Structure of Skeletal Muscle
We will begin our look at the structure of
muscle starting with the largest structures and
working our way down to the smallest
microscopic features found within the cell.
A whole muscle, like the biceps muscle of the upper
arm, is composed of groups of fasciculi
surrounded by a white connective tissue
called perimysium. Each fasciculus, in turn,
is made up of bundles of muscle cells (also
called muscle fibers). Within each cell there
are cylindrical bundles of myofibrils. These
myofibrils are composed of two types of
myofilaments, which are the actual contractile
elements of the cell. Let's have a closer look
at a muscle cell . . .
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Structure of Skeletal Muscle
Muscle cells (or fibers) are one of the few cells in
the body with more than one nucleus.
They are surrounded by the sarcolemma—the
muscle cell membrane—over which the action
potential is transmitted.
The sarcolemma has small tube-like projections
called transverse (T) tubules that extend down into
the cell.
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Structure of skeletal muscle
These T tubules conduct the action potential
deep into the cell where the contractile
proteins are located.
Within the muscle cell are long cylindrical
myofibrils that contain the contractile
proteins of the muscle.
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Structure of skeletal muscle
The myofibrils are surrounded by the sarcoplasmic
reticulum (SR).
This is a mesh-like network of tubes containing
Ca++, which are essential for contraction.
At either end of and continuous with the SR is the
terminal cisterna, which is close to the T tubule
where the action potential travels.
Let’s now have a look at the contractile proteins
found in the myofibrils.
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Thin Myofilaments
The myofibrils contain two types of
myofilaments.
The thin myofilaments are composed
predominantly of the globular protein actin.
Each actin molecule contains a special binding
site for the other contractile protein myosin.
Many actin molecules are strung together like
the beads on a necklace and then twisted to
form the backbone of the thin myofilaments.
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Thin myofilaments
Also found on the thin myofilaments are long protein
strands called tropomyosin. When the muscle is at
rest, the proteins cover the binding sites for the head
of myosin.
A third regulatory protein, called troponin, is made up
of three subunits—troponin , which binds with Ca++;
which binds to the tropomyosin; which then exposes
the binding site on actin. At rest, the troponin holds the
tropomyosin over the myosin binding sites.
As we will see later, when Ca++ bind to the troponin
unit, the tropomyosin is pulled off the myosin binding
sites by the troponin.
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Thick Myofilaments
The second type of myofilament—the thick
myofilament—is made up of the protein
myosin. This protein has a long, bendable tail
and two heads that can each attach to the
myosin binding sites on actin (as mentioned
on the previous page).
The heads also have a site that can bind and split
adenosine triphosphate (ATP).
As we will see, it is the splitting of ATP that
releases energy to the myosin that powers the
contraction of the muscle.
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Thick Myofilaments
The second type of myofilaments the thick myofilamentis made of the protein myosin. This protein has a long
bendable tail and two heads that can each attach to the
myosin binding sites on actin.
The heads also have a site that can bind and split
adenosine triphosphate. As we will see, it is the splitting
of ATP that releases energy to the myosin that powers
the contraction of the muscle. Many myosin molecules
are arranged to form one thick filament. Under a
microscope, the arrangement of the thin and thick
myofilaments gives the myofibril and the muscle cell a
banded appearance.
This is why skeletal muscle is called striated muscle.
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Actin / Myosin Relationship
Groups of thin (actin) myofilaments and
groups of thick (myosin) myofilaments are
arranged in a repeating pattern (thick, thin,
thick, thin, and so on) along the length of the
myofibril from one end of the cell to the other.
Each group of thin myofilaments extends
outward in opposite directions from a central Z
line, where they are anchored.
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Actin/myosin relationship
Similarly, groups of thick myofilaments
extend outward from a central M line, where
they are attached. Each myofilament is
parallel to the length of the myofibril and the
muscle cell.
The region from one Z line to another is
called a sarcomere. This is the smallest
functional contractile unit of the muscle cell.
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Excitation-contraction
Excitation-contraction this is the process by which an
action potential in the sarcolemma excites the muscle
cell to produce a muscle contraction.
The AP that was generated at the neuromuscular
junction will spread out over the sarcolemma and
down the T-tubules into the core of the muscle. This
AP reaches the SR and increases CA++, which will
bind to troponin causing tropomyosin to move
exposing the binding sites on actin.
Myosin will now be able to attach to the actin and a
power stroke will occur.
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Relaxation of Muscle
Once action potentials stop, Ca++ will no
longer diffuse out of the sarcoplasmic
reticulum (SR). Special calcium pumps rapidly
pump Ca++ back into the SR, up its
concentration gradient; this requires ATP.
Without the calcium present in the cytoplasm
of the muscle cell, the tropomyosin will cover
the myosin binding sites once again. Myosin
will be unable to bind to actin and power
strokes will not occur. The muscle will relax.
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Actin-Myosin and ATP Cycle
Here are the steps summarized
1. Myosin, which has been energized by the
splitting of ATP to adenosine diphosphate
(ADP) and inorganic phosphate (Pi), attaches
to actin* and forms a crossbridge.
2. A power stroke is initiated while ADP and Pi
are expelled from the myosin head.
3. Actin and myosin slide past one another.
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Actin-Myosin:
4. Actin and myosin are bound together until
a new molecule of ATP attaches to myosin;
the crossbridge is broken.
5. ATP is split to form ADP and Pi,energizing
the myosin molecule.
6. The cycle repeats as long as actin and
myosin can interact.
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The Motor Unit
A motor unit is a motor neuron and all of the
muscle cells/fibers it contacts. In almost all
situations, one motor neuron will contact (or
innervate) several muscle cells, but each
muscle cell is innervated by only one motor
neuron. The number of muscle cells
innervated by a motor neuron varies. A large
motor unit has a motor nerve in contact with a
large number of muscle cells, while a small
motor unit is one in which the motor neuron
contacts a few muscle cells.
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The Structure of the Neuromuscular
Junction
The neuron that contacts a muscle cell is
sometimes called a motor nerve fiber. The
membrane of the axon terminal contains
Ca++ voltage-gated channels. These channels
open in response to a nerve impulse.
The axon terminal of the motor cell/fiber also
contains synaptic vesicles that contain the
neurotransmitter acetylcholine(ACh).
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Neuromuscular -JCT
The basement membrane of the axon terminal
contains the enzyme acetylcholinesterase
(AChE).
The muscle cell membrane (also called the
sarcolemma) directly under the axon terminal
is thrown into folds. This region is called the
end plate.
The end plate contains receptors for ACh,
which are associated with gated ion channels.
The gap between the motor fiber and muscle cell
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is called the synaptic cleft.
Events at the Neuromuscular
1.The action potential on the motor nerve fiber
triggers Ca++ voltage-gate channels to open. Ca++
flow into the cell, down their concentration
gradient.
2.Ca++ trigger the fusing of synaptic vesicles to
the membrane and the release of ACh into the
synaptic cleft by exocytosis.
3.ACh diffuses across the synaptic cleft and
attaches to receptors on the muscle cell
membrane/sarcolemma.
4.The ACh is broken down by the enzyme AChE
and is taken back into the axon terminal to be
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recycled.
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