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PPT
Nerve Impulses
Developer Notes

Version
01
Date
2004/07/06
Who
sc
Revisions
Initial version
Goals
1. Students should understand that a stimulus generates a nerve response
2. Students should understand that a neuron is a specialized cell
3. Students should understand how resting potentials and action potentials are generated
4. Students should understand that neurons transmit messages through changes in cell potentials
Concepts & Skills Introduced
Area
Physiology
Concept
How nerves work
Standards Addressed
Time Required
Warm-up Question
Presentation
We have seen how the nervous system plays an important role in reaction time, stability and
balance. The nervous system contains special cells called neurons that respond to and send
messages. These “messages” are actually electrical. We can use our knowledge of physics to
understand how they are transmitted!
Different types of neurons respond to different stimuli. A stimulus is anything that generates a
nerve response. For example, light is a stimulus that generates a response in specific neurons
called photoreceptors (rods & cones). Neurons are peculiar looking cells with a main cell body
and a long tail called the axon. The entire cell is enclosed by a cell membrane.
When a neuron is not responding to a stimulus it is called a resting neuron. There is a difference
in charge between the inside and outside of a resting neuron. The inside of a resting neuron has a
net negative charge while the outside has a net positive charge. How can this be? Don’t
opposite charges attract? Wouldn’t the charges want to be in equilibrium? The difference in
charge results from different amounts of positive and negative ions (charged atoms) inside and
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outside the cell. This difference is maintained by energy using protein “pumps” that transport
ions across the cell membrane. Without the action of the pumps the amounts of positive and
negative ions inside and outside the cell would be equal; there would be no net charge.
However, with the action of such a pump, along with diffusion, 3 positive sodium ions (Na+) go
outside the cell while only 2 positive potassium ions (K+) come inside the cell. The inside of the
cell loses 3 positive charges and only gains 2. The outside of the cell only loses 2 positive
charges while it gains 3. Thus, there is a net negative charge inside the cell and a net positive
charge outside the cell. Transporting only positive ions may seem silly. Why not just separate
the individual positive and negative ions? The negative ions found in our cellular fluids are
relatively large and do not pass through cell membranes easily. The positive ions are much
smaller and easier to move!
Since there is a separation of charges, there is a difference in electric potential (a voltage!).
Think about it! Work has to be done to separate the charges, thus increasing potential energy. If
a small positive charge were put outside the cell it would have a high potential, it would be
repelled and travel toward the inside of the cell where it would lose potential energy and gain
kinetic energy. Since this difference in potential occurs in a resting neuron, it is referred to as the
resting potential.
A stimulus disrupts the resting potential of a neuron. At the location of the stimulus, channels in
the cell membrane open and allow Na+ to freely enter the cell. This causes a change in the
resting potential (voltage) at the area of stimulation. This change is called an action potential.
(The neuron is now active)! How does the signal then travel along the neuron to relay the
message? Na+ channels further along the neuron will open in response to a change in voltage
(they are called “voltage-gated channels”). So one action potential causes adjacent Na+ channels
to open, this causes another action potential that causes more channels to open…and the chain
reaction keeps going! The wave of action potentials along the neuron constitutes the “signal.”
What happens when the signal reaches the end of a neuron? The neuron will release a chemical
(neurotransmitter) that will act as a stimulus on the next neuron. (In the heart and digestive
system, one neuron can stimulate the next one directly with action potentials).
What happens after the action potentials have been generated? The neuron quickly re-establishes
its resting potential.
Assessment
Have students develop a model that illustrates nerve impulses.
Writing Prompts
Relevance
Answers to Exercises
1.
Answers to Challenge/ extension
1.
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Background
Problem
Materials
Procedure
Summary
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Reading
Exercises
1.
Challenge/ extension
1.
Glossary
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