Chapter 4 - The Action Potential Flashcards by Bridgette Marie

passive processes

diffusion

electrostatic gradient

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movement of electrical charge

I: current

measured in Amps (A)

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relative ability for the charge to migrate

g: Conductance

measured in Siemens (S)

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inability of the charge to migrate

R: Resistance

measured in Ohms

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difference in charge between anode and cathode

V: Electrical potential

measured in Volts (V)

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electrical current flow across a membrane

Ohms law

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Ohms law

I = gV

I = current 
g = conductance 
V = voltage (difference in charge)

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The action potential has four important qualities

1) There is a threshold for initiation
- the Vm changes as a function of Ohms law
- (V=Ig or V = IR)

2) The action potential is all-or-none
3) The action potential conducts without decrement
4) The action potential is followed by a brief refractory period

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cross threshold value for action potential to occur

All-or-none

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Chain of events that initiates the action potential

opens Na + permeable channels
Na+ influx ->
reaches threshold ->
action potential

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firing frequency reflects the _______

magnitude of the depolarizing current

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max Hz for action potential ____

1000 Hz

depends on the absolute and refractory periods

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I(ion) = ?

I (ion) = g(ion) [Vm-E(ion)]

I = current (Ionic current)
movement of electrical charge

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holds voltage of membrane constant, measures current.

Inward (down) and outward (up) current measurements

Voltage clamp

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the accumulation (or removal) of electrical charges on the electrode and in the salt solution

capacitive current

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persistant outward current

always letting ions leak through
independent of membrane potential

leakage current

leak channels

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selective voltage K+ channel blocker

TEA

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neurotoxin found in puffer fish

blockage voltage Na+ channels

TTX

another: saxitoxin (red tide)

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transient increase in g(Na)
influx of Na+ ions
- inside the membrane has negative electric potential so there is a large driving force on Na+ ions. Therefore, Na+ ions rush into the cell through Na+ channels, causing the membrane to rapidly depolarize
- rapid depolarization

rising phase

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increase in g(K)
efflux of K+ ions
- voltage-gated Na+ channels inactivate. Voltage-gated K+ channels open (1msec after depolarization). Greater driving force on K+ ions when membrane is strongly depolarized. K+ rushes out of the cell. Membrane potential gets more negative.
- rapid repolarization

falling phase

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With increasing depolarization using voltage clamp the K+ current _______

increases due to g(k) and (Vm - E(K))

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as the Vm begins to depolarize, the Na+ current becomes increasingly ______

inward
as Vm approaches E(Na) (+55mV) the inward I(Na) starts to decrease due to a decrease in driving force (Vm-E(Na))
at further depolarization (>55m V) the I(Na) will reverse

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after-hyperpolarization (AHP) phase

K+ driven outwards, and towards E(K)

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g(Na) turns on and off more ____ than potassium channels (gK)

rapidly

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what makes up the components of ion current?

conductance (how easily it moves) and | driving force

equilibrium state for membrane potential of K+

- 80 mV

equilibrium state for membrane potential of Na+

+ 55 mV

if we have a lipid bilayer with closed channels in the membrane, what can we say about current levels? conductance of K+?

no conductance - equation = 0 | no current

current clamp inject _____ and measure _____

current clamp injects current and measures voltage

voltage clamps inject ____ and measure _____

voltage clamp injects voltage and measures current

you can determine current by knowing ___ and ____

you can determine current by knowing resistance and current

holds voltage of membrane constant, measures current

voltage clamp

come down past membrane potential - drive for K+ to reach E(K+) after hyperpolarization stage

AHP (after-hyperpolarization)

what is faster, K+ or Na+ ?

Na+ is faster

Voltage-Gated Sodium Channel

structure-transmembrane domains and ion-selective pores - 4 domains - each domain = 6 transmembrane a-helices - pore loop-filter (what makes it slective; they recognize voltage changes) - pore closed at RMP - molecule twists to allow Na+ through the pore - voltage sensor (S4): senses changes in membrane voltage; makes it go from closed to open channel

senses changes in membrane voltage (voltage gated sodium channel) - makes it go from closed to open channel

