Free Connectome by Sebastian Seung

are red, violets are blue.” We should distinguish between the medium and the message. Is there something fundamentally primitive about chemical signals as a medium for communication? There are indeed several limitations, but the brain has found a way to circumvent all of them.
    Chemical signals are typically slow. If a woman walks into a room, you will usually hear her footsteps and see her clothing well before you catch a whiff of her perfume. A draft in the room might blow the scent toward you more rapidly, but it will still arrive more slowly than sound and light. Nervous systems, however, generate speedy reactions. When you suddenly jump away from a car piloted by a reckless driver, your neurons signal each other quickly. How can they accomplish this with chemical messages? Think of it this way: Even the slowest runner can finish a race in the blink of an eye if the racetrack is just a few strides long. Though chemical signals may move slowly, the distance that they have to travel across the synaptic cleft is extremely short.
    Chemical signals also seem crude because it is difficult to send them to specific targets.All the partygoers surrounding a woman can smell her perfume. Wouldn’t it be more romantic if her fragrance could be sensed only by her beloved? Alas, no inventor has managed to create a scent that is focused in this way. So what keeps the chemical messages at one synapse from spreading like perfume and being sensed by others? The answer is that a synapse “recycles” neurotransmitter by sucking it back up, or degrading it into an inert form, leaving the molecules with little chance to wander. It’s no trivial matter for the nervous system to minimize crosstalk—as engineers call the spreading phenomenon—because synapses are packed so close to each other. With a billion synapses to a cubic millimeter, the brain is far more crowded than Manhattan, and that island’s residents often complain about hearing conversations (and much else) from each other’s apartments.
    Finally, the timing of chemical signals is not easily controlled. A woman’s perfume may linger in a room long after she has left the party. The dawdling of neurotransmitter is averted by the same mechanisms of recycling and degradation that squelch crosstalk. This allows chemical messages between neurons to occur at precise times.
    These properties of synaptic communication—speed, specificity, and temporal precision—are not shared by other types of chemical communication inside your body. After you jump away from the car in the street, your heart races, you breathe heavily, and your blood pressure skyrockets. This is because your adrenal gland secreted adrenaline into your bloodstream, which was sensed by cells in your heart, lungs, and blood vessels. The reactions of the “adrenaline rush” may seem immediate, but actually they are tardy. They happened
after
you jumped away from the car, because adrenaline spreads through your bloodstream more slowly than signals jump from neuron to neuron.
    Secretion of hormones into the blood is the most indiscriminate type of communication, called broadcasting. Just as a television show is received by many households, and a perfume by everyone in a room, a hormone is sensed by many cells in many organs. In contrast, communication at a synapse is restricted to the two neurons involved, just as a telephone call connects the two people on the line. Such point-to-point communication is much more specific than broadcasting.
    In addition to chemical signals between neurons, there are also electrical signals in the brain. These travel
within
neurons. Neurites contain salty water rather than metal, but they nonetheless resemble, in both form and function, the telecom wires that crisscross the planet. Electrical signals can travel long distances by propagating through neurites, much as they move along wires. (Interestingly, the mathematical equations developed by