Question

Discuss ALL of the steps involved in the excitation and contraction of a myofiber, in correct order. Start with the arrival of the first action potential at the axon terminal, and end with the active transport of Ca+2 into the SR. You may use outline/ bulleted list/ flow chart formats if you prefer.  

Answer 1

Myofiber is a single muscle cell which containing multinucleus. Its excitation and contraction occur in following sequence

1. A nerve action potential reaches at presynaptic axon terminal of a somatic motor neuron in Neuromuscular Junction (junction between motor neuron and skeletal muscle cell).

2. This action potential opens the voltage gated calcium channel in the axon terminal and influx of calcium ions into the axon terminal occur

3. Calcium ions in the axon terminal attract the synaptic vesicle (contain acetylcholine) towards the presynaptic membrane of axon terminal

4. Fusion of Synaptic vesicle containing acetylcholine with presynaptic membrane and release of acetylcholine to the synaptic cleft, a small gap between axon terminal and muscle cell

  

5. This acetylcholine diffuse to the opposite site toward the motor end plate, muscle fiber part of NMJ which contain numerous acetylcholine receptors ( that is nicotinic receptor)

6. Acetylcholine binds to the nicotinic receptors present in Motor end plate and opens small cation channels (Na+ channels) in the motor end plate

  

7. Influx of Na+ ions occur this make cell Membrane more positively charged which trigger muscle fiber action potential in the motor end plate

8. This muscle action potential propagates along sarcolemma into transverse tubule (T-tubule) system

  

9. This action potential in T-tubules opens the calcium releasing channels in the SR (Sarcoplasmic reticulum)

  

10. That allows release of calcium from SR into sarcoplasm, cytoplasm of myofiber

  

11. Calcium ions binds to troponin, a regulatory protein component of thin filament and expose myosin-binding site on actin for myosin

  

12. Binding of myosin causes thin filament to be pulled towards the center of sarcomere leads to contraction of myofiber

13. After contraction, calcium are actively transported into the SR with Ca+/ATPase, this makes low calcium level in the sarcoplasm

14. Myosin binding site on action will be covered - relaxation of myofiber occur