Action Potentials Flashcards
Open when membrane is depolarized, slow inactivation, depolarizes after AP
Delayed rectifier K channel
Where is the selectivity filter of the VG Na channel?
The loop between subunits 5 and 6
What initiates closure of the inactivation gate in VG Na channels?
Outward movement of S4 (voltage sensor)
Many properties displayed by the AP are due to properties of this channel
VG Na channel
How does water play an important role in determining selectivity of the VG Na channel pore?
The size of a partially hydrated Na ion can fit but a partially hydrated K ion cant
What are used to record APs?
Intracellular and extracellular electrodes
What happens in the rising phase?
Na going into the cell, high Na conductance (how easy it is for ions to move), inside becoming positive
From puffer fish, block VG Na channel
Tetrodotoxin
What happens in the falling phase?
ABSOLUTE REFRACTORY. Na channels close and won’t open, K channels open, high K conductance
Sea anemone toxins are active here
Site 3
When the AP looks like it is jumping from noble to node
Saltatory conduction
Channel with selective ion conductance for Na
VG Na channel
When AP conduction is started from the end and moves toward cell body (rare)
Antidromic
Are the a helical segments of the VG Na channel
S1-S6
These two things help convey info about the neuron ion channels
Rate and pattern of firing
Set by K channels
Relative refractory period
These two VG channels structures are related
VG K and VG Na channels
Why cant an AP be faster than 1 ms?
The absolute refractory period
When do toxins have an effect on the VG Na channel?
When there is an AP. No effect if it is at rest
Opening the inactivation gate is also tied to doing what with it?
Closing it
What is deinactivation of the inactivation gate and when does it occur?
It is when the VG Na channel is not activated and the ball and chain falls out. It happens around -65 mV
Tetrodotoxin and saxitoxin are active here
Site 1
The voltage sensor of the VG Na channel that moves outward when the cell depolarizes
S4
What does tetraethyl-ammonium do to cells?
Blocks K current outward
What happens to channels behind the AP that the AP has already passed?
They are refractory bc Na channels are inactivated
When AP conduction is started from the cell body
Orthodromic
Block pore and selectivity filter, specific amino acids required
Tetrodotoxin and saxitoxin
What is length constant?
How far current flows before it leaks out, before it is reduced to 37%
Used to deduce the nature of the AP
Voltage clamp and squid giant axon
Segment of VG Na channels that mostly line the pore
S6
These don’t open right after depolarization, they are delayed
VG K channels
Composed of 4 subunits, each similar to one domain of the VG Na channel
VG K channels
Axon placed in saline solution, reference electrode in solution, electrodes hooked up to ultimately measure current at specific voltages
Voltage clamp
From a dinoflagellate, occurs in calms, shellfish, and mussels. Block VG Na channel
Saxitoxin
Na channels inactivated, cant be deainactivated until Vm is more negative
Absolute refractory period
This gate is less accessible after depolarization in the VG Na channel
Inactivation gate
What does I ion = g ion represent?
Current of the ion = conductance of the ion (Like Ohms law Vm - Eion)
What is needed to initiate an AP?
Depolarization of membrane due to generator potentials beyond some threshold (around -40 mV)
What are spike initiation zones?
In unmyelinated neurons, they are areas of high Na channel concentration
Part of the VG Na channel sensitive to proteases and antibodies
Inactivation gate
Used voltage clamp to determine ionic permeability changes during the AP
Hodgkin and Huxley
VG Na channels contain 4 of these
Four homologous domains
What is the maximum rate of an AP?
1000 Hz = 1 ms
Na channels inactivate, K channels open, large driving force for K to leave cell
Falling phase
Toxins that affect channel inactivation
Scorpion and sea anemone toxins
Techniques used to record electrical activity from single neurons or single channels
Patch clamp techniques
What are membrane conductance changes dependent on?
Time and voltage
Set by Na channels
Absolute refractory period
How does depolarization occur as the AP moves down the axon?
Successive patches of membrane are depolarized
These segments of VG Na channels move in specialized channels
S4
These areas contain many VG Na channels
Nodes of Ranvier
Mechanistic models of this channel were developed in the 1960s and 1970s
VG Na channels
These studies support S4 as voltage sensor because cysteine mutations showed outward movement
Mutagenesis studies
AP conduction moves down the cell in how many directions?
One direction
Sigma conotoxins are active here
Site 6
What happens to the concentrations of Na and K inside and outside the cell during an AP?
The relative concentrations stay the same even though Na flows in and K will flow out. K still higher in and Na still higher out
What does lidocaine do in the VG Na channel?
Blocks the inside channel (S6 segment)
This was used to examine the biophysical properties of individual Na channels
Patch clamp technique
What are generator potentials also known as and what do they do?
EPSPs. Bring the cell to threshold (-40 mV)
What are total Na and K currents composed of?
