2.1 Resting & Action Potentials

Question 1

Diffusion refers to the ______________ movement of molecules in a solution from high → low concentration:
• Equilibrium (and no net movement) is achieved after a period of time
• Useful for transport of substances over short distances

Answer 1

spontaneous and energy-independent

Question 2

What does flux refers to? (e.g. at time C above, net movement in both directions result in no net flux)

Answer 2

Flux refers to the number of molecules which cross a unit area per unit time

Question 3

What is the definition of voltage (V)?

Answer 3

Generated by ions (not electrons) to produce a charge gradient

Question 4

What is the definition of current (A)?

Answer 4

Movement of ions (not electrons) due to voltage/potential

Question 5

What is the definition of resistance (Ω)

Answer 5

Barrier which prevents the movement of ions (not electrons) → formed by cell membrane (may change depending on membrane permeability)

Question 6

Membranes are selectively permeable (allowing some ions to cross the barrier), causing the concentration of at least 1 permeant ion to be different on one side:
• Movement of ions across the membrane is carried out by specialised proteins (ion channels) which can change conformation to open or close
o Selective for different types of ions (e.g. K+, Na+, Cl-, Ca2+)
o May be voltage-dependent (open by changes in _____________) or voltage-independent (open ___________________)
• All cells possess a resting membrane potential (inside of cells are generally ______________ compared to the outside)

Answer 6

membrane potential;

all the time → generates membrane potential;

negatively charged

Question 7

Membrane potential is generated due to the presence of __________________ which make the membrane selectively permeable to some ions, causing ___________:
• If there are no channels, there is no diffusion across the membrane despite concentration gradients → no charge separation → membrane potential = 0

If membrane only permeable to K+: K+ crosses membrane down concentration gradient → charge separation (compartment 1 is positive; compartment 2 is negative) → some ions pulled back by electrical gradient

If membrane only permeable to Na+: Na+ crosses membrane down concentration gradient → charge separation (compartment 1 is negative; compartment 2 is positive) → some ions pulled back by electrical gradient

*The 2 scenarios above may have equal but opposite RMPs (due to the direction of movement of ions) → sign is different (e.g. one has RMP = -70mV, the other has RMP = +70mV).

Electrochemical equilibrium is achieved when the __________________________, generating a stable membrane potential:
• Electrical forces (pulling ions back) essentially counteract diffusion forces (pushing ions out) → only very small amounts of ions actually cross the barrier at this point
• Equilibrium potential is the potential which __________________

Answer 7

ion channels;

charge separation;

concentration gradient is balanced by the electrical gradient across the membrane;

prevents diffusion down the ion’s concentration gradient

Question 8

The Nernst equation calculates the potential for a given ion to reach equilibrium.

What is the formula?

Answer 8

Ex = (RT/ ZF) ln (C0/Ci)

• RT/F = 27 at 37°C
• Co = [X+] outside cell
• Ci = [X+] inside cell
• R= gas constant,
• T = temperature (in K),
• Z = charge on ion (-1 for Cl-, +2 for Ca2+),
• F = Faraday’s number (charge per mol of ion)

Question 9

What does the GHK voltage equation describe? What is it used for?

Answer 9

• resting membrane potential (Vm), with P being the permeability/channel open probability (0 = 100% closed, 1 = 100% open)
• Can be used to calculate the membrane potentials of the neurone at different times (e.g. during generation of action potential)

Question 10

What is the definition of depolarisation?

Answer 10

Vm becomes less negative and moves towards 0 (overshoot occurs when Vm becomes positive)

Question 11

What is the definition of repolarisation?

Answer 11

Vm returns back to RMP

Question 12

What is the definition of hyperpolarisation?

Answer 12

Vm becomes more negative than RMP

Question 13

Graded membrane potentials are a change in the membrane potential in response to stimulation, which may be depolarisation (excitatory) or hyperpolarisation (inhibitory):
• Degree of membrane polarisation differs depending on the ______________
• Response is not uniform (amplitude gets smaller the further away from stimulus) → movement of charge across membrane reduces (decremental spread)
• Occur at ________________ → contribute to initiating or preventing action potentials

Answer 13

stimulus strength;

synapses and sensory receptors

Question 14

Action potentials transmit information reliably and quickly over long distances:
• Controls the influx of Ca2+ by facilitating the _________________
• Not restricted to neurones (may occur in most excitable cells e.g. cardiovascular system, endocrine organs, smooth and skeletal muscles

Answer 14

opening of voltage-gated Ca2+ channels

Question 15

The action potential is generated due to changes in ion permeability:
1. Permeability depends on the conformational state of voltage-gated channels:
• Opened by __________
• Inactivated by __________
• Closed by membrane __________

