Threshold Potential

Threshold potential is the minimum shift in membrane potential (the potential difference between the outer and inner surfaces of the membrane) at which membrane depolarization reaches a critical level sufficient for the occurrence of a propagating action potential (AP). The depolarization threshold is the value of the membrane potential on the membrane, which is necessary for the occurrence of AP.

The threshold potential depends on the properties of the membrane and the length of the excitable tissue. In nerves and muscle fibers, threshold potentials are about -50 mV; in skeletal muscles they are slightly higher. In nerve fibers, the depolarization threshold is slightly higher than in muscle fibers, which is due to the properties of the membrane.

When tissue is exposed to an electrical stimulus, the threshold potential changes depending on the strength and duration of the stimulation. With a sufficiently long-term effect of the stimulus, the threshold potential can change towards hyperpolarization, which can lead to inhibition of neuronal activity.

There are many ways to study the properties of membranes, but the most important and informative of them is the method based on recording their electrical characteristics, especially the resting potential and action potential. Potentiometry is a method for recording the distribution of stationary potentials on a membrane when a constant electric field is applied to it and measuring the flow currents associated with it. Typical curves and characteristic graphs are plotted based on the dependence of the membrane potential drop on the applied field near the equilibrium potential. The initial sections of these dependencies are approximated by functions in which two parametric components are distinguished. The first is a normal (or diffusion) element in the form of an ordinary hyperbolic sine (tangent), normalized to the value of the membrane resistance, the second characterizes the magnitude of the field that causes the critical level of depolarization to reach the diffusion blocking region. Typically, the first parameter is called the membrane potential, and the second is called the threshold potential. The membrane potential measured between two membrane fragments in contact with each other is called intermembrane potential. In cases where the sensor is immersed in the internal contents of a cell or tissue (microelectrodes), it is called intracellular or interstitial. The main task of the threshold potential in membrane electrophysiological analysis is the establishment and comparative assessment of the relationship between membrane ion flux and membrane voltage.