Excitability and Ion channels I Flashcards
What is Electrical Excitability? (2)
Property of a cell (esp excitable cells - like neurons + muscle cells) that allow them to generate + propagate electrical signals known as A.P.’s (/nerve impulses/electrical signals)
What is Electrical Excitability closely related to? (2)
- movement of ions across cell membrane
- Changes in membrane potential
What are the 2 main factors regarding K+ electrical excitability? (2)
- K+ conc gradient
- positive charge of ion
What is a concentration gradient? (2)
ratio of ions inside and outside a cell - which helps determine the equilibrium potential
What does a high conc gradient inside a cell mean and vice versa? (2)
ions from a region of high conc to low conc, across a partially permeable membrane
so cells will leave the cell in this e.g., or cells will enter the cell in the opposite.
What does a positive charge mean and what does it do? (2)
The ions are attracted to the negatively charged interior of the cell (electrostatic attraction). This attraction will pull positive ions into the cell.
What is Ek? (2)
The equilibrium potential of the ion (i.e. K+ ek) = the net movement of ions across the membrane is 0
What is the Ek of K+ and why is it important? (3)
-70 to -90mv (in most excitable cells)
It is one of the key factors that establish the resting potential in excitable cells
plays a critical role in the generation + propagation of A.P.’S.
Define Reversible Tension/ Nerst Potential (3)
The membrane potential where the chemical and electrical gradients of ions, across a biological membrane, are in equilibrium.
There is no net flow of ions across the membrane.
It is written as Eion e.g. Ek for potassium equilibrium
How do you calculate the Equilibrium Potential? (2)
Using Nernst equation
Ex = (RT/zF)ln ((X)out)/(X)in)
What is Ex? (1)
The equilibrium potential for any given ion (i.e. ek = K+)
What is electrophysiology used for? (1)
It is used to measure the potential difference b/w an electrode inside a cell and an electrode outside a cell.
Whole cell patch clamping explained (2)
1) For these types of recordings, a single electrode is placed inside a cell where we can seal the electrode onto the outside of the cell and rupture the membrane = electrical control
2) measure membrane resting membrane potential + active/passive electrical properties of the cell (e.g. A.P. shape + firing patterns)
What maintains the Resting Membrane Potential? (2)
It is often maintained by potassium leak currents (K+ leak channels = often called the K2 or otherwise referred to as the K2P family)
but also the inwardly rectifying potassium channels
When a membrane potential reaches it’s threshold… (6)
voltage gated sodium channels are activated = cascade of effects = more voltage gated sodium channels are activation
After, the sodium channels inactivate = slower action = voltage gated potassium channels begin to open (causing an overshoot = afterhyperpolarisation)
Ends with re-established resting membrane potential
What is the resting membrane potential? (2)
The electrical potential difference across the cell’s plasma membrane when the cell is not actively transmitting signals.
It’s primarily determined by the selective permeability of membrane to different ions, specifically potassium, sodium and chloride.
What are the main components of a Voltage gated channel and what do they do? (10)
voltage sensor: identifies changes in the membrane potential
selectivity filter: determines which membranes determine which ions flow through the membrane
pore: allows the flow of ions
gate: can be opened and closed
anchor protein: vary slightly per each cell type
K2P family structure (‘leak channels’) (4)
4 transmembrane domains: M1-M4
2 pore domains: P1 + P2
2 extracellular cap helixes: C1 + C2
Functionally: K2P channels assembles dimers - contrast to other K+ channels that are tetramers
How are the K2P family regulated? (4 + 3)
- Mechanosensitivity (increased pressure opens channels) ==> contraction of the cell during hyperosmolarity leads to reduction in this current but expansion of cell by applying pressure leads to opening of these channels)
- Polyunsaturated fatty acids e.g. arachidonic acid (activate channels - but FA have X effect)
- Thermosensitivity (TREK1 activated - gradually + reversibly in response to the elevation to ambient temps (14-42°) - more steeply b/w 22-42°)
- pH, G proteins + partner proteins
What is different about K2P compared to K+, Na+ and Ca2+ channels? (2)
They don’t have a specialised voltage sensing domain + don’t show any activation during depolarisation voltage steps (except TWIK-2)
= * They are voltage insensitive (except TWIK-2)
What do K2P channels do? (1)
leak channels allow the bidirectional movement of ions up to the point of equilibrium potential (-70 to -90mv)
How many inwardly rectifying K+ channels (Kir/Kirk) are there and how are they regulated? (2)
15 channels
regulated by intracellular signalling molecules (e.g. G proteins or ATP)
KIRK channels properties + how does it work? (3)
each subunit consists of 2 transmembrane domains = form tetramer
predominantly allow the flow of K+ ions into the cell at negative membrane potentials (pushing k+ against electrical gradient = depolarise membrane)
= contribute to stabilising the rest of the membrane potential + controlling cell excitability
What are 2 well studied A.P. cells (2)
- CA1 pyramidal cells (glutamatergic ‘principal cell)
- Oriens-Lacunosum molecular interneuron (GABAergic interneuron)
what is the difference in the threshold of these 2 cells + why? (3)
CA1 threshold is more hyperpolarized (lower) than O-LM interneuron ==> reduced input needed to generated CA1 AP compared to OLM AP
CA1 contains 2 types of voltage gated sodium channels (rNav1.6 + rNav1.2);
OLM interneuron only one (rNav1.1).