2. Ions cross the membrane down their electrochemical gradient when permeability increases (changes the membrane potential towards the _______________)
3. Changes in membrane potential during action potential are not due to ion pumps

Answer 15

membrane;

depolarisation;

hyperpolarisation/repolarisation;

ionic equilibrium potential

Question 16

PHASES OF THE ACTION POTENTIAL
There are 5 main phases of the action potential generated in the axon:
1) Resting membrane potential: _________________ are closed (different from non-voltage gated channels maintaining the RMP) → RMP is closer to ______________

2) Depolarising stimulus: Stimulus depolarises the membrane potential (moves it positively towards the _____________ of -55mV)

3) Upstroke: Starts at the threshold potential with rapid opening of ___________ (rapid PNa increase) → Na+ enters cell down electrochemical gradient → rapid depolarisation:
• K+ leaves cell down electrochemical gradient (but less than Na+ influx) → membrane potential moves towards ______

4) Repolarisation: _______________ become inactivated (PNa drops rapidly) while more ___________ (PK increases further):
• K+ leaves cell down electrochemical gradient → membrane potential moves towards EK
• Absolute refractory period: _______________ cannot be triggered even with a strong stimulus (due to closing of the inactivation gate of Na+ channels) → start of repolarisation

5) Hyperpolarisation: Voltage-gated K+ channels are slow to close (remain open) → allows K+ to continue to leave the cell down the electrochemical gradient (membrane potential moves even more towards EK):
• As K+ channels close, the K+ efflux stops, and the original membrane potential is restored
• Relative refractory period: _______________ is required to trigger action potential (opening of the inactivation gate allows AP generation, but more negative potential requires larger stimulus to reach threshold)

Answer 16

Voltage-gated ion channels (Na+ and K+); EK as PK > PNa

threshold level;

voltage-gated Na+ channels; ENa

Voltage-gated Na+ channels; voltage-gated K+ channels open; new action potential;

stronger than normal stimulus

Question 17

REGENERATION OF ACTION POTENTIALS
Action potentials have an _____________ nature, so once the threshold potential is reached, a full-sized action potential is triggered:
• During the ____________, the neurone is unresponsive even to threshold depolarisation
• Once threshold is reached, the cycle continues via _________________
• Cycle continues until the ______________ (closed + voltage-insensitive) → membrane remains in the refractory state until channels recover from inactivation

During the action potential, Na+ enters the cell and K+ leaves the cell, but only a very small number of ions cross the membrane and change the membrane potential:
• Concentration change is extremely small (< 0.1%)
• Ion pumps are not directly involved in ion movements during action potential
• Electrochemical equilibrium is restored via _____________________________

Answer 17

all-or-none;

refractory state (absolute or relative);

positive feedback loop;

voltage-gated Na+ channels inactivate;

ions moving through non-voltage-gated ion channels with some ions exchanged through pumps (much slower; s vs ms)

Question 18

PROPAGATION OF ACTION POTENTIALS

Passive propagation of __________ potentials is when only resting K+ channels are open:
• Allows for K+ efflux across the membrane starting from the site of depolarisation
• Propagation distance and velocity are altered by the internal membrane resistance
• Results in __________________

Active propagation of action potentials occurs with the opening and closing of voltage-gated Na+ and K+ channels sequentially along the axon:
• Propagation of action potentials is a combination of active and passive processes → occurs unidirectionally due to ____________________________

Answer 18

subthreshold;

decremental spread ;

refractory state of voltage-gated Na+ channels in the previously depolarised region

Question 19

Saltatory conduction occurs in cells with _________________ (containing most of the voltage-gated ion channels) in between the highly myelinated sections of the axon:

• Action potentials are generated at the ________, followed by the passive propagation of charge along the membrane between nodes
• Highly myelinated sections of the axon improves the ____________ of the membrane → rapid movement of ions without need to generate new action potentials
o Sufficient depolarisation travels to the next node to reach threshold and trigger a second action potential
• Appears like the action potential is “jumping” from node to node

Answer 19

nodes of Ranvier;

node;

capacitance

Question 20

The movement of charge between nodes depends on _____________ (propagation/velocity of movement) and physical properties of the membrane (__________):
• Affected by axon diameter and myelination (120m/s velocity in large diameter, myelinated axons; 1m/s velocity in small diameter, non-myelinated axons)

• Axon diameter: Increases with axon diameter → less resistance to current flow
• Myelination: Higher in myelinated than non-myelinated → action potentials only need to be generated at the nodes of Ranvier
  • Conduction velocity is slowed by reduced axon diameter (e.g. re-growth after injury), reduced myelination (e.g. _____, _______), cold, anoxia, compression, some drugs (e.g. some anaesthetics)

Answer 20

internal resistance;

myelination;

multiple sclerosis, diphtheria