Lecture 5 notes
Lecture 5
Thursday, 22 September, 2011 3:33 PM
-Dendrites are not as good to conduct AP as axons. The potential decreases and diffuses somewhat on its way to the cell body. Thus, when it reaches the cell body, it rarely triggers an AP, and the neuron is capable of recieving mutiple signals, integrate them and decide if the sum is large enough to cause an AP down the axon. Passive electrical properties: properties of the membrane which allow neurons to conduct electrical impulses without the use of voltagegated ion channels (leak channels): 1. rm (membrane resistance): *most important* Amount of leakiness due to ion channels. It is inversly proportionnal to conductance, aka amount of ion channels open. 2. cm (membrane capacitance): Property of the lipid bilayer, can store charge, ions 3. ra (Intracellular(axial) resistance): resistance of the cytoplasm itself, property of ions which enter the cell to move through the axon. What is affected b passive properties 1. Magnitude of membrane potential change after current enters. 2. Time course of membrane potential change after current enters. Does it attenuate or keep its size over great distance. 3. Distance which the voltage change travels 4. Speed of AP propagation rm v = ir ; r = amount of open ion channels i = current, thus the v is dependant on r To determine rm, block AP with TTX and TEA and put current in the cell. Record the voltage change of varying amounts of current, and use them to determine resistence of the cell. Passive conductance: passive membrane resistance = amount of ion (leak) channels (not voltage-gated) open. These are the permeabilities which set resting potential in GHK equ. (1:0.04:0.45) Active conductance: active membrane resistance = amount of voltage-gated ion channels open during AP threshold. They are closed at rest. RM: specific membrane resistance, property of a given cell. units: Ohm x cm^2. resistance of a unit area rm: membrane resistance of a specific patch of membrane units: ohm rinput: term used by electrophysiologists. membrane resistance of the entire cell
membrane capacitance Membrane will charge, store energy, equal but opposite charges on both sides of membrane The reason why voltage change is not instantaneous is that the membrane will charge up before affecting the potential. C(F) = Q / V CM = 1 microFarad/cm^2 Ci (total membrane capacitance of a cell) = surface area (in cm^2) x CM For the average neuron = 10-50 pF Explanation of voltage change Current is injected into the cell --> plasma membrane charges up rapidly (cm shoots up) Current continues to enter the cell --> cm is fully charged and slowly discharges, current goes through the ion channels. Voltage slowly increases as rm increases Current stops --> cm drops and completely discharges, voltage and resistance slowly decrease π Β (membrane time constant): it determines the rate of change in Vm. = cm x rm = cin x rin time to charge membrane to 63% of max: (1 - 1 / e) x Vmax time to discharge from Vmax to 37% of Vmax: V = Vmax x e^(-t/π) so, large π Β means it takes longer for the membrane voltage to go up. Small π means the voltage foloows closely the amount of current injection Axial/internal Resistance: ra - Will determine how far and fast an impulse will travel - specific resistance of cytoplasm - diameter - Membrane length constant (Ζ(lambda)) = sqrt( rm / ra ): The distance it takes for the voltage to decrease by 37% of Vmax. So high rm, thus small amount of channels open means impulse will go further. high ra, thus current impeded, means impulse will go less far The opposites are also true. Why squid axon so fat? If cross-section of axon bigger, then ra will decrease. Resistance in cytoplasm decreases because there is more space for ions to flow through. Also, cross-sectional area increases faster then surface area. Less surface area, higher rm because not as many channels. This causes Ζ to get bigger, and the axon is more efficient Myelin makes Ζ much larger. how? - Where there is myelin, there are no channels. Thus, rm is very high, and ions have no choice but to move forward, thus increasing the speed of imoulse. - Myelin acts like a capacitor. when adding capacitors in series, the cap.tot. decreases, because they must share the charge. Thus if cm decreases (less charge goes to the membrane) then more charge is left to go forward. rm increases and cm decreases = π does not change. Integration