What is the voltage-gated sodium channels open in response
to depolarisation? (steps -7)
1) outside cell = more +ve ions and inside the cells = more -ve ions == cell becomes activated
2) channel opens = mass influx of Na+ ions into cell = further depolarisation
3) local depolarisation = opening of more voltage gated Na+ ions = more positively charged Na+ channels open
- Voltage gated Na+ channels rapidly inactivate after activation
- Voltage gated K+ channels are also activated but more slowly
- channels gradually close + return to resting state = hyperpolarisation
- Na/K pump
What determines the A.P. threshold? (1)
The properties of the cell = the key VG ion channels (e.g, VG na+ channel)
How do voltage patch clamps work? (2)
- allows researchers to control + monitor the voltage across cell membrane while measuring ionic currents
=crucial for understanding how ion channels open + close in response to changes in voltage
What does it mean when the current changes in response to the voltage reducing from -80 to -40mv? (1)
The channels are opening
What is GMax and how do you measure it? (3)
The maximum conductance of an ion channel, determined by the flow of ions through the channel when fully open
measured through: The examination of the proportional opening of the ion channels as the activity of the cell = reflection of the no. of channels activated at any certain voltage
What ion channels are essential for repolarisation of an AP? (1)
VG potassium
VG K+ channel info (4)
one of the largest families of ion channels
have 6 transmembrane domains
exist in both an open and closed state
when membrane potential becomes more depolarised = conformational change of opening ( = K+ ions leave cells = repolarisation of the membrane)
How does the Na+/K+ pump work with ATP + why? (6 + 1)
Ions are in the wrong place: sodium ions are in intracellular space + potassium has flowed into extracellular space
1) Cytoplasm Na binds to the Na/K pump along w/ TP
2) Na/K pump is phosphorylated w/ the ATP
3) pump changes its confirmation = Na release
4) extracellular potassium binds to the pump = dephosphorylation
5) pump returns to it original confirmation
6) K+ released from by the pump
What is Burst firing? (1)
The frequency of AP firing slows down with time
What controls high freq burst firing in OLM compared to CA1 cells? (3)
-A longer depolarization leads to a train of AP’s but they slow down over time
-higher AHP(afterhyperpolarisation) observed in OLM compared to CA1
-AHP controls a continuous high frequency firing in OLM (not present in CA1)
What are the 3 states of Na+ channels? (3)
-A closed/resting state
-An open state = Depolarizing stimulus → polarity of the membrane changes → gates are
opened
-An inactivated state→ closes the pore from the inside → inactivated channel
(can’t be activated)
most are closed or open
The shape of a waveform is dependent on what?
It can vary massively depending on cell type
Voltage gated sodium channels structure- each subunit (6)
Alpha subunit - comprise 6 TM domains: pore forming region in S5 + S6 segments, voltage sensing component located in S4 segment
Beta1 + 2 subunits: consist of large extracellular n-terminal, single TM segment + short intracellular segment
What is an inactivation gate and what does it do in a sodium channel?
