Analogy with the Brain

For pedagogical purposes, it is useful to make an analogy between the brain and an ANN. The following simplified overview of brain structure at the cellular level suffices to make the key points. 

we have been too cavalier with the words contact and connection. The contact between an axon and a dendrite is not a direct contact there is a small gap between them, which is called the synaptic junction or synapse. An electrical nerve signal moves down an axon to the synapse, where it is converted to a chemical signal, which crosses the gap via a neurotransmitter such as acetylcholine. The chemical signal is converted back to an electrical signal when the neurotransmitter reaches the dendrite side of the gap and is absorbed. The signal can go in only one direction—axon to dendrite—because the dendrite cannot release neurotransmitters. 

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