This lab explores how ion concentrations and ion channel activity generate action potentials through computer simulations and physical models. In part one, students model ion flow across a permeable membrane using water, food coloring, and a punctured cup to represent ions, channels, and the cell membrane. In part two, the simulation shows how injected current triggers the opening of sodium and potassium channels, allowing ions to flow down their gradients and change the membrane potential. Students then alter sodium concentrations outside the cell to model hyponatremia and observe how this affects action potential generation. The results demonstrate that lower sodium equilibrium potentials prevent action potentials, and by extension, neural signaling and heart contractions.

1. Can an action potential occur if there is not the
Modeling Ion MovementIn this lab we will explore the importance of ion balance in
neurotransmission. In part one you will model how ions move in and out of a neuron due to
channels opening. Then in part two you will use a computer simulation to look at how ion
distributions across the cell membrane (plasma membrane) make an action potential
possible.You will make a hypothesis that talks about how ion balance is important for
neuron transmission. Remember that transmission (communication from one neuron to a
another) can only happen when an action potential is triggered. Here are some questions to
consider when framing your hypothesis. Make a hypothesis that integrates these
questions.⦁ Can an action potential be triggered without ion flow occurring?⦁ Can an action
potential occur if there is not the correct concentrations of ions on the outside and the
inside of the cell?Now that you have a hypothesis we will test the hypotheses with some
modeling! In part one we are using physical models to look at how channels opening allow
ion flow down their concentration gradients. In part two we will use a computer model to
ask how important the precise ionic concentrations are to be able to trigger action
potentials.Lab 3 – Part 1:Understanding ion flow:Grab 1 clear plastic cups, 1 paper cups, and
a pin. Put an inch of water in the clear plastic cup, fill the paper cup with water. Put the
paper cup inside the clear plastic cup. Add 5-10 drops of food coloring the water inside the
paper cup. Now, lift the paper cup up, take the pin and puncture the side of the paper cup.
Now put it back into the clear plastic cup. Describe what happens.Insulation:Using two
magnets and a paper towel, allow the magnets to connect through the paper towel. Repeat
but this time fold the paper towel. Can the magnets still connect through the folded paper
towel. Continue folding the paper towel until the magnets cannot connect. What have you
demonstrated here?Answer these questions in your lab report in the methods section as
you are describing them. Remember to state the methods in a narrative form and not a
numbered list.⦁ What does the water inside the paper cup represent based on our lectures?⦁
What kind of channel does the pin hole represent (be specific!!)?⦁ What ion does the food
coloring represent?⦁ What does the paper towel represent?⦁ What do the magnets
represent?Lab 3 – Part 2:The knowledge you gained from the food coloring should help
build your hypothesis about how a cell’s membrane potential is controlled by changes in the
ion concentration. Form a hypothesis about what will happen as you decrease Na+
concentrations outside the cell. Remember this hypothesis as you will need it for the
lab!Next go to this Simulator for Neural Networks and Action Potentials (SNNAP)
https://nba.uth.tmc.edu/neuroscience/s1/labs/actpot/hhsimu.htmlby the University of

