Skeletal Muscle Flashcards
Types of Muscle
- Smooth
- Cardiac
- Skeletal
Smooth Muscle
- Involuntary
- In the walls of blood vessels and internal organs
Cardiac Muscle
- Controls itself with help from nervous and endocrine systems
- Only in the heart
Skeletal Muscle
- Voluntary muscle
- Over 600 throughout the body
Skeletal Muscle Chemical Composition
- Water = 75%
- Protein = 20% ((myosin, actin, and tropomyosin most abundant)
- Salts and other substances = 5%
Skeletal Muscle Organization (deep to superficial)
- Myofilaments combine to form…
- Myofibrils combine to form…
- Muscle Fibers AKA Muscle Cells combine to form…
- Fasciculi combine to form…
- Muscles
- Myofilaments are surrounded by the sarcoplasmic reticulum
- Myofibrils are surrounded by the sarcoplasm
- Muscle fibers are bound by the sarcolemma
- Muscle fibers are separated by endomysium
- Fasciculus are bound by perimysium
- Muscles are bound by epimysium
Sarcoplasm
Contains:
- Nuclei
- Mitochondria
- Specialized organelles
Myofibril
Any of the elongated contractile threads found in striated muscle cells
Sarcolemma
Surrounds each muscle fiber and encloses the fiber’s cellular contents
Muscle Fiber
- AKA muscle cell
- Bundle of myofibrils
Endomysium
Wraps each muscle fiber/cell and separates it from neighboring fibers
Fasciculus
A bundle of muscle fibers
Perimysium
Surrounds a bundle of fibers
Epimysium
Surrounds the entire muscle and then blends into the intramuscular tissue sheaths to form tendons
Sarcoplasmic Reticulum
- Surrounds myofibrils
- Provides structural integrity to the cell
Blood Supply to Muscle
- Enters and exits along the intramuscular connective tissue
- Adapts with training (how??)
Skeletal Muscle Ultrastructure
- A single muscle fiber has myofibrils that lie parallel to the fiber’s long axis
- Myofibrils contain smaller subunits called myofilaments
Myofilaments
- ~85% consists of actin and myosin
- Other proteins either serve a structural function or affect protein filament interaction during muscle action
Other proteins present in myofilament
- Tropomyosin
- Troponin
- Alpha-actinin
- Beta-actinin
- M protein
- C protein
Sarcomere
- Functional unit of the muscle fiber repeating between 2 z lines
- Lie in series
- Sarcomere length greatly determines the muscle’s functional properties
- I band
- A band
- H zone
- M band
- Z line
I Band
- Isotropic
- Represents lighter area
- Only actin is visible
- Gets smaller with contraction
A Band
- Anisotropic
- Represents darker area
- Actin and myosin overlap
- Remains the same size during contraction
- H zone
- M band
H Zone
- Center of A band
- Area of A band that only has myosin
M Band
- Bisects H zone
- Consists of protein structures that support arrangement of myosin filaments
Z Line
- Bisects I band
- Adheres to sarcolemma to provide structural stability
Thin Filament
- Actin
- Troponin
- Tropomyosin
Thick Filament
Myosin
Myosin Filaments
Bundles of molecules with polypeptide tails and globular heads
Actin Filaments
2 twisted monomer chains bound by tropomyosin polypeptide chains
Actin-Myosin Orientation
- Thousands of myosin filaments lie along line of actin filaments in muscle fiber
- 6 thin actin filaments encircle thicker myosin filament
- In a single fiber, this arrangement consists of approximately 16 billion thick filaments and 64 billion thin filaments
ATP Hydrolysis and Cross-Bridges
- ATP hydrolysis activates myosin’s 2 heads, placing them in an optimal orientation to bind actin’s active sites
- This pulls thin filaments and Z lines of sarcomere toward the middle
Steps of Cross-Bridge Cycle
- Binding of myosin to actin (ADP + P)
- Inorganic phosphate is released - Power stroke (ADP)
- Actin gets pulled toward the middle of the sarcomere
- ADP is released - Rigor (myosin is in low-energy form)
