Download Membrane Potential and the Action Potential

Lecture 2 Review
This review is intended as a brief summary of the lecture. I hope it will help you to
focus your studies. It is not intended to provide you with an exhaustive review of the
material that was covered. It is also not intended to provide a review of everything
you will need to know for the exam. If a topic that was covered in lecture is not
mentioned on this review, you should not take that to mean that you do not need to
know that material.
All cells in your body, not just neurons, have an electrical voltage, called the
membrane potential, that is the result of the differential distribution of ions across the
cell membrane and the differential permeability of the cell membrane to these ions. For
the most part, the membrane potential is the result of the high concentration of potassium
(K+) inside the cell and the high permeability of the cell membrane to K+ due to
potassium leak channels. As potassium leaves the cell down its concentration gradient
through the leak channels it leaves behind negatively charged protein ions. As a result,
the inside of the cell takes on a negative charge relative to the outside of the cell. If
potassium did not have a charge, it would continue to leak out of the cell until the
concentration of potassium inside the cell was equal to the concentration outside.
However, potassium carries a positive charge. So, as the negative charge builds up inside
the cell the potassium ions start to become attracted back into the cell. When the
concentration gradient forces pulling potassium out of the cell are balanced by the
electrical forces pulling it back into the cell there will be no net movement of potassium
ions across the membrane. The electrical potential across the membrane at this balance
point is called the potassium equilibrium potential (Ek+). Because the movements of
ions is governed by physical laws, the equilibrium potential for potassium can be
calculated if you know the concentration of potassium across the membrane. The
equation that takes into account the physical laws that govern the movement of ions is
called the Nernst Equation. You should be prepared to use this equation to calculate the
equilibrium potentials of any ion, if you are provided with the concentration of the ion
inside and outside of the cell. You should understand that the equilibrium potential for an
ion is not a set number, but will depend on the concentration of the ion inside and outside
of the cell. For example, if the concentration of potassium outside of the cell were to
increase or decrease, the potassium equilibrium potential would change. In the brain a
stable potassium concentration is maintained by the blood-brain-barrier, and by
potassium spatial buffering.
If the cell membrane were only permeable to potassium then the membrane
potential would be equal to the equilibrium potential of potassium. However, the
membrane is also somewhat permeable to other ions, such as sodium (Na+). Sodium is
more concentrated outside the cell than inside, so it leaks into the cell. Because the
membrane is permeable to both potassium and sodium ions, the actual membrane
potential lies somewhere in between the equilibrium potentials for these two ions. The
membrane is 40X more permeable to potassium than it is to sodium and as a result, the
membrane potential is closer to the potassium equilibrium potential. If you know the
concentrations of potassium and sodium inside and outside of the cell and the relative
membrane permeabilities of each of these ions you can calculate the membrane potential
using the Goldman Equation. You should be prepared to use this equation to calculate
the membrane potential of a cell during the exam.
The concentration gradients for potassium and sodium ions are maintained by a
protein pump called the sodium/potassium pump. This pump moves 3 sodium ions out
of the cell for every two potassium ions that it brings into the cell. Therefore, it
contributes a small amount to the electrical potential across the cell membrane. This is
why it is described as and electrogenic pump.The sodium/potassium pump moves these
ions against their respective concentration gradients and so requires energy from the
hydrolysis of ATP.
Neurons are unique among cells in that they are able to change their membrane
potential by opening and closing ion channels. You should understand the definitions of
the terms: depolarization, repolarization, and hyperpolarization, and the opening and
closing of ion channels and the movement of ions that can cause each of these states.
Neurons use the capacity to change membrane potential to generate electrical signals.
The two electrical signals that neurons generate are action potentials and post-synaptic
potentials. I compared and contrasted these two types of signals during lecture.
Action potentials are generated by depolarization of the axon hillock (trigger
zone). This part of the axon has an abundance of voltage-sensitive sodium and voltagesensitive potassium channels. These channels open when depolarization of the axon
hillock reaches a certain amplitude. This depolarization amplitude is called the threshold
amplitude or just threshold. When the axon hillock is depolarized to or above threshold
the voltage-sensitive channels open. This results in a change in the membrane
permeability to sodium and potassium that produces the action potential. You should
know in detail the changes in ion permeability and the movement of ions that produce the
three phases of the action potential as discussed in lecture. You should also know the
origin of the absolute refractory period and the relative refractory period. In lecture I
also discussed the conduction of the action potential down the axon. You should be
prepared to describe this process and the factors that determine the speed of action
potential conduction. You should also understand the role that the myelin sheath plays in
increasing the speed of action potential conduction.