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56 Cards in this Set

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Generator potential
The term for local depolarizations when they result in an Action Potential
Physiological examples of generator potentials:
1. Receptor potentials (local changes in membrane potential at sensory receptors)
2. Local Em changes at postsynaptic cell synapses (EPPs, neurons)
What is the core conductor model for an electrically excitable cell membrane?
-Succussion of parallel transmembrane electrical elements
What are the electrical elements in the core conductor model of the membrane composed of?
Membrane Resistance and capacitance
Which resistance is taken into account in the core conductor model?
That of intracellular longitudinal axon resistance
4 Assumptions of the core conductor model:
1. Passive elements only - NO batteries (no membrane pots!)
2. Resistance and capacitance values for each length of membrane are CONSTANT.
3. Values of the passive elements do not change with application of an external local subthreshold or hyperpolarizing voltage
4. Infinitely long axon
What are the requirements for the voltage applied in the core conductor model?
SUBTHRESHOLD - will not close an inactivation gates. B/c we want to study an electrically excitable cell at resting.
What does the core conductor model illustrate about a resting membrane?
When applied at a single spot, a subthreshold voltage will decrement as it moves along the membrane.
Why does an externally-applied subthreshold voltage decremate as it flows along the membrane?
Because the current splits successively at each junction into 1) flow through a resistor/capacitator component, AND 2)along the in/out membranes
Which membrane resistance matters more?
Intracellular - Ra
Extracellular R doesn't really have enough magnitude to count.
How exactly does current flow longitudinally at an axon membrane?
First along Rm (thru the memb)
Then along Ra (intracellular side)
How (mathematically) does current decremate with increasing distance travelled?
What is the equation for exponential decay of a subthreshold voltage applied to an axon membrane?
Decay rate = Initial voltage times e to the neg x / lambda
What is lambda?
The length constant

L=Square root of Rm/Ra
-Denominator of the exponent in determining Subthreshold voltage decrement
What are the dimensions of lambda?
How much will an applied subthreshold voltage decay along an axon if the distance = lambda?
What is the equation for determining how much the voltage will change in a given time?
V = initial voltage times (1 - e to the -t/tau)
What is tau?
The time constant for a nerve membrane = rmcm
How much will an externally applied subthreshold voltage decrease when t = tau?
1-37% = 63%Vo
What is the difference between 67%Vo and 37%Vo?
67%Vo is how much it will change during buildup of capacitor

37%Vo is how much it will falloff during discharge
What happens to tau if the subthreshold depolarization approaches threshold?
It no longer applies because the decay time course is no longer a simple exponential.
Why does the time course of decay change with increasing depolarization?
Because some channels begin to open and the membrane becomes no longer passive.
What is "Electrotonic Potential"?
The passive subthreshold transmembrane potential that can be elicited by a subthreshold depolarizing (or hyperpolarizing) voltage.
What causes an action potential to be PROPAGATED?
Passive conductance of depolarization along the membrane.
What is the 'name' of the current that flows during AP propagation?
Action currents
What direction do Action currents flow in?
From neg (polarized) SOURCES ahead of the AP, to pos SINKS (depolarized) within the AP.
How do action currents propagate the AP?
They take away pos charge from the outside face of memb ahead of AP, and replace it with neg.
Hence DEPOLARIZING the membrane.
What do the local membrane depolarizations caused by Action currents do?
Cause the membrane depolarizatn to exceed threshold, open Na channels, and propagate the AP.
In what direction do Action currents flow from a membrane depolarization?
In both directions
So why doesn't an AP flow in both directions?
Because K channels open behind the sink and cause the pos flow to go right thru the membrane (efflux).
2 Critical factors that regulate the velocity of AP propagation:
1. Time factor
2. Membrane Length factor
2 Electrical properties that regulate the time required for an Action current ahead of the AP to depolarize the membrane segment to threshold:
1. Total pathway resistance
2. Membrane capacitance
What is total pathway resistance equal to?
Ra + Ro + Rm
(if you're not ignoring Ro)
How does resistance affect time of AP propagation?
The greater the axoplasmic resistance (Ra) the slower the propagation.
What nerve fibers have higher Ra?
So what has a faster propagation velocity; a small or large unmyelinated nerve fiber?
How will changing capacitance increase the propagation velocity?
Decreasing capacitance decreases the time of charging up and hence using the stored charge.
How does myelinating axons affect capacitance?
Myelination = decreased capacitance; hence LARGE, MYELINATED axons have the fastest AP propagation velocity!
How does the value of the length constant affect AP propagation?
The larger lambda is, the longer the AP has to go before it decremates; so better.
How do you increase lambda?
By decreasing axoplasmic resistance (Ra)
How do you decrease Ra?
By increasing axon diameter!
So 2 effects of increased axon diameter:
Decreased Ra (shorter time)
Increased lambda (longer length of staying full force)
So what are the 2 strategies for increasing AP propagation velocity?
1. Increased diameter
2. Myelination
What exactly is the effect of myelination?
Increased Rm - increased length constant; helps electrotonic potential to flow between nodes.
Where does an AP propagate faster; at a node of ranvier or inside an ensheathed internode?
In the internode.
Why is AP propagation along myelinated axons called saltatory?
Because at each node of ranvier the membrane takes longer to reach threshold b/c its capacitance is higher.
What does Saltatory mean?
To dance (latin) - how AP's move from node to node, pausing at each one where AP generation takes longer to generate.
2 Demyelinating diseases:
-Multiple sclerosis
-Guillian-Barre syndrome
What are the 3 main consequences of demyelinating disease?
-Decreased Action Current Density
-Slow AP propagation
-Block of propagation eventually
What is the effect of demyelination on the core conductor model?
Increased cm (increased tau)
Decreased Rm (decreased lambda)
What is the definition of Temporal Summation?
Addition of successive EPSPs or IPSPs at a particular point on a cell membrane.
What is required for temporal summation to work?
A long enough Tau - time constant - so APs don't decremate before subsequent ones are added.
2 Biochemical factors that control the duration of tau:
1. Rate of NT inactiation
2. Time for postsynaptic ion channel inactivation
What is Spatial summation?
Summation of IPSPs or EPSPs arriving simultaneously in a critical surface area of membrane.
What affects the critical SA of membrane necessary for spatial summation to work?
The space constant
What is the space constant?
Lambda - the length constant (rmcm) but in 2D