Skeletal Muscle Physiology
Structure of Skeletal Muscle
- Muscle fiber (cell): elongated, multinucleated, formed from myoblast fusion
- Myofibril: subunit of a muscle fiber
o Made of thick (myosin) and thin (actin) filaments
- Sarcomere: contractile unit of myofibril
o Z-line to z-line
- Connective tissue layers (outermost to innermost)
o Epimysium: surrounds the entire muscle
o Perimysium: surrounds fascicles (bundles of fibers)
o Endomysium: surrounds individual fibers
- Tendons: attach muscle to bone
Sarcomere Structure
- Z-line: anchors actin
- M-line: middle of the sarcomere
- A-band: length of myosin (dark)
- I-band: actin only (light)
- H-zone: myosin only; no overlap
Thick vs Thin
- Thick: myosin
o Contains actin and ATP binding sites
- Thin: actin, tropomyosin, and troponin
o Tropomyosin blocks myosin binding sites on actin
o Troponin binds Ca2+ to shift tropomyosin, exposing myosin binding sites
Action Potential
- Initiated by an alpha motor neuron (AMN)
- Travels along sarcolemma and down T-tubules
, - Triggers Ca2+ release from sarcoplasmic reticulum via terminal cisternae
Motor unit
- One motor neuron + all muscle fibers it innervates
- All fibers contract when the neuron fires
Phases of a Twitch Contraction
1. Latent period
a. Time between the AP arriving at the muscle and the start of contraction
b. Excitation-contraction coupling occurs
i. AP spreads
ii. Ca2+ is released from SR
iii. Troponin binds to Ca2+ à tropomyosin shifts à binding sites exposed
c. No tension yet; short delay
2. Contraction phase
a. Cross-bridge cycling begins
b. Myosin bind actin, performs power strokes, and pulls thin filaments
c. Tension increases as more cross-bridges engage
d.
Duration depends on fiber type
3. Relaxation phase
a. Ca2+ is pumped back into SR by Ca2+ ATPase
b. Troponin releases Ca2+ and tropomyosin re-blocks binding sites
c. Cross-bridge cycling stops and tension falls back to 0
d. Typically longest phase because Ca2+ removal takes time
Structure of Skeletal Muscle
- Muscle fiber (cell): elongated, multinucleated, formed from myoblast fusion
- Myofibril: subunit of a muscle fiber
o Made of thick (myosin) and thin (actin) filaments
- Sarcomere: contractile unit of myofibril
o Z-line to z-line
- Connective tissue layers (outermost to innermost)
o Epimysium: surrounds the entire muscle
o Perimysium: surrounds fascicles (bundles of fibers)
o Endomysium: surrounds individual fibers
- Tendons: attach muscle to bone
Sarcomere Structure
- Z-line: anchors actin
- M-line: middle of the sarcomere
- A-band: length of myosin (dark)
- I-band: actin only (light)
- H-zone: myosin only; no overlap
Thick vs Thin
- Thick: myosin
o Contains actin and ATP binding sites
- Thin: actin, tropomyosin, and troponin
o Tropomyosin blocks myosin binding sites on actin
o Troponin binds Ca2+ to shift tropomyosin, exposing myosin binding sites
Action Potential
- Initiated by an alpha motor neuron (AMN)
- Travels along sarcolemma and down T-tubules
, - Triggers Ca2+ release from sarcoplasmic reticulum via terminal cisternae
Motor unit
- One motor neuron + all muscle fibers it innervates
- All fibers contract when the neuron fires
Phases of a Twitch Contraction
1. Latent period
a. Time between the AP arriving at the muscle and the start of contraction
b. Excitation-contraction coupling occurs
i. AP spreads
ii. Ca2+ is released from SR
iii. Troponin binds to Ca2+ à tropomyosin shifts à binding sites exposed
c. No tension yet; short delay
2. Contraction phase
a. Cross-bridge cycling begins
b. Myosin bind actin, performs power strokes, and pulls thin filaments
c. Tension increases as more cross-bridges engage
d.
Duration depends on fiber type
3. Relaxation phase
a. Ca2+ is pumped back into SR by Ca2+ ATPase
b. Troponin releases Ca2+ and tropomyosin re-blocks binding sites
c. Cross-bridge cycling stops and tension falls back to 0
d. Typically longest phase because Ca2+ removal takes time
