Why do the 100 billion highly interconnected neurons in our brain need 60 different kinds of neurotransmitters to carry out the simple chemical segment of its electro-chemical transmission process?

Neurons are beautifully organized to carry out their functions. (For relevant illustrations, see Neuron and Synaptic Region in the BrainConnection Image Gallery.) The human arm provides a simple model of the neuron's three functional parts: the fingers represent the neuron's multiple dendrites that receive touch information; the hand represents the neuron's cell body that processes the information; and the arm represents the (normally single) axon that transmits touch information along the information chain to our brain.

Neurotransmitter molecules are synthesized within a neuron. Their shape and electrical properties determine the nature of their chemical message. The type and amount of incoming chemical messages determine whether a neuron will initiate an outgoing response. The activation of a neuronal message is akin to an electrical charge that travels from the cell body to the axon terminal, where it releases neurotransmitters into the narrow synaptic gap that separates two neurons. Released neurotransmitters attach to receptors that are typically located on a dendrite of the receiving neuron.

Think of deciding to write and mail a letter to a friend. The postal service transports your letter to your friend's house where it is placed into the mailbox. Synthesis of message—transportation of message—reception of message.

Brain messages and human behavior are typically either excitatory or inhibitory. A brain either receives or doesn't receive sensory information. It either relays or doesn't relay the received information. It either does or doesn't carry out an action. Excitatory neurotransmitters enhance the transmission of information and inhibitory neurotransmitters reduce or stop the transmission.

But why does our brain need dozens of different kinds of neurotransmitters to send what's essentially a simple on/off message? For the same reason that our language has dozens of different kinds of verbs to express the basic message—move your body by moving your legs, when the word walk expresses the essence of the action. Verbs such as run, hop, skip, and jog describe variants of the basic message, and adverbs such as slowly, quickly, and enthusiastically add additional qualities to the action.