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2010 - Lecture 6 Chapter 4
Neural Condition: Synaptic Transmission
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Chemical Transmission
1. Transmitting Step: presynaptic cell release NT
2. Receptive Step: NT binds postsynapticallyto chemically gated (ion) channel
presynaptic postsynaptic
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Resting membrane potential: AT REST
The differences in electrical charges between the inside of the cell and the outside cell
“Polarized”
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Why Negative You ASK?
IONS
Charged MoleculesNa+
Cl-
A-
K+
Na+: Cation (+)
K+: Cation (+)
Cl-: Anion (-)
A-: Anion (-)
Ratio of negative to positive is greater inside than outside
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Why Unequal Distribution?
4 Factors Equalizers
Unequalizers
1. Concentration Gradient (passive)
Random movement of ions Ions travel “down” their [ ] gradient higher concentration moves to region of lower concentration Stops when ions are =
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2. Electrostatic Pressure (Passive)
Charges Opposites attractLike charges repel
Na+ Cl-
Hey baby..What’s your sign?
Na+K+
Hey baby..What’s your sign?
Talk to theMembrane..
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3. Selectivity of Membrane (Passive)
Cell membrane is permeable to certain
Ions – controls in & out
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Phospholipids
<------- Phosphate head
hydrophilic
<------- Lipid tails
hydrophobic
Phospholipid Bilayer: Cell Membrane
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Phospholipid Bilayer
Hydrophilic heads ----->
Hydrophobic tails ----->
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4. Sodium Potassium Pump (Energy-Active)
For every 2 K+ in 3 Na+ out (transport system)
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End Result…
Polarization
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IONIC BASIS OF THE RESTING MEMBRANE POTENTIAL
General overview
Resting membrane potential is a characteristic feature of ALL cells in the body
Membranes of neurons contain channels that allow the movement of ions into or out of the cell
The permeability of the membrane is determined by the state of these channels – i.e. whether they are open or closed
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Neurons have the ability to “gate” their ion channels
(permeability of the membrane to selected ions)
3 basic types of ion channels:
(i) Passive – each passive channel is identified by the specific ion it allows through it(e.g. K+, Na+ channel) resting membrane potential
(ii) Voltage gated – open or close based on the membrane potential action potentials (AP)
(iii) Chemically gated – transmitters binding on sites (receptors) on the channels. These channels are important in synaptic transmission
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Depolarize the Cell
Make more positive
How do we get the cell to
depolarize?
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How do we get the cell to depolarize?
EPSP: excitatory post synaptic potential
Increases the likelihood that postsynaptic cell will fire
NT binds to chemical gated channel Ion Channel opens (NA+) Flood of Na rushes in…why? Cell becomes depolarize (–70mV) If reaches “threshold” (-65 mV) Action Potential (AP)! Will be triggered!!!!
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NT binds to chemical gated channel Ion Channel opens (NA+) Flood of Na rushes in…why? Cell becomes depolarize (–70mV) If reaches “threshold” (-65 mV) Action Potential (AP)! Will be triggered!!!!
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How do we reach threshold?
Neural Integration:Cell “adds” up signals over space
& time
GRADED RESPONSE
Spatial Summation:Many synaptic inputs adds up to threshold
Temporal Summation: One Input that fires quickly in time serving to build on each otherTo reach threshold
AP triggered
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Integration of Signals
Figure 8-25: Locations of synapses on a postsynaptic neuron
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Figure 3.4 Temporal and spatial summation
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Postsynaptic and Action Potentials
Excitatory Postsynaptic Potentials (EPSP) Graded depolarizations
Inhibitory Postsynaptic Potentials (IPSP) Graded hyperpolarizationsRelationship between EPSP’s, IPSP’s and AP????
All postsynaptic potentials are added together and if enough EPSP’s occur to cause cell to cross threshold, an action potential occurs
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Hits Threshold All-or-Nothing!
Action Potential: firing of a neuronmassive momentary change in the membrane potential from –70mV to ~ +50mV
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Figure 3.3 Recordings from a postsynaptic neuron during synaptic activation
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Voltage Gated Channels on Axon
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Getting back resting membrane potential
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Properties of the action potential
•AP is triggered by depolarization •Depolar. must exceed threshold value to trigger AP •AP is all-or-none •AP propagates without decrement •AP involves reversal ("overshoot") of membrane potential •AP is followed by refractory period
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Voltage Gated Channels in axon open serving to “Propogate” the AP down the axon
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What Happens When Axon is Myelinated
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AP generated at every single spot all the way down the axon
Voltage gated Channels only at Nodes of Ranvier
AP only generated at “Nodes of Ranvier” less depolarizing “Saltatory Conduction”