Read Online Connectome by Sebastian Seung - Free Book Online Page A

100,000 times smaller than the tree’s football-field height. A branch of a neuron, called a
neurite,
can extend from one side of the brain to the other, yet can also narrow to 0.1 micrometer in diameter. These dimensions differ by a factor of one
million.
In its relative proportions, a neuron puts a redwood to shame.
    But why do neurons have neurites? And why do they branch to look like trees? In the case of a redwood, the reason for branches is obvious: The redwood’s crown captures light, which is a source of energy. A passing sunbeam will almost surely collide with a leaf rather than travel all the way to the ground. Likewise, a neuron is shaped to capture contacts. If a neurite passes through the branches of another neuron, it will likely collide with one of them. Just as a redwood “wants” to be struck by light, a neuron “wants” to be touched by other neurons.
    Â 
    Every time we shake hands, caress a baby, or make love, we may be reminded that human life depends on physical contact. But why do neurons touch? Suppose that the sight of a snake causes you to turn and run. You respond because your eyes are able to communicate a message to your legs:
Move!
That message is conveyed by neurons, but how?
    Neurites are much more densely packed than the branches of a forest or even a tropical jungle. Think instead of a plate of spaghetti—or microscopically fine capellini. Neurites entangle much like the jumbled strands on your plate, allowing one neuron to touch many others. Where two neurons touch, there can be a structure called a
synapse,
a junction through which the neurons communicate.
    But contact alone does not make a synapse, which most commonly transmits chemical messages. A molecule known as a
neurotransmitter
is secreted by the sending neuron and sensed by the receiving neuron. Secretion and sensing are performed by still other types of molecules. The presence of such molecular “machinery” signifies that a contact point is actually a synapse, as opposed to a place where one neurite just goes past another.
    These telltale signs are blurred in an ordinary microscope, which uses light to make images, but show up nicely with a more advanced microscope based on electrons rather than light. The image shown in Figure 14 is a highly magnified (100,000×) view of a cut through brain tissue. There are two large, round cross-sections of neurites (marked “ax” and “sp”).These are like the cut ends of strands that would be exposed if you sliced through spaghetti. The arrow points to a synapse between the neurites, which are separated by a narrow cleft. Now we see that the term
contact point
is not entirely accurate, as the neurites come extremely close to each other but do not really touch.
    Â 
    Â 
    Â 
Figure 14. A synapse in the cerebellum
    Â 
    On either side of the cleft is the molecular machinery for sending and receiving messages. One side is dotted with many little circles, tiny bags called vesicles that store neurotransmitter molecules ready for use. On the other side the membrane holds a dark fuzz called the
postsynaptic density,
which contains molecules known as
receptors.
    How does this machinery transmit a chemical message? The sender secretes by dumping the contents of one or more vesicles into the cleft. The neurotransmitter molecules spread out in the salty water there. They are sensed by the receiver when they encounter receptor molecules embedded in the postsynaptic density.
    Many types of molecule are used as neurotransmitters. Each is assembled from atoms bonded to each other, as in the examples shown in Figure 15. (In these “ball-and-stick” models, each ball represents an atom and each stick a chemical bond.) You can see that each type of neurotransmitter has a characteristic shape determined by the specific arrangement of its atoms, a fact that will become important shortly.
    Â 
    Â 
    Â 
Figure 15. “Ball-and-stick”