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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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The Action PotentialThe Action Potential
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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The Action PotentialThe Action Potential
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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The Action PotentialThe Action Potential
Hyperpolarization
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The Action PotentialThe Action Potential
Refractory Period
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The Action PotentialThe Action Potential
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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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