The Nature Of The Synaptic Connection

Synapses are specialized intercellular junctions that connect pre- and postsynaptic neurons (and presynaptic neurons to postsynaptic effector cells such as muscle cells). As intercellular junctions, synapses are typical in that they display membrane specializations on both sides of the junction, and in that a uniformly spaced cleft separates the two cells at the junction. Synapses are different from other intercellular junctions in that synaptic junctions are highly asymmetric, with clusters of vesicles on the presynaptic side5. Intercellular junctions (and junctions between cells and the extracellular matrix) usually have three functions, to couple two cells mechanically to each other (for example, in creating the architecture of a tissue), to signal between cells, and to organize the spatial organization of intracellular membrane traffic in participating cells. Synapses are no different. Their signaling and intracellular membrane trafficking functions have grown enormously compared to other types of intercellular junction, but they still also act to mechanically connect two cells, for example, in fixing axons and dendrites in space.

At a synapse, a fast signal is transferred from the presynaptic to the postsynaptic neuron in the form of a chemical neurotransmitter that is released from the presynaptic nerve terminal, and recognized by postsynaptic receptors. In addition, several slow anterograde and retrograde signals regulate synapse properties and size. The probability of synaptic transmission for each individual synaptic contact is always less than 1, sometimes considerably less. As a result, whenever a synaptic connection has to always elicit a postsynaptic response for every presynaptic action potential, neurons elaborate a multitude of individual contacts on the target cell (instead of a single larger contact). For example, in the Calyx of Held synapse a presynaptic terminal forms ~600 individual contacts with the same postsynaptic neuron; approximately a third of these contacts transmit a synaptic signal for each action potential, thereby ensuring that ~200 synaptic inputs on the postsynaptic neuron are activated per action potential6.

Apart from the typical differences between excitatory and inhibitory synapses, no major structural differences exist between typical central synapses that would allow prediction of their often quite dramatically different functional properties. This was for example shown for cerebellar parallel fiber and climbing fiber synapses that in spite of very different release probabilities exhibit quantitatively similar ultrastructural features7. Although the size of synapses varies, it only varies over a relatively small range. It should be noted that these statements only to standard central synapses; the structure of the neuromuscular junction8 or of ribbon synapses9 is dramatically different, as are their properties.

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