FREE ESSAY ON ANATOMY OF A MUSCLE CELL

ANATOMY OF A MUSCLE CELL

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.