- New ATP binds to myosin head - Unbinding of myosin and actin
- ATP is hydolyzed - Cocking of the myosin head (myosin is in high-energy form) (ADP + P)
- Repeat
Other Endosarcomeric Proteins
- Titin
- M-line proteins
- Alpha-actinin
Titin
- Links end of thick filament to the Z-disc
- Keeps myosin in position
M-line Proteins
- Runs along the middle of the H-zone
- Important for spatial organization
Alpha-Actinin
Strong attachment of actin molecules at the Z-disc
Intracellular Tubule Systems
- Lateral end of tubule channel within muscle fiber terminates in saclike vesicle that stores Ca2+
- T-tubule system runs perpendicular to myofibril
- T-tubule functions as a transportation network by spreading action potential from outer membrane
Role of Calcium in Muscle Action
- Ca2+ is released from vesicle and diffuses a short distance to “activate”actin filaments
- Muscle action begins when myosin filament crossbridges attach to active sites on actin
- Electrical excitation ceases, Ca2+ concentration in cytoplasm decreases, and the muscle relaxes
Sequence of Muscle Action Events
- ACh is released from neuron into neuromuscular junction, diffuses across synaptic cleft, and attaches to ACh receptor on sarcolemma
- AP depolarizes transverse tubules
- T-tubule system depolarization causes Ca2+ release from sarcoplasmic reticulum lateral sacs
- Ca2+ binds to troponin-tropomyosin complex, releasing inhibition of myosin combining with actin
- Actin joins myosin ATPase to split ATP, provides energy release, which produces myosin crossbridge movement
- Muscle shortening occurs when new ATP binds to myosin, which breaks the actin-myosin bond and allows crossbridge dissociation from actin and sliding of thick and thin filaments
- Ca2+ removal restores troponin-tropomyosin inhibitory action. With ATP present, actin and myosin remain in the dissociated state.
- When muscle stimulation ceases, Ca2+ moves back into the sarcoplasmic reticulum lateral sacs through active transport
- Crossbridge activation continues when Ca2+ concentration remains high (from membrane depolarization) to inhibit troponin-tropomyosin action
Muscle Fiber Type Differences
Fiber types differ in:
- Primary mechanisms they use to produce ATP
- Type of motor neuron innervation
- Type of myosin heavy chain expressed
4 Characteristics of Fast-Twitch Fibers (Type II)
- High capability for transmission of APs
- High myosin ATPase activity
- Rapid Ca2+ release and reuptake (efficient sarcoplasmic reticulum)
- High rate of crossbridge turnover
Properties of Fast-Twitch Fibers
- Rapid energy production for quick, powerful muscle action
- 3-5x faster than slow-twitch ribers
- Relies on well-developed, short-term glycolytic system for energy transfer
Type II Fiber Subtypes
- Type IIa
- Type IIx (previously referred to as type IIb)
- “New Type IIb
Type IIa
Represents fast-oxidative-glycolytic fibers (FOG)
Type IIx
Midway between a and b types in physiologic and metabolic characteristics
“New” Type IIb
- Possesses greatest anaerobic potential and most rapid shortening velocity
- Represents “true” fast-glycolytic fiber
4 Characteristics of Slow-Twitch Fibers
- Low myosin ATPase activity
- Slow calcium handling ability and shortening speed
- Less well-developed glycolytic capacity
- Large and numerous mitochondria
Properties of Slow-Twitch Fibers
- Generate energy for ATP resynthesis through aerobic system
- Highly fatigue resistance
- Slow shortening speed
Contribution of fiber types during near-max exercise
Both slow- and fast- twitch contribute during near-max exercise
All-or-Nothing Rule
- For a motor unit to be recruited the potential must pass threshold, otherwise there’s no muscle action
- If threshold is reached, all muscle fibers in the motor unit act maximally
- More force is produced by activating more muscle units
