Myelination and Saltatory Conduction in Neurons Explained

An action potential has distinct phases: (1) Resting potential at -70 mV; (2) Depolarization when membrane potential rises to +30 mV; (3) Repolarization when membrane potential returns toward resting values; (4) Hyperpolarization when membrane potential briefly becomes more negative than resting; (5) Return to resting potential. The action potential threshold is the minimum membrane potential (approximately -55 mV) that must be reached to trigger an action potential. When the membrane potential reaches this threshold, voltage-gated sodium channels open, allowing sodium ions to flow into the cell and generate the action potential. Voltage-gated sodium channels have a gating mechanism that responds to membrane potential changes. Voltage-gated potassium channels open in response to membrane depolarization but with a delay compared to sodium channels. When these channels open, potassium ions flow out of the cell, following their concentration gradient. This efflux of potassium ions causes repolarization of the membrane, returning the potential toward the resting value. After an action potential, the membrane returns to its resting potential through the activity of voltage-gated potassium channels and the sodium-potassium pump. Leak channels are always open and allow passive ion movement according to concentration gradients.

Action potentials are generated by voltage-gated sodium and potassium channels. When the threshold potential is reached, voltage-gated sodium channels rapidly open, allowing sodium ions to rush into the cell due to both concentration and electrical gradients. This causes rapid depolarization to about +30 mV. Simultaneously, voltage-gated potassium channels begin opening more slowly. Critically, sodium channels then inactivate (close) at this peak, preventing continuous sodium influx. Potassium channels remain open longer, causing repolarization as potassium ions exit the cell.

An action potential is a wave of depolarization and repolarization caused by voltage-gated sodium and potassium channels opening and closing in sequence: when a neuron reaches threshold (-55 mV), sodium channels open causing rapid depolarization (inside becomes positive), followed by potassium channels opening causing repolarization (restoring negative charge), and finally hyperpolarization occurs before the refractory period returns the neuron to resting potential (-70 mV).

Action potentials are generated by specific ion movements across the neuronal membrane. Depolarization is caused by the rapid and significant entry of sodium ions (Na+) into the cell through voltage-gated sodium channels. This sodium influx makes the inside of the cell more positive, reducing the negative charge difference across the membrane. Repolarization and hyperpolarization are caused by the slower and prolonged exit of potassium ions (K+) from the cell through voltage-gated potassium channels. The sodium ions enter quickly and in large amounts, while potassium ions exit more slowly and over a longer period, creating the characteristic shape of the action potential. Voltage-gated ion channels are specialized proteins that open and close in response to changes in electrical potential. These channels are crucial for action potentials because they allow ions to cross the membrane only when the neuron is actively firing. The sodium channels open in response to depolarization, allowing sodium ions to enter and continue the depolarization process. The potassium channels open more slowly, allowing potassium ions to exit and cause repolarization.

An action potential is a rapid, all-or-none electrical signal generated when a stimulus depolarizes the membrane to threshold (-55 mV), causing voltage-gated Na+ channels to open and Na+ influx depolarizes the membrane; subsequently, Na+ channels close and voltage-gated K+ channels open, causing K+ efflux that repolarizes the membrane back toward resting potential (-90 mV), with the Na-K ATPase pump continuously restoring ion concentration gradients; voltage-gated Na+ channels contain both activation and inactivation gates that control Na+ flow during depolarization and repolarization phases, and the generation involves a positive feedback loop where initial Na+ entry intensifies further Na+ entry, while the concentration gradient changes per action potential are negligible, allowing millions of action potentials before significant gradient reduction occurs.

In unmyelinated fibers, action potentials propagate through continuous conduction. Depolarization at one point creates a local circuit that depolarizes adjacent membrane, triggering a new action potential. This process repeats sequentially along the fiber. The action potential itself does not move; rather, each segment generates its own action potential. This is called continuous conduction.

In unmyelinated nerves, the action potential propagates continuously from point to point along the entire length of the axon. This is called continuous or point-to-point conduction. Each segment of the axon membrane undergoes depolarization and repolarization as the signal passes through. The local current flow from the depolarized region triggers voltage-gated channels in adjacent segments, propagating the signal.

