Free Connectome by Sebastian Seung Page A

Lord Kelvin in the nineteenth century to describe electrical signals in undersea telegraph cables have been used in the modeling of neurites.)
    In 1976 the legendary engineer Seymour Cray unveiled one of the most famous supercomputers in history, the Cray-1 (see Figure 16). Some called it the “world’s most expensive loveseat,”and indeed its sleek exterior could have graced the living room of a 1970s playboy. Its interior was anything but sleek, containing 67 miles of tangled wirein lengths spanning 1 to 4 feet. This looked like a chaotic mess to the casual observer, but actually it was highly ordered. Every wire transmitted information between a specific pair of points chosen by Cray and his design team from locations on thousands of “circuit boards” holding silicon chips. As is common in electronic devices, the wires were wrapped with insulating materialto prevent crosstalk.
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Figure 16. The Cray-1 supercomputer, exterior
(left)
and interior
(right)
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    You may think the Cray-1 looks complex, but it’s laughably simple compared with your brain. Consider that
millions
of milesof gossamer neurites are packed inside your skull, and they are branched rather than straight like wires. The tangle in your brain is far worse than that of the Cray-1. Nevertheless, the electrical signals in different neurites—even adjacent ones—interfere with each other very little, just as in insulated wires. Transmission of signals between neurites occurs only at specific points, those junctions called synapses. Similarly, signals cross from one wire to another in the Cray-1 only at locations where the insulation is removed and the metals come directly into contact.
    I’ve spoken of neurites generically up to now, but many neurons have two types of neurite—dendrites and axon. The dendrites are shorter and thicker. Several emanate from the cell body and branch in its vicinity. A single axon,long and thin, travels far from the cell body and branches out at its destination.
    Dendrites and axons not only look different but play different roles in chemical signaling. Dendrites are on the receiving end of synapses. Their membranes contain the receptor molecules. Axons send signals to other neurons by secreting neurotransmitter at synapses. In other words, the typical synapse is fromaxon to dendrite.
    The electrical signals of dendrites and axons also differ. In axons, electrical signals are brief pulses known as
action potentials,
each lasting about a millisecond (see Figure 17).Action potentials are informally known as “spikes,” owing to their pointy appearance, so let’s use this nickname for convenience. Neuroscientists often say, “The neuron spiked,” much as a financial reporter writes, “The stock market spiked on bank profits.” When a neuron spikes, it is said to be “active.”
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Figure 17. Action potentials, or “spikes”
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    Spikes are reminiscent of Morse code, which you’ve probably heard in old movies as a sequence of long and short pulses generated by a telegraph operator pressing a lever. In early telecom systems, pulses were just about the only type of signal that could be heard clearly above the static.Signals tend to become more corrupted by noise as they travel farther. That’s why Morse code was still used for long-distance communication even decades after the telephone became popular for local calls. Nature “invented” the action potential for much the same reason, to transmit information over long distances in the brain. Thus spikes occur mainly in the axon, the longest type of neurite. In small nervous systems like that of
C. elegans
or a fly, neurites are shorter and many neurons do not spike.
    So how are these two types of neural communication, chemical and electrical, related? Simply put, a synapse is activated when a passing spike triggers secretion.On the other side of the synapse,