<P> Several types of cells support an action potential, such as plant cells, muscle cells, and the specialized cells of the heart (in which occurs the cardiac action potential). However, the main excitable cell is the neuron, which also has the simplest mechanism for the action potential . </P> <P> Neurons are electrically excitable cells composed, in general, of one or more dendrites, a single soma, a single axon and one or more axon terminals . Dendrites are cellular projections whose primary function is to receive synaptic signals . Their protrusions, known as dendritic spines, are designed to capture the neurotransmitters released by the presynaptic neuron . They have a high concentration of ligand - gated ion channels . These spines have a thin neck connecting a bulbous protrusion to the dendrite . This ensures that changes occurring inside the spine are less likely to affect the neighboring spines . The dendritic spine can, with rare exception (see LTP), act as an independent unit . The dendrites extend from the soma, which houses the nucleus, and many of the "normal" eukaryotic organelles . Unlike the spines, the surface of the soma is populated by voltage activated ion channels . These channels help transmit the signals generated by the dendrites . Emerging out from the soma is the axon hillock . This region is characterized by having a very high concentration of voltage - activated sodium channels . In general, it is considered to be the spike initiation zone for action potentials, i.e. the trigger zone . Multiple signals generated at the spines, and transmitted by the soma all converge here . Immediately after the axon hillock is the axon . This is a thin tubular protrusion traveling away from the soma . The axon is insulated by a myelin sheath . Myelin is composed of either Schwann cells (in the peripheral nervous system) or oligodendrocytes (in the central nervous system), both of which are types of glial cells . Although glial cells are not involved with the transmission of electrical signals, they communicate and provide important biochemical support to neurons . To be specific, myelin wraps multiple times around the axonal segment, forming a thick fatty layer that prevents ions from entering or escaping the axon . This insulation prevents significant signal decay as well as ensuring faster signal speed . This insulation, however, has the restriction that no channels can be present on the surface of the axon . There are, therefore, regularly spaced patches of membrane, which have no insulation . These nodes of Ranvier can be considered to be "mini axon hillocks", as their purpose is to boost the signal in order to prevent significant signal decay . At the furthest end, the axon loses its insulation and begins to branch into several axon terminals . These presynaptic terminals, or synaptic boutons, are a specialized area within the axon of the presynaptic cell that contains neurotransmitters enclosed in small membrane - bound spheres called synaptic vesicles . </P> <P> Before considering the propagation of action potentials along axons and their termination at the synaptic knobs, it is helpful to consider the methods by which action potentials can be initiated at the axon hillock . The basic requirement is that the membrane voltage at the hillock be raised above the threshold for firing . There are several ways in which this depolarization can occur . </P> <P> Action potentials are most commonly initiated by excitatory postsynaptic potentials from a presynaptic neuron . Typically, neurotransmitter molecules are released by the presynaptic neuron . These neurotransmitters then bind to receptors on the postsynaptic cell . This binding opens various types of ion channels . This opening has the further effect of changing the local permeability of the cell membrane and, thus, the membrane potential . If the binding increases the voltage (depolarizes the membrane), the synapse is excitatory . If, however, the binding decreases the voltage (hyperpolarizes the membrane), it is inhibitory . Whether the voltage is increased or decreased, the change propagates passively to nearby regions of the membrane (as described by the cable equation and its refinements). Typically, the voltage stimulus decays exponentially with the distance from the synapse and with time from the binding of the neurotransmitter . Some fraction of an excitatory voltage may reach the axon hillock and may (in rare cases) depolarize the membrane enough to provoke a new action potential . More typically, the excitatory potentials from several synapses must work together at nearly the same time to provoke a new action potential . Their joint efforts can be thwarted, however, by the counteracting inhibitory postsynaptic potentials . </P>

The speed of the action potential is determined by what two anatomical properties