EXSC 510
CASE STUDY ESSAY QUESTION 3
Muscles are required for movement in the body due to their contractile
abilities. There are three main groups of muscles: skeletal, cardiac, and
smooth (Powers & Howley, 2021). Skeletal muscles are used for voluntary
movements in the body. Cardiac muscles comprise the heart. Smooth
muscles are needed for involuntary movements and those without conscious
thought. Muscles are required for many bodily functions including
movement, stability, expression of oneself, bodily openings, production of
heat, life sustaining processes, and protection (Powers & Howley, 2021).
Myocyte is the scientific name used for a muscle fiber or muscle cell.
The myofibril is the smallest component of these cells. Myofibrils are
composed of lined up end-to-end sarcomeres. These are made of the
contractile actin and myosin proteins (Liberty University, Muscle Architecture, 2021).
Therefore, myofibrils are important in helping the muscle cell contract. A group
of myofibrils is called a muscle fiber or muscle cell. The endoplasmic
reticulum of a muscle cell is called the sarcoplasmic reticulum (SR). The SR
has a very important job in muscle contraction and relaxation because it
stores calcium. The cell wall of a muscle cell is called the sarcolemma. The
sarcolemma is in charge of being a border that determines what materials
are allowed into the cell. The sarcolemma wraps around and between each
myofibril. The encircling of each myofibril by the sarcoplasm is called
transverse tubules. A group of muscle fibers is called a fascicle (Liberty
University, Muscle Architecture, 2021). A bundle of fascicles makes up the muscle.
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The perimysium covers each individual fascicle, the endomysium surrounds
each muscle fiber, and the epimysium (outermost layer) surrounds the entire
muscle (Liberty University, Muscle Architecture, 2021). All three of these layers are
made of connective tissue. Endomysium, perimysium, and epimysium run
through the muscle, and all connect together to form the tendon. Tendons
are responsible for connecting muscle to bone. All of these components,
explained in order from smallest to largest, comprise the muscle cell and aid
in its structure and function.
The sarcomere is the smallest contractile functional unit. Sarcomeres
contain actin and myosin proteins. Actin is the thin filaments and myosin is
the thick filaments (Liberty University, Muscle Architecture, 2021). These myosin
proteins compose the centralized A-band of a sarcomere. The length of the A-
band remains constant during contraction. Actin makes up the two laterally
located I-bands (one on each side) which shortens or lengthens during
muscle use. I-bands from two neighboring sarcomeres meet at the Z-line.
The center of the A-band is called the M-line. The H-zone is where there is no
overlapping of the A-band and I-band. The sliding filament theory says that
the movement of the A-band myosin along the I-band actin enables the
entire sarcomere to lengthen and shorten for muscle contractions. This
theory suggests myosin filaments “walk” along actin filaments.
In order to begin muscle activation, there must be motor neuron
activation. The efferent motor neurons send action potential down to the
muscle being activated. At the neuromuscular junction, acetylcholine (ACh)
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will be released into the synapse from the motor neuron. The receptors on
the muscle cell are ligand-gates sodium (Na+) channels. When the ACh binds
to these receptors, it sends a rush of Na+ into the sarcolemma of the muscle
cell. This addition of Na+ depolarizes the muscle cell. The action potential
travels down the t-tubules and excites the voltage-gated calcium (Ca2+)
channels of the cell’s sarcoplasmic reticulum. When these channels are
opened calcium rushes into the muscle cell. This calcium will be used to
move the proteins from actin in the sliding filament model. The excitation-
contraction coupling is described as the calcium ions entering the muscle cell
and allowing the myosin to bind with actin (excitement) and the constant
binding, unbinding and rebinding of myosin heads to actin (contraction).
At rest, actin and myosin do not touch within the sarcomere. Actin
binding spots are covered by a rope-like tropomyosin and troponin proteins.
Calcium and ATP are needed in order to move these proteins and allow the
myosin globular heads to attach to the actin binding sites (Liberty University,
Muscle Contraction, 2021). Troponin has an affinity for binding with calcium. The
binding of troponin and calcium causes the tropomyosin to be pulled away
from the binding spots of the actin. In order to bind to these newly exposed
sites, the myosin head must have energy from ATP. The ATP is broken down
into ADP and a phosphate (Liberty University, Muscle Contraction, 2021). This
addition of energy to the myosin head allows it to stretch into an extended or
“cocked” position. Then, the myosin binds to the actin and forms a cross-
bridge. The ATP energy is used for this attachment to occur. The myosin tilts
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and pulls on the actin strand which contracts the sarcomere and the muscle
(Liberty University, Muscle Contraction, 2021). This is called a “power stroke.” This
pulling of actin inwards causes the Z-lines to get closer to one another, the
muscle to contract, and muscle force is generated. After using the ADP and
phosphate then unbind with the myosin head and a new ATP is able to bind.
When the new ATP binds, the myosin head changes shape and unbinds from
the actin. The new ATP must be broken down into ADP and a phosphate
again in order to repeat the process and produce another “power stroke”
(Liberty University, Muscle Contraction, 2021). This process is happening in several
myosin heads simultaneously and repeatedly to cause contraction. The
calcium pumps are actively replenishing calcium stores in the sarcoplasmic
reticulum by pulling the Ca2+ back out of the sarcoplasm (Liberty University,
Muscle Contraction, 2021). The calcium will unbind from the troponin which
places the tropomyosin back over the binding sites and ending possibility for
muscle contracting from myosin and actin binding. The lack of neural signal
and stimulation from efferent neurons halts contraction in the muscle and
the muscle enters a state of rest again (Liberty University, Muscle Contraction, 2021).
During the muscle contraction, several aspects of the original
sarcomere’s structure change. The Z-lines are pulled closer to one another
and the M-line. The I-band gets smaller. Myosin heads are pulling themselves
closer to the Z-line as they slide across actin. Finally, the A-band does not
change in with since it is composed of the myosin filaments. Actin filaments
are the ones being physically moved in this theory.
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References
Liberty University. (2021). Muscle Architecture. Retrieved from
https://canvas.liberty.edu/courses/315530/pages/watch-muscle-architecture?
module_item_id=41914304.
Liberty University. (2021). Muscle Contraction. Retrieved from
https://canvas.liberty.edu/courses/315530/pages/watch-muscle-contraction?
module_item_id=41914311.
Powers, S. K., Howley, E. T., & Quindry, J. (2021). Exercise physiology: Theory and application
to fitness and performance. McGraw Hill LLC.
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