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outside+ Neurophysiology inside + + + ++ + - - -- - - - Opposite electrical charges attract each other In case negative and positive charges are separated from each other, their coming together liberates energy Thus, separated opposing electrical charges carry a potential energy Neurophysiology • Voltage (V) measure of differences in electrical potential energy generated by separated charges • Current (I) the flow of electrical charge between two points ++ + ++ + • Resistance (R) + outside hindrance to charge flow inside - - -- - - - Ohm’s law + + + + + + + + + + Current: ions - - - - - - outside inside Voltage: potential across the membrane Resistance: membrane permeability + + + + + + + + + + - - - - - - Resistance: membrane permeability outside inside How can ions move across the membrane? Ion channels Ion channels 1) Leak channels 2) Chemically (ligand) – gated channels - Can be ion-specific or not (e.g. the Acetylcholine receptor at the neural-muscular junctions is permeable to all cations) 3) Voltage – gated channels - Ion selective - Gates can open (and close) at different speeds 4) Mechanically – gated channels - Found in sensory receptors The driving force: the electrochemical gradient outside + + + + + + + + - - - - - - - inside + + The driving force: the electrochemical gradient K+ Na+ K+ Na+ Cations are the key players here , as anions are actually negatively charged proteins that cannot move through channels Potassium wants to go out, but also wants to go in Potassium will diffuse via leak channels until balanced (higher concentrations INSIDE) Potassium wants to go out Sodium wants to go in K+ Na+ K+ Na+ Na+/K+ pump - The neuronal membrane is much less permeable to Na+ than to K+ . The result- Na+ stays out - How do we keep this gradient? The sodium/potassium pump acts to reserve an electrical gradient - Requires ATP - Throwing 2 K+ in, while throwing 3 Na+ out The resting membrane potential is Negative K+ Na+ K+ Na+ The Membrane is Polarized Depolarization Making the cell less polarized Hyperpolarization Making the cell more polarized This is the resting membrane potential How can we change it? Stimulus How can we depolarize a cell? Example A chemical stimulus Cell body Axon Dendrites Sodium channels opening leads to depolarization -70 mV - Generation of a graded potential measure of differences in electrical potential energy generated by separated charges The graded potential is increased with a stronger stimulus Think about a membrane with 50 channels Stimulating them with 4 ligand molecules or 40 will make a difference A graded potential can spread locally - Cations will move towards a negative charge - The site next to the original depolarization event will also depolarize, creating another graded potential Membrane potential - Graded potentials measure of differences in electrical potential energy generated by separated charges - Graded potentials spread locally but die out Membrane potential Who said you have to depolarize? A stimulus can lead to hyperpolarization How would that occur? • Graded potentials - Proportional to the stimulus size - Act locally, starting from the stimulus site - Attenuate with distance - Spread in both directions - Take place in many types of cells • Action potentials do/are NOT - Proportional to the stimulus size - Act locally - Attenuate with distance - Spread in both directions - Take place in many types of cells Action potential can be generated and propagated ONLY in: - Neurons (only at the axon) - Muscles Why only there? Function follows form Voltage - gated channels are found mainly on the axon and the axon hillock Axon hillock (trigger zone) Axon Dendrites Cell body Voltage - gated channels are found mainly on the axon and the axon hillock Axon hillock (trigger zone) Axon Dendrites Cell body