Neuroscience - Synaptic Transmission & Neural Circuits

The electrical potential difference across the plasma membrane of a neuron at rest, typically around $-70mV$. It's maintained by the differential distribution of ions ($N{a}^{+}$, $K^{+}$, $C{l}^{-}$) and the action of the $N{a}^{+}/K^{+}$ pump.

The $N{a}^{+}/K^{+}$ pump actively transports $3N{a}^{+}$ ions out of the cell and $2K^{+}$ ions into the cell for every ATP molecule hydrolyzed. This creates and maintains the concentration gradients essential for the RMP and action potentials.

Depolarization is a decrease in the absolute value of a cell's membrane potential, making the inside of the cell less negative (or more positive). It typically occurs due to the influx of positive ions, such as $N{a}^{+}$, into the cell.

The threshold potential is the critical level of depolarization (typically around $-55mV$) that must be reached for an action potential to be initiated. Once reached, voltage-gated $N{a}^{+}$ channels open rapidly, leading to a regenerative depolarization.

The rising phase is characterized by rapid depolarization. Upon reaching threshold, voltage-gated $N{a}^{+}$ channels open, causing a massive influx of $N{a}^{+}$ ions into the cell, driving the membrane potential from negative to positive (e.g., up to $+30mV$).

Repolarization is the phase where the membrane potential returns to a negative value. It occurs due to the inactivation of voltage-gated $N{a}^{+}$ channels and the opening of voltage-gated $K^{+}$ channels, leading to an efflux of $K^{+}$ ions out of the cell.

Hyperpolarization, or the undershoot, is a brief period where the membrane potential becomes even more negative than the resting potential. It occurs because voltage-gated $K^{+}$ channels close relatively slowly, allowing excess $K^{+}$ efflux before returning to the RMP.

The 'all-or-none' principle states that if a stimulus reaches the threshold potential, an action potential will fire with its full, characteristic amplitude. If the threshold is not reached, no action potential will occur. The strength of the stimulus does not affect the amplitude of the action potential, only its frequency.