This is the 2007 version. Click here for the 2017 chapter 02 table of contents.

How Neurons Communicate

You have heard the word nerve used in everyday conversation. Technically, a nerve is a bundle of axons outside the central nervous system (that is, outside the brain and spinal cord). A bundle of axons wholly contained within the central nervous system is called a tract or pathway.

What is a nerve? What is a tract or pathway?

Neurons communicate over long distances by sending signals called nerve impulses through the axons which make up a tract or nerve. Because each axon may branch into a whole tree, and because nerve impulses go down each branch when an axon divides, a single neuron may send signals to thousands of other neurons. Meanwhile, the dendrites and cell body (and often the axon) of that single neuron may receive nerve impulses from thousands of other neurons. So the nervous system is one big network of neurons, with each cell having inputs and outputs that may connect it to thousands of other nerve cells.

What is a synapse?

The output from an axon arrives at an area called a synapse (SIN-apse is the American pronunciation, SINE-apse the British pronunciation). At a synapse , two neurons are separated by a tiny gap called the synaptic cleft. When a nerve impulse reaches the end of an axon, it stimulates chemicals called transmitters or neurotransmitters to flow rapidly across the synaptic cleft, producing an output from the axon and an input on the dendrite of the following neuron. Each neuron might stimulate thousands of other neurons this way.

How is each neuron like a pattern recognizer?

When transmitters flow across a chemical synapse, they have one of two effects on the post-synaptic neuron (the neuron that comes after the synapse). They either excite it (make it more likely to fire a nerve impulse itself) or inhibit it (make it less likely to fire a nerve impulse itself). Each neuron responds to many such inputs and, based on the pattern of activity and how recently it has fired an impulse, either fires another nerve impulse or not. In that sense, each neuron is like a pattern recognizer, responding to the pattern of inputs (and timing of inputs) from other cells. One neuron might fire when you smell garlic, another might fire when you see a familiar face, or have a particular memory, and so forth. The overall pattern of firing in the nervous system determines your state of mind, moment by moment. Most of this activity is unconscious, but some small part of it composes your conscious thought process. "That art thou."


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