An Introduction to Muscle Tissue
Muscle Tissue
A primary tissue type, divided into
Skeletal muscle
Cardiac muscle
Smooth muscle
Skeletal Muscles
Are attached to the skeletal system
Allow us to move
The muscular system
Includes only skeletal muscles
Functions of Skeletal Muscles
Produce skeletal movement, Maintain body position, Support soft tissues, Guard openings, Maintain body
temperature, Store nutrient reserves.
Skeletal Muscle Structures
Muscle tissue (muscle cells or fibers)
Connective tissues
Nerves
Blood vessels
Organization of Connective Tissues
Muscles have three layers of connective tissues
Epimysium:
– exterior collagen layer
– connected to deep fascia
– Separates muscle from surrounding tissues
Perimysium:
– surrounds muscle fiber bundles (fascicles)
– contains blood vessel and nerve supply to fascicles
Endomysium:
– surrounds individual muscle cells (muscle fibers)
– contains capillaries and nerve fibers contacting muscle cells
– contains myosatellite cells (stem cells) that repair damage
Muscle attachments
Endomysium, perimysium, and epimysium come together:
– at ends of muscles
– to form connective tissue attachment to bone matrix
– i.e., tendon (bundle) or aponeurosis (sheet)
Nerves
Skeletal muscles are voluntary muscles, controlled by nerves of the central nervous system (brain and spinal cord)
Blood Vessels
Muscles have extensive vascular systems that
Supply large amounts of oxygen
Supply nutrients
Carry away wastes
Skeletal Muscle Fibers
Are very long ; Develop through fusion of mesodermal cells (myoblasts); Become very large ; Contain hundreds of
nuclei
Internal Organization of Muscle Fibers
The sarcolemma
The cell membrane of a muscle fiber (cell)
Surrounds the sarcoplasm (cytoplasm of muscle fiber)
Transverse tubules (T tubules)
Transmit action potential through cell; Allow entire muscle fiber to contract simultaneously; Have same properties as
sarcolemma
Myofibrils
Lengthwise subdivisions within muscle fiber
,
Made up of bundles of protein filaments (myofilaments)
Myofilaments are responsible for muscle contraction
Types of myofilaments:
– thin filaments: made of the protein actin
– thick filaments: made of the protein myosin
Sarcoplasmic reticulum (SR)
A membranous structure surrounding each myofibril
Helps transmit action potential to myofibril
Similar in structure to smooth endoplasmic reticulum
Forms chambers (terminal cisternae) attached to T tubules
Triad
Is formed by one T tubule and two terminal cisternae
Cisternae: concentrate Ca2+ (via ion pumps) , release Ca2+ into sarcomeres to begin muscle contraction
Sarcomeres
The contractile units of muscle; Structural units of myofibrils ; Form visible patterns within myofibrils
Muscle striations
A striped or striated pattern within myofibrils:
– Alternating dark, thick filaments (A bands) and light, thin filaments (I bands)
Sarcomeres
M line: the center of the A band ; at midline of sarcomere
Z lines: the centers of the I bands; at two ends of sarcomere
Zone of overlap: the densest, darkest area on a light micrograph ; where thick and thin filaments overlap
The H Band: the area around the M line; has thick filaments but no thin filaments
Titin: are strands of protein ; reach from tips of thick filaments to the Z line; stabilize the filaments
Transverse tubules encircle the sarcomere near zones of overlap
Ca2+ released by SR causes thin and thick filaments to interact
Muscle Contraction
Is caused by interactions of thick and thin filaments
Structures of protein molecules determine interactions
Four Thin Filament Proteins
F-actin (Filamentous actin) : Is two twisted rows of globular G-actin; The active sites on G-actin strands bind to
myosin
Nebulin: Holds F-actin strands together
Tropomyosin: Is a double strand; Prevents actin–myosin interaction
Troponin: A globular protein; Binds tropomyosin to G-actin; Controlled by Ca2+
Initiating Contraction
Ca2+ binds to receptor on troponin molecule
Troponin–tropomyosin complex changes
Exposes active site of F-actin
Thick Filaments: Contain twisted myosin subunits, Contain titin strands that recoil after stretching
The mysosin molecule- Tail: binds to other myosin molecules; Head: made of two globular protein subunits; reaches
the nearest thin filament
Myosin Action: During contraction, myosin heads
Interact with actin filaments, forming cross-bridges
Pivot, producing motion
Skeletal Muscle Contraction
Sliding filament theory
Thin filaments of sarcomere slide toward M line, alongside thick filaments
The width of A zone stays the same
Z lines move closer together
The process of contraction
Neural stimulation of sarcolemma:
– causes excitation–contraction coupling
Cisternae of SR release Ca2+:
– which triggers interaction of thick and thin filaments
– consuming ATP and producing tension
The Neuromuscular Junction
Is the location of neural stimulation
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