Lecture series 3 - Ion channels and Excitability

Lecture 1 - the phospholipid layer

what analogy is used for phospholipid layer?

how permeable is it?

• phospholipid layer functions like a dam, separating intracellular and extracellular space

• it is impermeable to most things especially iOS

basic structure of phospholipid bilayer in terms of compartments and gradient

• 2 different compartments with different concentration of ions separated by a membrane

• neuron produces ion gradient using ATP

ion channels

• movement down or against electrochemical gradient

• do they open/close

• can movement of ions cause change in membrane voltage

• can movement of ions cause change in concentration of ions

• what do they need to move across membrane

• allow for on demand passive movement of ions down an electrochemical gradient

• ion channels open and close in response to specific stimuli to conduct ions = action potential

• movement of ions can cause change in membrane voltage of neuron e.g. action potential/post synaptic potential

• movement of ions needed to cause a signal isn’t enough to impact concentration of ions

• ions can’t move across membrane without specific protein

singular phospholipid draw and label

• hydrophilic ( water loving) head. = polar - hydrogen bonds with water = charged

• glycerol linker = uncharged fatty acid tail = hydrophobic(water fearing) = non polar

• usually one tail saturated and one unsaturated creating a kink in the tan

assembly of phospholipid bilayer

assembly of phospholipid bilayer 2

assembly of phospholipid bilayer continued…

• and how to remember

• amphiphilic ( both water loving and hating) nature of phospholipid means there is energetic prohibition against free edges

• only way this can be avoided s to turn it into a ball which also gives it a self healing property

• hospremember like bread butter bread ( bread absorbs water)

phospholipid bilayer resistor or conductor?

because it doesn’t let ions in - it is an electrical resistor

what can cross phospholipid bilaye and how easily?

ion concentration gradient in neurons

• cover sodium, potassium, calcium and chlorine

• extracellular Na+ conc is high but intracellular is low

• extracellular K+ conc is low but intracellular is high

• intracellular Ca2+ conc is kept very very low

• extracellular Cl- conc is high but intracellular is low

can remember as put salt (NaCl) on outside of chip so extraceullar is high

energy and the brain

• neurons generate lots of ATP which required oxygen through oxidative phosphorylation which is very efficient but not always enough oxygen

• astrocytes form support network to shuttle lactate into rum to help them produce ATP

primary active transport

• use ATP directly through ATP hydrolysis by pump to move an ion across cell membrane against its conc gradient rp

primary active transport - Na+/K+ ATP ase

primary active transport = use of ATP directly through ATP hydrolysis by a pump to move ions across a cell membrane against its concentration gradient

• 2 K+ go in and 3 Na+ go out.

• (catalytic binding sites for Na+ and ATP on intracellular surface and K+ on extracellular)

what is palytoxin?

I dont know if he will test this dont stress if dont know

lethal marine toxins that works by collapsing concentration gradient so pumps Na+/K+ willy nilly

Ca2+ pump/ Ca2+ ATPase

• calcium used as an important secondary

• maintain a very low cytoplasmic Ca2+ where free calcium is a lot lower than extracellular conc

secondary active transport

• what is it

• what does active transport allow..

• explain Na+-Ca2+ transporter

• does NOT directly use ATP hydrolysis. instead uses energy from ion gradient established already from primary active transport (to move another ion across a cell against conc gradient )

• active transport allows for storage of energy in form of concentration gradient

Na+-Ca2+ exchanger

• Ca2+ transported out of cell without atp

• because Na+and Ca2+ transported in opposite directions, exchanger= antiporter

• basically hydrolysis of ATP from na+-K+ pump also provides energy for this because uses Na+ gradient

how does transport of Cl- change upon development

developing neuron

• low KCC2

• high NKCC1

adult neuron

• high KCC2

• low NKCC1

note

KCC2 = pumps chlorine out

NKCC1 = pumps chlorine in

Lecture 2

what is ohms law

I = V/R

I= current ( amount of charge moved per unit time)

V=voltage (potential energy that can be exerted on a charge carrier)

measure between 2 points^^

R= resistence

• what does membrane voltage mean ?

• what charge is inside of neuron?

