Ion Channels And Synapses Flashcards
What is the flow of ions across channels at Equilibrium (Eion)?
no ion movement
What is the driving force for ion movement across chanells?
the difference between the membrane potential (Vm) and the equilibrium potential (Eion)
- driving force = Vm - Eion
What determines the size of ion flow across channels (current)?
- driving force
2. no. of ion channels (conductance/ gion)
How can you calculate the Current for an ion?
- current = conductance x (driving force)
- driving force = membrane potential - equilibrium
Iion = gion x (Vm x Eion)
During an action potential (depolarisation), what happens to the conductance of the sodium ions?
- conductance increases -> sodium influx… occurs down sodium conc. gradient
- sodium channels open due to increasing MP
What is the conductance increase for potassium vs sodium?
- conductance increases more slowly and for longer
- occurs during re-polarisation/ hyper-polarisation phases
What acts as a stimulus for the opening of voltage-gated ion channels?
- the increasing membrane potential
- rate of channel opening depends on rate of membrane potential depolarisation
resting MP caused by?
leak K+ channels (Few Na+)
whats rising phase… and falling phase?
RISING
inward Na+ current (VG Na+ channels)
FALLING
outward K+ current (VG K+ channels)
how do Na, K, Ca cannels behave at negative MP?
pore (channel) closed…
open by depolarisation (inc in Vm stimulus)
What is the voltage clamp method.. shows what?
how ion conductance changes with membrane potential and time.
What is the importance of the voltage clamp method?
- changing MP changes amount of active VG channels open (changed driving force)
each can -> inc/dec in ion flow movement
so..Vm has to be fixed for stable measurement of channel activity
voltage clamp:
whats size of current affected by?
driving force and conductance, g.
for VG channel, gion is dependant on Vm
voltage clamp: what happens after 1. electrode controls intracellular Vm?
hows current measured (indiectly)
when ions flow through channels, equal + opposite current injected (via Im) to maintain Vm
if Im measured, we know the channel current!
What is the current clamp method- measures what?
intracellular voltage difference across cellular membrane while injecting constant positive ions into the cell.
What is the process of Current clamp method?
o Electrode placed inside membrane to measure MP. VG Na+ channels also placed in the membrane
o current (+ve ions) injected into the cell by electrode -> depolarization and is stimulus for the Na-channels to open
o Na+ now moves down conc. gradient into the cell –> further increases MP
o change in MP can be recorded in neurons during AP
What are the issues with the voltage clamp method?
- doesn’t identify which ion moves
- however ion flow determines whether current is positive or negative
– Positive (up) deflection = +ve ions leaving cell (efflux)
– Negative (down) deflection = +ve ions entering cell (influx)
. time dependant
. pharmacological separation
how do voltage dependant ion channels react to the method?
technique allows change in Vm to be applied to cell
voltage dependant channels will respond to a depolarisation
if depol large enough,
=> Na+ (INa) + K+ (IK) current (summed)
Which pharmacological agents can be used to identify individual currents?
o Tetrodotoxin (TTX) – blocks Na-channels (only K flow is evident)
o Tetraethylammonium (TEA) – blocks potassium channels (only Na flow is evident)
What can be measured after analysis of the voltage clamp experiments?
- the relationship between the flow of current and the membrane potential
(max current in/out measured at each Vm) - this can be plotted on a current vs voltage graph
show how size of current peak varies w Vm
What can be seen from a current vs voltage graph for sodium and voltage gated sodium channels?
- votage increase -> rapid influx of sodium (steep line)
- eventually these channels close
- Channels close as they are voltage gated – as more channels close the membrane potential returns to normal (negative)
down then up
What can be seen from a current vs voltage graph for potassium and voltage gated potassium channels?
- voltage increase -> efflux of potassium
- potassium current also increases - suggesting more K channels opening leads to K efflux
- this is a slower and prolonged process (flatter line)
p676
Why does current depend on membrane potential? (Vm)
incraesed membrane potential:
- opens more channels (increased stimulus) depolarisation
- leads to a change in driving force (Vm - Eion)
How does depolarisation effect the flow of current for potassium ions?
- the equilibrium for poatassium = -80 mV
- in depolarisation the driving force is increased as we (Vm) are moving away from the equilibrium
- this increases the opening of K channels and the outward flow of potassium
- current reaches plateau quickly
How does depolarisation effect the flow of current for sodium ions?
- equilibrium = +55mV
- during depolarisation driving force is decreased as we move closer to equilibrium
- this leads to less sodium channels open and slower influx of sodium
but when Vm > +55, (Vm - ENa) = +ve
What is the final current dependant on?
the combinations of the sodium and potassium currents
What is Ohm’s Law?
voltage is equal to the current multiplied by resistance
V = IR
How to calculate conductance using Current and Resistance?
