Given the following steps: 1. voltage gated potassium channels close; 2. threshold; 3. undershoot; 4. stimulus channels open; 5. voltage gated sodium channels close, pick the option that places them in the correct sequence starting with a neuron at its rmp.

4, 2, 5, 3, 1

Explanation:

The neuron remains at RMP unless stimulated. The stimulus results in opening of stimulus channels, this causes depolarization. if the extent of depolarization reaches to the level of threshold, the sodium and potassium channels begin opening but potassium channels are slow to open. Hence at first sodium goes out causing further depolarization until it reaches the peak at which the potassium channels open and the sodium channels close. This causes the potassium to rush our causing repolarization i.e. return of the membrane potential to RMP but the potassium channels are slow to close and it leads to hyperpolarization (undershoot) making the membrane potential more negative due to excessive movement of potassium outside the cell. Once the potassium channels close, the leak channels and Na K pump acts to return the potential to RMP.  

Explanation:

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.