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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,
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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
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