The sum of individual currents
What does the undershoot represent?
Relative refractory
How does the voltage sensor (S4) fit in each domain?
Slides within a channel in each domain
From frog, channels open at more negative voltages, stay open too long
Batrachotoxin
Vm (membrane potential) moves towards EK (eq pot for K), hyperpolarizing cel, little permeability to Na, opening of Ca activated K channels
Undershoot
Segments of VG Na channel that form inner pore
S6
SLIDES 14-18 GRAPHS
SLIDES 14-18 GRAPHS
What allows the cell to depolarize?
The faster opening of Na channels than K channels so K cant dominate and the cell becomes positive
What does myelin reduce?
Membrane capacitance
The falling phase of APs is also due to the opening of these, not just the closing of Na channels
Opening of K channels
Maintained by Na-K-ATPase
Gradients
An important type of K channels in APs and hyperpolarization
Delayed rectifier channels
Used to see what current is flowing at specific voltages
Voltage clamp
Many types of these, 80 subunit genes, multiple families
K channels
Are a brief reversal of charge (not gradients) in which the inside becomes briefly more positive than the outside
AP
What makes up the VG Na channel?
One protein with 4 domains that make it look like 4 proteins, this is the alpha subunit and there are B1 and B2 subunits on the ends
Also known as spike, discharge, nerve impulse
APs
What is the goal of an AP?
Ultimately stimulate the release of NTs
Is a wave of positive charge transmitted down the neuron
AP
Vm hyperpolarized until K channels close
Relative refractory period
The time when more current is required to fire AP
Relative refractory period
Local anesthetics block in these segments of the VG Na channel
S6
How does myelin help propagation?
Increasing length constant (how far current flows before it leaks out - reduced to 37%)
Frequency and pattern of these encode info
APs
AP conduction is propagated without doing what?
Decreasing strength
Used to look at the electrical activity of one channel
Patch clamp technique
Due to outward K current
Falling phase
Channel with two gates, including an inactivation gate that closes after about 1 ms and only opens when the cell returns to a more negative voltage
VG Na channels
What is required for fast inactivation in the VG Na channel?
3 key amino acids
What does an extracellular electrode oscilloscope display?
A dip to more negative (Na in), then a spike to above the resting to be a little positive (K out with overshoot), and then back to resting (different than normal AP curve)
Toxins used to isolate a, B1, and B2 subunits in mammalian brain, a and B1 in skeletal muscle, eel a in this channel
VG Na channels
These are used to study the function and structure of the VG Na channel
Toxins
What does the amount of depolarization control?
The firing frequency. More depolarization = more APs (all are always the same size)
What does tetrodotoxin do to cells?
Block Na current inward
Channel with rapid inactivation (open 1 ms)
VG Na channel
All chemical transmission depends on these
APs
What happens 1 ms after Na channels open in the AP?
Na channels close and the K channels open causing the cell to rapidly hyperpolarize
What is conductance?
How easy it is for ions to move
Vm (membrane potential) approaches ENa (eq pot for Na) greater than 0 mV
Overshoot
What does the inactivation gate form with the gate receptor in the VG Na channel?
It forms a hydrophobic interaction with it
Why was the squid axon used to study APs?
It is 800 um in diameter compared to humans 2 um. Easy enough to see and manipulate
What are AP firing patterns due to?
Type/combination of ion channels present
Why cant the membrane depolarize the area before an AP it just passed over?
It is in the absolute refractory period
Reentrant loop forms pore here in the VG Na channel
Between 5 and 6 of each domain
How does the inactivation gate of the VG Na channel work?
Closes after 1 ms and only opens when the cell returns to a more negative voltage
Na ions enter cell due to large driving force
Rising phase
What does an intracellular electrode oscilloscope display?
A spike up to a positive charge and a fall back down to negative with a little undershoot (Na in, K out, normal looking AP curve)
What are the B subunits required for in the VG Na channels?
Required for normal kinetics and voltage dependence of gating
Channel with voltage dependent activation (around -40 mV)
VG Na channel
Biochemical approaches led to the discovery of the proteins that make up this channel in 1980
VG Na channels
Caused by influx of Na ions through VG Na channels open for about 1 ms
Depolarization
Vm (membrane potential) at which Na channels open, more permeable to Na
Threshold
At what voltage does the VG Na channel open?
-40 mV
Caused by efflux of K ions through VG K, brings cell back to -65 mV
Repolarization
Produced by entry of positive charges into the cell
Generator potential (small changes in voltage)
Contain immunoglobulin like folds (look like antibodies) in VG Na channel
B1 and B2
What are intracellular and extracellular recordings measured in?
Intra = mV Extra = uV
Brevetoxins and ciguatoxins are active here
Site 5
What does a more excited cell have in terms of APs?
More APs, more frequency, not bigger APs
Due to inward Na current
Rising phase
