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_Anatomy of a Muscle cell _
By: John Centore
John Centore1 Anatomy and Physiology Dr. Jain Anatomy of Muscle Cells There
are three types of muscle tissue in the human body. These muscle tissues are
skeletal muscles, smooth muscles and cardiac muscles. Each of these muscle
tissues has it very own anatomical makeup, which vary from muscle to muscle.
The muscle cells in a muscle are referred to as muscle fibers, these fibers
are skeletal muscle fibers, smooth muscle fibers and cardiac muscle fibers.
The anatomy of a skeletal muscle fiber is formed during embryonic development.
Skeletal muscle fibers arise from a hundred or more small mesodermal cells
called myoblasts. The mature skeletal muscle fiber has a hundred or more
nuclei. Once fusion occurs the skeletal muscle fiber will lose the ability to
undergo cell division. This means that the number of muscle fibers is set
before birth and most of these fibers will last a lifetime. The muscle growth
that occurs after birth is a result of the enlargement of these existing
muscle fibers. The mature muscle fibers have a few myoblasts, which remain as
satellite cells. These myoblasts retain the capacity to join with one another
or with damaged muscle fibers in order to regenerate these muscle fibers. John
Centore2 Dr. Jain Anatomy &Physiology The many nuclei of skeletal muscle fiber
are located underneath the sarcolemma, which is the fiber’s plasma membrane.
Thousands of invaginations of the sarcolemma, which are called T Tubules,
Tunnel from the surface to the center of the muscle fiber. These T Tubules are
open to the outside of the fiber and are filled with extra-cellular fluid.
Muscle action potentials propagate along the sarcolemma and through the T
tubules and quickly spread through the muscle fiber. This process ensures that
all parts of the muscle fiber become excited by an action potential virtually
simultaneously. The sarcoplasm is located inside the sarcolemma. Sarcoplasm is
the cytoplasm of a muscle fiber, it contains a good amount of glycogen, which
is used for ATP synthesis. The sarcoplasm also contains myoglobin, a red
colored, oxygen binding-protein, that is found only in muscle fibers. The
myoglobin binds oxygen molecules, which are needed for ATP production within
the mitochondria. The Mitochondria lie in rows throughout the muscle fiber,
strategically close to the proteins that use ATP during contraction. The
sarcoplasm is filled with little threadlike structures. These structures are
contractile elements of skeletal muscles called myofibrils. The myofibrils are
about two micrometers in size and extend the length of the muscle fiber. The
striations appear to make the muscle fiber look striated. John Centore3
Anatomy &Physiology Dr. Jain The sarcoplasmic reticulum is a fluid filled
system of membranous sacs. This system of sacs is similar to smooth endplasmic
reticulum in non-muscle cells. In a relaxed muscle fiber the sarcoplasmic
reticulum store calcium ions, the release of these calcium cells, trigger
muscle contraction. There are two types of structures which are even smaller,
they are known as thick and thin filaments. The filaments inside of a
myofibril do not extend the entire length of a muscle cell, instead they are
arranged in compartments called sarcomeres. The darker middle portion of a
sarcomere is called the A-band. The A-band extends the entire length of the
thick filaments. The A-band is a zone of overlap. The I-band is a lighter less
dense area that contains the rest of the thin filaments, but has no thick
filaments. A Z-disk passes through the center of each I band. The next form of
muscle fiber is smooth muscle fiber, these fibers are considerably smaller
than skeletal muscle fibers. A single smooth muscle fiber is thirty to two
hundred micrometers long, thick centered and tapered at the ends. These smooth
muscle fibers have a single nucleus that is centrally located and oval in
shape. Smooth muscle fiber contains thick filaments and thin filaments. These
filaments are in ratios of 1:10 and 1:15, but are not arranged in order
sarcomeres as in striated muscle. John Centore4 Anatomy &Physiology Dr.Jain
Smooth muscle fibers also contain intermediate filaments. Various filaments
have no regular pattern of overlap, therefor smooth muscle fibers do not
exhibit striations. As a result the muscle gets the name smooth. Smooth
muscles also lack transverse tubules and have only scanty sarcoplasmic
reticulum for storage of calcium. In smooth muscle fibers, intermediate
filaments attach to structures called dense bodies, which are very similar to
the Z-disks in striated muscle fiber. Some dense bodies are dispersed
throughout the sarcoplasm, others are attached to the sarcolemma. The bundles
of intermediate filaments stretch from one dense body to another. During
contraction, the sliding filament mechanism involving thick and thin filaments
generates tension that is transmitted to the intermediate filaments. These
pull on the dense bodies attached to the sarcolemma, causing a lengthwise
shortening of the muscle fiber. Also, the shortening of the muscle fiber
produces a bubble-like expansion of the sarcolemma. A smooth muscle fiber is
known to contract like a corkscrew turns, the fiber twists in a helix as it
contracts and rotates in the opposite direction as it relaxes. Smooth muscle
fibers also contract in response to nerve impulses, hormones and other local
factors. These muscle fibers can also stretch considerably and still maintain
their contractile function. John Centore5 Anatomy &Physiology Dr. Jain The
last of the three groups of muscle fiber is cardiac muscle fiber. The cardiac
muscle fibers have the same arrangement of actin and myosin and the same
bands, zones, and Z-disks as skeletal muscle fibers. However, the ends of
cardiac muscle fibers connect to adjacent fibers by irregular transverse
thickenings of the sarcolemma called intercalated disks. These disks contain
desmosomes, which hold the fibers together, and gap junctions, which allow
muscle action potential to spread from one cardiac muscle to another. In
cardiac muscle fibers, calcium ions enter the sarcoplasm both from the
sarcoplasmic reticulum and from extracellular fluid. The mitochondria in
cardiac muscle fiber are larger and more numerous than in skeletal muscle
fiber. Cardiac muscle fibers can also use lactic acid produced by skeletal
muscle fibers to make ATP, a benefit during exercise.
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