AP Biology—Unit 9
Nerve Impulse—Conduction
Name:
Background:
How are the nerve impulses that allow us to sense and react to changes in the environment created? The key is
IONS! The distribution of ions in a neuron’s cytoplasm and extracellular fluid creates an electrical environment
that is the driving force behind the nerve impulse. Sodium ions (NA+) are almost 30 times more plentiful outside
the plasma membrane of a neuron than potassium (K+) ions. Potassium ions are 10 times more plentiful on the
interior of the neuron than sodium ions. There is also a difference in the distribution of negatively charged ions
(anions) since there are large organic anions in the cytoplasm that cannot diffuse through the plasma membrane.
This uneven distribution of charge causes the interior of the neuron in its resting state to be much more negatively
charged than its surrounding environment.
Conduction of an electrical signal along an axon is similar, in some ways, to the way The Wave moves through a
crowd as spectator seated in a sporting arena. At a sporting event, one group of spectators stands up while at the
same time raising their arms. This action triggers the group next to the starting group to stand up while at the same
time raising their arms. While the second group is rising from their seats, the first /starting group is sitting down.
The Wave then passes from the second group to a third with each group stimulating the motion of the next group
standing up and sitting down in rapid succession until The Wave passes completely around the stadium.
Similar to the sporting arena analogy, an electrical signal traveling from the neuron cell body to the axon triggers
sodium gated channels in the neuron cell membrane to open. Sodium ions then diffuse through the gates into the
cell. The movement of the sodium ions into the cell triggers the opening of sodium channels down the length of the
axon. Like The Wave, the sodium channels close very shortly after they open even as sodium channels continue to
open in rapid succession down the length of the axon. The influx of positively charged sodium ions into the cell
reduces the electrical potential across the plasma membrane causing the interior of the cell to temporarily become
less negative and more positively charged. This process is called depolarization.
The cell makes up for this polarization by opening potassium-gated channels allowing potassium ions to flow from
the interior of the neurons to the outside environment. Allowing positively charged potassium ions out of the cell
causes the interior of the cell to become more negatively charged once again. This process is called
repolarization.
Repolarization returns the neuron to its pre-depolarized state. However, in its repolarized state, sodium and
potassium ion concentration on either side of the membrane are out of balance. That is where the sodium
potassium pump enters the picture! Sodium potassium pump are membrane bound proteins that use ATP to
actively transport sodium and potassium across the cell membrane reestablishing ion concentrations in appropriate
locations on both sides of the plasma membrane of a neuron in its resting state.
To model the conduction of an electrical signal in a neuron, complete the following lab activity.
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Resting Potential of the Neuron
1. Compare your model with the list below to make sure all of the parts are there.
a. ATP/ADP (2)
b. Phosphate (1)
c. Sodium ion channel gate (1)
d. Potassium ion channel gate (1)
e. Protein pump (1)
f. Sodium ions (27)
g. Potassium ions (27)
2. Illustrate the distribution of sodium and potassium ions when a nerve cell is in its resting state by:
a. Placing 24 sodium ions and 1 potassium ion on the neuron mat in the region labeled Extracellular
Fluid.
b. Placing 22 potassium ions and 2 sodium ions on the neuron mat in the region labeled Cytoplasm.
3. Your neuron mat is now at resting potential. Draw a picture of your neuron at resting potential. Be sure to
have the correct distribution of ions on either side of your membrane.
4. The distribution of ions creates a negative resting potential in the cytoplasm, usually between -60 and -90
millivolts (mV). This negative charge is created by the presence of many large impermeable anions in the
cytoplasm. Plot the resting potential (-70 mV) at Tim 0 on your nerve impulse graph.
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Action Potential and Depolarization
The electrical impulse that travels from the cell body of the neuron through the axon to the synapse is created by an
exchange of sodium and potassium ions between the cytoplasm and extracellular fluid. An action potential is a
TEMPORARY (think milliseconds—it’s that fast!) reversal of electrical charge in a specific spot of the neural
membrane and is caused by a change in the polarity of the plasma membrane of the neuron.
As an electrical impulse travels down an axon, it triggers the sodium ion channels in the plasma membrane to
open. This permits the sodium ions to diffuse into the interior of the axon.
1. To stimulate this process, open the sodium ion channel in your nerve impulse model by sliding the sodium
voltage gate so that the sodium channel is open. Now move most of your sodium ions through the sodium
ion channel into the cytoplasm of your axon.