2. Texas Health Science Center at Houston. You will land on a page showing the one below.The
blue line Vm represents the membrane potential that is calculated using this equation.In
order to understand anything about this equation you will need to know what the terms in
the equation mean. As stated above, the Vm is the membrane potential, Cm is the membrane
capacitance (capacitance is the ability of a membrane to store a charge and is determined by
physical properties of the membrane), Iinj is the injected current, gNa is the
Na+ conductance, gK is the K+ conductance, and gl is the leakage conductance. ENa and EK
are the equilibrium potentials of sodium (Na) and potassium (K) respectively. There are
also a few variables that model activation of ion channels. Recall that ion channels are what
gives the membrane permeability to the ions. Ions themselves will not pass-through
membranes. There had to be a hole in the cup to let the ions (dye molecules) out. The
conductance of the Na+ channel is governed by an activation variable m and an inactivation
variable h, gNa = gNamax m3h and the conductance of the K+ channel is governed by a
single activation variable n, gK = gKmax n4Causing and Action Potential with Channel
Opening.Now that we understand the terms, we can start manipulating some of them. The
default settings here are set to use an electrical impulse (Current Injection) to trigger an
action potential (blue line). The orange and green lines model the increase in permeability
of sodium and potassium ions due to the opening of sodium and potassium channels due to
an electrical impulse. Notice that there is a term for Current Injection. This is the stimulus
that will trigger the opening of the ion channels in the membrane because here we are
modeling voltage gated ion channels. When you set this Current Injection to zero, there will
be no permeability of the membrane to ions (all channels are closed).Answer these
questions in your results section.⦁ What happens to the membrane potential when the
Current Injection is zero?⦁ Alter the Current injection to be 5 nA/cm2. Do you see an action
potential being fired now?⦁ Step up the Current Injection from 5 nA/cm2 one step at a time
leaving all other parameters set to their default. At what current do you see the action
potential fire?Answer these questions in your results and discussion section.⦁ Knowing that
Current Injection causes ion channels to open, explain what has happened when the current
reaches the level needed to cause the action potential. Make sure to mention channels
opening and increased permeability of ions.Altering Ion ConcentrationsWe know from the
Nernst and Goldman equations mentioned in the beginning of this lab that the equilibrium
potential for ions can be simplified (over simplified) to a ratio of the concentration of an ion
outside a cell to that on the inside of the cell [ion]outside/ [ion]inside. Mathematically this
means that as the concentration of the ions outside a neuron is lowered so will the
equilibrium potential of that ion. Reducing the concentration of sodium ions outside side of
a neuron would mimic what happens in cases of hyponatremia (low sodium and too much
water) that we saw in “The Agony of Ecstasy MDMA and the Kidney” paper.Notice that you
can also change Equilibrium potential of both sodium and potassium. In this exercise we
will manipulate the equilibrium potential of sodium to model hyponatremia. When we
reduce the equilibrium potential of sodium, we are effectively decreasing the amount of
sodium outside the cell and breaking down the concentration gradient of sodium that exists
across the neuron membrane. Recall that ions will move from areas of high concentration to
areas of low concentration (flowing down the concentration gradient).The default settings

3. on our simulator models a standard sodium gradient that you would see under normal
conditions. This results in an Equilibrium potential of sodium of 55 mV. To model
hyponatremia, change this to 0 mV while keeping all other parameters the same.Answer
these questions in your results section.⦁ Where you able to trigger an action potential while
the equilibrium potential of sodium was zero? Note that the action potential trace is still in a
dotted blue line so you can see what it would look like.⦁ Note the membrane potential at the
peak (apex of blue line) when the Equilibrium potential of sodium is 0 mV and check that it
matches what is on the chart below. Determine the membrane potential at the peak for four
more points of sodium equilibrium potential levels anywhere between zero and 55 and fill
in the chart below.Sodium Equilibrium Potential (mV) Membrane Potential (mV)0 -41.5For
your graphics section.⦁ Make a scatter plot of Membrane Potential vs. Sodium Equilibrium
Potential. Since Sodium Equilibrium Potential is the independent variable it should go on
the x-axis (Abscissa) and since the Membrane Potential is the dependent variable it should
go on the y-axis (Ordinate). Add a trendline and determine the coefficient of determination
(R2). For more information on the R2 and what it can tell us ⦁ look here. What trend do you
notice? How can you tell if it is significant?In your discussion section answer the following.⦁
Now let’s imagine a scenario like the one in the JAMA article about ““The Agony of Ecstasy
MDMA …” where the sodium outside the neuron is really low and thus causing the
equilibrium potential of sodium to be low. Based on your plot above do you think that
action potentials (depolarization of the membrane) will be more difficult in hyponatremia?⦁
How might this change in action potentials affect consciousness?⦁ How might this change in
action potentials affect your heart’s ability to beat?⦁ Why does taking ecstasy sometimes
result in this?Introduction: Introduce the lab and your hypothesis. Give pertinent
background information as needed that explains what you are doing.Methods : Talk about
what you did for the lab to collect the data and/or information as well as the analyses you
did.Results : Talk about the data you collected and the results of the analysis without
analyzing it.Discussion : Analyze the data in your results and relate that to your hypothesis.
The discussion section will also be where you talk about how to improve your research and
why this type of study matters.Summary of what was found and why it matters. Make sure
to answer the questions given in the lab report for this section.Clear statement of whether
the hypothesis was supported or rejected and why.Statement(s) of what the strengths of the
experimental approach were.Statement(s) of what the weaknesses of the experimental
approach were including possible sources of error or bias.Graphics :Give a completed chart
of your values from part 2 and an XY scatter plot of these values.Appropriate labels are
provided for each graphic. This include chart titles and axis labels.