Lec 4 - Electrical Signaling

Lec 4 - Electrical Signaling - 1 Electrical Signaling The...

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Unformatted text preview: 1 Electrical Signaling The nervous system uses electrical signals and chemical signals to do its job When we discussed the evolution of nervous systems, we saw that electrical signaling predates the development of nervous systems For example, the calcium action potential of Paramecium: Signaling in the Nervous System As an example of electrical and chemical signaling in the nervous system, lets consider one of the simplest forms of behavior controlled by the nervous system: the patellar stretch reflex (knee-jerk reflex) This simple example embodies all of the basic principles used by the nervous system for more complicated tasks 2 The Patellar Reflex The Patellar Reflex 3 The Electrical Signal Carried by the Sensory Neuron Membrane Potential To understand how the nervous system uses electrical signals to transmit and process information, we must first understand the basic chemical and physical principles that govern the electrical behavior of cells All cells have an electric field across their plasma membranes, called the membrane potential (potential being used in the electrical sense, to denote a voltage difference) The membrane potential arises from the different ionic compositions of the intracellular fluid (ICF) and extracellular fluid (ECF) and from the differential permeability of the plasma membrane to different ions in the ECF and ICF (see recorded lecture on membrane potential at Blackboard). 4 Compositions of ICF and ECF Substance Internal concentration (mM) External concentration (mM) Can it cross the plasma membrane? Equilibrium potential (mV) K + 125 5 Yes -81 Na + 12 120 Yes +58 Cl- 5 125 Yes -81 A 1.2- 108 0 No N.A. H 2 O 55,000 55,000 Yes N.A. The major anion inside cells (A 1.2- ) is a diverse group of impermeant molecules, including proteins, charged amino acids, sulfate, phosphate, etc. These have a charge of -1, some -2, -3, or more. Taken as a group, the average charge per molecule is approximately -1.2 Resting membrane potential in a neuron -70 mV (inside of cell negative with respect to the outside) The Goldman Equation The equation that gives the quantitative relation between membrane potential (abbreviated E m ) on the one hand and ion concentrations and permeabilities on the other is the Goldman Equation (also called the constant-field equation ): E m = RT F ln p K [ K + ] o + p Na [ Na + ] o + p Cl [ Cl ] i p K [ K + ] i + p Na [ Na + ] i + p Cl [ Cl ] o In the Goldman equation, the concentration of each ion is simply multiplied by its permeability, p. R = gas constant; T = absolute temperature; F= Faradays constant 5 A Simplified Form of The Goldman Equation The contribution of chloride to the resting membrane potential of most neurons is not significant. So, we can drop chloride....
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Lec 4 - Electrical Signaling - 1 Electrical Signaling The...

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