Bio 2311 Muscle Tissue Notes

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