Connectome by Sebastian Seung

extend and branch toward the surface of the body, they are known as the peripheral nervous system (PNS). The axons in nerves come from cell bodies in the CNS or in little outposts of neurons known as peripheral ganglia. The CNS and the PNS together make up the nervous system, defined alternatively as the set of all neurons and the cells that support them.The emphasis on nerves in the term
nervous system
is perhaps misleading, as the brain and spinal cord are its predominant parts.
    Now let’s return to the question posed earlier: How does the sight of a snake cause you to turn and run? The rough answer is that your eyes signal your brain, which signals your spinal cord, which signals your legs. The first step is mediated by the optic nerve, a bundle of a million axons from the eye to the brain. The second step happens through the pyramidal tract, a bundle of axons from the brain to the spinal cord. (A bundle of axons in the CNS is known as a tract rather than a nerve.) The third step passes through the sciatic and other nerves, which connect your spinal cord to your leg muscles.
    Let’s consider the neurons at the beginning and end of the pathways mediated by these axons. At the back of your eye is a thin sheet of neural tissue called the retina. Light from the snake strikes special neurons in the retina called photoreceptors, which respond by secreting chemical messages, which in turn are sensed by other neurons. More generally, every one of your sense organs contains neurons that are activated by some type of physical stimulus. Sensory neurons kick off the journey along neural pathways from stimulus to response.
    These pathways end when axons in nerves make synapses onto muscle fibers,which respond to secretion of neurotransmitter by contracting. The coordinated contraction of many fibers causes a muscle to shorten and produce a movement. More generally, every one of your muscles is controlled by axons that come from motor neurons. The English scientist Charles Sherrington, who won a Nobel Prize in 1932 and coined the term
synapse,
emphasized that muscles are the final destination of all neural pathways: “To move thingsis all that mankind can do . . . for such the sole executant is muscle, whether in whispering a syllable or felling a forest.”
    Between sensory and motor neurons there are many pathways, some of which we will consider in detail in later chapters. It’s clear that these pathways exist; if they didn’t, you wouldn’t be able to respond to stimuli. But exactly how do signals travel along pathways?
    When California joined the United States in 1850, communicating with the eastern states took weeks. The Pony Express was created in 1860 to speed up mail delivery. Along its two-thousand-mile route from California to Missouri were 190 stations.A mailbag traveled day and night, switching horses at every station and changing riders every six or seven stations. After reaching Missouri, messages traveled by telegraph to states farther east. The total transit time for a message between the Pacific and the Atlantic was reduced from twenty-three to ten days. The Pony Express operated for only sixteen months before being completely replaced by the first transcontinental telegraph, which in turn was succeeded by telephone and computer networks. The technology may have changed, but the underlying principle has not: A communication network must have a means of relaying messages from station to station along pathways.
    It’s tempting to think of the nervous system as a communication network that relays spikes from neuron to neuron. A neural pathway would behave like dominoes, with each spike igniting the next spike in the pathway in the same way that each falling domino tips over the next one in the chain. This would explain how your eye tells your legs to move when you see a snake. But in fact it’s not that simple. While it’s true that an axon relays spikes from the cell body to