Neuroscience - Action Potentials, Neurotransmitters & Synaptic Plasticity

Repolarization: Inactivation gates of voltage-gated $ext{Na}^{+}$ channels close and delayed rectifier ${\text{K}}^{+}$ channels open, releasing ${\text{K}}^{+}$ outward.
Hyperpolarization (Undershoot): Slow closing of voltage-gated ${\text{K}}^{+}$ channels drives $V_m$ temporarily closer to $E_{\text{K}}$ before returning to resting state.

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

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

Calculates the resting membrane potential ($V_m$) considering the relative permeabilities ($P$) and concentration gradients of multiple monovalent ions (${\text{K}}^{+}$, ${\text{Na}}^{+}$, ${\text{Cl}}^{-}$):

An electrogenic primary active transporter that hydrolyzes $1\text{ ATP}$ to pump $3{\text{ Na}}^{+}$ out of the neuron and $2{\text{ K}}^{+}$ into the neuron. It maintains the essential ion concentration gradients that drive resting potential and action potentials.

Typically around $-70\text{ mV}$ in neurons. RMP is mainly determined by high baseline permeability to ${\text{K}}^{+}$ via continuous open leak channels ($E_{\text{K}}\approx -90\text{ mV}$), along with minor ${\text{Na}}^{+}$ leak and the electrogenic contribution of the ${\text{Na}}^{+}/{\text{K}}^{+}$ ATPase.

Initiated when membrane potential reaches threshold (approx. $-55\text{ mV}$), opening fast voltage-gated ${\text{Na}}^{+}$ channels. ${\text{Na}}^{+}$ influx shifts $V_m$ toward $E_{\text{Na}}\approx +60\text{ mV}$ through a positive feedback mechanism (Hodgkin cycle).

Absolute Refractory Period: Time during depolarization/early repolarization when no new action potential can fire because voltage-gated ${\text{Na}}^{+}$ channels are inactivated.
Relative Refractory Period: Time during hyperpolarization when an action potential can fire, but requires a stronger-than-normal stimulus because ${\text{K}}^{+}$ channels remain open.

The rapid propagation of action potentials along myelinated axons. Voltage-gated ${\text{Na}}^{+}$ channels are densely clustered at the Nodes of Ranvier, allowing depolarization to 'jump' electronically from node to node between insulated myelin segments.