Comprehensive Study Guide on Neuronal Membrane Potentials and Action Potential Propagation

Resting Membrane Potential (RMP) Characteristics - The resting membrane potential is defined as the electrical potential difference across the plasma membrane of a neuron during periods when it is not actively firing or transmitting signals. - The typical value for RMP is approximately $-70\,mV$. - This negative value indicates that the internal environment of the cell is more negative relative to the extracellular environment. - Major contributors to the maintenance of RMP include: - 1. Unequal Ion Distribution: - There is a high concentration of potassium ions ($K^+$) inside the cell. - There is a high concentration of sodium ions ($Na^+$) outside the cell. - Large negatively charged proteins ($A^-$) are trapped inside the cell. For the purposes of this specific class, these are referred to as Negative Collagen Fibers. - 2. Selective Membrane Permeability: - At rest, the neuronal membrane is significantly more permeable to $K^+$ than it is to $Na^+$ . - The presence of $K^+$ leak channels allows potassium to diffuse out of the cell down its chemical gradient. - While small amounts of $Na^+$ leak into the cell, this movement is far less substantial than the outward movement of $K^+$ . - 3. Electrochemical Gradient Dynamics: - The Chemical Gradient acts to push $K^+$ out of the cell. - The Electrical Gradient acts to pull $K^+$ back into the cell due to the attraction to the negative interior. - The stabilization of the membrane potential near $-70\,mV$ occurs due to the balance between these opposing forces. - 4. Sodium-Potassium ($Na^+/K^+$) ATPase Pump: - This active transport mechanism moves $3\,Na^+$ out of the cell for every $2\,K^+$ it moves into the cell. - The pump is essential for maintaining the established concentration gradients. - It provides a slight additional contribution to the negative charge inside the cell. # Electrochemical Driving Forces at Specific Potentials - Ion movement analysis at $-70\,mV$ (Resting Potential): - Sodium ($Na^+$): The chemical gradient is strongly inward and the electrical gradient is also inward because the negative interior attracts the positive ions. This creates a strong net driving force for $Na^+$ influx if channels open. - Potassium ($K^+$): The chemical gradient is outward while the electrical gradient is inward. The net effect is a moderate outward movement because the chemical gradient is the dominant force. - Ion movement analysis at $+30\,mV$ (Peak Depolarization): - Sodium ($Na^+$): The chemical gradient remains inward, but the electrical gradient is now outward because the positive interior of the cell repels the $Na^+$ ions. This results in a significantly reduced inward force. - Potassium ($K^+$): Both the chemical gradient and the electrical gradient are directed outward. This results in a strong net driving force for outward efflux, making $K^+$ the major contributor to the repolarization phase. # Detailed Steps of an Action Potential - 1. Resting State: Both voltage-gated $Na^+$ channels and voltage-gated $K^+$ channels are closed. The membrane remains at approximately $-70\,mV$. - 2. Graded Potential and Initial Depolarization: A neurotransmitter binds to ligand-gated $Na^+$ channels. This allows $Na^+$ to flow into the cell, making the membrane potential less negative. If this depolarization reaches the threshold of approximately $-55\,mV$, an action potential is triggered. - 3. Rapid Depolarization: Voltage-gated $Na^+$ channels open in response to reaching the threshold. This causes a massive influx of $Na^+$ , causing the membrane potential to rise rapidly until it peaks at approximately $+30\,mV$. - 4. Sodium ($Na^+$) Channel Inactivation: The inactivation gate of the voltage-gated $Na^+$ channels closes, preventing further entry of $Na^+$ . This event marks the beginning of the absolute refractory period. - 5. Repolarization: Voltage-gated $K^+$ channels open, though they exhibit a slower response time than sodium channels. $K^+$ flows out of the cell down its gradient, and the membrane potential begins to return toward a negative value. - 6. Hyperpolarization (Undershoot): Because the $K^+$ channels remain open slightly longer than necessary to reach the resting potential, the membrane becomes more negative than its resting state, reaching between $-80\,mV$ and $-90\,mV$. The relative refractory period occurs during this phase. - 7. Return to Resting State: Voltage-gated $K^+$ channels finally close. The combined action of leak channels and the $Na^+/K^+$ pump restores the original resting ion distributions and stabilizes the membrane at $-70\,mV$. # Refractory Periods and Conduction - Absolute Refractory Period: This period occurs during the time of $Na^+$ channel inactivation. During this phase, no stimulus, regardless of strength, can trigger another action potential. This mechanism ensures that the action potential propagates in only one direction. - Relative Refractory Period: This period occurs during the hyperpolarization phase. A second action potential can be triggered, but it requires a stronger-than-normal stimulus due to the ongoing efflux of $K^+$ making the cell more negative. - Saltatory Conduction in Myelinated Axons: - Myelin serves to increase membrane resistance and decrease membrane capacitance. - Voltage-gated $Na^+$ channels are not distributed evenly but are clustered at the nodes of Ranvier. - Action potentials essentially jump from one node to the next, which dramatically increases the velocity of signal conduction. - Impact of Demyelination: The loss of myelin results in current leakage, reduced efficiency of the signal, and either slowed or completely failed propagation of the action potential.