Action Potentials and Neuronal Conduction

Fundamentals of Resting Membrane Potential

• The neuronal membrane exhibits selective permeability, primarily to potassium ions ($K^+$) at rest.

• Two opposing forces govern ion movement across the neuronal membrane:

  • Chemical Concentration Gradient (Diffusion): Drives $K^+$ from areas of high concentration inside the cell toward areas of low concentration outside the cell.

  • Electrical Potential Difference (Voltage): Attracts positively charged $K^+$ ions back into the negatively charged cell interior.

• Equilibrium state distribution:

  • Potassium concentration ($[K^+]$) is significantly higher inside the cytoplasm than in the extracellular fluid.

  • Sodium concentration ($[Na^+]$) is significantly higher in the extracellular fluid than inside the cytoplasm.

• Resting membrane potential is negative, typically maintained at approximately $-65\,\text{mV}$ to $-70\,\text{mV}$ in human neurons.

• Increasing membrane permeability to sodium ions ($Na^+$) allows $Na^+$ influx down its concentration and electrical gradients, shifting the membrane potential from negative toward positive values.

Electrophysiological Investigation via Voltage Clamp

• The voltage clamp technique enables researchers to control membrane potential at a fixed voltage level while measuring the resulting ion currents across the membrane.

• Developed and utilized by Alan Hodgkin and Andrew Huxley on the squid giant axon to systematically elucidate changes in membrane permeability as a function of membrane potential.

• Hodgkin and Huxley were awarded the Nobel Prize in Physiology or Medicine in 1963, sharing the honor with John Eccles.

• Hyperpolarization Responses:

  • Hyperpolarizing the membrane (e.g., stepping potential from $-65\,\text{mV}$ to $-130\,\text{mV}$, representing a $65\,\text{mV}$ hyperpolarization) produces a brief transient capacitive current (reaching $-1\,\text{mA/cm}^2$ at $t = 0\,\text{ms}$) with no sustained ionic current flow.

• Depolarization Responses:

  • Depolarizing the membrane (e.g., stepping potential from $-65\,\text{mV}$ to $0\,\text{mV}$, representing a $65\,\text{mV}$ depolarization) triggers a multi-phase electrical response:

  • A brief initial capacitive current.

  • A transient inward current that peaks rapidly.

  • A delayed outward current that develops slowly and persists for the duration of the depolarization step.

Ionic Basis of Action Potential Currents

• Identification of Sodium Current ($Na^+$):

  • Removal of extracellular $Na^+$ reverses the early transient inward current into an outward current, demonstrating that the transient inward current is driven by $Na^+$ influx.

• Identification of Potassium Current ($K^+$):

  • Experiments utilizing radiolabeled potassium ($^{42}\text{K}^+$) demonstrated that the delayed outward current corresponds directly to $K^+$ efflux from the cell.

• Pharmacological Channel Blockers:

  • Tetrodotoxin (TTX): A neurotoxin that selectively blocks voltage-gated $Na^+$ channels. Application of TTX completely eliminates the transient inward $Na^+$ current without altering the delayed outward $K^+$ current.

  • Tetraethylammonium (TEA): A chemical compound that selectively blocks voltage-gated $K^+$ channels. Application of TEA completely eliminates the delayed outward $K^+$ current without altering the transient inward $Na^+$ current.

Threshold Dynamics, Refractory Periods, and All-or-None Behavior

• Threshold Dynamics:

  • An action potential is initiated when local membrane depolarization reaches a specific threshold potential.

  • Typical neuronal threshold involves a shift from a resting potential of $-70\,\text{mV}$ up to approximately $-55\,\text{mV}$.

• All-or-None Principle:

  • Action potentials are binary events: an action potential either fires completely or fails to fire at all.

  • Subthreshold stimuli elicit only passive electrotonic responses that decay exponentially across distance and time.

  • Action potentials do not vary in amplitude or intensity based on stimulus magnitude; every full action potential generated by a neuron achieves the same peak voltage.

  • Stimulus intensity and information are encoded by the frequency (firing rate) of action potentials rather than their amplitude.

• Refractory Periods:

  • Absolute Refractory Period: Time window immediately following action potential initiation during which no second action potential can be generated, regardless of the intensity of the depolarizing stimulus.

  • Relative Refractory Period: Period following the absolute refractory period during which a second action potential can be elicited, but requires a stronger depolarizing stimulus than normal to reach threshold.

Voltage-Gated Channel Kinetics and Feedback Loops

• Positive Feedback Loop ($Na^+$ Channels):

  • Depolarization to threshold opens voltage-gated $Na^+$ channels.

  • $Na^+$ ions flow into the cell, causing additional local depolarization.

  • This increased depolarization opens a greater number of adjacent voltage-gated $Na^+$ channels, driving rapid runaway depolarization toward the $Na^+$ equilibrium potential.

• Dual-Gate Structure of Voltage-Gated $Na^+$ Channels:

  • Activation Gate: Closed at resting membrane potential; opens rapidly in response to membrane depolarization.

