<Tr> <Td> Postsynaptic density Voltage - gated Ca channel Synaptic vesicle Neurotransmitter transporter Receptor Neurotransmitter Axon terminal Synaptic cleft Dendrite </Td> </Tr> <P> Most axons carry signals in the form of action potentials, which are discrete electrochemical impulses that travel rapidly along an axon, starting at the cell body and terminating at points where the axon makes synaptic contact with target cells . The defining characteristic of an action potential is that it is "all - or - nothing"--every action potential that an axon generates has essentially the same size and shape . This all - or - nothing characteristic allows action potentials to be transmitted from one end of a long axon to the other without any reduction in size . There are, however, some types of neurons with short axons that carry graded electrochemical signals, of variable amplitude . </P> <P> When an action potential reaches a presynaptic terminal, it activates the synaptic transmission process . The first step is rapid opening of calcium ion channels in the membrane of the axon, allowing calcium ions to flow inward across the membrane . The resulting increase in intracellular calcium concentration causes vesicles (tiny containers enclosed by a lipid membrane) filled with a neurotransmitter chemical to fuse with the axon's membrane and empty their contents into the extracellular space . The neurotransmitter is released from the presynaptic nerve through exocytosis . The neurotransmitter chemical then diffuses across to receptors located on the membrane of the target cell . The neurotransmitter binds to these receptors and activates them . Depending on the type of receptors that are activated, the effect on the target cell can be to excite the target cell, inhibit it, or alter its metabolism in some way . This entire sequence of events often takes place in less than a thousandth of a second . Afterward, inside the presynaptic terminal, a new set of vesicles is moved into position next to the membrane, ready to be released when the next action potential arrives . The action potential is the final electrical step in the integration of synaptic messages at the scale of the neuron . </P> <P> Extracellular recordings of action potential propagation in axons has been demonstrated in freely moving animals . While extracellular somatic action potentials have been used to study cellular activity in freely moving animals such as place cells, axonal activity in both white and gray matter can also be recorded . Extracellular recordings of axon action potential propagation is distinct from somatic action potentials in three ways: 1 . The signal has a shorter peak - trough duration (~ 150μs) than of pyramidal cells (~ 500μs) or interneurons (~ 250μs). 2 . The voltage change is triphasic. 3 . Activity recorded on a tetrode is seen on only one of the four recording wires . In recordings from freely moving rats, axonal signals have been isolated in white matter tracts including the alveus and the corpus callosum as well hippocampal gray matter . </P>

What is the other name of a nerve cell