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CSB332H1 (52)
Lecture 7

# CSB332 Lecture 7 Notes

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School
University of Toronto St. George
Department
Cell and Systems Biology
Course
CSB332H1
Professor
Francis Bambico
Semester
Winter

Description
CSB332 Lecture 7 Slide 1 - When there is no net movement of ions through the plasma membrane, then it is similar to a state of the neuron called the resting membrane potential. At resting membrane potential, there is no net flow of ions. - Equilibrium potential is reflective of the resting membrane potential. - Is the equilibrium potential of K+ ions always equivalent to the resting membrane potential of a given neuron? Is the resting membrane potential solely dependent on the equilibrium potential of K+? Slide 2 - The resting membrane potential is not only dependent on the equilibrium potential of K+. The resting membrane potential of a typical neuron is dependent on a number of factors. There are other ions and their respective equilibrium potentials that are involved in the generation and the maintenance of the resting membrane potential. - The resting membrane potential of a neuron is dependent on Na+, K+, and the ATPase pump. - The currents that contribute to the resting membrane potential are of two entities. o K+ leak current  Voltage-gated K+ channels are open when the resting membrane potential is reached. o Na+ leak current  NALCN channels are the predominant Na+ leak current channels. - HCN channels are non-selective for K+ and Na+. Slide 3 - Nernst equation o K+ dictates the resting membrane potential of a typical neuron because there is a greater concentration of K+ inside the neuron and there is a high number of resting K+ channels on the plasma membrane. o The resting membrane potential would always be equivalent to the equilibrium potential for K+, but this is if only K+ ion channels are important in maintaining the resting membrane potential. o The equilibrium potential for K+ is dependent on the external concentration and the internal concentration of K+. o There is a direct linear relationship between the external concentration of K+ and the membrane potential of the neuron. The membrane potential changes by 58 mV per 10-fold change in the external concentration of K+ ions. - The equilibrium potential for K+ is not sufficient to determine the resting membrane potential of a neuron. - Hodgkin and colleagues were able to record the resting membrane potential by using a squid giant axon to verify if (or not) Vr is equivalent tK E . o The axon is about 800 microns in diameter, which is 1000X larger than the diameter of a typical mammalian neuronal axon. o They stuck electrodes in to the giant axon. You can manipulate the external concentrations of K+ in the bath solution and record the resulting change in membrane potential. You can examine the relationship of external concentration to the change in membrane potential. o There is a clear deviation from the expected equilibrium potential based on the Nernst equation. The membrane potential deviates significantly from expected equilibrium potential when the concentration of K+ is less than 100 mM. - Therefore, the resting membrane potential is not always equivalent to the equilibrium potential of K+. o Na+ contributes to the generation of resting membrane potential. o Cl- does not contribute to the resting membrane potential. Slide 4 - (A) o This is the condition when only resting K+ channels are present in the plasma membrane. K+ is at equilibrium, so there is no net movement of K+ ions in and out of the neuron. The membrane potential is equivalent to the equilibrium potential of K+. - What if only leak Na+ channels are present? o If only leak Na+ channels are present, then Vr =Na . - (B) o There is a large concentration of Na+ outside of the neuron. The membrane potential that is being recorded at that moment is the membrane potential that defines equilibrium potential for K+. o The internal potential is -85 mV. It is more negative inside the neuron. There is a larger concentration of Na+ ions outside of the neuron. Therefore, there is an inward movement of Na+ ions because there is a large net driving force that is composed of the chemical driving force (e.g., concentration gradient) and the electrical driving force (e.g., more negative potential inside the neuron attracts positive io
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