bioelectricity provides basis for “irritability” or “excitability fundamental property of all...
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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
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Membrane Channels:
Leak Channels
Gated Channels
Chemical (Ligand)
Voltage
Mechanical
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Potassiumchannel
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Look at squid giant axon:
Vm = -70 mV
[Pr-] = 415 mM
+25mV 440ENa = ln = + 54.4 mV
+1 50
[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
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What determines the value of the membrane voltage?
Goldman-Hodgkin-Katz Eq.
PNa [Na+] o + PK [K+] o + PCl [Cl-] i
Vm = + 25 mV • lnPNa [Na+] i + PK [K+] i + PCl [Cl-] o
If the membrane were only permeable to Na:
PNa [Na+] o Vm = + 25 mV • ln
PNa [Na+] i
//
= ENa = + 54.4 mV
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Goldman-Hodgkin-Katz Eq.
PNa [Na+] o + PK [K+] o + PCl [Cl-] i
Vm = + 25 mV • lnPNa [Na+] i + PK [K+] i + PCl [Cl-] o
If the membrane were only permeable to K:
PK [K+] o Vm = + 25 mV • ln = EK = - 74.9 mV
PK [K+] i
//
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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 bypermeability!
In the resting neuron, PK & PCl >> PNa
(~ 20-50 x)
Therefore, Vm is close to EK & ECl
with very little contribution from ENa
//
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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
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mV
S
1
2
S 1 2
Time (ms)
Graded Potential
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mV
S
1
2
S 1 2
Time (ms)
Graded Potential
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S 1 2
mV
S
1
2
Time (ms)
threshold voltage
Action Potential
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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+