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.
The three types of molecular verbs and adverbs described below can thus transmit complex messages about the external and internal environment.
Amino Acids. Four chemically simple amino acids form one class of neurotransmitters. Glutamate and aspartate are excitatory neurotransmitters. GABA (gamma-amniobutyric acid) and glycine are inhibitory neurotransmitters. Glutamate and GABA are the principal verb-like neurotransmitters in the cerebral cortex (send, don’t send). Glycine is a major neurotransmitter in the brainstem and spinal cord. These neurotransmitters play a key verbal role in processing specific information about the outside environment (such as color, shape, sound, and movement), and in activating the specific motor system neurons that regulate a movement.
The monoamine and peptide neurotransmitters described below play a key adverbial role in transmitting information about the changing state of our body and brain—how we currently feel about what is occurring within and without. (They modulate the actions of the amino acid neurotransmitters the way adverbs modulate the actions of verbs). Each type of monoamine and peptide is synthesized in a single brainstem or limbic system source and its circuitry spreads widely from there throughout the brain (think of a small lawn sprinkler that distributes water to a large lawn area).
Monoamines. The six types of monoamine neurotransmitters are acetylcholine, dopamine, histamine, norepinephrine (or noradrenalin), epinephrine, and serotonin. They are chemically modified amino acids that act more slowly than the amino acid neurotransmitters. The interaction of a monoamine neurotransmitter with its postsynaptic receptor helps to determine the nature of the message.
Peptides. The largest and most complex neurotransmitters are the peptides (or neuropeptides), such as oxytocin and endorphin. They are composed of chains of 2 to 39 amino acids. Most of the dozens of types of neurotransmitters in our brain and peripheral nervous system are peptides, but their concentrations are much lower than that of the amino acids and monoamines (which are also known as the classical neurotransmitters). Many neurons that distribute a classical neurotransmitter also distribute a peptide that increases or decreases the postsynaptic neuron’s receptivity of the neuron’s primary transmitter—thus modulating its effect.
In addition to moving within neural networks, peptides use our circulatory system and air passages to move throughout our body/brain to modulate our broad range of pleasure and pain. A peptide action that occurs simultaneously in a large number of related cells can powerfully affect the decisions we make within the continuum of emotionally charged approaching and retreating behaviors, such as to drink-urinate, agree-disagree, buy-sell, and marry-divorce. In effect, the shifts in the body/brain levels of these molecules allocate our emotional energy: what to do, when to do it, and how much energy to expend on the activity.
A peptide’s message can vary in different body/brain areas, just as a two-by-four can be used in many different ways in the construction of a house. For example, angiotensin is a peptide that activates the seeking and conserving behaviors that regulate our body’s fluid levels. In our brain it does this principally by activating feelings of thirst and the consequent behaviors that seek water. In our body it causes kidneys to conserve water. The situation is similar with many drugs. For example, alcohol can excite or sedate, depending on the amount ingested and the drinker’s emotional state. Endorphins can similarly reduce intense pain and increase euphoria.
A lot of chattering is continuously occurring within our brain—all mediated by awesomely complex but elegant simple chemical verbs and adverbs!