Excitable cells
All cells have a membrane potential, but only excitable cells can generate and propagate action potentials. The most common types of cells in these tissues are neurons and muscle cells (skeletal, cardiac, and smooth).
Important to understand that most of the cells are NOT excitable. Some slow, graded changes can occur across their membrane, but they lack the ability to rapidly change the membrane's charge leading to action potential. Those cells include - epithelial cells, fibroblasts, adipocytes, hepatocytes, glandular cells, blood cells, and more.
Excitable cells have the ability to convert electrical gradients into rapid signaling via ion channels. Those changes occur mainly due to the Na⁺/K⁺ pump, voltage-gated, ligand-gated, and leak ion channels.
An action potential is a series of events that excitable cells undergo when a stimulus reaches threshold. This process, caused by the mentioned ion channels, has a few phases:
- Resting membrane potential - Important to note that this stage is present in almost all living cells, except pacemaker cells, which are said to have no resting membrane potential or an unstable one. This is meaning, that almost all cells, has a negative inside environmat relative to extracellular fluid.
- Depolarization - Present only in excitable cells
- Repolarization - Present only in excitable cells
- Hyperpolarization - Present only in excitable cells
- Return to the RMP - Present only in excitable cells.
The excitable cells genratly follow the “all-or-none principle”. Once the threshold is reached, an action potential will follow. If the stimulus is not strong enough, there will be no reaction.
