An<unk>  is a "spike" of electrical discharge that travels along the membrane of a cell.<unk> s are an essential feature of animal life, rapidly carrying information within and between tissues. They also occur in some plants.<unk> s can be created by many types of cells, but are used most extensively by the nervous system for communication between neurons and for transmitting information from neurons to other body tissues such as muscles and glands.<unk> s are not the same in all cell types and can even vary in their properties at different locations in the same cell. For example, cardiac<unk> s are significantly different from the<unk> s in most neurons. This article is primarily concerned with the "typical"<unk>  of axons.
There is always a difference in electrostatic<unk>  between the inside and outside of a cell, i.e. the cell is polarized. This membrane<unk>  is the result of the distribution of ions across the cell membrane and the permeability of the membrane to these ions. The voltage of an inactive cell remains close to a resting<unk>  with excess negative charge inside the cell. When the membrane of an excitable cell becomes depolarized beyond a threshold, the cell undergoes an<unk>  (it "fires"), often called a "spike" (see Threshold and initiation).
An<unk>  is a rapid change of the polarity of the voltage from negative to positive and then vice versa, the entire cycle lasting on the order of milliseconds. Each cycle — and therefore each<unk>  — has a rising phase, a falling phase, and finally an undershoot (see Phases). In specialized muscle cells of the heart, such as cardiac pacemaker cells, a plateau phase of intermediate voltage may precede the falling phase, extending the<unk>  duration into hundreds of milliseconds.<unk> s are measured with the recording techniques of electrophysiology and more recently with neurochips containing EOSFETs. An oscilloscope recording the membrane<unk>  from a single point on an axon shows each stage of the<unk>  as the wave passes. These phases trace an arc that resembles a distorted sine wave; its amplitude depends on whether the<unk>  wave has reached  that point on the membrane or has passed it and if so, how long ago.
The<unk>  does not dwell in one location of the cell's membrane, but travels along the membrane (see Propagation). It can travel along an axon for long distances, for example to carry signals from the spinal cord to the muscles of the foot. In large animals, such as giraffes and whales, the distance traveled can be many meters. After traveling the whole length of the axon, the<unk>  reaches a synapse, where it stimulates the release of neurotransmitters. These neurotransmitters can immediately induce an<unk>  in the next neuron to propagate the signal, but the response is usually more complex.
Both the speed and complexity of<unk> s vary between different types of cells, but their amplitudes tend to be roughly the same. Within any one cell, consecutive<unk> s are typically indistinguishable. Neurons are thought to transmit information by generating sequences of<unk> s called "spike trains". By varying both the rate as well as the precise timing of the<unk> s they generate, neurons can change the information that they transmit.
Action potential
Saturation 
Concentration
Emotion
Electric current
Face time
Extraction ratio
Psychodynamics