calcium ions channels and sodium channels in the pre
synaptic nerve and it creates a high concentration of [Ca+2] and
causes release of calcium ions into axon terminals and a result the
neurotransmitter vesicles are fused with the presynaptic that lead to
release of neurotransmitter. The short delay that is 1-2 sec delay
from time when the action potential reaches to the muscle fibers
which is still in its tension condition, this delay is called the latent
period of muscle fibers and this latent period causes a temporary
tension action known as twitch response ( Hall 2015)(Figure 4) . As
latent period finished a muscles tension is developed, created the
power struck effect, which is the movement of Actin and myosin to the
center of the sarcomeres, which lead to shorting the sarcomeres
(Figure 5a).However;(Figure 5-b), As the curve gets to the peak, Ca+2
is transported from the cytosol into the Sacromatic Reticulum (SR) in
the skeletal muscle.Where as in Figure 5-c) sarcomeres is returning
into it’s relaxing state; where the muscle is not on it’s contracting
state.
Muscle twitches do not have refractory period that causes
inactivation voltage gated Na+2 channels by being closed and
inhibition of the action potential takes place. The rising of the curve in
the twitching graph is caused via the fluting of Ca+2 into to the
cytosol causing action potential. This action potential occurred
around 2 ms ago. This incident means that in order to get another
action potential after the first one and more Ca+2 will be moving to
the cytosol which results in more attention creating twitch summation
creating more decreasing of the same sarcomeres and shorting it even
more ( Khurana 2018).
Figure3: The pathway of action potential causing the release of
Neurotransmitter (acetylcholine) in the synaptic cleft.
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