NEUR30080 L1 SQ ion channels & permeability

L1 learning objectives - answer these Qs

the lipid bilayer is a perfect insulator

• ions have a shell of water, removing it has thermodynamic cost

• bare diffusion is effectively zero (never) - an Na+ ion crosses about 1x in hours

• therefore needs channels and transporters

ion movement

• signals in the NS are ions crossing the memb

• Na+, K+, Cl- and Ca2+ sit unequally at either side of memb

• opening channels releases stored potential energy (stimulus changes permeability, opening channels, balance shifts, voltage moves)

• depol = reduced voltage

4 types of channels & transporters

ion conc and locations at rest at rest

ions at rest diagram

aqueous pore

a water-filled path through the memb

selectivity filter

the narrowest point dictates permeability

gates

the moving part that opens/closes the channel

sensor

the domain that detects the stimulus: voltage, ligand, force or light

diagram of channel: pore, selectivity filter, gate, sensor

why K+ fits and Na+ does not

• the filter is a ring of backbone carbonyls

  • O2 points inwards, precisely spaced

• K+ sheds its water

  • carbonyl O2s perfectly surrogate their water molecules, energetically compensating for dehydration

• Na+ too small

  • cannot simultaneously contact the pire O2s. the thermodynamic debt of dehydration is not repaid, therefore rejected

• result

  • 1000-fold selectivity

K+ vs Na+ selectivity diagram

hetero-oligomers

• GABA-A receptors

• nACH receptors (muscle)

• voltage-gated K channel

• NMDA receptors

• AMPA receptors

homo-oligomers

• nACH receptors (alpha-7, brain)

• AMPARs (GluA1)

• 5-HT3A receptors

• P2X receptors

hetero-oligomers diagrams

homo-oligomers diagrams

Na and Ca channels

• one chain with four repeats - single polypeptides with 4 homologous domains

• each domain contains 6 transmemb segments, folding for form a single central pore

• two rounds of evolutionary duplication generated a single, continuous transcript

• the four domains are asymmetrical, an evolutionary structural requirement for fast inactivation

Na and Ca channels diagram

Na and Ca channels - the sensor

• S4 is the voltage sensor - an alpha-helix carrying reg spaced +charged Arg residues

• depolarisation electrostatically repels S4 outward against massive electric field (10⁷ V/m)

• this physical movement of amino acids generates a gating charge - a tiny measurable capacitive current that precedes actual ion flow (conformational change)

• sensing and gating are mechanically coupled - S4 translocation physically pulls the activation gate open

the sensor image

3 states

1. closed - poor blocked but the protein is primed to respond to a stimulus

2. open - conducting

3. inactivated - a secondary gate occludes the pore. The channels cannot reset until the memb repolarises (Na channels only

4. kinetic diversity - Na channels activate and inactivate rapidly, K channels activate with a delay

*inactivation structurally distinct from closure - dictates the refractory period

kinetic states image

conductance: Ohmic conduction

• current scales linearly with voltage

• the slope of the I-V curve equals conductance

• only flows one direction instead of 2

conductance: rectification

• asymmetrical conductance

• ions flow more readily in one direction, yielding a non-linear I-V curve

• rectification allows the cell to maintain a stable resting potential without leaking K+ during an AP

• mainly K+, rectifying brings K+ out and wont let it back in

ohmic vs rectifying image

inward rectifier K+ channels (Kir)

conduct K+ inward current, restrict efflux

the polyamine plug

IC Mg2+ and polyamines occlude (block) the pore during depolarisation

leak channels

• resting, constitutively active at rest - no stimulus needed, no gate

• overwhelmingly K+ - more permeable to K+ than anything else at RMP

• they set the RMP

gated channels

• open only on a stimulus - voltage, ligand, light etc.

