<P> The electrical properties of a cell are determined by the structure of the membrane that surrounds it . A cell membrane consists of a lipid bilayer of molecules in which larger protein molecules are embedded . The lipid bilayer is highly resistant to movement of electrically charged ions, so it functions as an insulator . The large membrane - embedded proteins, in contrast, provide channels through which ions can pass across the membrane . Action potentials are driven by channel proteins whose configuration switches between closed and open states as a function of the voltage difference between the interior and exterior of the cell . These voltage - sensitive proteins are known as voltage - gated ion channels . </P> <P> All cells in animal body tissues are electrically polarized--in other words, they maintain a voltage difference across the cell's plasma membrane, known as the membrane potential . This electrical polarization results from a complex interplay between protein structures embedded in the membrane called ion pumps and ion channels . In neurons, the types of ion channels in the membrane usually vary across different parts of the cell, giving the dendrites, axon, and cell body different electrical properties . As a result, some parts of the membrane of a neuron may be excitable (capable of generating action potentials), whereas others are not . Recent studies have shown that the most excitable part of a neuron is the part after the axon hillock (the point where the axon leaves the cell body), which is called the initial segment, but the axon and cell body are also excitable in most cases . </P> <P> Each excitable patch of membrane has two important levels of membrane potential: the resting potential, which is the value the membrane potential maintains as long as nothing perturbs the cell, and a higher value called the threshold potential . At the axon hillock of a typical neuron, the resting potential is around--70 millivolts (mV) and the threshold potential is around--55 mV . Synaptic inputs to a neuron cause the membrane to depolarize or hyperpolarize; that is, they cause the membrane potential to rise or fall . Action potentials are triggered when enough depolarization accumulates to bring the membrane potential up to threshold . When an action potential is triggered, the membrane potential abruptly shoots upward and then equally abruptly shoots back downward, often ending below the resting level, where it remains for some period of time . The shape of the action potential is stereotyped; this means that the rise and fall usually have approximately the same amplitude and time course for all action potentials in a given cell . (Exceptions are discussed later in the article). In most neurons, the entire process takes place in about a thousandth of a second . Many types of neurons emit action potentials constantly at rates of up to 10--100 per second . However, some types are much quieter, and may go for minutes or longer without emitting any action potentials . </P> <Table> <Tr> <Td> </Td> <Td> This section needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (February 2014) (Learn how and when to remove this template message) </Td> </Tr> </Table>

Where does action potential start in a neuron