Skeletal muscle
[Ref: WG21:Chp3]
Action potential
- RMP: -90mV
- Duration: 2-4ms
- Absolute refractory period: 1-3 ms
- Conduction velocity: 5 m/s
- Ionic changes are similar to nerve
* Depolarisation: Na+ influx
* Repolarisation: K+ efflux
- Depolarisation starts at neuromuscular end-plate
Muscle twitch
Starts 2ms after start of depolarisation, before repolarisation is complete
- Fast muscles: Twitch lasts as short as 7.5ms
- Slow muscles: Twitch lasts up to 100ms
Excitation-contraction coupling
In resting muscle
- Troponin I is tightly bound to actin
- Tropomyosin covers the sites where myosin heads bind to actin (thus inhibiting preventing interaction between myosin and actin)
Contraction
AP travels down nerve
--> Acetylcholine at neuromuscular junction (NMJ)
--> Acetylcholine binds to nicotinic acetylcholine receptors on muscle end-plate
--> Depolarisation of the end-plate
--> AP propagated over muscle membrane
--> AP travels along T-tubules
--> Triggers release of Ca2+ from terminal cisterns
* via dihydropyridine receptors
--> Ca2+ binds to troponin C
--> Weakens binding of troponin I to actin
--> Tropomyosin moves laterally
--> Binding site on actin (for myosin) exposed
When binding site on actin is exposed
--> Myosin head binds to actin
--> Myosin head bends and utilises one ATP
--> Sarcomere shortens by about 10nm
--> Myosin head detaches
Dihydropyridine receptor
- Channel protein for voltage-dependent L-type Ca2+ channel
- High affinity to dihydropyridine (antagonist)
- In cardiac muscles, AP triggers Ca2+ influx via these channels which in turn triggers release of more Ca2+ from SR (via a non-voltage-gated Ca2+ channel on SR (ryanodine receptor))
- In skeletal muscles, it senses voltage changes and directly opens ryanodine receptor channel, causing Ca2+ release from SR
- In skeletal muscles, influx of Ca2+ from ECF via dihydropyridine is not necessary to trigger Ca2+ release from SR
Ending contraction
--> Sarcoplasmic reticulum actively transport Ca2+ back into its longitudinal portion
* by Ca-Mg ATPase pump
--> Ca2+ diffuse back into terminal cisterns
--> Ready for release when AP arrives
--> Once Ca2+ outside the reticulum dropped enough, contraction ceases
NB:
- ATP is required for both contraction and relaxation