- Current divided by resistance
- g = I / R
How to calculate voltage using Current and Conductance?
- Current divided by Conductance (g)
- V = I/ gI
g V
BUT ionic current depends on driving force, not just V
bc current = NOT 0 when Vm = 0!!!
thus for Na+…
INa = gNa x (Vm = ENa)
What are the different types of Calcium channels (voltage gated)
- low voltage Activated (LVA): T-type (CaV3)
- High voltage Activated (HVA)
p680
families of ionic currents!
Characeteristics of Low voltage activated Calcium channels?
- mainly T-type
- opened by small depolarisations
- rapidly inactivate
Characteristics of High voltage activated Calcium channels?
- found in muscle (L-type)
- found in Neurons (N, P/Q, R types)
- opened by larger depolarisations
- identified pharmacologically
What is the role of Calcium channels in Neurotransmitter release?
· Action Potential opens Ca2+ channels
· Ca2+ influx triggers release of vesicles containing NT
exocytosis
experimental recording of elec activity: describe current clamp
electrode put into cell- monitor voltage
charge inected via leecterode = change in Vm (depol/ hyperpol)
depol: opens VG ion channels: Na, K, Ca … further change sin Em - not measured by electrode (i.e. voltage NOT clamped)
current clamp (IClamp) recording, whats seen with…
- brief current injection
- long current injection
single AP
.. can test drugs, modify struc + half life
multiple AP (Vm stays above threshold)- gradually slows down bc threshold exhaust K+ ions run out
SYNAPSES
2 types of receptor? and what do they each mediate? direct/indirect
ligand gated ion channels (IONOTROPIC REC)
… mediate DIRECT synaptic transmission
GPCR (METABOLIC REC)
… mediate INDIRECT transmission
electrical effects can be either? (2)
excitatory (depol due to Na+ influx USUALLY!)
inhibitory (hyperpol due to Cl- influx)
What is the main neurotransmitter in the CNS for excitatory synaptic transmission?
Glutamate
- ionotropic and excitatory
What are the binding sites for Gluatmate?
- 3 ionotropic receptors (NMDA, AMPA and Kainite)
- 1 metabotropic receptor
What are examples of Non-NMDA receptors?
- AMPA and Kainite receptors
synapse situation?
EPSP
excitatory postsynaptic potential
What are cationic channels?
- typically sodium/potassium channels (similar to Ach channels)
- NMDA and Non NMDA receptors are all cationic channels
What are the extra characteristics of NDMA receptors vs Non-NMDA receptors?
o also permeable to calcium ions
o can be blocked by extracellular Mg2+ ions and other anesthetics
o Mostly all blocked at -60 mV (positive charge attraction)
o Depolarization relieves block – as Mg2+ is unbound from channel
What is the Excitatory Postsynaptic Current/ EPSC?
sodium influx due to glutamate binding to receptor
how does NMDA receptor change once activated?
Na+ can get thru as Glu stuck on and activated
Mg2+ there and can block
2nd messenger= Ca enter cell= enz cascade!
What is excitatory postsynaptic potential (EPSP)?
- the change in membrane when sodium enters the cell
- this can be measure via the voltage clamp method
What is the process of the voltage clamp method?
- stimulate AP generation in the afferent (presynaptic neurons)
- The AP stimulates release of glutamate
- post synaptic receptors opened -> flow of ions into cell (EPSC)
- => small EPSP in post synaptic membrane – can measure change in potential in postsynaptic neuron
- can redo this experiment using a compound called APV – this blocks NMDA channels from opening
What are the effects of APV on EPSC
- APV has no effect at -60mV (NMDA receptors are already closed)
- effect only (blockage) present at -40mV and above
Why are NMDA receptors important?
- detect BOTH activation of the synapse and depolarization of membrane
- allow for the influx of calcium when those 2 conditions are met
How do NMDA receptors contribute to EPSC?
- when the Vm is greater than -40 mV magnesium block is LOST
- calcium influx occurs
- Increased calcium allows for the placement of more AMPA receptors in the membrane (Increasing Na permeability)
- calcium also increases the conductance of INDIVIDUAL AMPA receptors
- Occurs during single synaptic activation
What is the effect of repeated synaptic activation?
- causes summation of EPSP’s (increases depolarization)
- Depolarization reduces Mg block
– 0 Mg block —> non-NMDA channels contribute to EPSP + calcium influx
What is temporal summation?
