Tim Berners-Lee, a creator of the World Wide Web, likens the brain’s complexity to the nearly infinite capacity for Web sites to connect to each other. “A piece of information is really defined only by what it’s related to,” he said. “The structure is everything. There are billions of neurons in our brains, but what are neurons? Just cells. The brain has no knowledge until connections are made between neurons. All that we know, all that we are, comes from the way our neurons are connected.”


Communicating with another cell, neurotransmitters journey across a synapse.
Communicating with another cell, neurotransmitters journey across a synapse.

Transmissions between neurons take place in two stages. The first is electrical. An electrical discharge travels the length of an axon. When it reaches the axon terminal that abuts the synaptic space, it sets the second stage in motion. This button, like the rest of the nerve cell, has an outer wall called a mem-brane. Its envelope contains a solution of messenger chemicals. These electrically charged chemicals move in the solution, constantly poised to respond to an impulse and exit through small openings of the membrane and into the synapse. When an electrical impulse arrives from the axon, if it is of sufficient strength it trips a trigger that releases one of the messenger chemicals, called a neurotransmitter, from storage in the button.


The neuro transmitting chemical then enters the synapse. Like a ferryboat crossing a small stream, the neurotransmitter traverses the synaptic cleft and attempts to link up with the dendritic membrane of a receptor cell. The journey across the synapse takes only a thousandth of a second. The receptor cell’s surface contains specially shaped docking sites, so particular neurotransmitters can dock only at the appropriate places, just as a key needs exactly the right shape to fit into a lock. The neurotransmitter either excites the receptor cell into action or dampens it into inaction.

Once the receptor cell has been stimulated by the neuro transmitting chemical, the communication reverts to an elec- trical signal. It travels the length of the new cell until it reaches the synapse of another receptor cell, and starts the process all over again. After they have done their job in the synaptic space between nerve cells, neuro transmitting chemicals are reabsorbed by the transmitting neuron and prepared for rerelease (a process known as reuptake) or broken down and metabolized by enzymes in the synaptic space. It sounds like a lot of work, but neurons can repeat the electrochemical firing process up to a thousand times a second.

WAKING IN THE middle of the night on the eve of Easter, 1921, German-born pharmacologist Otto Loewi (1873-1961) recalled an inspiring dream that gave him an idea for an experiment that would shatter scientists’ conception of neural communication.


Most turn-of-the-century brain Scientists believed nerves sent impulses via electric waves, firing sparks across the synaptic gap, neuron to neuron. In this way, they thought, motor intentions born in the cerebral cortex could be transmitted to receptor muscles and organs throughout the body. Only a handful of scientists-most notably Loewi and his English counterpart, Henry Daleargued that chemical neurotransmitters are released at the synapse. An accelerant, noradrenaline, causes the heart to beat more quickly, Dale said. An inhibitor, acetylcholine, induces the opposite. Yet Dale was unable to extract either chemical organically, and lacking proof, his case remained dormant.

Then, as Loewi recalled, a fateful frog experiment flashed to him in a dream, and he dashed to his laboratory. He began with two frogs’ hearts. Stimulating the vagus nerve of one to slow its beating, he applied a residual solution from this donor to a second heart, from which he’d severed the vagus nerve. The second heart immediately slowed, as if discouraged by an unseen force. Loewi’s hypothesis was correct: A neurotransmitter (acetylcholine) had slowed the first heart, leaving a trace fluid-enough to slow the second, isolated heart.

Precursors to axons and dendrites, in yellow and blue, respond to nerve growth stimulation.
Precursors to axons and dendrites, in yellow and blue, respond to nerve growth stimulation.

The brain devotes huge amounts of neural circuitry to the hands, lips, and tongue.

Dozens of neurotransmitters have been identified, and more discoveries are expected. Certain neurotransmitters make muscles contract, help regulate sleep, and block pain. Research into the role of neurotransmitters in mental and physical health is constantly expanding, and neurotransmitter disorders have been linked to Parkinson’s disease, depression, Alzheimer’s disease, schizophrenia, and a host of other illnesses.

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