Resting Potentials

Question 1

What role do the cytosol and extracellular fluid play?

Answer 1

water containing dissolved ions
water is polar as oxygen has a greater affinity for electrons that hydrogen in the covalent bond
this makes it an effective solvent of polar/ hydrophilic compounds but not hydrophobic compounds
as charged particles have a stronger attraction for ions than each other (law of solubility)
as each ion breaks away from the crystal its surrounded by a hydration shell that insulates ions from one another
Ions are the main major charge carriers in the conduction of electricity in biological systems.
Specifically in neurophysiology: Na+, K+, Ca2+ and Cl-

Question 2

what is the role of the phospholipid membrane?

Answer 2

made of phospholipds that have hydrophobic tails and hydrophilic heads

the hydrophobic tails line up to form lipid bilayers

mosaic of components (cholesterol, proteins and carbohydrates) that are able to diffuse laterally

impermeable to ions and organic molecules so the cell can maintain its own intracellular environment

protein pumps and channels enable the selective movement of molecules across the membrane

acts as an insulator between two conductors because it separates ionic charges in the intracellular and extracellular fluids
- this enables charge to be stored on the neuronal membrane (capacitor)

Question 3

what is the role of proteins?

Answer 3

Ion channels
Functional channels require 4-6 similar protein molecules assemble to form a pore in the middle.
Subunit composition varies; specifying their different properties: pore diameter, R group lining, ion selectivity (become selectively permeable to a specific ion), gating (ability to be opened or closed by changes in microenvironment)

Ion pumps
Enzymes spanning the membrane that use energy from ATP hydrolysis to transport ions across the membrane
Important in neuronal signalling by transporting Na+ and Ca2+ out of neuron.
Pumps establish ionic concentration gradients across the neuronal membrane

Question 4

How do ions move across the membane?

Answer 4

electrochemical gradient
diffusion - high conc -> low conc
electricity - opposite charges attract like charges repel

Question 5

What is membrane potential?

Answer 5

The voltage across the neuronal membrane at any moment (Vm)
Electrical charge is unevenly distributed across the neuronal membrane
Negative charge inside the neurone compared to outside.
Resting potential: hyperpolarised membrane potential (-50 to -90 mV).

Question 6

What are equilibrium potentials?

Answer 6

Eion: the electrical potential difference that exactly balances an ion concentration gradient

equilibrium state achieved when diffusion and electrical forces are equal

net difference in electrical charge occurs at the inner and outer surfaces of the membrane as its thin enough for ions either side to interact electrostatically

ions are driven across the membrane at a rate proportional to the difference in membrane potential and equilibrium potential (Vm-Eion)

Question 7

What is the Nernst Equation?

Answer 7

Each ion has its own equilibrium potential that would be achieved if the membrane was only permeable to that ion.

can deduce whether the inside of the cell would be positive or negative if we know ionic change and difference in concentration across membrane.

Nernst Equation uses ionic charge, temperature and ratio of external and internal ion concentrations to calculate the value of the equilibrium potential for any ion.
Nernst potential for that ion
Eion =2.303 {RT}/{zF} log 10 [ion outside cell]/[ion inside cell]
neurons are permeable to more than one type of ion
doesn’t account for relative permeabilities
why mV is - 65

Question 8

What is the Goldman equation?

Answer 8

GHK equation accounts for relative permeability of membrane for different ions so determines membrane potential
average of Nernst potentials for each permeable ion weighted by their relative permeabilities
Vm= 61.54mV log Pk[K+outside] + PNa[Na+ouside] / Pk[K+in] + PNa [Na+in]

Question 9

what is the transmembrane ion gradient?

Answer 9

cells maintain an asymmetric distribution of ions across their membrane
higher [K+] in
higher [Na+] and [Ca2+] out
this is not sufficient alone to generate membrane potentials as the negative and positive charges cancel out

Question 10

How do sodium-potassium pumps work?

Answer 10

The protein pump is an enzyme that hydrolyses ATP in the presence of internal Na+.
This drive the pump to exchange 3 Na+ out for 2 K+ in against concentration gradient.
Keeps K+ concentrated inside and Na+ concentrated outside.
electrogenic - due to imbalance in movement of charge directly causes a small hyperpolarisation of membrane

Question 11

How does the Na+-Ca2+ pump work?

Answer 11

secondary active transport
enzyme moves Ca2+ out of the cytosol across the cell membrane
exchanger where the flow of Na+ down its electrochemical gradient into the cell is used to expel Ca2+
(Intracellular calcium-binding proteins and organelles (e.g mitochondria and ER) decrease intracellular [Ca2+])

Question 12

What is the significance of leak channels?

Answer 12

predominantly selective for K+
explains hyperpolarised resting membrane potential
efflux of positive K+ inside the cell resulting in net negative charge inside the cell
initially diffusion gradient is stronger than opposing electrical gradient until electrochemical equilibrium is reached
weakly permeable to other ions (about 1/1000) - why experimental values differ from Nernst and GHK equation needs to be used.

Question 13

How are ion channels selective?

Answer 13

specific to size - but K+ ions are larger than Na+ ions
to pass through pore as a single ion hydration shells have to be shed.
at the selectivity filter the water molecules are replaced by interactions with oxygen atoms from carbonyl or hydroxyl groups of the proteins
the spatial configuration of these oxygen atoms makes the interactions more favourable for K+ ions over Na+, enabling selectivity

Question 14

How does K+-Cl- pump work?

Answer 14

co-transporter
secondary active transport
direction of transport is the same for both
K+ and Cl- move out of the cell

Question 15

What happens if Na+-K+ pump is blocked?

Answer 15

selectively blocked using oubain
small depolarisation due to the lost electrogenic activity
the resting membrane potential does not collapse
suggests leak conductance is most important
however neuronal signalling depends on ion flux across the membrane and glia use active transport to maintain ion gradients and uptake neurotransmitters
in active neuronal networks much higher demands are placed on transporters to maintain the intra/extracellular environments
applying oubain can lead to rapid collapse of membrane potentials

Question 16

What is the importance of osmosis?

Answer 16

as water moves down its water potential gradient it exerts hydrostatic pressure on the membrane
the system reaches equilibrium when hydrostatic pressure = osmotic pressure

the accumulation of ions inside the neuron
influx of water.
increase in hydrostatic pressure 
cellular swelling
decreased volume of extracellular space
change in concentration and diffusion of molecules in extracellular fluid
altered neuronal excitability
(e.g in ischemic stroke)

aquaporins are water channels that facilitate osmosis
deleting gene Aquaporin4 in mice is associated with 7x reduction in cell plasma membrane water permebility
mice do appear physiologically normal

Question 17

RC circuit

Answer 17

the electrical properties of the neuronal membrane can be represented as an electronic circuit
leak channels - resistor (RLeak)
able to conduct current (g) specified in terms of resistance (g=1/RL)
ionic grandients - battery
try to clamp membrane at the Nernst potential of leak channels (EL)
in series with resistor
membrane - capacitor
in parallel
to change the voltage across the membrane circuit charge has to be stored on the membrane capacitor

Question 18

What controls resting potential?

Answer 18

Ionic permeability - leak channels
Ion gradients - ion pumps
Osmosis