This section explains how action potentials propagate along unmyelinated axons through a continuous process. When an action potential occurs in one region, it creates local currents that flow from depolarized regions to adjacent regions. These local currents cause gradual depolarization in adjacent regions. If this depolarization reaches threshold, it triggers new action potentials in the adjacent region. The process repeats sequentially along the axon, with each region undergoing depolarization, repolarization, and hyperpolarization in sequence. This mechanism ensures the signal travels from the initial segment to the axon terminals.

In unmyelinated axons, action potentials propagate through continuous conduction where voltage-gated sodium channels open sequentially along the axon membrane, allowing sodium ions to enter and depolarize adjacent segments, followed by potassium channels that repolarize the membrane; this process occurs at speeds below 3 meters per second and is essential for transmitting sensory information related to pain, temperature, and pressure, as well as motor signals to autonomic organs.

In unmyelinated axons, action potentials propagate through continuous diffusion of ions. As sodium enters one area, it diffuses to adjacent areas within the axon. When sodium concentration increases enough to reach threshold in these adjacent areas, sodium gates open wide, sodium rushes in, causing depolarization and generating a new action potential. This process continues down the length of the axon as sodium diffuses through the ectoplasm.

This section establishes the foundational framework for analyzing electrical phenomena in biological systems. Key concepts include: (1) Current variables for cylindrical conductors—current I flowing along the exterior, current I_in flowing internally, and transmembrane current flowing perpendicular across the membrane; (2) Current density J = I/(2πaL) representing current per unit surface area; (3) Current per unit length K = I/L; (4) Ohm's law V = IR extended to biological contexts using impedance (complex values accounting for capacitive membrane behavior); (5) Electric field E = V/distance, with E = ρI/(2πrL) for cylindrical geometry; (6) Resistivity ρ as a material property (Ohm-meters) and conductivity σ = 1/ρ (S/m); (7) Current density formulation J = σE for analyzing current flow through biological tissues.

Current refers to the flow of charged particles—ions in the body, electrons in electrical cords. Resistance represents the friction charged particles experience moving through tubes or vessels, caused by rubbing against walls. Resistance is expressed as a constant multiplied by length divided by cross-sectional area. This concept applies not only to electrical conduction in axons but also to blood pressure in blood vessels. Understanding resistance is crucial for comprehending how electrical signals conduct through the nervous system and how physical dimensions affect signal transmission.

Electrical resistance is a fundamental unit of electricity alongside voltage and current. Resistance represents opposition to current flow when electrons attempt to travel between two points. Current is the flow of electrons through a conductor, which can encounter either easy or difficult paths. Conductors (like metals) have low resistance, allowing current to flow easily. Insulators impede current flow, allowing only minimal current even under high voltage. The key difference is the quantity of current that can pass through each material type.

Electric current is the rate of flow of electric charge, defined by I = Q/t, with SI units of Coulomb for charge, second for time, and Ampere for current. Electric potential is the work done to bring a unit positive charge from infinity to a point, while potential difference is the work done to move a unit charge between two points. Conductors allow current flow easily (silver is best), while insulators prevent it. Electrical resistance is the property opposing current flow, denoted by R and measured in Ohms. Resistance depends on four factors: (1) Material nature - different materials have different resistances, (2) Temperature - resistance increases with temperature as atoms vibrate more and cause more electron collisions (direct relationship), (3) Length - resistance is directly proportional to length (longer conductors have higher resistance), and (4) Cross-sectional area - resistance is inversely proportional to area (thicker conductors have lower resistance). These concepts form the foundation for understanding electrical circuits.

Electricity is built upon three fundamental pillars: current (I), voltage (V), and resistance (R). Current is the flow of electric charge measured in Amperes using an ammeter connected in series. Voltage is the work done to move unit charge between two points, measured in Volts using a voltmeter connected in parallel. Resistance is the opposition to current flow, measured in Ohms. Conductors consist of positive ions fixed in place and free electrons that move throughout the material. When voltage is applied, electrons move in a specific direction, creating current. Resistance arises from collisions between moving electrons and the atoms/ions of the conductor. These collisions impede electron flow, creating opposition to current. Resistance has both harmful effects (unwanted heat generation) and beneficial effects (used in heaters, light bulbs).