You measure levels of Ca2 in various locations within a moto

You measure levels of Ca²⁺ in various locations within a motor neuron and a skeletal muscle fiber when the motor neuron is depolarized, and the muscle fiber is actively shortening. Where do you expect to find higher levels of Ca²⁺? (Select all that apply.)

Btw there is more than one answer! I need the ANSWERS for this question cant figure out which one I\'m missing.

1. diffusing into the axon terminus of a motor neuron through specialized channels in the plasma membrane

2. bound to troponin

3. within the sarcoplasmic reticulum

4. in vesicles within the motor neuron

5. binding to the receptors at the motor endplate of the muscle cell

6. within the T-tubules

7. bound to myosin

8. bound to tropomyosin

Solution

Calcium plays crucial role in muscle fibre contraction.

When the depolarizing current reaches the presynaptic terminal, it causes membrane depolarization and that initiates opening of voltage gated calcium channels (specialized channels through plasma membrane). Thus high calcium level at the axon terminus.

The neurotransmitter causes the change action potential along the surface of muscle fibre. At the opening of each Transverse tubule onto the muscle fibre surface, the action potential spreads inside the muscle fibre. At each point where a t-tuble touches part of sarcoplasmic reticulum, it causes sarcoplasmic reticulum to release Calcim ions. Thus high level of calcium observed at t-tubule and sarcoplasmic reticulum.

The calcium ions then flow into the cytoplasm and bind to the troponin and tropomyosin molecule. Thus high level of calcium bound to troponin and tropomyosin.

Binding of calcium allows troponin to change shape and expose the actin-myosin binding site.Myosin then connects to actin, and the formed complex forms a crossbridge that opens and closes and allows muscle cells to contract. Though actin-myosin complex is formed in presence on calcium ions, Calcium ion does not bind to myosin