- series of subthreshold EPSPs in one excitatory fibre produce ONE AP in the postsynaptic cell synapse
- EPSPs are added before membrane has returned to resting state
multiple activation of one synapse= multiple increasing peaks on curve
- additive effect, doesnt return to normal
What is spatial summation?
> 1 presynaptic neuron is activated simultaneously, until sufficient NT is released to activate an AP in the postsynaptic neuron
What are the inhibitory Neurotransmitters for inhibition of synaptic transmission? 2
- Glycine (found in spinal cord)
- GABA (found in brain)
How do the inhibitory Neurotransmitters inhibit synaptic transmission?
- bind to receptors causing a hyperpolarization
- move Vm away from threshold
- occurs via influx of chlorine ions via channels (GABA and Glycine receptors)
How can alcohol/ barbituates affect inhibitory activity?
- receptors can also be bound by alcohol/barbiturates
- increases inhibitory effect + extent of hyperpolarization
- using alcohol + barbiturates simultaneously = respiratory depression
what carries IPSC (inhib postsynaptic current)?
Cl-
how does voltage clamp (of post synaptic memb) work?
how does it cause-> hyperpolarisation i.e. inhibition
stim presynaptic axon + record IPSCs
- 0 current @ -70mV (this is Ecl!)
at resting membrane potential (-65mV)
- Cl- influx
- outward current!- same elec effect as + ions leaving cell
- Vm change hyperpolarisation i.e. inhibition
What is an example of a metabotropic receptor?
where are they found?
- GABA-B receptor
- G-protein coupled receptor
ANS: heart, SM, glands,
CNS
How do G coupled protein receptors work?
· activated by NTs – these trigger 2nd messenger signals inside the cell
· receptors modulate ion channel activity via indirect effects on electrical properties
· work more slowly than ionotropic receptors at 100s of milliseconds
What is the MOA of a G-Coupled receptor?
p700!
- ligand binds receptor causing conformational change releasing energy (GTP to GDP)
- Energized G-protein complex dissociates from receptor and moves across membrane activating an enzyme in the membrane
· enzyme activates another molecule which triggers the production of cAMP from ATP
· cAMP acts an intracellular second messenger - which activates enzymes in the cytoplasm
· enzyme activation can change the permeability of membranes - may open other ion channels and trigger influx of ions
What are some functions of cAMP?
- activate enzymes in cell
- may cause a change in gene activity in cells
– upregulate protein synthesis (receptors etc.)
How can metabotropic receptors affect other proteins?
- modulate activity of ionotropic or voltage gated ion channels
- affect release of NT from presynaptic neurons
- can affect neuronal excitability and AP characteristics of post synaptic neurones
Where are the presynaptic metabotropic receptors located? role?
in the post ganglionic sympathetic neurons
affect release of NT
Where do the presynaptic metabotropic receptors synapse?
- neuromuscular junction
- NAd is the major neurotransmitter
functional effects of postsynaptic receptors?
affect neuronal excitability and AP characteristics
What proteins are located on the presynaptic metabotropic receptors?
- Calcium channels
- a2-adrenoceptors
What is the effect on the presynaptic calcium channels when the a2-adrenoceptors are activated?
calcium channels are inhibited
What is the consequence of the inhibition of the calcium channel on the presynaptic neuron? whats the mechanism?
Lack of calcium influx prevents the exocytosis of NTs (i.e. -ve feedback) -> postsynaptic firing is slowed down
G protein By signalling from adrenoceptor -> Ca2+ channel
What is the role of the presynaptic metabotropic receptors?
regulation of Synaptic transmission (via inhibition of NT release)
NTs that activate presynaptic metabotropic receptors come from where?
Same OR different neuron
What are the post-synaptic metabotropic receptors Responsible for?
· Modulation of Pattern of AP Firing in the Autonomic Ganglia
- using Ach as a NT
What are the 2 types of EPSP in Postsynaptic Metabotropic Receptors?
- Fast EPSP
- Slow EPSP
What are fast EPSPs?
- caused by Nicotinic ACh receptors
- Cause a Large depolarization. (EPSP) leading to a single AP (not shown)
What is a slow EPSP?
- caused by Muscarinic ACh receptor (mAChR)
- Releases 2nd messenger (DAG)
- DAG causes inhibition of M-type K+ channels - (decreased K+ efflux)
- Causes a Small depolarization (slow EPSP) due to leak Na+ channels (no AP!)
What is the functional consequnece of Slow EPSPs?
- Cell is more excitable during slow EPSP
- If synapse is stimulated again at this time, multiple APs are generated
- This is because the cell is already slightly depolarized – there is less depolarizing to do to fire an AP
- This is an additive effect
