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top speed is 3 million times slower than that of electricity flowing through a wire. We
                                                measure brain activity in milliseconds (thousandths of a second) and computer activity in
                                                nanoseconds (billionths of a second). Unlike a computer’s nearly instantaneous reaction,
                                                your response to a sudden event, such as a book slipping off your desk during class, may
                                                take a quarter-second or more. Your brain is vastly more complex than a computer but
                                                slower at executing simple responses.
                                                   Like  batteries,  neurons  generate  electricity  from  chemical  events.  In  the  neuron’s
                                                chemistry-to-electricity process, ions (electrically charged atoms) are exchanged. The fluid
                                                outside an axon’s membrane has mostly positively charged ions; a resting axon’s fluid
                                                interior has a mostly negative charge. This positive-outside/negative-inside state is called
                                                the resting potential. When a neuron fires, the first section of the axon opens its gates,
                                 Distributed by Bedford, Freeman & Worth Publishers. Not for redistribution.
                                                rather like a storm sewer cover flipping open, and positively charged ions (attracted to the
                                                negative interior) flood in through the now-open channels. The loss of the inside/outside
                                                charge difference, called depolarization, causes the next section of axon channels to open,
                                                and then the next, like a line of falling dominos. This temporary inflow of positive ions is
                                           Copyright © Bedford, Freeman & Worth Publishers.
                                                the neural impulse — the action potential.
                                                   Each neuron is itself a miniature decision-making device performing complex calcula-
                                                tions as it receives signals from hundreds, even thousands, of other neurons. The mind bog-
                                                gles when imagining this electrochemical process repeating up to 100 or even 1000 times a
                                                second. But this is just the first of many astonishments.
                                                   “What one neuron tells another neuron,” noted Nobel laureate Francis Crick (1994),
                                                “is simply how much it is excited.” Indeed, most neural signals are excitatory, somewhat
                                                like pushing a neuron’s accelerator. Others are inhibitory, more like pushing its brake. If
                                                excitatory signals exceed the inhibitory signals by a minimum intensity, or threshold, the
                                                combined signals trigger an action potential. (Think of it as a class vote: If the excitatory
                                                people with their hands up outvote the inhibitory people with their hands down, then
                                                the vote passes.) The action potential then travels down the axon, which branches into
                   threshold  the level of
                   stimulation required to trigger a   junctions with hundreds or thousands of other neurons or with the body’s muscles and
                   neural impulse.              glands.
                   refractory period  in neural    Neurons need short breaks (a tiny fraction of an eyeblink). During a resting pause called
                   processing, a brief resting pause   the refractory period, subsequent action potentials cannot occur until the axon recharges
                   that occurs after a neuron   and returns to its resting state. Then the neuron can fire again.
                   has fired; subsequent action    Increasing the level of stimulation above the threshold will not increase the neural
                   potentials cannot occur until the   impulse’s intensity. Instead, the neuron’s reaction is an all-or-none response (also known
                   axon returns to its resting state.
                                                as the all-or-nothing principle): Like mechanical mousetraps, neurons either fire or they don’t.
                   all-or-none response  a      How, then, do we detect the intensity of a stimulus? How do we distinguish a gentle touch
                   neuron’s reaction of either firing
                   (with a full-strength response) or   from a big hug? A strong stimulus can trigger more neurons to fire, and to fire more often.
                   not firing.                  But it does not affect the action potential’s strength or speed. Triggering a mousetrap with a
                                                firmer push won’t make it snap harder or faster.







                         ®
                       AP  Science Practice           Check Your Understanding
                    Examine the Concept                                 Apply the Concept
                    ▶ ▶Explain the functions of the dendrites, the axon, and the cell   ▶ ▶Explain how our nervous system allows us to experience the
                    body.                                               difference between a slap and a tap on the back.
                    ▶ ▶Explain the refractory period.                   ▶ ▶Explain how the all-or-none response is like a mousetrap.
                    Answers to the Examine the Concept questions can be found in Appendix C at the end of the book.




                 30   Unit 1  Biological Bases of Behavior






          03_myersAPpsychology4e_28116_ch01_002_163.indd   30                                                                   15/12/23   9:21 AM
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