General Characteristics

• Mesodermal Origin
• Cells specialized in contraction
• Composed by elongated cells (muscle fibers)
• Present great amount of cytoplasmic filaments
• Have special denominations:
-Sarcolemma(membrane)
-Sarcoplasm(cytoplasm)
-Sarcoplasmic Reticulum (smooth endoplasmic reticulum)
-Sarcosomes(mitochondrium)

Smooth Muscle

• Long Spindle-shaped Cells
• Organized in Layers
• Found above all in hollow organs

• Mononuclear cells, central nucleus
• Involuntary contraction in waves and normally slow
• Cells structure
-Don’t have a T system
-Have pinocytosis vesicles that regulate the flow of calcium

Striated skeletal muscle

• Elongated cylindrical cells
• Multinuclear cells, peripheral nuclei

• Present transverse striations
• The connective tissue has two important rolls:
• Maintain the fibers united, allowing the contraction of each fiber to act on the entire muscle
• Allows the contraction to act on other structures such as tendons and ligaments.
• Organization of the connective tissue
- Epimysium: Membrane of connective tissue that involves the muscle
- Perimysium: Septa branch out from the epimysium and divide the fibers into bundles
- Endomysium: Thin layer that involves each fiber
• Rapid and Voluntary Contraction
• In the Sarcoplasm we find:
-Glycogen that works as an energy deposit
-Myoglobin that serves as an oxygen deposit

Cell Structure

Myofibrils: Formed by thin and thick filaments
• Thin: actin, troponin and tropomyosin
• Thick: myosin
• The thin and thick filaments organize themselves and form cross striations

Sarcomere: morphofunctional unit of striated muscle fibers
• The sarcomeres are positioned between two Z lines
• A sarcomere possesses two types of bands
I- Anisotropic (A band)
-We find myosin myofilaments as well as actin.
-There is a pale line, the H zone, which is formed by myosin
II- Isotropic (I band)
-We only find actin myofilaments

Sarcoplasmic Reticulum: Regulates specifically the flow of calcium ions

Transverse tubules (T system):
• Complex network of tubular invaginations of the sarcolemma
• Responsible for the uniform contraction of each fiber
• Formation of the Triad (One T tubule and two expansions of the sarcoplasmic reticulum)

Striated Cardiac Muscle

• Formed by elongated cells that anastomose themselves
• Present transverse striations
• Present one or two central nuclei
• Present junctions: the intercalated discs
• These present three junction specializations:
-Zonula Adherens
-Desmosomes
-GAP Junctions
• Present an involuntary and rapid contraction
• Present diads ( one T tubule and one cisternae of the sarcoplasmic reticulum)
• Impulse Generating and Conducting System of the Heart
• Purkinje Fibers present
-They are modified cardiac cells
-Have an important roll in the conduction of the cardiac stimulation
• The cardiac cells auto stimulate themselves(SA and AV nodes)
• The nervous system is simply regulatory

Muscular Contraction

• The motor end plate is where the nerve inserts itself in the muscle fiber
• Synaptic vesicles with acetyl choline(neurotransmitter) are present
• Release occurs when a nervous stimulation is present
• Increase of the permeability of the sarcolemma to sodium
• Depolarization of the membrane
• Calcium ions that were concentrated in the cisterns are released
• These connect themselves to the troponin-tropomyosin complex
• Myosin binding site is unblocked
• Formation of bridges between actin and myosin
• Once depolarization is ended, calcium returns to cisterns through an active transport