• shut most of the time - why a resting neuron is stable and quiet

• they CHANGE the potential - synaptic potentials, APs, everything transient

leak and gated channels link

leak channels explain why Vm = -65 mV

gated channels explain why it moves

studying a protein’s physics: use target mRNA

• neuron has dozens of channel types, a mixture

• microinject target mRNA into a Xenopus oocyte

• translocation and trafficking: the oocytes machinery translates and inserts the protein into the memb

• the macro-scale cell (1 mm) expressing a pure, isolated population of the target channel

isolate one variable, then measure it

single molecules: binary transitions

rectangular current steps reflect absolute open or closed states, there are no partial openings

single molecules: stochastic gating

identical voltage stimuli yield different opening latencies on every single trial

single molecules: Na+ kinetics

brief, early openings followed by inactivation

every channel will produce some current (consistent\)

single molecules: K+ kinetics and ensemble averaging

summing thousands of these stochastic, delayed openings yields the macroscopic, analogue current

NA+ channel image

K+ channel image

conformational change made visible

5 ways to open a channel

few hundred genes lead to many more channels

toxins in animals: voltage-gated Na+ channels

toxins in animals: voltage-gated K+ channels

toxins in animals: voltage-gated Ca2+ channels

poison dart frogs

blue ringed octopus & pufferfish

scorpion (Na)

tarantulas (Na)

green mambas

sea anemone

scorpions (K)

tarantulas (K)

black mambas

cone snails

scorpions (Ca)

funnel web spider

animal toxins table DONT MEMORISE

other function of channels

• electrical signalling - APs

• vesicular release - Ca-dependent exocytosis in neurons and glands

• excitation-contraction coupling - Ca influx triggering muscle fibre contraction

• intracellular coupling - direct and metabolic electrical continuity via connexons

• epithelial transport - driving osmotic gradients for secretion and absorption

Q: the selectivity of the K+ channel for K+ over Na+ is explained by ____

precise substitution of K+’s hydration shell by carbonyl oxygens in the filter

Dravet syndrome (epilepsy type)

• SCN1A → Nav1.1, loss of function:

  • usually a de novo mutation, 80% Dravets cases

• Nav1.1 sits in inhib interneurons:

  • fast-spiking PV cells depend on it to fire at high rates

• lose it and inhibition fails first:

  • less Na+ current in the brake, not in the accelerator

• seizures from a LOSS of excitability:

  • in the wrong cell, loss of function looks like gain of function

Dravet syndrome image

Nav1.7 - the volume knob for pain

• SCN9A encodes Nav1.7

• in nociceptors

• loss of function: no pain at all

  • congenital insensitivity - otherwise normal sensation

• gain of function: erythromelalgia

  • burning pain in the hands and feet, triggered by warmth

• a major analgesic target right now, an on/off switch has already validated in humans

Nav1.7 image

the pump

primary active transport

• the Na+/K+-ATPase exports 3 Na+ for every 2 K+ imported per ATP hydrolysed

electrogenic nature

• it yields a net export of one positive charge per cycle, contrib to roughly -5 mV to the resting potential

gradient maintenance

• primary physiological role is sustaining the massive concentration gradients required for signalling

up to 1/3 of a neuron’s ATP goes here, the resting potential is not free

the pump image

spending what the pump saved

• the Na+/Ca2+ exchanger (NCX): imports 3 Na+ to export 1 Ca2+ against its massive gradient, without directly hydrolysing ATP

• symporters (co-transporters): Glut reuptake rides the Na+ gradient to clear NTs from the syn cleft

• primary transport: energy is derived directly from local ATP hydrolysis

• secondary transport: energy is derived from the established Na+ electrochemical gradient - a delayed expernditure of the pump’s work

pumps image

channels signal, transporters maintain

classic proof of maintenance

• Hodgkin and Keynes tracked radiolabelled Na+ efflux from the giant quid axon

metabolic dependence

• efflux ceases when ATP is depleted or the pump is blocked, and resumes upon restoration

temporal disparity

• channels conduct millions of ions per second, transporters cycle hundreds of times per second

functional dichotomy

• channels transmit information, transporters restore the biophysical conditions required to transmit it

a neuron can fire a thousand times before the pump has restored what a single second of firing cost

channels signal, transporters maintain image

learning objectives