Layman's guide to synapses

What are synapses? Where are they located?
The nervous system coordinates the behaviour of animals and humans: it mediates perception of information (through the different senses), integration and processing of this information and, as a result, coordinates stimulation of muscles and glands. The nervous system is composed of different types of nerve cells which carry out these functions: sensory neurons (perception), interneurons (integration, processing) and motorneurons (stimulation).
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These different nerve cell types are organised into complex networks. To this end the neurons have formed long processes making specific contacts with other nerve cells, muscles or glands. All neuronal surfaces display a voltage of about 70mV. Nerve impulses (action potentials) are 'electrical messages' in form of focal depolarisations (reduction of this voltage to about 0mV, followed by quick recovery or "repolarisation") which flow along the neuronal processes (yellow zigzag arrows; see more explanations here >>>). The paths in which these electrical messages can flow are defined by the spatial arrangements in which neurons have established contact.
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The points of contact at which these electrical messages are passed on to other neurons, muscles or glands are the synapses (red circles). The following 3 pages will guide through aspects of function and importance of synapses in neuronal circuits, as outlined below:

Page 2: The transfer of information from signal sending (presynaptic) to signal receiving (postsynaptic) cell is a complex process, called synaptic transmission. Synaptic transmission can be used to manipulate the transferred signal (enhancing it, reducing it, holding it back). The degree or kind of this manipulation can even be changed over time (plasticity) - an essential feature underlying phenomena like learning and memory.

Page 3: Different modes of synaptic transmission exist. Transmission can be excitatory (inducing electrical signals in the postsynaptic cell), or inhibitory (suppressing postsynaptic electrical signals). These different modes are carried out by different types of synapses. In addition, one postsynaptic cell can receive input from different presynaptic neurons. Synchronised incoming information from more than one synapse leads to integration of the message in the postsynaptic cell, rendering information processing in neuronal circuits even more complex.

Page 4: Therefore, information flow within neuronal circuits is determined by the spatial arrangements of contacts established between neurons (hardwiring), but also the types of synapses formed at precise positions within neuronal networks (signal processing). These complex arrangements equip neuronal circuits with enormous functional capabilities which can be used to co-ordinate behaviour.
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What is synaptic transmission? How can it be used to influence information flow in neuronal circuits? >>>