Voltage sensor (S4)

measures current through a single channel hold voltage, measure current through channel on end of electrode - studies ionic current single channels

``` Patch clamp (like voltage clamp) - can look at how membrane functions at different voltages ```

largest channels

voltage-gated channels

several stimuli control the opening/closing of ion channels

- ligand-gated - phosphorylation gated (cells that change_ - voltage-gated (largest channels) - stretch (mechanical) - (autotory/sematosensory system)

direction of current is dependent on ____

direction of current is dependent on membrane voltage

Plot current versus the potential difference of membrane I is linearly related to voltage - you can look at and calculate current by conductance or resistance

Ohmic channel

positive charge flowing into the cell, or negative charge flowing out of the cell

inward current

positive charge flowing out of the cell, or negative charge flowing into the cell

downward current

sodium channel inactivation

1) close channel 2) depolarization - open; voltage change, Na+ comes in 3) Inactivation - (globular) protein blocks channel 4) deinactivation: takes time until it dissociates from channel - cannot activate channel when protein is there/ blocks neuronal firing.

functional properties of the Na+ channel

- open with little delay (fast firing) - stay open for about 1 millisecond - cannot be open again by depolarization

channels are inactivated (blocked by protein) cannot be opened, even at high stimulation - Na+ channels inactivated. Membrane potential goes back to a more negative value closer to threshold value.

absolute refractory period (voltage-gated Na+ channels)

some channels are inactivated, mane are deinactivated neurons can fire with increased stimulation - Membrane potential stay depolarized until K+ channels close.

relative refractory period (voltage-gated Na+ channels)

delayed opening channels that repolarize the membrane - another term for the voltage-dependent K+ channels that open ~ 1msec after the onset of an action potential to repolarize the membrane

delayed rectifiers

types of voltage dependent K+ channels

1) delayed rectifiers 2) Ca 2+ activated K+ channel 3) a fast-transient K+ channel 4) M-type K+ channel - slow, inactivated by ACh

proposed voltage sensory

S4 region

impulse traveling in the normal direction in a nerve fiber | action potential travels in one direction

orthodromic

impulse traveling in the opposite direction to that normal in a nerve fiber backward propagation

antidromic

duration of action potential

~ 2 msec

typical conduction velocity

~ 10-12 m/sec

two directions ions can go

down the cytoplasm | out the membrane into extracellular space

facilitates current flow - effectively increases size of membrane 100x - increases resistance - channels (V-D and leak) only at the Nodes of Ranvier

myelin

myelinating cells

Schwann cells in PNS | oligodendroglia in CNS

a gap in the myelin sheath of a nerve, between adjacent Schwann cells

Nodes of Ranvier

is the propagation of action potentials along myelinated axons from one node of Ranvier to the next node, increasing the conduction velocity of action potentials. - high density of Na+ V-D at Nodes of Ranvier

Saltatory conduction

autoimmune degeneration of myelin in CNS without myelin, the spread of + charge is diminished

multiple sclerosis

the transfer of information from one neuron to the other

synaptic transmission

because relative permeability of membrane greatly favors Na+, the membrane potential goes to a value close to Ena, membrane potential is above 0 mV (positive) - inside is negative with respect to outside

overshoot

K+ channels are at resting K+ membrane permeability. Membrane potential goes towards Ek which causes hyperpolarization until K+ channels close - hyperpolarization

undershoot

_______ of action potential reflects the magnitude of depolarizaing current

firing frequency

maximum firing frequency

1000 Hz

membrane potential at which enough voltage-gated Na+ channels open so that they relative ionic permeability of the membrane favors over K+ - critical level of depolarization that must be crossed to trigger action potential

threshold

diseases caused by disturbed function of ion channel subunits or the proteins that regulate them. These diseases may be either congenital (often resulting from a mutation or mutations in the encoding genes) or acquired (often resulting from autoimmune attack on an ion channel).

channelopathy

Voltage-gated Potassium channels

4 subunits | pore loop: channel selectively permeable to K+ ions

Importance of regulating extracellular K+

at rest - membrane mostly permeable to K+, membrane potential is closer to Ek - sensitive to changes in [K+]o - increase in extracellular K+ will lead to depolarization of membrane ( x10 [K+]o 5-> 50mV; Vm: -65 -> -17mV) depolarization - blood brain barrier: limits K+ movement into external cellular environment - K+ spatial buffering (astrocytes) - K+ pumps take up K+, K+ channels.

another term for axon hillock Its the area where the soma ends and the axon starts in a typical brain or spinal neuron, depolarization of the dendrites and soma caused by synaptic input leads to the generation of action potential if the membrane of the axon hillock is depolarized beyond threshold

spike initiation zone

Direction of current (Im) is dependent on

Vm (membrane potential)

Current is linearly related to

voltage

current is linearly related to voltage | because you can go through, look at and calculate current by conductance or resistance

Ohmic channel

increasing size of membrane increases

resistance