Neuroscience - Synaptic Transmission, Neural Pathways & Action Potentials

Calculates the equilibrium potential ($E_x$) for a single permeable ion across a membrane:

At physiological body temperature ($37^{\circ }\text{C}$), this simplifies to:

Determines the overall resting membrane potential ($V_m$) by accounting for concentrations and relative membrane permeabilities ($P$) of monovalent ions:

Triggered when a stimulus brings membrane potential to threshold ($\approx -55\text{ mV}$). Fast voltage-gated $N{a}^{+}$ channels open rapidly, driving an influx of $N{a}^{+}$ down its electrochemical gradient towards $E_{Na}(\approx +60\text{ mV})$, peaking around $+30\text{ mV}$.

Voltage-gated $N{a}^{+}$ channels undergo inactivation (inactivation gate closes), halting $N{a}^{+}$ influx. Delayed rectifier voltage-gated $K^{+}$ channels open, causing a rapid efflux of $K^{+}$ down its gradient to restore negative intracellular charge.

Slow closing kinetics of voltage-gated $K^{+}$ channels permit continued $K^{+}$ efflux, temporarily driving $V_m$ lower than resting potential, closer to $E_K(\approx -90\text{ mV})$. Baseline potential is restored by $N{a}^{+}/K^{+}$-ATPase and leak channels.

Exists in three distinct states:
1. Closed/Resting: Activation gate closed, inactivation gate open (primed for depolarization).
2. Open: Both gates open upon reaching threshold.
3. Inactivated: Activation gate open, inactivation gate closed via ball-and-chain mechanism (prevents reactivation).

• Absolute Refractory Period: No new action potential can be fired because voltage-gated $N{a}^{+}$ channels are inactivated.
• Relative Refractory Period: A suprathreshold stimulus can induce an AP because $N{a}^{+}$ channels have reset, but membrane is hyperpolarized and $K^{+}$ conductance remains elevated.

Myelin wraps axons to increase membrane resistance ($R_m$) and decrease capacitance ($C_m$). Action potentials jump between unmyelinated gaps called Nodes of Ranvier, where voltage-gated $N{a}^{+}$ channels are clustered, drastically increasing conduction velocity.