Muscle fiber

Muscle Fibers: Structure & Function

Motor neuron activity triggers sarcolemmal depolarization via NMJ
Action potential generated
- Travels along muscle surface membrane
- Enters transverse tubule system (T-tubules)
- Signal transmitted from T-tubules to terminal cisternae at triads
Ca⁺⁺ ions released from sarcoplasmic reticulum: Regulated by 2 membrane protein complexes
- Dihydropyridine receptor (DHPR)
  - Located in junctional t-tubules
  - Voltage sensing: α1 subunit of DHPR activated by membrane depolarization
  - Activated DHPR interacts with ryanodyne receptor
- Ryanodyne receptor (RyR)
  - Located in junctional sarcoplasmic reticulum (SR)
  - RyRs release Ca⁺⁺ ions into cytosol after interaction with activated DHPR
  - Other RyRs in SR membrane without DHPR interactions
    - Activated by excess Ca⁺⁺ ions released by neighboring RyRs
    - Amplify & propagate Ca⁺⁺ signal
- Related molecules: CASQ; Triadin; ATP2A1; STAC3; SPEG
Contractile apparatus activated
- Ca⁺⁺ binds to troponin complex
- Tropomyosin binding to contractile apparatus changes
- Actin allowed to bind to myosin heads
- Muscle contraction occurs via myofilament sliding

Juxtaposition of: T-tubule + 2 Terminal cisternae
Invaginations of sarcolemma
- Terminal cisternae
- T-tubule
Triad proteins
- Dihydropyridine receptor (DHPR): T-tubule
- Ryanodine receptor (RYR): Sarcoplassmic reticulum
Triad functions
- Maintain Ca⁺⁺ homeostasis
- Excitation–contraction (EC) coupling
EC coupling disorders (Triadopathies)
- RYR1
- CACNA1S
- ORAI1
- STAC3
- STIM1
- MTM1
- DNM2
- BIN1
- JPH1

From UCSD

From M Ferenczi

From M Ferenczi