Synaptic transmission

     The effect of an action potential in a nerve terminal depends on the type of synapse that form the axon with its target cells. There are electrical synapses and chemical synapses. The former are formed by channels which connect the intracellular medium of the neuron with one of the target cell, so that the action potential propagates directly from one to the other. This type of synapse has the advantage of very rapid transmission, but it does not offer the possibility of plasticity What chemical synapses.
     Chemical at a synapse, the arrival of an action potential produced a complex sequence of events. The axon terminal contains small vesicles loaded with molecules that will ensure the transmission of neural activity through the synaptic space (hence the name of neurotransmitter). Depolarization of a synaptic terminal by an action potential causes the opening of calcium-permeable channels,and the input of this ion in the termination. Spurred on by the influx of calcium vesicles fuse with the cell membrane and release their contents into the extracellular medium. Then y is the neurotransmitter freely diffuse and reach the cell membrane of the target to which the nerve terminal is adjusted. The first stage of transmission is therefore to convert an electrical signal (action potential) into a chemical signal (neurotransmitter).

     The target cell has on its membrane receptor proteins which neurotransmitter molecules come join specifically, like a key in a lock. The binding of these neurotransmitter receptor proteins will generally cause the opening (or, more rarely closing) of channels in the neuronal membrane downstream. Depending on the type of ion channels that are permeable, it will result in a depolarization or hyperpolarization of the target cell. This second stage of synaptic transmission is therefore to convert a chemical signal into an electrical signal. However, this second conversion is not systematic: in some cases the binding of certain neurotransmitter receptor proteins produced no effect electric but only metabolic changes in the target cell.
There is a wide range of neurotransmitters from small molecules (such as glutamate, glycine, oxyacetylene, dopamine, etc..) To amino acid chains . Each neurotransmitter can match several types of receptor proteins whose action is fast or slow, excitatory or inhibitory. the operation of the chemical synapse requires a large number of different proteins, including channels permeable to calcium, the proteins that control the vesicle fusion and receptor proteins. Neurons can thus modulate synaptic transmission by modifying the properties of each of these proteins. The richness of this diversity may explain the predominance of synapses chemicals on electrical synapses in the nervous system of mammals evolved.