2. What effect does this change have on the electrical charge of the internal environment of the neuron?
3. Draw your membrane with the appropriate distribution of ions and label it Depolarization.
4. As the positive sodium ions diffuse into the neuron the resting potential of -70 mV changes to the action
potential of +50 mV. Show this change on your graph. Note the broken line on your graph. This line
represents the threshold voltage required to trigger the action potential. Between 0 milliseconds and 2.5
milliseconds show the change in voltage to the threshold of -55 mV. From 2.5 msec to 3.5 msec, show
change to +50 mV.
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Repolarization
When the inside of the cell reaches approximately +50 mV, inactivation gates of the sodium channels close and the
gates of the potassium channels open. Consequently, sodium stops diffusing into the cell, but potassium starts
diffusing out of the cell to the extracellular fluid.
1. Close the sodium ion channels. Open the potassium gate so that potassium ions can diffuse from the
cytoplasm to the extracellular fluid. Allow most of the potassium ions to exit the cell.
2. How does the movement of potassium ions out of the cell affect its electrical charge?
3. Sketch your membrane with the appropriate distribution of ions and label it Repolarization.
As the number of potassium ions in the cytoplasm increases, the electronegativity of the axon interior will increase
and the voltage will again drop to the resting potential or in some cases, to less than -70 mV as the potassium ion
channels allow the escape of potassium ions. If the voltage decreases below -90 mV, it is identified as
hyperpolarization. As potassium channels close, the voltage is restored to the resting potential.
4. Show this change in voltage from +50 mV to -95 mV on your graph. Start at 3.5 msec and end at 5 msec.
From 5.1 msec to 8.0 msec, show the return of the voltage to the resting potential of -70 mV.
5. Label each section of the graph, resting potential, action potential, repolarization, hyperpolarization,
and resting potential.
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Sodium Potassium Pump Reestablishes Ion Distribution
Sodium potassium pumps are carrier proteins embedded in the plasma membrane. Their function is to maintain
appropriate distributions of sodium and potassium ions on both sides of the plasma membrane. This is
accomplished through active transport. Energy from the hydrolysis of ATP powers the pump as it transports three
sodium ions from inside the axon to the surrounding extracellular fluid while at the same time, transporting two
potassium ions from the extracellular fluid back into the axon. At top speed, the sodium potassium pump can
transport about 450 sodium ions and 300 potassium ions per second.
1. Activate the pump:
a. Move the ATP molecule to the cytoplasm side of the sodium potassium pump.
b. Attach a loose phosphate to the pump.
c. Flip the ATP over to its ADP side.
d. Move the ADP molecules off to the side.
2. Bind and transport the sodium ions from the cytoplasm to the extracellular fluid:
a. Rotate the sodium potassium pump pieces so that the interior of the pump is open to the cytoplasm.
b. Place three sodium ions on the green side of the pump for a ride out of the cell.
c. Rotate the pump pieces transporting the sodium ions out of the cell.
3. Bind and transport the potassium ions from the extracellular fluid to the cytoplasm:
a. Place two potassium ions found in the extra cellular fluid on the blue side of the pump for the ride
across the membrane and back into the cell.
b. Rotate the pump pieces transporting the potassium ions back into the cell and release these ions into
the cytoplasm.
4. Repeat steps 1-3 until the original ion concentrations are restored. (See your original resting potential
diagram for appropriate concentrations.)
Conduction of Action Potentials
The action potential will travel down the neuron axon because the rising phase of the action potential causes and
electrical current that depolarizes the neighboring region of the axon membrane. The depolarization is large
enough to reach the threshold, causing the action potential to be initiated there. This process repeats itself as the
action potential travels down the axon. The impulse only travels in one direction DOWN the axon because the
area behind the action potential is repolarizing.
Speed of Conduction
1. Wider axons allow for faster conduction.
2. Myelinated axons in vertebrates allow for faster conduction while allowing the axon to be thinner.
Conduction in myelinated axons is called Saltatory Conduction. Depolarization / repolarization only
occur in the Nodes of Ranvier; consequently, the impulse jumps from node to node. (Much Quicker!)
Summary Questions
1. Describe the distribution of ions and the electrical gradient of the neuron when it is at resting potential.
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2. When does depolarization result in an action potential?
3. What is happening during repolarization?
4. Why are sodium potassium pumps important in the neuron?
5. Why does the action potential only travel in one direction?
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