Lecture 2

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24 Feb 2011
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Lecture 2
Synaptic Transmission
Electrochemical Potential
oWhen ions diffuse across the membrane they carry a net charge
and generate an electrical potential
oNet flow = 0 (equilibrium potential) = Nernst equation
calculates the potential required to balance the concentration
gradient
Membrane Potential
oFactors contributing to membrane potential:
oDistribution of ions across the membrane
oRelative permeability of the ions
oCharges of the ions
oGoldman equation for the calculation of membrane potential
(Em) permeability included in this equation; will be somewhere
between the potential of all three ions (Na, K and Cl)
Conduction with Decrement graded potentials
oChange in membrane potential due to the flow of ions
oMembrane is degraded with the distance from the synapse
Spatial Summation
oThe two different synaptic potentials can be summed up
oSummation added effect of the synaptic input
Temporal Summation
oToo far apart no temporal summation
Action Potentials
oOccur only when membrane potential at axon hillock reaches
threshold
oThree phases:
Depolarization
Repolarization
Hyperpolarization MP more polarized
oAbsolute refractory period
Cell capable of generating a new AP
oRelative refractory period
More difficult to generate new AP
Voltage-Gated Channels responsible for AP
oNa channels open first depolarization
oK channels open slowly repolarization
oNa channels close
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Absolute refractory period caused by deactivations of Na
channels
oK channels close slowly
Relative refractory period cause by open K channels
Na Channels Have Two Gates
oThe time is takes the channel to go back to the other
conformation is the relative refractory period
oDelay until the recovery (inactivation)
Action Potentials Travel Long Distances
oAll-or-None
Occurs or does not occur
All APs are same magnitude
oSelf Propagating
An AP triggers the next AP in adjacent areas of
membrane without degradation
oElectronic Current Spread
Charge spreads along membrane
oRegnerative Cycle
Ion entry electronic current spread triggering of AP
o**Na that enters the axon through voltage-gated Na channels
induces a local depolarization, this local depolarization spreads
along the axon via electronic conduction, triggering additional
APs further down the axon; this process continues down the
axon each AP is essentially the same as the preceding ones,
resulting in conduction without decrement
Myelination
oVertebrate neurons are myelinated
oMyelin
Insulating layer of lipid-rich Schwann cells wrapped
around axon
Reduce leakage of charge across membrane
Schwaan cells are a type of Glial cell
Cells other than neurons that support neuron
function
Better conduction
oNodes of Ranvier propagation occurs only here, internode
flow of current occurs only here (glia cell)
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