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The Central Nervous System
STANDARD 6.7 understand the relationship between the brain and behavior.
• To identify the parts of the nervous system and
explain how they work
The Main Parts of the Nervous System
For more information, go to
The nervous system:
• Has two main parts: the central nervous system and
the peripheral nervous system.
• BOTH are composed of neurons, or nerve cells, that
transmit messages to different parts of the body.
• Neurons have three main parts: cell body (produces
energy), dendrites (DELIVERS info to the cell body),
and axons (carries info AWAY from the cell body.)
• Some neurons are several FEET long!
• A neuron has many dendrites but only one axon,
which tells you that your brain SENDS more
messages to our bodies than it receives from our
The Purpose of Neurons
A neuron exists to perform three tasks:
1. To receive information (in the form of
electrochemical impulses) from other neurons that
feed into it
2. To carry this information down its length
3. To pass the information to the next neuron in line
This transmission of an electrochemical impulse is
called “firing.” They can either fire, or not. This is
called the “all-or-none principle.” A neuron always
fires with the same intensity regardless of the
stimulation from the dendrites.
The Neural Chain
• The neural chain describes the path
information follows as the nervous system
processes it.
• Your nervous system “collects” information
using receptor cells, specialized cells in the
sensory systems of the body.
• These receptor cells convert energy into
impulses (action potentials) your brain can
understand. We will discuss these cells in
detail when we study sensation and
Types of neurons
• Sensory nerves carry information from the
sense receptors cells to the brain and
spinal cord.
• Interneurons are the billions of neurons in
your brain and spinal cord that process
• Motor nerves carry information from your
brain and spinal cord to your muscles and
The Role of the Brain
• Once it receives the impulses, the brain make
appropriate decisions, using its interneuron.s
• The brain can make the decision, but it still has to
communicate the information to the body’s muscles
and glands through motor neurons.
• Ex. When you have a tension headache, sensory
neurons transmit a pain message to your brain.
Motor neurons transmit from the brain to your
muscles and glands, telling them to RELAX. They
might even tell you to massage your temples or
close your eyes!
How does this work?
• Dendrites receive information and deliver impulses
to the cell body to tell the cell body (soma) how to
• Axons carry messages away from the cell body to
other neurons, muscles, or glands.
• BOTH dendrites and axons jump from neuron to
neuron via synapses that connect dendrite to axon
to cell body. (We will diagram this later.)
• New synapses form when neurons that were not
previously connected create connections.
Click on the picture for a link to a Discovery Channel
video explaining how neurons work.
Dendrites and Axons at work
How does a neuron fire?
The Neural Impulse
• When a neuron fires, it creates a neural
impulse called an action potential, which
carries the electrical charge from the dendrites
to the axon terminals, referred to as firing.
• After firing, each action potential is followed
by a brief recharging period, known as the
refractory period, when it can’t fire.
• When the cell is capable of firing again, it has
reached its resting potential, meaning it’s
relaxed and ready to fire again. Like YOU after
a nap or a vacation!
Analogy of how a neuron fires
• Like a neuron, a toilet has an action potential. When
you flush, an “impulse” is sent down the sewer or
septic pipe.
• Like a neuron, a toilet has a refractory period. There
is a delay after flushing when the toilet cannot be
flushed again because the tank is being refilled.
• Like a neuron, a toilet has a resting potential. The
toilet is “charged” when the tank is filled and can be
flushed again.
• Like a neuron, a toilet operates on the “all-or-none
principle” – it flushes with the same intensity no
matter how much force you apply to the handle.
Critical Thinking Activity
• With one or two of your neighbors,
brainstorm other analogies for the neural
• Outline each step compared to the neuron
just like the toilet example.
• Write down each step.
Synaptical Connection
Synapse between
axon and dendrite
• Neurotransmitters travel
across the synapse to carry
the information from one
neuron to the next. The
neurotransmitter influences
whether the next neuron will
generate an action potential
or not. Neurotransmitters
can only fit in receptor sites
that fit their shape, referred
to as “lock and key.”
The central nervous system
• The central nervous system is composed of two
parts: the brain and the spinal cord.
• The spinal cord is a column of nerves. If you’ve
ever “pinched a nerve,” then you know nerves
are important.
• The spinal cord is also involved in spinal reflexes
(simple, automatic responses), such as touching a
hot stove immediately sends a message to
remove your hand. Sometimes the reflex occurs
BEFORE the pain even registers!
Put your dendrites & axons to work!
• Line up around the room from the table next
to the white board to my desk area.
• The first person faces the rear of the room,
and everyone else puts their back to the
person behind them.
• Wait for further instructions . . .
Works Consulted
Kasschau, Richard A. Understanding Psychology.
Columbus, Ohio: Glencoe/McGraw-Hill, 2003.
6-33. Print.
Rathus, Spencer A. Holt Psychology: Principles in
Practice. Austin, TX: Holt, Rinehart and
Winston, 2007. 2-23. Print.
Blair-Broeker, Charles T., and Randal M. Ernst.
Thinking about Psychology. New York: Worth
Publishers, 2013. 59-92.