Neurons and Neurotransmitters
Brain cells are called neurons. Each neuron has two kinds of projections emanating from its cell body: branch-like message-receivers called a dendrites; and spindly “fingers” of different sizes called axons, which are message transmitters. Each neuron’s dendrites and axons are intertwined with those of other neurons, with each neuron establishing between 1,000 and 30,000 discrete links to others, resulting in a complex web of interconnection.1
Neurons communicate through the exchange of minuscule molecules called neurotransmitters which travel in the microscopic spaces, called synapses, between dendrites and axons. Neurotransmitters are three-dimensional “keys” that fit into “locks,” called “receptors.” Only a particular neurotransmitter can fit any individual receptor. There are multiple types of receptors, even for a single neurotransmitter. For example, 5 receptors have been identified for dopamine, while serotonin docks with 14 receptors.
Axons, the neuron’s message transmitters, are packed with tiny sacs or “vesicles,” each containing particular neurotransmitters. When the neuron sends the appropriate message, the vesicles release their neurotransmitters, flooding the synaptic gaps between axons and dendrites. A neurotransmitter locating its match attaches both physically, like a key, and electro-chemically, resulting in transmission of an electrical charge to the neuron. These exchanges, from the release of neurotransmitters to docking with receptors, transmission of its electro-chemical message, and freeing of its receptor, take place in the tiniest fraction of a second and continue millions of times within a group of neurons until the supply of neurotransmitter is exhausted or counterbalancing systems mop up any excess.
The electro-chemical exchange initiated by neurotransmitters triggers the neuron to respond with its own electrochemical reaction. Neurons exchange information in chain reactions of electro-chemical messaging along interconnected bundles of nerve cells called “neural networks.” Boosting neurotransmitters stimulates or inhibits neural firing rates, depending on the neurotransmitter.
The most common neurotransmitters in the brain are the glutamate/GABA (gamma-aminobutyric acid) pair. Glutamates stimulate neural activity, while its opposite, GABA, depresses it. Dopamine stimulates the brain’s reward center to experience pleasure and is the primary neurotransmitter manipulated by addictive drugs. It’s also centrally involved in learning, memory and motivation. Other neurotransmitters implicated in addiction include: opioids, the body’s natural pain killers; serotonin, which is involved in mood regulation; cannabinoids, implicated in appetite regulation, among other things; and corticotropin-releasing factor, or CRF, which helps regulate stress.
1. How the Mind Works: Revelations, New York Review of Books, Jun. 26, 2008.