The synapse was named decades before anyone could actually see one
Charles Sherrington coined the word synapse in 1897 for a gap between nerve cells that no microscope of the day could show. Only in the 1950s did electron microscopes reveal the cleft, roughly 20 nanometres wide, across which nearly every thought and movement is relayed.
The idea came first. Santiago Ramon y Cajal argued that neurons are separate cells that nonetheless communicate, a view known as the neuron doctrine. Sherrington, writing for Michael Foster's physiology textbook, wanted a word stressing a union of two distinct parts. A classical scholar and friend of Foster, Arthur Woollgar Verrall, suggested one from Greek roots meaning to fasten together. For half a century the gap stayed theoretical, because the finest light microscopes could not resolve such a narrow separation.
Two designs dominate. In a chemical synapse, an electrical pulse opens calcium channels in the sending cell, which then releases neurotransmitter molecules stored in tiny vesicles. These cross the cleft, bind receptors on the receiving cell and nudge it toward firing or away from it. Glutamate usually excites, GABA usually inhibits, and some messengers, such as acetylcholine and dopamine, can do either depending on the receptor they meet. Afterwards the transmitter is swiftly cleared, recycled by the sending terminal, absorbed by neighbouring glial cells or broken down by enzymes.
Electrical synapses skip the chemistry. Channels called gap junctions link the two cells' interiors so current flows directly, giving speed and helping neurons fire in synchrony, though the coupling works both ways, so the direction of a signal cannot always be defined. Some sites combine both, pairing a fast electrical component with a slower chemical one. A neuron can even synapse onto itself, a structure called an autapse.
Inhibition matters as much as excitation. Opening chloride or potassium channels makes a cell harder to excite, and the toxin strychnine shows what happens when that brake fails: by blocking glycine receptors it triggers spasms and convulsions. Most mammalian synapses join an axon to a dendrite, but axons can also contact cell bodies, other axons or even the bloodstream. Supporting astrocytes respond to synaptic activity and help regulate it, and recent work suggests hippocampal synapses need mechanical tension to fire.
Source: Synapse
