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Muscle cell/myocytes are the smallest subunits of muscular tissues and organs across the body. It's in the myocyte that the steps of muscle contraction and where the pathophysiology of many muscular diseases happen.
pathophysiology: study of abnormal changes in body functions
Humans have 3 muscle cell types:
Smooth muscle tissues - Found in visceral (internal) organs; stomach, liver, or pancreas, where it pushes fluid and other materials over.
Cardiac muscle tissues - Only found in the heart with unique properties to pump and make up the myocardium, the heart's wall.
Skeletal muscle tissues - Most muscles in the body with roughly 640 in total. They're both striated and voluntary.
Striated: Long, thin lines, or strips of color.
Each muscle tissues are mostly made of muscle tissues with connective, blood vessels, and nerve fibers.
Skeletal Muscles
Skeletal Muscles
Skeletal muscles (progenitor myocytes) are made of overlapping thick and thin filaments called myofilaments arranged longitudinally into sarcomeres.
A muscle is covered by layers called "epimysium". It protects muscle from other muscles and bones. It continues at the end of the muscle to form the tendons at the bones with other protective tissues. Strong muscles like the quadriceps have more fibers. Hands' muscles would have less.
The next layer is perimysium
Each muscle, under perimysium, is made of a bundle of muscle fibers called fasciculi.
Under, fasciculi have endomysium, which covers fibers.
Under endomysium, is the sarcolemma, the muscle fiber cell membrane.
Under, is the sarcoplasm.
Under sarcoplasms si the myofibril.
Each myofibrils are made of protein bundles called actin and myosin.
Finally, each sarcomere are broken down into sarcomeres, the basis for the most popular muscle contraction theory, called "sliding filament model" describes sarcomere shortening by recurrent myosin/actin interactions. Per each interaction, the myosin heads work to bring adjacent actin-free ends closer to the center of the sarcomere.
Therefore, the shortening or contraction of skeletal muscle fibres is a result of sarcomere shortening.
Thick filaments are made of myosin, a protein polypeptide. Each myosin molecule has two globular heads which are involved in the contraction through binding thin filaments.
Thin filaments have actin (contains a binding site for myosin heads), tropomyosin and troponin (has three subunits: troponin T, troponin I and troponin C). These sarcomere structures give skeletal muscle its striated appearance and are readily visible on electron microscopy.
Skeletal muscles also have structures called T tubules, extensions of the myocyte plasma membrane open to the extracellular space and function to carry depolarizing potentials to the intracellular space, allowing coordinated contractions. T tubules contain dihydropyridine receptors, which are essential for contraction after myocyte excitation.
Fasiciles
Fasiciles
Myofribils build up muscle fibers/cells, which are composed of mitochondria, countless nucleis, and cellular membranes called sarcolemma.
It then forms the fasiciles, then muscles.
Fasciles forms the muscle
Each muscle tissues contain a few type of connective tissues when physical work is done.
In "actin" we can recall it has "tin" syllabe like "thin" to help remember.
Around the myofibril, is the Sarcoplasmic reticulum, where calcium is stored for muscle contraction.
Myofibrils
Myofibrils
Countless of tiny parallel threads called myofibrils form muscle fibers and are divided into sarcomeres by zig-zag lines called Z-lines/discs.
Like cardiac muscles, sarcomeres create striations (stripes) and contain two kinds of myofilament (protein); actin and myosin.
Actins are thin filaments and myosin are thick ones. Z-lines connect actins. Myoses are connected by M-lines/discs in the middle called H-zone.
Muscle contracting is when Z-lines and actins approach the M-lines.
By closer scale, this is done by countless myosin heads hydrolyzing ATPs, creating ADPs and phosphates, allowing it to attach to the actin, a process called "cross-bridge".
The myosin head then does a power stroke - it slides toward the M-line before a new ATP finds that myosin head and so on.
Hydrolize: Water molecule breaks compounds.
Each muscle tissues contain a few type of connective tissues when physical work is done.
Smooth Muscles
Smooth Muscles
TBA
Both muscle cells also have sarcomere that create striations.
But unlike skeletal fibers, cardiac ones are shorter and aren't straight lines.
Cardiac Muscles
Cardiac Muscles
Also called cardiomyocytes/cardiocytes, building the myocardium, the heart's wall, allowing it to pump blood, oxygen, and nutrients on an involuntary basis (conscious control). It has fewer nucleis than skeletal fibers.
Involuntary basis: Not by one's own choice.
Involuntary: Done without will/automatic.
Basis: Bottom/base of something standing.
Both cardiac and skeletal muscle cells are surrounded and divided by connective tissue called endomysium.
Endomysium etymology:
"endo" = "within" in Greek
"mysium" = "muscle" in Greek.
Zigzag lines called intercalated discs connect cardiac cells and have three types: desmosome, fasciae adherens, and gap junctions
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