  • Inactivation Gate: Open at resting membrane potential; closes automatically via a time-dependent mechanism shortly after depolarization occurs.

  • Closing of the inactivation gate halts $Na^+$ influx and limits runaway depolarization.

• Negative Feedback Loop ($K^+$ Channels):

  • Membrane depolarization activates voltage-gated $K^+$ channels with delayed kinetics.

  • Efflux of positively charged $K^+$ ions removes positive charge from inside the cell.

  • $K^+$ efflux reverses depolarization, restoring the membrane potential to negative values (repolarization and hyperpolarization), which ultimately leads to the closing of the voltage-gated $K^+$ channels.

Sequential Phases of the Action Potential Cycle

• 1. Resting State:

  • Membrane potential remains stable at $-70\,\text{mV}$.

  • Voltage-gated $Na^+$ and $K^+$ channels are closed.

  • Membrane permeability to $Na^+$ and $K^+$ is low; excess positive ions reside outside the plasma membrane and excess negative charges reside inside cytoplasm.

• 2. Depolarizing Phase:

  • A depolarizing stimulus brings the membrane potential to threshold ($-55\,\text{mV}$).

  • Voltage-gated $Na^+$ activation gates open rapidly.

  • A massive influx of $Na^+$ reverses membrane polarity, driving the membrane potential up toward positive values (e.g., $+30\,\text{mV}$).

• 3. Repolarizing Phase:

  • Voltage-gated $Na^+$ channel inactivation gates close, rendering the membrane impermeable to $Na^+$ again.

  • Delayed voltage-gated $K^+$ channels open fully.

  • Efflux of $K^+$ down its electrochemical gradient removes positive charge from the cytoplasm, driving the membrane potential back toward negative values.

• 4. Repolarization Continuation and Hyperpolarization:

  • Continued $K^+$ efflux temporarily hyperpolarizes the membrane past the resting potential toward the $K^+$ equilibrium potential.

  • Voltage-gated $K^+$ channels close, and leak currents return the cell to its resting potential.

Active Ion Transport via the Sodium-Potassium Pump

• Role of the $\text{Na}^+/\text{K}^+$ Pump:

  • Repeated action potentials lead to intracellular accumulation of $Na^+$ and extracellular accumulation of $K^+$.

  • The $\text{Na}^+/\text{K}^+$ ATPase pump actively transports $Na^+$ out of the cell and $K^+$ into the cell against their respective concentration gradients to restore resting ionic distributions.

  • Metabolic requirement: Requires active ATP hydrolysis and accounts for $20\%$ to $40\%$ of total brain energy consumption.

• Enzymatic Mechanism Steps:

  1. $\text{Na}^+$ Binding: Three intracellular $Na^+$ ions bind to high-affinity sites on the cytosolic side of the pump.

  2. Phosphorylation: ATP donates a phosphate group ($\text{P}_i$) to the pump, forming ADP and phosphorylating the protein.

  3. Conformational Change and $\text{Na}^+$ Release: Phosphorylation triggers a conformational change that opens the pump to the extracellular side, reducing affinity for $Na^+$ (releasing $3\,\text{Na}^+$) and increasing affinity for extracellular $K^+$.

  4. $\text{K}^+$ Binding, Dephosphorylation, and Release: Two extracellular $K^+$ ions bind to the pump, inducing dephosphorylation. Loss of the phosphate group causes the pump to revert to its original conformation, releasing $2\,\text{K}^+$ ions into the cytoplasm.

Propagation Dynamics and Saltatory Conduction

• Mechanism of Propagation:

  • $Na^+$ influx during an action potential causes passive local current flow inside the axon cytosol.

  • This current depolarizes adjacent membrane regions to threshold, triggering voltage-gated $Na^+$ channels in neighboring segments.

  • Refractory periods of recently activated channels prevent retrograde signal propagation, ensuring strictly unidirectional conduction along the axon.

• Determinants of Conduction Velocity:

  • Axon Diameter: Larger axon diameters decrease internal resistance to passive current flow, increasing conduction velocity.

  • Myelination: Fatty insulating sheath wrapped around axons by glial cells.

• Comparison of Conduction Speeds:

  • Unmyelinated axons: Conduction velocity ranges from $0.7\,\text{m/s}$ to $2.3\,\text{m/s}$.

  • Myelinated axons: Conduction velocity ranges from $70\,\text{m/s}$ to $120\,\text{m/s}$.

• Saltatory Conduction:

  • Passive electrical current spreads rapidly down the axon cytoplasm beneath the myelin sheath, but decays over distance.

  • To prevent complete decay of the signal, action potentials are regenerated periodically at uninsulated gaps in the myelin sheath known as Nodes of Ranvier.

  • Nodes of Ranvier contain dense clusters of voltage-gated $Na^+$ and $K^+$ channels.

  • By restricting time-consuming channel gating and ion fluxes strictly to the Nodes of Ranvier, myelin allows the action potential to effectively jump from node to node, maximizing conduction speed.