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Neuronal Potentials • • • • Resting membrane pot. ~ -70 mV Action pot. (“spike”) ~ +40 mV (all-or-none) Threshold ~ -60 mV Graded pot. : Excitatory post-synaptic pot. (EPSP) Inhibitory post-synaptic pot. (IPSP) Polarization, depolarization, hyperpolarization Absolute and relative refractory periods MEMBRANE POTENTIALS V ~ -70 mV Ions: cation : +V ; anion : -V Sodium - Na+ ( higher outside ) Potassium - K+ ( higher inside ) Chloride - Cl- ( higher outside ) Amino acids - A- ( higher inside ) (protein) Resting membrane potential 1. Concentration gradients: ( high to low) 2. Electrical gradients - electrostatic pressure ( +/- attract ; equal repel ) 3. Na+ and K+ “pumps” Resting Level (~ -70mV ): Na+ --- (a) hi concentration outside > driven into cell (concent. gradient) (b) inside negative > Na+ attracted (electrical gradient) BUT: (a) membrane channels closed (voltage gated) (b) Na+ pumps to outside K+ -- (a) driven out by hi inside concentration (concent. grad.) BUT: (a) stopped by inside negative charge ( attract; elect. grad) (b) K+ pumps to inside Resting Potential Cl- Na+ Sodium channels ar closed (voltage gate Sodium cannot move into the cell a readily A- “Pumps” Na+ - out K+ - in mV Axonal Propagation of Action Potential: Electrotonic Current Electrotonic current Spread of Na+ to adjacent polarized region, slightly depolarizing it ( -60mV = Threshold); removes voltage gate on Na+ channels, action potential (“spike”) occurs. Electrotonic current becomes smaller as distance from spike increases (decremental conduction)