Action Potencial

Action Potential

Electrical potentials exist across the membranes of all cells of the body. Some cells, such as nerve and muscle cells, generate rapidly changing electrochemical impulses at their membranes, Action Potential. In other types of cells, such as glandular cells, macrophages, and ciliated cells, local changes in membrane potentials also activate many of the cells’ functions.

Signals are transmitted by action potentials, which are rapid changes in the membrane potential that spread rapidly along ,for example, the nerve fiber membrane. Each action potential begins with a sudden change from the normal resting negative membrane potential to a positive potential and ends with an almost equally rapid change back to the negative potential. To conduct a nerve signal, the action potential moves along the nerve fiber until it comes to the fiber’s end.

Overview

Resting membrane potential

The resting membrane potential (RMP) is the stable, negative electrical charge across a cell membrane when it is not actively sending signals. It is primarily maintained by the $Na^{+}/K^{+}$ ATPase pump, which keeps Na⁺ high outside and K⁺ high inside, and by the high permeability of the membrane to ions.

$Na^{+}$/ $K^{+}$ pumps can take up to 70% of available cellular energy (ATP) in some cells, in order to keep the electrochemical gradient at RMP. Common in neuronal cells.

Passive K⁺ movement occurs through leak channels, with K⁺ moving along its electrochemical gradient. During hyperpolarization (which will be discussed later), K⁺ is pulled back into the cell. This is fast and happens immediately, making it the primary mechanism for restoring the RMP.

The Goldman-Hodgkin-Katz equation is used to calculate the resting membrane potential (RMP) with contributions from multiple ions.

Depolarization

Depolarization is a cellular process where the internal charge of a cell becomes less negative (more positive).

To cause depolarization, the net charge must overcome the threshold. It is the critical level of membrane charge that must be reached to trigger an action potential in excitable cells. The summation of stimuli, of a neurotransmitter, ligand, or a mechanical stimulus, that acts on the cell causes a rapid influx of positive ions. This is referred to as a graded potential. Sodium voltage-gated channels are opened due to the depolarization and cause a rapid influx of sodium into the cell.

Transpolarization is a transient state of membrane polarity during an action potential, where the intracellular side becomes briefly more positive relative to the extracellular side. It occurs only at >0mv

Repoliraztion

Repolarization is the phase of an action potential where the membrane potential returns to a negative value after the depolarization peak. This is driven by the opening of voltage-gated potassium channels and closure of sodium channels, causing an efflux of ions, reducing the positive charge inside the cell.

Hyperpolarization

Hyperpolarization is a change in a cell's membrane potential that makes the inside more negative than its resting state, decreasing the likelihood of firing an action potential. Primarily driven by efflux or influx of ions, it serves as an inhibitory mechanism in excitable cells and follows action potentials. In neurons, it is characterized by the efflux of potassium toward its electrochemical equilibrium.

Plateau

The plateau phase is a period during which the membrane potential of cardiac myocytes remains relatively stable around 0 mV. The Inward Ca²⁺ current through calcium channels and outward K⁺ current through potassium channels causes these opposing currents balance each other, keeping the membrane potential from changing much. It prolongs ventricular depolarization, allowing sufficient time for contraction.

Propagation of Action Potential

Action potentials are initiated at the trigger zone (axon hillock in neurons, or the initial segment in muscle fibers). Once initiated, the AP propagates along the membrane of the excitable cell. In unmyelinated neurons or muscle fibers, AP spreads continuously along the membrane. In myelinated neurons, AP travels rapidly via saltatory conduction .Under the myelin, the voltage drops slightly, but the AP is regenerated at each node of Ranvier, ensuring fast and reliable transmission.

Refractory periods

Absolute refractory period

No action potential can be intiated, no matter how strong the stimulus, because Na⁺ channels are inactivated.

Relative refractory period

Stronger than usual stimulus can trigger a new AP, as some Na⁺ channels have reset but K⁺ efflux is still repolarizing the membrane.

A stronger stimulus can generate additional APs during this period. In muscle fibers, more APs lead to more frequent calcium release from the sarcoplasmic reticulum, resulting in stronger or summated contractions.

Sources

Guyton and Hall textbook- membrane potential and action potential