Action Potentials and Synaptic Transmission

7/28/2019 Action Potentials and Synaptic Transmission

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Ionic Equilibrium & Membrane PotentialIonic Equilibrium & Membrane Potential

z 2 forces affecting ion movement:

concentrational and electrical

z Electrochemical potential () quantifies thecontribution of each to the movement of an ion

z If is +, ions move from A to B If is -, ions move from B to A If = 0, no movement


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TheThe NernstNernst EquationEquation

z

At electrochemical equilibrium, = 0 Electrical force = concentrational force

zNow, we can compute the difference in

potential (across the membrane) responsible

for this balance.

EA EB = -60/z * log ([X+]A/[X+]B)z Can be used to predict which way ions will

flow


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Resting Membrane PotentialResting Membrane Potential

z The Nernst potential of each ion contributes to

overall resting potential Weighted average of the Nernst potentials of all

permeant ions


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NernstNernst PotentialPotential vsvs ActualActual

PotentialPotentialz Nernst potential = actual membrane potential

No movement

z Nearnst potential is the same sign, but larger

Direction of flow determined by concentrational forcez Down concentration gradient

z Nernst potential is the same sign, but smaller

Direction of flow determined by electrical force

z Opposite to concentration gradient

z Nernst potential is of the opposite sign

Ion is not in equilibrium; flow determined by both

electrical and concentrational forces


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Resting Membrane PotentialResting Membrane Potential

z Resting membrane potential can changewhen a stimulus is applied

Depolarization decreases potential difference;

brings it closer to zero

Hyperpolarization increases potential

difference; brings it further away from zero


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The Action PotentialThe Action Potential

zNothing happens untilthese graded potentials

exceed a thresholdpotential

20-30 mV abovebaseline

Unless threshold

reached, potentialreturns to baseline


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z

Voltage-sensitive Na+ channels in the cellmembrane start to open causing...

z The voltage to become more positive causing...

z Even more voltage-sensitive Na+ channels to

open

z gNa ; Na+ rushes into the cell

membrane potential grows to +25-35 mV


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Depolarization


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The Action PotentialThe Action Potentialz Na+ stops flooding in

High concentration inside cell limits diffusion

Positive voltage inhibits entry of +ions

z K+ gates open

z

gK; K+ leaves the cell Hyperpolarization back to resting potential


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Hyperpolarization


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Refractory Period


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Action Potential SummaryAction Potential Summary

1. Resting potential

2. Influx of Na+

3. K+ gates open

4.Outflow of K+

5. Refractory period


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Action Potential ConductionAction Potential Conduction

z Depolarized region causes current flow to adjacent

areas of the membrane

Depolarization continues to next segmentsz Electrotonic conduction

z AP regenerated; keeps same size and shape


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Conduction VelocityConduction Velocityz Speed is very

important

Depends on electricalproperties of themembrane

z Conduction velocity w/ fiber diameter Resistance

z Mylenation

membrane of Schwanncells wrap aroundnerve fiber

Act as insulation


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How doesHow does mylenationmylenation

increase conductionincrease conductionvelocity?velocity?

z Myelin sheath:

length constant ofthe fiber the capacitance of

the fiber

Restricts APgeneration toNodes ofRanvier

z Under the myelinsheath:

Depolarization more

rapid Membrane resistance

z APs localized to

Nodes of Ranvier saltatory conduction


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CellCell--Cell Transmission ofCell Transmission of

Information: The SynapseInformation: The Synapsez Action potential ends

at end of axon

z At end of axon there is

either

Another nerve cell

(dendrites)

A muscle

z Small gap in between

Synapse


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Synaptic TransmissionSynaptic Transmissionz Action potential triggers release of Ca++

z Presence of Ca++ triggers release of specialchemicals stored in synaptic vescicles

Neurotransmitters

z Acetylcholine (ACh) most important for nerve-muscletransmission

z Excitatory

z Inhibitory

z Neurotransmitters diffuse across synapse


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Synaptic TransmissionSynaptic Transmission

z Receptors on other end of synapse

pick up neurotransmitters

z If enough excitatory neurotransmitters

are received, gNa and gKincrease

depolarization (EPP) will occur

z New action potential in areas adjacent

to endplate

z More action potentials received

more neurotransmitters released


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InputInput--Output RelationsOutput Relationsz One-to-one a single AP in the presynaptic cell

evokes a single AP in the postsynaptic cell (i.e.neuromuscular junction)

z One-to-many a single AP in the presynaptic cell

elicits many AP in the postsynaptic cell (i.eRenshaw cells)

z Many-to-one many, simultaneous APs from the

presynaptic cells are necessary to elicit one AP inthe postsynaptic cell

Some exitatory; some inhibitory


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Integration of InputsIntegration of Inputsz Permits fine control of

neuronal firing patternsz Spatial Summation

addition of two APs that

arrive almost simultaneously

z Temporal Summation

occurs when 2 APs arrive

in rapid succession

Causes stepwise change in

postsynaptic cell


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Action Potentials and Synaptic Transmission