The action potential and the graduated potential are two types of changes in the membrane potential that can occur in an excitable cell (neuron, muscle cell, etc). Although both suppose a change of voltage, each one has characteristics of intensity and different speed.
The intensity of the graduated potential is weaker and depends on the stimulus received, while the action potential is more intense and is always of the same magnitude, it does not depend on the intensity of the stimulus.
The graduated potential
The graduated potential , also called stepped potential or gradual potential , is a change in the membrane potential of variable magnitude that decreases with distance. They are a consequence of the sum of the individual activity of ion channels regulated by ligand , not including voltage-dependent channels.
A very important characteristic of potential graduates is that they are directly proportional to the magnitude of the stimulus ; the greater the number of ion channels involved, the greater the magnitude of the potential.
The stimulus that triggers a graduated potential can be the action of a neurotransmitter or a mechanical stimulus that affects the ionic permeability of the membrane; for example, stretching, temperature, etc.
A potential graduate can be depolarizing(EPSP – Excitatory Postsynaptic Potential ) or hyperpolarizing (IPSP – Inhibitory Postsynaptic Potential ). In the case of potential depolarizers, they can trigger an action potential if they reach an intensity above the threshold potential that is usually at a membrane potential of -55 mV. It can occur in postsynaptic dendrites or in the cells of skeletal muscle, smooth muscle or cardiac muscle.
The action potential
The action potential, unlike the graduated potential, is of the “all or nothing” type , in other words, its magnitude is always the same regardless of the intensity of the stimulus . The action potential can travel long distances without losing intensity and involves the opening of ion channels regulated bysodium and potassium voltage .
An action potential can be generated by the opening of sodium channels, which generate an action potential that usually lasts 1 millisecond, or can be generated by the opening of calcium channels , which generates an action potential that can last up to 100 milliseconds
Action potentials can occur in neurons, muscle cells and endocrine cells. In neurons they have a fundamental role in intercellular communication and are also known as nerve impulses . In the remaining excitable cells, the action potential is the signal for the initiation of intracellular processes. For example, in muscle cells it triggers muscle contraction, or in the beta cells of the pancreas it causes the release of insulin.