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one. He placed two frog hearts in two vessels joined by a
    narrow tube. One heart had the vagus nerve still attached. When he electrically stimulated
    the heart with the nerve attached, it slowed down. Then, after some delay, the second heart
    began to slow down as well. This simple experiment demonstrated the existence of what he
    unpoetically called Vagusstoff , a substance ( stoff ) that comes out of the vagus nerve of one
    frog heart to slow the beat of the other heart. Vagusstoff , now called acetylcholine, is one
    of dozens of neurotransmitters that neurons use to communicate with one another.
    It’s odd that synapses are small enough to be flaky, but this appears to be a widespread
    phenomenon. Synapses reach a similar minimum size in the brains of various animals, including mice
    and people. No one is sure why individual synapses have evolved to be small and unreliable, but one
    possible reason is that the brain may work better if it’s packed with a tremendous number of them.
    This may be a trade-off that stuffs the most function into the smallest possible space.
    For the brain to accomplish its many duties, neurons have to take on very specific tasks. Each
    neuron responds to a small number of events, such as hearing a particular sound, seeing someone’s
    face, carrying out a certain movement—or other processes that aren’t observable from the outside. At
    any given moment, only a small fraction of your neurons, distributed all over your brain, are active.
    This fraction is ever shifting; the whole game of thinking depends on which neurons are active and
    what they are saying to each other and to the world.
    Neurons in all animals are organized into local groups that serve the same broad purpose, such as
    detecting visual motion or planning eye movements. In our own brains, each division can have
    billions of neurons, with many subdivisions; in a rat, millions, with fewer subdivisions; in a squid or
    insect, thousands of neurons (though in these tiny creatures’ brains, different parts of individual
    neurons may do multiple things at once). Each of these divisions contains its own distinctive types of
    neurons, particular patterns of connection, and connections with other brain structures.

    Scientists first learned about the functions of different parts of the brain by studying people with
    brain damage. Sadly, World War I was an especially rich source of data. Soldiers often survived
    head wounds because high-velocity bullets cauterized their wounds, preventing a fatal loss of blood
    or even infection. But the soldiers exhibited a baffling range of symptoms, which depended on the
    location in the brain that was damaged. Modern neurologists still publish papers on patients who
    have brain damage, most commonly from strokes. Indeed, a few patients with very rare types of
    damage actually support themselves by participating in paid studies.
    Scientists can also figure out what a neuron does by tracking its activity under different
    conditions, by stimulating it, or by tracing its connections to other brain areas. For example, motor
    neurons in the spinal cord receive signals from neurons in the cortex that generate basic movement
    commands. In turn, these spinal cord neurons send signals to the muscles, causing them to contract. If
    scientists electrically stimulate only the spinal cord neurons, the same muscles contract. Together,
    these results make it clear that spinal cord motor neurons are responsible for executing movement
    commands that are generated at higher levels of the brain, although there is still plenty of controversy
    over exactly what aspect of the movement is specified by these commands.
    To learn to get around in your brain, you need a quick tour of its parts and what they do. The
    brainstem , as the name suggests, is at the very bottom of the brain, where it attaches to your spinal
    cord . This region controls basic functions that are critical for life, like reflexive movements of the
    head