Physiology Essay: Action Potentials

[Na⁺] outside cell > inside cell
[K⁺] outside cell < inside cell
Na⁺/K⁺ ATPase actively pumps 3Na⁺ out and 2K⁺ into cell.
The cell membrane is more permeable to K⁺ than it is to Na⁺, so there’s a higher tendency for K⁺ to diffuse out than for Na⁺ to diffuse in.
This gives the cell a RMP of around -70mV.

A stimulus opens the Na⁺ gates at the axon of hillock, so Na⁺ quickly diffuse into the cell, causing depolarisation.
→ The axon of hillock is an area where the density of voltage-gated Na⁺ channels is the highest.
A threshold of around -50mV must be reached for an AP to be generated as APs are “all-or-nothing” events. This prevents overstimulation of neurons.
More Na⁺ channels open because of positive feedback loop, making the cell become more positive.
The peak is reached at around +40mV to +50mV. This change in potential causes K⁺ channels to open.

Na⁺ channels close but K⁺ channels remain open.
This causes the potential to slowly return to its resting state (by becoming more negative again).

Since K⁺ channels open and close very slowly (compared to Na⁺channels), the cell becomes more negative than its initial RMP.
The Na⁺/K⁺ ATPase pumps rapidly correct for tiny quantities of Na⁺ that were gained and K⁺ that were lost by pumping Na⁺ out and K⁺ into the cell → RMP restored.

After Na⁺ gates close, they’re blocked for a short period, meaning no stimulus of any size can cause them to open.
This ensures that the signal travels in only 1 direction, preventing it from going backward and stimulating the previously activated region.
It also limits the frequency of APs along a neuron.

Have myelinated axons → Schwann cell growing around axon which forms a fatty layer around it.
This acts as an insulator and increases speed of conduction.
This means APs don’t have to be formed at every point along an axon - instead, the APs jump from 1 Node of Ranvier to another.

Is the process by which an AP moves step-by-step along an unmyelinated axon, depolarising each successive segment of the axonal membrane.
This is slower than saltatory conduction due to the continuous involvement of every part of the axon membrane.