Action Potentials

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FIGURE
The Neuron
Chapter 2 Biology and Behavior
2.2
Part A shows fibers extending outward
from the cell body of a neuron, which is a
nervous system cell. These fibers are called
axons and dendrites. Part B shows an enlarged drawing of the neuron’s cell body.
The cell body of a neuron includes an outer
membrane, a nucleus, and mitochondria.
Dendrites
Cell body
Nucleus
Mitochondria
Outer
membrane
Axon
axon A fiber that carries signals away
from the cell body.
dendrites Fibers that receive signals
A typical neuron
Cell body of a neuron
(A)
(B)
Every cell in the body has a skin, called an outer membrane; a cell body that (with
the exception of red blood cells) contains a core called the nucleus; and tiny “engines,”
called mitochondria (pronounced “my-toh-CON-dree-uh”). Neurons are no different.
A neuron’s outer membrane acts like a screen, letting some substances pass in and
out while blocking others. In the neuron’s cell body, the nucleus (or center) carries
genetic information that tells the cell what to do. And neurons’ mitochondria turn
oxygen and glucose into energy. This process is especially vital to brain cells, because
although the brain accounts for only 2 percent of the body’s weight, it uses more
than 20 percent of the body’s oxygen. All of this energy is required because brain
cells transmit signals among themselves to an even greater extent than do cells in the
rest of the body.
Neurons have special structural and chemical features that allow them to communicate with each other. Let’s first examine their structure. Although neurons come in
many shapes and sizes, they all have long, thin fibers that reach outward from the cell
body like arms (see Part A in Figure 2.2). When these fibers get close to other neurons,
communication between the cells can occur. The interweaving of these fibers with fibers
from other neurons allows each neuron to be close to thousands or even hundreds of
thousands of other neurons.
Fibers extending from the cell body are called axons and dendrites. As shown in Figure 2.2, each neuron generally has only one axon, whose function is to carry signals
away from the cell body. An axon may have many branches along its stem, much like
a tree. Axons can be short or long. In the brain, they may extend no more than a fraction of an inch, but the axon from your big toe to your spine is more than three feet
long! Dendrites are the fibers that receive signals from the axons of other neurons and
carry those signals to the cell body. As you can see in Figure 2.2, a neuron can have
many dendrites, each of which usually has many branches. Remember that axons carry
signals away from the cell body, and dendrites detect those signals.
from the axons of other neurons.
action potential The electrochemical
Action Potentials
impulse or message that is sent down
an axon and stimulates release of a
neurotransmitter.
The communication signal between neurons begins with an electrochemical pulse
called an action potential, which shoots down the axon. This is an “all-or-nothing”