Review & outline APs & Synapses Chapters 2 & 3 Cells of the nervous system The blood brain barrier Membrane potential (at rest = -70mV) Sodium potassium pump Concentration Electrical Action potentials Synapse 1/17/11 1 1/17/11 2 Na Na More Sodium (Na) outside than inside Sodium-Potassium pump maintains (uses energy; stops with death) Thought experiment Let us assume that the membrane is permeable ONLY to ions. K More Potassium (K) inside than outside 1/17/11 3 1/17/11 4 1
Na Na K K A - leaves the cell due to the K channels enters the cell due to the electrical Equilibrium Potential: -100mV 1/17/11 5 Basic mechanisms Concentration due to sodiumpotassium pump Selective permeability of the membrane Two forces: Concentration Electrical Equilibrium potential: when 2 forces are balanced 1/17/11 6 Specific equilibrium potentials Potassium: E = -100mV Sodium: E Na = 40mV Chloride: E Cl- = -70mV Calcium: E Ca = 100mV 1/17/11 7 K Na leaves the cell due to the electrical Na channels Equilibrium Potential: 40mV enters the cell due to the 1/17/11 8 2
Summary If membrane permeable ONLY to K, equilibrium potential (E K ) = -100mV If permeable ONLY to Na: 40mV In vivo membrane is permeable to both, so in between = -70mV It s closer to E K because membrane is more permeable to K than to Na 1/17/11 9 Large because lots of K channels open at rest electrical Resting Potential: -70mV electrical - 70 Not as large as for K because there are less Na channels open at rest 1/17/11 10 Resting Potential Resting Potential Where does it come from? Selective permeability of the membrane: What happens if: We suddenly increase the permeability to Na? tug-of-war between ions controlled by the permeability to each ion 1/17/11 11 1/17/11 12 3
1- Increase in Na permeability 1- Increase in Na permeability electrical Na electrical - 70 1/17/11 13 electrical 3-forces due to electrical are reduced electrical - 10 2- Voltage moves toward positive values 1/17/11 14 Resting Potential What happens if: We suddenly increase the permeability to Na? We suddenly increase K outside the cell (For example, by adding potassium chloride to the outside of cell)? electrical 1- Increase in K outside electrical - 70 1/17/11 15 1/17/11 16 4
Na 1- Increase in K outside electrical - electrical - 10 70 2- Voltage moves toward positive values 1/17/11 17 Na 1- Increase in K outside electrical 3- electrical forces change direction - electrical 10 2- Voltage moves toward positive values 1/17/11 18 Hyperpolarization/Depolarization Excitation & Inhibition A decrease in membrane potential is called an hyperpolarization (inhibition) An increase in membrane potential is called a depolarization (excitation) Opening K channels hyperpolarizes the cell Opening Na channels depolarizes the cell How about Cl-? 1/17/11 19 1/17/11 20 5
Chloride Cl- Equilibrium potential 1/17/11 21 C C C C C C C C C More Chloride outside than inside C C C C C C Cl- C Cl- leaves the cell due to the electrical Cl- channels - Cl- enters the cell due to the Equilibrium Potential: -70mV 1/17/11 22 Chloride Cl- Chloride Cl- What happens if we open chloride channels when cell is at rest (-70mV)? Membrane potential remains at -70mV! Because Cl- pulls the membrane potential toward -70mV, but we re already there! 1/17/11 23 What happens if we open chloride channels when cell is already depolarized (say to 0mV)? Cl- pulls the potential down toward -70mV: hyperpolatization/inhibition Therefore Cl- channels are inhibitory, even though we don t see it at rest: silent inhibition 1/17/11 24 6
Chloride Cl- What happens if we open Cl- channels when cells is already hyperpolarized (say -100mV)? Membrane potential goes up (!) because Cl- pulls it toward -70mV Depolarization/Hyperpolarization At resting potential (-70mV) : Opening of Na or Ca channels: depolarization (excitatory channels) Opening of K channels: hyperpolarization (inhibitory channels) Opening of Cl- channels has no effect BUT if the potential has already been depolarized, say to 20mV, the result is a hyperpolarization (silent inhibition) 1/17/11 25 1/17/11 26 Energy consumption Brain: 2% of body mass, 20% of energy consumption in adults, 50%(!!) in kids Stage 3 sleep: 40% decrease in metab rate Half this energy used to maintain membrane potential Two main sources of energy consumption are 1- propagating APs 2- postsynaptic processing In other words, communication is costly! 1/17/11 27 7