Thursday, 22 September, 2011 3:33 PM
-Dendrites are not as good to conduct AP as axons. The potential decreases and diffuses somewhat on its way to the cell body. Thus, when it reaches the cell body, it rarely triggers an AP, and the neuron is capable of recieving mutiple signals, integrate them and decide if the sum is large enough to cause an AP down the axon. Passive electrical properties: properties of the membrane which allow neurons to conduct electrical impulses without the use of voltagegated ion channels (leak channels): 1. rm (membrane resistance): *most important* Amount of leakiness due to ion channels. It is inversly proportionnal to conductance, aka amount of ion channels open. 2. cm (membrane capacitance): Property of the lipid bilayer, can store charge, ions 3. ra (Intracellular(axial) resistance): resistance of the cytoplasm itself, property of ions which enter the cell to move through the axon. What is affected b passive properties 1. Magnitude of membrane potential change after current enters. 2. Time course of membrane potential change after current enters. Does it attenuate or keep its size over great distance. 3. Distance which the voltage change travels 4. Speed of AP propagation rm v = ir ; r = amount of open ion channels i = current, thus the v is dependant on r To determine rm, block AP with TTX and TEA and put current in the cell. Record the voltage change of varying amounts of current, and use them to determine resistence of the cell. Passive conductance: passive membrane resistance = amount of ion (leak) channels (not voltage-gated) open. These are the permeabilities which set resting potential in GHK equ. (1:0.04:0.45) Active conductance: active membrane resistance = amount of voltage-gated ion channels open during AP threshold. They are closed at rest. RM: specific membrane resistance, property of a given cell. units: Ohm x cm^2. resistance of a unit area rm: membrane resistance of a specific patch of membrane units: ohm rinput: term used by electrophysiologists. membrane resistance of the entire cell
membrane capacitance Membrane will charge, store energy, equal but opposite charges on both sides of membrane The reason why voltage change is not instantaneous is that the membrane will charge up before affecting the potential. C(F) = Q / V CM = 1 microFarad/cm^2 Ci (total membrane capacitance of a cell) = surface area (in cm^2) x CM For the average neuron = 10-50 pF Explanation of voltage change Current is injected into the cell --> plasma membrane charges up rapidly (cm shoots up) Current continues to enter the cell --> cm is fully charged and slowly discharges, current goes through the ion channels. Voltage slowly increases as rm increases Current stops --> cm drops and completely discharges, voltage and resistance slowly decrease π Β (membrane time constant): it determines the rate of change in Vm. = cm x rm = cin x rin time to charge membrane to 63% of max: (1 - 1 / e) x Vmax time to discharge from Vmax to 37% of Vmax: V = Vmax x e^(-t/π) so, large π Β means it takes longer for the membrane voltage to go up. Small π means the voltage foloows closely the amount of current injection Axial/internal Resistance: ra - Will determine how far and fast an impulse will travel - specific resistance of cytoplasm - diameter - Membrane length constant (Ζ(lambda)) = sqrt( rm / ra ): The distance it takes for the voltage to decrease by 37% of Vmax. So high rm, thus small amount of channels open means impulse will go further. high ra, thus current impeded, means impulse will go less far The opposites are also true. Why squid axon so fat? If cross-section of axon bigger, then ra will decrease. Resistance in cytoplasm decreases because there is more space for ions to flow through. Also, cross-sectional area increases faster then surface area. Less surface area, higher rm because not as many channels. This causes Ζ to get bigger, and the axon is more efficient Myelin makes Ζ much larger. how? - Where there is myelin, there are no channels. Thus, rm is very high, and ions have no choice but to move forward, thus increasing the speed of imoulse. - Myelin acts like a capacitor. when adding capacitors in series, the cap.tot. decreases, because they must share the charge. Thus if cm decreases (less charge goes to the membrane) then more charge is left to go forward. rm increases and cm decreases = π does not change. Integration