Drives the mechanism in the sodium channel: by stopping the flow of ions through it
At negative membrane potentials what happens to most Na+ channels? (1)
They are in the closed = available state
What happens to voltage gated na+ channels during depolarisation + then over time? (3)
Many channels are activated and shift into the open state = causes an inward current (seen in voltage clamp recordings)
Over time: the activated channels rapidly shift into inactivated closed state (channels unavailable to open)
Recovery from inactivation depends on… (2)
Time! Channels will open again proportionally to the amount of time passed since the 1st depolarising stimulus
+ voltage steps:
- need to re-depolarise the membrane at a more positive level = shows a smaller current in voltage clamp
Inactivation comparison of OLM and CA1 cells (7)
APs in OLM cells are narrower = membranes spend less time at depolarised potentials == less inactivation
APs in OLM cells have larger AHP = pushes membrane to more hyperpolarised potentials == faster recovery from inactivation
The faster Na+ channel recovery = promotes high frequency repetitive AP firing
What’s the role of VG K+ channels (esp KV3) in AHP’s? (6)
-both pyramidal and interneurons express different ones
- interneurons express high levels of KV3 subtypes
Generally:
- fast activating + fast deactivation (@ v depolarised potentials)
-produce large post spike AHP
- allow for v fast repolarisation of AP
- maximise quick recovery of resting conditions of membrane
What causes/ what other channels regulate burst firing? (2)
HCN channels
VGCC
HCN info (6)
Similar to VGK+C
Tetramer
Activated by hyperpolarisation + cyclic nucleotides
Mixed permeability to Na+ and K+
= allow for range of functions in CNS
- in brain: molecular correlate of hyperpolarisation activated current called IH
What is IH current important for? (3)
Regulation of input of resistance
pre and post synaptic properties
Rhythmic oscillatory patterns
What is unique about HCN channels compared to other VG channels? (2)
They can be activated by hyperpolarisation compared to other channels being activated due to depolarisation.
The structure of the pore is similar but the voltage sensors in HCN channels are in a depolarised confirmation
What is the other activator of HCN channels + how does it work? (4)
They’re VG but also ligand gated ====
Cyclic AMP - it’s the primary agonist of HCN channels
(But HCN channels can also be activated by cyclic GMP + cyclic CMP)
Binding occurs CMBD= by causing the protein to roll + twist => causing pore to open
How does HCN affect the excitability of a cell? - experiment (2)
Experiment:
- cell’s current is stepped from -200 picoamps to +100 picoamps ( allows measurement of intrinsic properties eg SAG + firing properties of cell eg threshold)
- used HCN blocker (ZD7288) = following use of blocker, more current ended to push the cell to threshold drastically increases
What is voltage SAG? (2)
The difference between peak hyperpolarisation + the amount of rebound
Important for biological function
VGCC structure (3)
- Comprised of a Gamma unit
- 2 alpha units: alpha1 has 4 repeated domains - each w/ 6 TM segment + TM associated loop b/w S5 + S6
- beta subunit
VGCC family (4)
3 subfamilies classified by a difference in alpha subunit:
- CaV: key role in processing excitations, contraction, coupling, reg of transcription + visual conduction
- CaV2: involved inin neurotransmitter release + dendritic calcium transients
- CaV3: conduct T type currents
Burst firing patterns mediated by CaV3 (5)
- depolarised cell @ -55mv = regular firing pattern
- depolarised cell @ -75mv = diff firing pattern (channels are in their inactivate state = able to open = leads to the depolarisation = leads to the pushing the cell to threshold = burst firing effect)
Short bursts => VG channels go into their inactive state
Why are VGCC referred to a low threshold burst firing? (1)
They are activated in a more negative membrane potentials than eg sodium channels
What is the role of burst firing/role of APs? -grading (3)
1) Role of AP: to release neurotransmitters from the synapse
2) synaptic events then measure in post synaptic cell as EPSC’s (graded)
- small = few synapses activated
- bigger = more synapses activated
- even bigger = even more synapses activated + AP firing
Allows us to distinguish between small + bigger events
How does your auditory nerve distinguish between 2 sounds if amplitude of AP is the same? (1)
The amplitude (size) of AP doesn’t change but the frequency (number) does!
Place cells info (2)
They are neurons within a structure in the hippocampus which fire AP’s only when humans/rats are in a specific location in space.
Place cells are able to encode geometric space using a rate code (experiment of rats running around = turned into a heat map)
Neurons have extensive axons. Why is this a problem? (3)
These axons are incredibly thin (1micronm diameter in adult) = v fine
=problematic electronically because they are made up of membrane capacitors = they store charge
The electrical resistance in these thin axons in much higher because of positively charged ions sitting on negatively charged membrane === charged ions find it harder to flow down an axon = AP propagation is much too slow to carry out ANY action
How do humans get around the slow conductivity causes by thin axons? (3)
Myelination:
Fatty membrane that insulates axon from external -ve charge = decreases membrane capacitance + increases resistance across membrane = faster transmission
But myelinated axon still has some capacity to store charge = signal degrades over v long axons
What is the solution to myelination problems too? (4)
Nodes of Ranvier (breaks/gaps in multinational of membrane)
- high density of VG na and K channels found here
- new AP initiated at these nodes
- nodes allow for saltatory conduction (leap)
What generates the myelin sheath in PNS + CNS? (2)
PNS: Shwann cells
CNS: oligodendrocytes
