Free Connectome by Sebastian Seung Page B

receptors sense neurotransmitter and then make electrical current flow. In more abstract terms, a synapse convertsan electrical signal into a chemical signal and then back into an electrical signal.
    Conversion between signal types is common in our everyday technologies. Imagine two people conversing by telephone. Electrical signals travel between them along a continuous wire. (Let’s ignore the fact that modern telephone networks additionally use light signals in optical fibers.) But electrical signals do not traverse the narrow gap of air between the handset and the ear; instead, they are converted into acoustic signals. After a journey of a thousand miles as electricity, it is sound that makes the leap to the listener’s eardrum. Similarly, an electrical signal may travel far in the brain along an axon, but it does not reach the next neuron directly. Rather, it is converted into a chemical signal, which jumps across the synaptic cleft to the other neuron.
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    If one neuron can signal a second neuron through a synapse, the second neuron can signal a third, and so on. A sequence of such neurons is known as a
pathway.
This is how neurons can communicate with one another even if they are not directly connected by a synapse.
    Unlike the mountain paths that we hike, neural pathways are directional. This is because synapses are one-way devices. When there is a synapse between two neurons, we say that they are connected to each other, like two friends talking on the telephone. But the metaphor is flawed, because a telephone transmits information in both directions. At any given synapse, the messages travel one way: One neuron is always the sender, the other always the receiver. This is not because one neuron is “talkative” or the other “taciturn.” Rather, it has to do with the structure of the synapse. The machinery for secreting neurotransmitter is on one side and that for sensing neurotransmitter on the other.
    In principle, neurites are two-way devices along which electrical signals can travel in either direction. In practice, a spike normally travels along an axon away from the cell body, and electrical signals travel along dendrites toward the cell body.Synapses impose this directionality onto neurites. In your circulatory system, blood flows in your veins toward your heart. If a vein were simply a tube, blood could potentially flow in either direction. But a vein also contains valves, which prevent blood from flowing backward. Valves impose directionality on veins in much the same way that synapses impose it on neural pathways.
    So a pathway in the nervous system is defined by stepping across synapses from neuron to neuron, respecting the direction of each synapse (see Figure 18). Inside one neuron, electrical signals flow from dendrites to cell body to axon. Chemical signals jump from the axon of this neuron to the dendrite of another neuron. Inside this neuron, electrical signals again flow from dendrites to cell body to axon. They are converted into chemical signals to jump to another neuron, and the process continues. Because the synaptic cleft is extremely narrow, almost all of the distance spanned by the pathway is actually within neurons rather than between neurons. Furthermore, most of this distance runs through axons, which are much longer than dendrites.
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Figure 18. Multineuron pathway in the nervous system
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    If you’ve eaten poultry, you may have spied bundles of axons on your dinner plate. They are called nerves, and can be recognized as soft whitish strings. They are not to be confused with tendons, which are tougher, or blood vessels, which are darker. Dissecting an uncooked nerve with a very sharp tool causes it to fray, much as a rope unravels into many threads when cut. The “threads” of a nerve are its axons.
    Nerves are rooted to the surface of the brain or spinal cord, together known as the central nervous system (CNS). Because most nerves