Neuroscience - Synaptic Transmission, Action Potentials & CNS Anatomy

The membrane potential ($E_{ion}$) at which the electrical gradient balances the concentration gradient for a specific ion across a selectively permeable membrane.

At $37^{\circ }\text{C}$, the simplified form for monovalent cations ($z=+1$) is:
$$E_{ion}=61.5{\log }_{10}\left(\frac{[ion{]}_{out}}{[ion{]}_{in}}\right)$$

Calculates the overall resting membrane potential ($V_m$) by considering the concentration gradients and relative membrane permeabilities ($P$) of multiple monovalent ions ($K^{+}$, $N{a}^{+}$, $C{l}^{-}$):

The steady-state baseline potential of a neuron, typically around $-70\text{ mV}$. It is maintained primarily by:
1. Leak $K^{+}$ Channels: High baseline resting permeability to $K^{+}$ ($P_K\gg P_{Na}$).2. $N{a}^{+}/K^{+}$-ATPase: Electrostatic pump pushing $3N{a}^{+}$ out for every $2K^{+}$ brought in.

Occurs when a local stimulus reaches threshold potential (typically $\sim -55\text{ mV}$):
• Rapid opening of voltage-gated $N{a}^{+}$ channels ($Nav$).
• Influx of $N{a}^{+}$ down its electrochemical gradient drives $V_m$ rapidly toward $+40\text{ mV}$ ($E_{Na}$).

Restoration of a negative internal potential following peak overshoot:
• Inactivation gates of $Nav$ channels close, terminating $N{a}^{+}$ influx.
• Delayed rectifier voltage-gated $K^{+}$ channels ($Kv$) open, leading to rapid $K^{+}$ efflux down its electrochemical gradient.

A temporary state where $V_m$ becomes more negative than the resting potential (approaching $E_K\approx -90\text{ mV}$).
• Caused by slow closing kinetics of $Kv$ channels, keeping $P_K$ elevated.
• Returns to baseline resting potential as $Kv$ channels close completely and $N{a}^{+}/K^{+}$-ATPase maintains gradients.

• Absolute Refractory Period: Time during depolarization and early repolarization when no action potential can be fired regardless of stimulus strength, because $Nav$ channels are open or inactivated.
• Relative Refractory Period: Time during hyperpolarization when an action potential can be fired, but requires a stronger-than-normal stimulus due to partial $Nav$ recovery and elevated $K^{+}$ conductance.

The rapid propagation of action potentials along myelinated axons.
• Myelin Insulation: Decreases membrane capacitance and increases transverse resistance.
• Nodes of Ranvier: Unmyelinated gaps enriched in high densities of $Nav$ channels, allowing the action potential to 'jump' from node to node, increasing conduction velocity while saving energy.