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LAB 6 - Action Potential Experiments


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Much of what we know about how neurons work comes from experiments on the giant axon of the squid. This giant axon extends from the head to the tail of the squid and is used to move its tail. The purpose of this lab was to use an axon potential stimulating program to learn how action potentials work in the giant squid axon.

Resting Potential

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When a neuron is not sending a signal, it is said to be at rest. When a neuron is at rest, the inside of the neuron is negative relative to the outside. Although the concentrations of the different ions attempt to balance out on both sides of the membrane, they cannot because the cell membrane allows only some ions to pass through ion channels. The difference in the voltage between the inside and outside of the neuron gives you the resting potential. An actual squid axon in seawater has a resting potential of about –50mV.

Na+ /K+ Pumps

Nerve cells actively pump Na+ out and pump K+ in using the Na+ and K+ pump. Since three Na+ ions enter the cell for every two K+ ions that leave the cell, three Na+ ions must leave for every two K+ that enter.

Action Potentials
The action potential indicates what happens when the neuron transmits information from one cell to another.The cause of the action potential is an exchange of ions across the neuron membrane.

The first step in the generation of an action potential is to depolarize the cell by injecting current into the axon. This will partially depolarize the cell membrane, causing it to become less negative and this change in membrane potential triggers voltage gated Na+ to open. Na+ ions are now free to pass through this channel, resulting in a relatively massive influx of Na+ inside the axon. Since the membrane is now overwhelmingly permeable to Na+ the membrane potential at the top of the spike will be driven close to the Na+ Nernst potential of 55+mV. Voltage gated K+ channels also open as a response to depolarization but they only do so after the opening of the Na+ channels allowing a relatively large amount of K+ to leave the axon. As the voltage gated K+ channels open, the voltage gated Na+ channels now close preventing additional Na+ from entering the axon. So much positive charged K+ leaves the axon under these conditions that the membrane potential temporarily becomes hyperpolarized at a value of -64mV. Voltage gated channels are now closed and the membrane potential had returned to its normal resting potential.

Nernst Equation - The Nernst equation can be used to calculate the membrane potential. The Nernst potential for potassium = -93mV and for sodium = +55mV.

K+(Nernst equation) = Ek = RT In [K+]o
ZF [K+]i
R = gas constant
F = faraday constant
Z = valance of K+
T = absolute temperature in degrees Kelvin
Ek = equilibrium membrane potential for K+. That is, the value in mV at which K+ ions is at equilibrium on either side of the axon membrane.

Hear some action potentials in the Sounds of Neuroscience gallery.