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Structure of a Muscle Cell
This page follows on from the
more general page about the structure
of muscle.
Skeletal muscles consist of 100,000s
of muscle cells (also known as "muscle fibers")
that perform the functions of the specific muscle
of which they are a part.
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Above: Diagram of a Muscle Cell
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The diagram above shows some detail of the
structure of a muscle cell.
This illustrates the distinctive structure of muscle cells,
including striated myofibrils (components of muscle
cells only).
It is also important to remember that the components essential
for all cells (not all of which are illustrated
in the diagram above) are also present in muscle
cells.
Information about the components of cells generally,
which are called "organelles"
is available on the page about Cell Structure.
General components of Muscle Fibres/cells:
The components of skeletal muscle cells that are specific
to muscle tissue are myofibrils.
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Each muscle fibre ("muscle
cell") is covered by a plasma membrane
sheath which is called the sarcolemma. |
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Tunnel-like extensions from the sarcolemma pass through the muscle fibre from one side
of it to the other in transverse sections
through the diameter of the fibre.
These tunnel-like extensions are known as transverse
tubules ("T
Tubules") - not shown in diagram above. |
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The nuclei of muscle fibres ("muscle
cells") are located at the edges of
the diameter of the fibre, adjacent to the sarcolemma.
As illustrated, a single muscle fibre may
have many nuclei. |
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Cytoplasm is present in all living cells.
The cytoplasm present
is muscle fibres (muscle cells)
is called sarcoplasm. |
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The sarcoplasm present in muscle
fibres contains very many mitochondria,
which are the energy-producing units within the cell.
These mitochondria produce large
amounts of a chemical called "Adenosine
Triphosphate", which is usually referred
to in abbreviated form as "ATP".
(The cellular activities for which ATP is
required include contracting muscles, moving chromosomes
during cell division, moving structures with cells,
transporting substances across cell membranes, and
synthesizing larger molecules from smaller ones.
To understand the function
of ATP for the actions of muscle
fibres/cells,
remember that ATP is
necessary for muscle contraction, and is produced
by the mitochondria within the muscle
cell/s). |
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Sarcoplasmic
reticulum is a network of membrane-enclosed tubules
similar to smooth
endoplasmic reticulum (SER). Sarcoplasmic
reticulum is present in muscle fibres/cells
and extends throughout the sarcoplasm of
the cell. The function of the sarcoplasmic
reticulum is to store calcium ions, which
are necessary for muscle contraction. |
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Myoglobin is
also present in the sarcoplasm of muscle fibres/cells.
This is a reddish pigment that not only results
in the distinctive colour of skeletal muscle,
but also stores oxygen - until it is required
by the mitochondria for the
production of ATP. |
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More specialised components
of Muscle Fibres/cells:
The components of skeletal muscle cells that are specific
to muscle tissue are called myofibrils.
These are cylindrical structures (illustrated above) that extend along
the complete length of the muscle fibre/cell.
Each myofibril consists of two types of
protein filaments called "thick
filaments", and "thin
filaments".
These two types of filament have different structures
- as illustrated on the page about labeled
diagrams of muscle filaments.
Here, it is sufficient to say that the thick filaments and
the thin filaments within myofibrils overlap
in a structured way, forming units called sarcomeres.
Sarcomeres are sections of myofibril that are separated from each other by areas of
dense material called "Z
discs".
The sarcomeres are also described
in terms of the bands/zones within which one or
both of the two filaments occur.
These bands/zones are illustrated in the diagram below:
The "A
band" is a relatively darker area within
the sarcomere that extends along the total length
of the thick filaments.
The "H
zone" is at the centre of the A
band of each sarcomere.
As shown below, this is the region in which there
are only thick filaments, and no thin filaments.
The "I
band" is the region between adjacent A bands, in which there are only thin filaments, and no thick
filaments.
( Each I band extends across two adjacent sarcomeres.)
In the diagram below the Z
discs are represented by the zig-zag
lines that form the boundaries between adjacent sarcomeres.
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Diagram
of one sarcomere (within a muscle cell).
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The next page describes the structure
of muscle filaments ...
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