October 11, 2026

To produce a behavior, a stretch reflex for example, each participating sensory and motor nerve cell must generate four different signals in sequence, each at a different site within the cell.

1. A receptive component for producing graded input signals.
2. A summing or integrative component that produces a trigger signal.
3. A conducting long-range signaling component that produces all-or-none conducting signals.
4. A synaptic component that produces output signals to the next neuron in line or to muscle or gland cells.

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Every cell, including a neuron, maintains a certain difference in the electrical potential on either side of the plasma membrane when the cell is at rest. This is called the resting membrane potential.

In a typical resting neuron, the voltage of the inside of the cell is about 65 mV more negative than the voltage outside the cell. Because the voltage outside the membrane is defined as zero, we say the resting membrane potential is –65 mV.

The resting potential in different nerve cells ranges from –40 to –80 mV; in muscle cells, it is greater still, about –90 mV.

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The resting membrane potential results from two factors:

1. The unequal distribution of electrically charged ions, in particular the positively charged Na+ and K+ ions.
2. The selective permeability of the membrane.

Most neurons have four functional regions in which different types of signals are generated.

The functional organization of most neurons, regardless of type, can be represented schematically by a model neuron. This model neuron is the physiological expression of Ramón y Cajal’s principle of dynamic polarization. The input, integrative, and conductive signals are all electrical and integral to the cell, whereas the output signal is a chemical substance ejected by the cell into the synaptic cleft. Not all neurons share all of these features; for example, some local interneurons lack a conductive component.

The unequal distribution of positively charged ions on either side of the cell membrane is maintained by two main mechanisms. Intracellular Na+ and K+ concentrations are largely controlled by a membrane protein that actively pumps Na+ out of the cell and K+ back into it. This Na+-K+ pump keeps the Na+ concentration in the cell low (about one-tenth the concentration outside the cell) and the K+ concentration high (about 20 times the concentration outside). The extracellular concentrations of Na+ and K+ are maintained by the kidneys and the astroglial cells, also known as astrocytes.

The otherwise impermeable cell membrane contains proteins that form pores called ion channels. The channels that are active when the cell is at rest are highly permeable to K+ but considerably less permeable to Na+. The K+ ions tend to leak out of these open channels, down the ion’s concentration gradient. As K+ ions exit the cell, they leave behind a cloud of unneutralized negative charge on the inner surface of the membrane, so that the net charge inside the membrane is more negative than that outside. With this state of affairs, the membrane potential is typically maintained at around –65 mV relative to outside of the neuron, and the neuron is said to be at rest.

The resting state is unsettled when the cell begins to take up Na+ (or Ca2+), which are at a higher concentration outside the cell. The inward movement of these positively charged ions (inward current) partially neutralizes the negative voltage inside the cell.

A cell, such as nerve and muscle, is said to be excitable when its membrane potential can be quickly and significantly altered. In many neurons, a 10-mV change in membrane potential (from –65 to –55 mV) makes the membrane much more permeable to Na+ than to K+. The resultant influx of Na+ further neutralizes the negative charge inside the cell, leading to even more permeability to Na+. The result is a brief and explosive change in membrane potential to +40 mV, the action potential.

This potential is actively conducted down the cell’s axon to the axon’s terminal, where it initiates an elaborate chemical interaction with postsynaptic neurons or muscle cells. Since the action potential is actively propagated, its amplitude does not diminish by the time it reaches the axon terminal. An action potential typically lasts approximately 1 ms, after which the membrane returns to its resting state, with its normal separation of charges and higher permeability to K+ than to Na+.

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A reduction in membrane potential, called depolarization, enhances a cell’s ability to generate an action potential and is thus excitatory.

In contrast, an increase in membrane potential, called hyper-polarization, makes a cell less likely to generate an action potential and is therefore inhibitory.

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Each of the neuron’s four signaling components produces a characteristic signal.