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Neurons, Synapses and

Signaling

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Review of last class

What is the CNS? Struc and function of Neurons?

What are Glia? Types?

How do neurons transmit signals?

How does ion transport occur across the cellmembrane?

Pumps? Channels? Transporters?

Resting membrane potential

Nernst Equation

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Equilibrium potential

Flux due to concentration gradient is equal to flux due to

electrical gradient Equilibrium potential for an ion = Nernst Equation

EK= 2.3 RT log [K+]right

nF [K+

]left R = Universal gas constant

T = Absolute temperature

n= Number of charges on the ion

F = Faradays constant (96500 coulombs/mole)

At 20C and n=1, the equation becomes-

EK= 58mvolts X log [K+]right

[K+]left

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Membrane Potential (Emem) in animal cells depends largely on

Potassium ion movement through open resting K+channels

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Resting Membrane Potential (Emem)

Resting Membrane Potential = GHK Equation

Eresting= 2.3 RT log PK[K+]outside + PNa[Na+]outside

F PK [K+]inside+ PNa[Na

+]inside

For cations (+) use o/i

For anions (-) use i/o

R = Universal gas constant

T = Absolute temperature

F = Faradays constant (96500 coulombs/mole)

P = Relative Membrane Permeability for an ion

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X

-

represents proteins that have a negative charge at theneutral pH of blood and cells

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The Na+ / K+ATPase Pump

Essential for setting the Resting Membrane Potential

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Generation of an Action

Potential

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Introduction to Action Potentials

The average neuron receives inputs from about 10,000synapses.

Inputs are integrated in space and time

If sum of input reaches threshold potential (~65mV)then neuron fires an action potential down its axon.

All axon potentials have same strength but coded by

different frequencies.

Signal frequency ranges from 1-100 action potentials/s

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Ionic Basis for the Action Potential:

Voltage gated Na+Channels

Action potential= rapid and transient permeabilityto Na+and K+

Upstrokeis due to opening of Voltage gated Na+

Channels

According to GHK equation in permeability to Na+

causes Vmto move towards ENa.

Small local depolarization causes influx of Na+which

causes more voltage gated Na+Channels to open =

Feedback loop

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OpenVoltage-gated ionchannels

Influx ofNa+

Depolarization

Positive feedback loop for activation of

Voltage-gated Na+channels

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Thresholdis the voltage at which enough Na+Channels are

open Feedback loop

All-or-none : feedback loop continues till all Na+Channels

are activated

Inactivation of Na+Channels: Immediately after opening Na+

Channels inactivate

Inactivated channels have to close to become active again.

Na+Channels require 5-20msec at -60mV to go from

inactivated to closed state

Ionic Basis for the Action Potential:

Voltage gated Na+Channels

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Initial stimulus: Na+ influx

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Are activated by depolarization

2 Differences between Na+and K+ Channels

Activation of K+Channels is slower than activation of Na+Channels

Activation requires stronger depolarization than activation of a few

Na+Channels therefore K+Channels are activated at the peak of

action potential.

Voltage gated K+Channels in nerve axons do not inactivate.

Absolute refractory period is due to activation and inactivation of Na+

Channels.

No action potentials can be initiated during this period

Relative refractory period is due to recovery of Na+Channels from

inactivation and activation of voltage gated K+Channels

Requires a greater stimulus since membrane is closer to EKand

hyperpolarized.

Ionic Basis for Action Potential:

Voltage gated K+Channels

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Resistance is inversely proportional to thickness of wire

R = l

r2

R = Resistance

= Resistivity (depends on the medium)

l = length of wire (or axon)r = radius of wire (or axon)

Speed of action potential

1) Increase diameter of axon

2) Increase insulation so that electrotonic ion flow in

cytoplasm can extend further - Myelination

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Summary

Function of Action Potentials Neurons: Cell-Cell Communication

Muscle: Contraction

Beta cells of Pancreas: Release of Insulin