Question

Walk through a signal propagation from beginning to end. Also, include +30mV, -60mV, -70mV, -80mV, resting, depolarization, repolarization, hyperpolarization, absolute refractory & relative refractory period, all-or-none principle, neurotransmitter, threshold.

Answer 1

1. RESTING STAGE:
The membrane is in polarised state with potential in nerve about - - 85mV.
Sodium and potassium channels are closed. Na+ are more outside the cell; K+ are concentrated inside the cell. Potential difference: - 80mV.

2. Depolarization STAGE:
When neurotrasmittr binds to the receptor, ligand gated Na channels are open that causes local potential to touchthe threshold level (-70 mV) it opens up sodium channel in the membrane causing the abrupt rising of membrane potential towards positivity. It overshoot beyond the zero level and become positve (+ 30 mV).
This is because of entry of Positive Sodium ions into the cell

3.Stage of Repolarization:
Within fractions of second the sodium channel begins to close and potassium channels open up causing diffusion of positive potassium to outside the cell.
This results in decreasing of membrane potential towards negativity.

4. Resting Conditions Re-established
Na+ and K+ channels are closed. Sodium-potassium exchange pump moves Na+ out and K+ in. Resting potential difference: -85 mV

Role of Sodium voltage gated channels:
These channels are essential for Depolarization stage.
If these channels are blocked, the Depolarization will not occur and the nerve will be hyperpolarised.
If these channels are stimulated then it results in more rapid functioning of the nerve.

Role of potassium voltage gated channels :
These channels are essential for Repolarization of the nerve potential
If the get blocked the nerve will be in sustained state of Depolarization.
If these channels are stimulated. Action potential will cease more rapidly and there will occur a hyperpolarization state.

Absolute refractory period is the duration in action potential at which any strength of stimulus won't be able to elicit a new action potential. This period corresponds to initial phase of repolarization during which sodium inactivation gates are closed.

Relative refractory period is the duration in the action in which a strong stimulus will only be able to evoke an action potential.

Action potential creates a local current in the membrane which result in depolarizaton of the successive forward site on the membrane.

All or nothing : All or None phenomenon :
action potential follows all or none phenomenon which means that if action potential occurs it will happen to its all extent it won't happen.