Chapter 4 - The Action Potential Flashcards
passive processes
diffusion
electrostatic gradient
movement of electrical charge
I: current
measured in Amps (A)
relative ability for the charge to migrate
g: Conductance
measured in Siemens (S)
inability of the charge to migrate
R: Resistance
measured in Ohms
difference in charge between anode and cathode
V: Electrical potential
measured in Volts (V)
electrical current flow across a membrane
Ohms law
Ohms law
I = gV
I = current g = conductance V = voltage (difference in charge)
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
cross threshold value for action potential to occur
All-or-none
Chain of events that initiates the action potential
- opens Na + permeable channels
- Na+ influx ->
- reaches threshold ->
- action potential
firing frequency reflects the _______
magnitude of the depolarizing current
max Hz for action potential ____
1000 Hz
depends on the absolute and refractory periods
I(ion) = ?
I (ion) = g(ion) [Vm-E(ion)]
I = current (Ionic current)
movement of electrical charge
holds voltage of membrane constant, measures current.
Inward (down) and outward (up) current measurements
Voltage clamp
the accumulation (or removal) of electrical charges on the electrode and in the salt solution
capacitive current
persistant outward current
always letting ions leak through
independent of membrane potential
leakage current
leak channels
selective voltage K+ channel blocker
TEA
neurotoxin found in puffer fish
blockage voltage Na+ channels
TTX
another: saxitoxin (red tide)
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
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
With increasing depolarization using voltage clamp the K+ current _______
increases due to g(k) and (Vm - E(K))
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
after-hyperpolarization (AHP) phase
K+ driven outwards, and towards E(K)
g(Na) turns on and off more ____ than potassium channels (gK)
rapidly
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
