Synapses

An impulse passes from one neuron to the next at a synapse. A synapse is formed by the membranes of the neuron sending the signal (pre-synaptic) and the one receiving it (post-synaptic). There may or may not be a small gap, the synaptic cleft, between them.

There are two kinds:

• Electrical synapses: the two membranes are very close, and the electric current flows directly from one neuron to the next, just as it flows along a single axon. This makes them always faster than chemical synapses. But they are rare in our body.
• Chemical synapses: the membranes are separated by a fluid-filled synaptic cleft. The signal is carried across by chemicals, the neurotransmitters. Since the message has to cross the gap, they are a little slower.

How a chemical synapse works:

• An impulse arrives at the axon ending.
• It moves the synaptic vesicles to the membrane; they fuse with it and release their neurotransmitters into the cleft.
• The neurotransmitters cross the cleft and bind to specific receptors on the post-synaptic membrane.
• This opens ion channels, letting ions in, which creates a new potential in the next neuron. It may be excitatory (starting an impulse) or inhibitory (preventing one).