The number of different pathways between neurons (nerve cells) in the human brain is astronomical. This is because neurons have so many interconnections; one neuron may connect up with over one hundred thousand others. “The figure of possible connections within our modern brain is as good as infinite,” states Anthony Smith in his book The Mind. It is larger “than the total number of atomic particles that make up the known universe,” says neuroscientist Thompson.

But how does one neuron pass information to another? Creatures with a simple nervous system have many nerve cells that are joined together. In such a case, an electrical impulse crosses the bridge from one neuron to the next. The crossing is called an electrical synapse. It is fast and simple.

Strange as it may seem, most neurons in the human body pass messages via a chemical synapse. This slower, more complex method can be illustrated by a train that reaches a river without a bridge and has to be ferried across. When an electrical impulse reaches a chemical synapse, it has to stop because a gap separates the two neurons. Here the signal is “ferried” across by the transfer of chemicals. Why this complex electro-chemical method of passing nerve impulses?

Scientists see many advantages in the chemical synapse. It ensures that messages pass one way. Also, it is described as plastic because its function or structure can easily change. Here signals can be modified. Through use, some chemical synapses get stronger while others disappear because of disuse. “Learning and memory could not develop in a nervous system that had only electrical synapses,” states Richard Thompson in his book The Brain.

Science writer Smith explains in his book The Mind: “Neurons do not just fire and not fire . . . they must be capable of passing on much more subtle information than yes or no. They are not just hammers hitting the next nail, either more frequently or less so. They are, to complete this analogy, a carpenter’s kit, with screwdrivers, pliers, pincers, mallets​—and hammers. . . . Each neural impulse is transformed along the way, and nowhere else than at the synapses.”