Since neurons form a network of electrical activities, they somehow have to be interconnected. This connection is not a simple continuity of cytoplasm, so that every neuron has electrical continuity with all others, as happens with simple wiring, but is carried out by very specialized and complex structures called synapses. A synapse is the place where two neurons join in such a way that a signal can be transmitted from one to the other. The typical and overwhelmingly most abundant type of synapse is the one in which the axon of one neuron activates a second neuron, usually making a synapse with one of its dendrites or with the cell body. There are two ways in which this can happen, one is by the coupling of ion channels at the synapse, creating a passage way for the traveling ionic flux of the action and membrane potentials, which is called an electrical synapse, and the other is by a much more complicated way called a chemical synapse. In the case of the chemical synapse, the two neurons are not in strict contact, but have a small gap between them called the synaptic cleft. The signal is transmitted when one neuron releases a chemical (called neurotransmitter) into the synaptic cleft which is detected by the second neuron thru activation of receptors placed exactly opposite to the release site. The binding of the neurotransmitter to the receptors causes a series of physiological changes in the second neuron which constitutes the signal. Usually the release from the first neuron (called presynaptic) is caused by a series of intracellular events evoked by a depolarization of its membrane, and almost invariably when an action potential takes place. The signal that is evoked in the second (postsynaptic) neuron is in the form of a depolarization of its membrane.
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