T3 -P1-Propagacion del Potencial de Acción - BCM - Profesor Biofísica Dr. Gonzalo Ferreira de Mattos

Propagation of Action Potential

Introduction to Action Potential

• The class focuses on studying the propagation of action potential, building on the previous lesson about how an active action potential is generated.

• The action potential is generated at the axon hillock (cone of initiation), where a change in cell polarity occurs, making the interior positive relative to the exterior.

Mechanism of Action Potential Generation

• Once the threshold is crossed, local current circuits are established, allowing for self-regeneration of action potentials towards the presynaptic terminal.

• Excitability relies on selective permeability for two types of ions: potassium and sodium, which have different electrochemical equilibrium potentials.

Ion Conductance and Membrane Dynamics

• Potassium currents are slower and outward while sodium currents are rapid and inward; this creates a peak in current that leads to depolarization.

• Selective permeability changes based on ion conductance; as sodium permeability increases, it drives membrane potential toward its equilibrium.

Role of Ion Channels

• Ion channels can open or close, altering membrane conductance. Sodium conductance peaks when all channels are open during an action potential.

• The relationship between ion current and voltage is crucial; maximum conductance occurs at peak sodium influx due to driving force.

Phases of Action Potential

• During repolarization, sodium conductance increases temporarily before inactivation occurs. This dynamic affects overall membrane behavior.

• If depolarization exceeds threshold, sodium conductance surpasses other membrane conductances leading to further polarization until reaching sodium's equilibrium potential.

Inactivation and Recovery Phase

• Sodium channels undergo intrinsic inactivation while voltage-gated potassium channels open slowly without inactivating immediately.

• As potassium conductance becomes greater than sodium during late phases post-action potential, membrane potential shifts closer to potassium's equilibrium state.

Summary Insights

• Understanding how action potentials generate and propagate reveals key characteristics defining excitability and rapid information transmission within neurons.

Membrane Capacitance and Sodium Conductance in Resting State

Understanding Membrane Dynamics

• The resting state of a membrane is characterized by capacitance and sodium conductance being inactive, leading to a simple RC circuit model.

• When the threshold is reached, an inward sodium current begins to flow, causing depolarization in adjacent elements due to their cylindrical structure.

• The incoming sodium current can polarize neighboring elements; if they reach the threshold, action potentials are regenerated.

• This process creates a self-regenerative wave of depolarization that requires the interior of the cell to become positively charged relative to the exterior.

Electromotive Forces and Action Potentials

• At rest, membrane resistance remains constant with an electromotive force equal to the resting potential; this results in no effective current circulation as opposing forces balance out.

• When an action potential occurs, it shifts towards sodium's equilibrium potential (+63 mV), altering current flow direction due to changes in electromotive forces.

• The positive charge inside during an action potential causes local currents that circulate differently compared to resting states.

Propagation of Action Potentials

• Localized currents generated during action potentials lead to capacitive currents initially before transitioning into ionic currents once thresholds are surpassed.

• The total membrane current comprises both capacitive and ionic components, distinguishing between localized (propagated) versus point potentials observed previously.

Circuitry and Clinical Implications

• Propagated action potentials involve multiple local circuits rather than uniform activation across membranes; this complexity allows for nuanced understanding of neuronal signaling.

• Localized currents can be recorded using extracellular electrodes positioned at various locations, enabling clinical applications such as monitoring brain activity for conditions like seizures.

• Variations in sodium and potassium conductances during propagated action potentials illustrate dynamic cellular responses essential for neural communication.

Summary of Key Concepts

Electroencephalography and Cardiac Activity

Understanding Electroencephalograms (EEG)

• The electroencephalogram (EEG) captures the activity of numerous neurons simultaneously, using extracellular electrodes to record local currents without penetrating cells.

• Similar to EEG, cardiac activity is recorded through a method known as the "triangle of Einthoven," which uses strategically placed electrodes to derive electrical signals from the heart.

Recording Cardiac Electrical Activity

• Electrodes positioned on the body surface detect local currents generated by action potentials in myocardial tissue, leading to an electrocardiogram (ECG).

• The propagation of action potentials creates local current circuits that can be detected by these extracellular electrodes, forming a basis for clinical techniques used in disease detection.

Action Potential Propagation

• At the activation front of an axon, there is a shift in charge distribution: positive outside and negative inside at rest; during action potential, this reverses.

• As the dipole formed by this charge distribution approaches an electrode, it generates detectable changes in potential due to its position relative to positive and negative charges.

Extracellular Recordings

• An extracellular recording of action potentials involves two active electrodes; they do not penetrate the axon but measure voltage changes based on ionic movements.

• By applying pressure on a nerve trunk, one can observe how stimulation affects potential propagation and amplitude through phenomena like recruitment.

Dynamics of Action Potentials

• When stimulating a compressed nerve trunk, increased stimulus leads to higher amplitude waves due to restricted propagation at the compression site.

• Local currents regenerate action potentials over time; recordings show variations at different milliseconds post-stimulation indicating phases of depolarization and repolarization.

Myelination and Signal Propagation Speed

• Myelination enhances action potential propagation speed; myelin sheaths are formed by Schwann cells in peripheral nerves creating nodes of Ranvier.

• These nodes allow for saltatory conduction where impulses jump between nodes rather than traveling continuously along unmyelinated sections.

Structure of Myelin Sheath

• In peripheral nervous systems, Schwann cells wrap around axons multiple times creating layers devoid of cytoplasm that insulate electrical signals effectively.

Understanding Membrane Resistance and Capacitance

The Role of Lipids in Membrane Resistance

• The structure of lipids does not create a series connection of membrane elements; instead, the equivalent resistance is the sum of all resistances, significantly increasing membrane resistance.

Capacitance in Series

• In a series configuration, the equivalent capacitance decreases as it is calculated by taking the inverse sum of individual capacitances, leading to reduced membrane capacity.

Impact on Lambda Constant

• As membrane resistance (rm) increases, the lambda constant (λ), which represents space constant, also increases. This results in prolonged polarization effects over greater distances.

Action Potential Propagation

• Increased λ allows action potentials to regenerate over larger distances in myelinated nerves compared to unmyelinated ones, enhancing nerve conduction speed.

Experimental Insights into Nerve Conduction

Electrode Placement and Action Potentials

• Experiments with extracellular electrodes positioned at various nodes revealed that action potentials are only observable at specific nodes due to high internal resistance preventing current leakage.

Saltatory Conduction Mechanism

• The concept of saltatory conduction explains how action potentials jump from node to node along myelinated fibers, minimizing current loss and significantly increasing propagation speed.

Comparative Analysis of Nerve Types

Evolutionary Adaptations for Speed

• Comparing giant squid axons with rabbit myelinated nerves illustrates evolutionary adaptations: squids increase cylinder area to reduce internal resistance while rabbits develop myelin sheaths for faster signal transmission.

Information Transmission Efficiency

• Myelin minimizes structural requirements for rapid information transfer in multicellular organisms, avoiding the need for excessively large communication structures.

Velocity of Action Potential Conduction

Diameter Influence on Conduction Speed

• The relationship between fiber diameter and conduction velocity resembles a parabolic fraction; as diameter increases, so does speed due to enhanced λ values derived from rm/r ratios.

Peripheral vs Central Nervous System Differences

• In peripheral nervous systems, conduction speed increases linearly with fiber diameter until reaching a minimum threshold necessary for Schwann cell wrapping around fibers.