monday april 9, 2014. n ervous system and biological electricity ii 1. p re -lecture quiz
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Monday April 9, 2014. N ervous system and biological electricity II 1. P re -lecture quiz 2. A review of resting potential and Nernst equation 3. Goldman equation 4. Action potential. Information flow through neurons. Nucleus. Dendrites Collect electrical signals. Cell body - PowerPoint PPT PresentationTRANSCRIPT
Monday April 9, 2014.
Nervous system and biological electricity II
1. Pre-lecture quiz2. A review of resting potential and Nernst equation3. Goldman equation4. Action potential
Information flow through neurons
Nucleus
DendritesCollectelectricalsignals
Cell bodyIntegrates incoming signalsand generates outgoingsignal to axon
AxonPasses electrical signalsto dendrites of anothercell or to an effector cell
Outside of cell
Inside of cell
Microelectrode 0 mV
– 65 mV
K channel
Increasingly negative charge inside the neuron
Increasing [K+] outside the neuron
Equilibrium!
at 20° C
The Nernst equation can be used to calculate the equilibrium potential of a given ion
Inside cell Outside cell
[K+] 400 mM 20 mM
[Na+] 50 mM 440 mM
[Cl-] 51 mM 560 mM
Squid have axons about 1,000 X wider than humans. This allowed them to do the early experiments that provided critical insights into how neurons work.
Andrew HuxleyAlan Hodgkin
Squid Neuron - ContinuedImportant Point #1: They measured actual membrane potential (E-membrane) for the squid axon.
voltage meter
SW
nerve1mm diameter
axon0.1mm diameter
Emembrane-measured = -65 mV
Squid Neuron - ContinuedImportant Point #2: They measured the concentrations of Na+, K+, and Cl- inside thesquid neuron and outside of it.
voltage meter
SW
nerve1mm diameter
axon0.1mm diamter
Emembrane-measured = -65 mV
In Out
[K+] 400 mM 20 mM
[Na+] 50 mM 440 mM
[Cl-] 51 mM 560 mM
Squid Neuron - ContinuedImportant Point #2: They measured the concentrations of Na+, K+, and Cl- inside thesquid neuron and outside of it.
In Out
[K+] 400 mM 20 mM
[Na+] 50 mM 440 mM
[Cl-] 51 mM 560 mM
What is the predicted membrane potential based on each of these ions?
To answer . . . we simplify the Nernst equation to the following for Na+ and K+.
[ ]58 *log
[ ]membrane
outE mV
inside
For Cl-, we alter the ratio due to the negative charge (valence). The formula is the following . . .
Remember: -log (x) = log (1/x)
What's the e-membrane potential based on K+?
In Out
[K+] 400 mM 20 mM
[Na+] 50 mM 440 mM
[Cl-] 51 mM 560 mM
[ ]58 *log
[ ]membrane
outE mV
inside
A. -75mVB. +75 mVC. -173mVD. -1.3 mVE. +173mV
Squid Neuron - ContinuedImportant Point #2: They measured the concentrations of Na+, K+, and Cl- inside thesquid neuron and outside of it.
Emembrane-measured = -65 mV
In Out
[K+] 400 mM 20 mM
[Na+] 50 mM 440 mM
[Cl-] 51 mM 560 mM
Emembrane -K+ = -75 mV
E membrane -Na+ = 55 mV
Emembrane- Cl- = -60 mV
Predicted E-membrane from Nernst
Measured E-membrane
Squid Neuron - SolutionSolution: We need a way to consider the effects of all 3 ions on the membrane potential. Will the sum of these predicted values equal the measured membrane potential?
Emembrane-measured = -65 mV
In Out
[K+] 400 mM 20 mM
[Na+] 50 mM 440 mM
[Cl-] 51 mM 560 mM
Emembrane -K+ = -75
E membrane -Na+ = 55
Emembrane- Cl- = -60
Emembrane-sum= -80
Predicted E-membrane from Nernst
Measured E-membrane
[ ] [ ] [ ]58 *log
[ ] [ ] [ ]
K o Na o Cl i
membrane
K i Na i Cl o
P K P Na P ClE mV
P K P Na P Cl
at 20° C
The Goldman Equation extends the Nernst Equation to consider the relativepermeabilities of the ions (P): Ions with higher P have a larger effect on Emembrane
Calculating the total resting potential – the Goldman Equation
Permeabilities change during an action potential and how this allows neurons to “fire”.
More key points on equilibrium & membrane potential
• The equilibrium potential for an ion is the voltage at which the concentration and electrical gradients acting on that ion balance out.
• The Nernst equation is a formula that converts energy stored in a concentration gradient to the energy stored as an electrical potential. This is calculated independently for each ion.
• The Goldman equation calculates a membrane potential by combining the effects of key individual ions.
The Action Potential Is a Rapid Change in Membrane Potential
1. Depolarization phase
2. Repolarization phase
3. Hyperpolarization phase
Resting potential
Threshold potential
Outside of cell
Inside of cell
Microelectrode 0 mV
– 65 mV
K channel
Increasingly negative charge inside the neuron
Increasing [K+] outside the neuron
Equilibrium!
Voltage-gated sodium channels allow the action potential to occur
• https://www.youtube.com/watch?v=ifD1YG07fB8
Voltage-gated channels
How voltage-gated channels work
At the resting potential, voltage-gated Na+ channels are closed.
Conformational changes openvoltage-gated channels whenthe membrane is depolarized.
Two important types:1.) Na+ voltage gated channels2.) K+ voltage gated channels
Patch Clamping Allows Researchers to Record from Individual Channels
Currents through isolated channels can be measured duringan action potential.
Na
+ infl
ow K
+ outflow
Inwardcurrentfrom Na+
channels
Outwardcurrentfrom K+
channels
Initial Depolarization - Some Na+ channels open. If enough Na+ channels open, then the threshold is surpassed and an action potential is initiated.
Action Potentials Propagate because Charge Spreads down the Membrane
PROPAGATION OF ACTION POTENTIAL
NeuronAxon
1. Na+ enters axon.
2. Charge spreads;membrane“downstream”depolarizes.
Depolarization atnext ion channel
3. Voltage-gatedchannel opens inresponse todepolarization.
Action Potentials Propagate Quickly in Myelinated Axons
Action potentials jump down axon.
Nodes of Ranvier Schwann cells (glia)wrap around axon,forming myelin sheath
Axon
Schwann cell membranewrapped around axon
Action potential jumpsfrom node to node