• membrane voltage is difference in potential between the inside and the outside of a neuron

• inside of a neuron is negative

what is RMP of a neuron

resting membrane potential approx = -70mV

extra note( not to memorise) :

if resistance is high, current is low

if voltage is low, current is low

• why is brain different to a circuit

• when we talk about voltage in the brain what do we mean

• different to circuit because instead of electrons being only charge carrier we have ions of different charges and different concentration gradients

• voltage is electrochemical gradient (V)of an ion and conductance( 1/resistance of flow of membrane). so each ion can produce its own membrane current

define concentration gradient

if there is a difference in conc of an uncharged chemical species across a permeable membrane then there will be a net directional flow of that species.

net flow : high conc → low conc

Nerst Equation

• what is equation

• what does each ‘thing’ mean

Eion = electrical potential that counteracts conc gradient for the ion

= equilibrium potential

R= 8.314

T= temp in K ( degrees+273)

Z= charge of ion

F= 96485

[X] = ion conc

^^ may not be tested just understand

what is the equation of driving force in context of ohms equation

*confirmed to be in assessment

I = V/R

I ion = (Vm-Eion)/R

driving force = (Vm-Eion)

the driving force is the difference between membrane potential of cell and equilibrium potential of ion

key equilibrium potential takeaways of following ions:

• na+

• K+

• Ca2+

• Cl-

• Na+ Eion (equilibrium potential) very depolarised relative to rmp

• K+ Eion hyper polarised relative to RMP

• Intracellular Ca2+ conc kept very low

• Eion for Cl- close to RMP

lecture 3

what do each of these mean

• driving force =0

• driving force<0

• driving force>0

driving force = 0

• no net movement of ions

• so at equilibrium potential, no net movement ( current) of ions

driving force<0

• for positive ion , it will want to flow into cell

• for negative ion, it will want to leave cell

driving force>0

• for positive ion, it will want to leave cell

• for negative ion, will want to flow into cell

• what does g stand for in R=1/G

• what values are a depolarising current

• what values are a hyperpolarizing current

• g = conductance

• depolarising current = -70 to more POSITIVE value

• hyper polarising current = -70 to more NEGATIVE value

• what is an inward current

• what is an outward current

• inward current = net entry of a positively charged ion into the cell causing depolarisation or net exit of negatively charged ion

• outward current = net exit of a positively charged current out of cell causing hyperpolarisation or net entry of negatively charged ion

are these currents ( inwards and outwards) requiring ATP ?

these currents are passive and do not directly require ATP. ions flow due to driving force( flowing down electrochemical gradient)

what are the requirements for a working membrane current

driving force AND conductance

• because phospholipid bilayer is a resistor, current in form of ion movement cannot free flow

• therefore ion channels are transmembrane proteins that allow ions to flow across all membrane

what are the 3 important properties of ion channels

1. most open and close in response to specific stimuli e.g. voltages gated ion channels

2. recognise and select specific ions

3. conduct ions across a membrane

what is the exception to this rule?

leak potassium channels

• dont need stimuli to be opened

• open at rmp and are reason for higher resting k conductance

but they do recognise and select specific ions ( k only ) and conduct ions across channels

*RMP is closer to Ek because of these leak potassium channels

voltage gated ion channels

• have voltage sensor often containing positively charged amino acids acid residue

• this ‘senses’ membrane voltage of cell

• at given membrane voltage , voltage sensor is translocated inducing conformational change in protein that enables opening of activation gate of ion channel pore

mostly channel opening favours making inside of membrane more positive ( depolarisation)

lecture 4

what is action potential?

action potential = rapid depolarisation followed by an almost equally rapid depolarisation and an even longer afterhyperpolarisation

dont memorise but be familiar with fundamentals of Hodgkin - Huxley study 5 steps

1. dissect out squid giant axon

2. peel axon from surrounding tissue

3. cut axon and insert cannula

4. suspend axon vertically in sea water

5. insert microelectrode through cannula into axon

I membrane equation

function of voltage gated sodium channels

positive feedback loop - likely on exam

key things to remember abound sodium conductance (GNa)

• low Na conductance at rmp

but massive increase in conductance due to positive feedback loop

• voltage gated Na channels activated by depolarisation

• this activates more sodium channels causing more sodium conductance and depolarisation

• these depolarisations are always explosive but short lived because channels deactivate quickly

key things to remember voltage gated k channels

• closed at rmp

• delayed deactivation after depolarisation and many have slow or no inactivation

• voltage gated k channels activated by depolarisation

• there is high potassium conductance at RMP due to leak NOT voltage gated k channels