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Transcript
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.