Neurons’ synaptic transmission.
This is a colorized scanning electron microscope picture of a nerve ending that has been broken open to reveal the synaptic vesicles (orange and blue) beneath the cell membrane.
At a synapse, the plasma membrane of the signal-passing neuron (the presynaptic neuron) comes into close apposition with the membrane of the target (postsynaptic) cell.
Neuron ends with axon, a long, slender projection that typically conducts electrical impulses away from the cell body and that represents the presynaptic site, while the postsynaptic site is represented by dendrites, branched projections that recieve the impulses from other neural cells and that conduct them to the cell body, or soma, of the neuron from which the dendrites project.
There are two different types of synapses:
- Chemical synapse, where electrical activity in the presynaptic neuron is converted (via the activation of voltage-gated calcium channels) into the release of a chemical called neurotransmitter, usually located in vescicles, that binds to receptors located in the postsynaptic cell, usually embedded in the plasma membrane. The neurotransmitter may initiate an electrical response or a secondary messenger pathway that may either excite or inhibit the postsynaptic neuron. Because of the complexity of receptor signal transduction, chemical synapses can have complex effects on the postsynaptic cell.
- Electrical synapse, where the presynaptic and postsynaptic cell membranes are connected by special channels called gap junctions that are capable of passing electric current, causing voltage changes in the presynaptic cell to induce voltage changes in the postsynaptic cell. The main advantage of an electrical synapse is the rapid transfer of signals from one cell to the next.
(Picture by The Cell Image Library).