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Bioelectricity

Bioelectricity. Provides basis for “irritability” or “excitability Fundamental property of all living cells Related to minute differences in the electrical potential across a cell ‘resting potential’

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Bioelectricity

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  1. Bioelectricity • Provides basis for “irritability” or “excitability • Fundamental property of all living cells • Related to minute differences in the electrical potential across a cell ‘resting potential’ • Resting potential values can range from 20 to 100 mv, with inside of membrane negative to outside • Any stimulus which evokes a response in a cell is associated with a change in this potential

  2. Membrane Channels: Leak Channels Gated Channels Chemical (Ligand) Voltage Mechanical

  3. Potassium channel

  4. +25mV 440 ENa = ln = + 54.4 mV +1 50 Look at squid giant axon: Vm = -70 mV [Pr-] = 415 mM [Na+] = 50 mM [K+] = 400 mM [Cl-] = 35 mM [Na+] = 440 mM [K+] = 20 mM [Cl-] = 560 mM EK = - 74.9 mV ECl = - 69.3 mV

  5. PNa [Na+] o + PK [K+] o + PCl [Cl-] i Vm = + 25 mV • ln PNa [Na+] i + PK [K+] i + PCl [Cl-] o PNa [Na+] o Vm = + 25 mV • ln PNa [Na+] i What determines the value of the membrane voltage? Goldman-Hodgkin-Katz Eq. If the membrane were only permeable to Na: / / = ENa = + 54.4 mV

  6. PNa [Na+] o + PK [K+] o + PCl [Cl-] i Vm = + 25 mV • ln PNa [Na+] i + PK [K+] i + PCl [Cl-] o / / PK [K+] o Vm = + 25 mV • ln = EK = - 74.9 mV PK [K+] i Goldman-Hodgkin-Katz Eq. If the membrane were only permeable to K:

  7. PCl [Cl-] i Vm = + 25 mV • ln = ECl = - 69.3 mV PCl [Cl-] o If the membrane were only permeable to Cl: / / Thus, the GHK eq. is a sum of the Nernst eqs. weighted by permeability! In the resting neuron, PK & PCl >> PNa (~ 20-50 x) Therefore, Vm is close to EK & ECl with very little contribution from ENa

  8. Neuronal Physiology Neurons = nerve cells; fundamental unit of nervous system Surrounded by plasma membrane that possesses an electrical potential (resting potential = -70 mv) Membrane potential due to uneven distribution of ions on either side of membrane Nerve (and muscle) cells can make special use of this potential (i.e., excitable tissues) – use changes in the potential to create signals and hence transmit information or bring about contractions

  9. S mV 1 2 S 1 2 Graded Potential Time (ms)

  10. S mV 1 2 S 1 2 Graded Potential Time (ms)

  11. S mV 1 2 S 1 2 Action Potential threshold voltage Time (ms)

  12. No “Resting” Potential for the Weary Use the Goldman, Hodgkin, Katz equation to calculate the resting potential (Vm) for a cell having the following features: Na+ concentration is 150 mM on the outside and 17 mM on the inside K+ concentration is 5 mM on the outside and 143 mM on the inside Cl- concentration is 165 mM on the outside and 20 mM on the inside The cell membrane is 25 times more permeable to K+ and Cl- than to Na+

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