Action Potentials

Question 1

What are action potentials?

Answer 1

Neurons respond to stimulation with all or nothing spikes of electrical activity which self-propogate along axons.

Question 2

what are the characteristic features of an action potential?

Answer 2

Rising phase - rapid depolarisation of the membrane (to about 40mV)
Overshoots 0mV so there is a period where the inside of the neuron is positively charged with respect to the outside.
Falling phase - rapid hyperpolarisation
undershoot (after-hyperpolarisation)- more negative than the resting potential
absolute refractory period
relative refractory period
must be due to changes in the selective permeability to specific ions

Question 3

what is the sodium hypothesis?

Answer 3

the upstroke of the action potential can be explained by an increase in Na+ permeability
[Na+] out > [Na+]in
increasing permeability drags the membrane potential towards the positive Nernst potential for Na+ ions
positive feedback loop: depolarisation increases Na+ permeability which increases depolarisation etc
plotting action potential peak against log[Na+]out reveals a straight line slope predicted by Nernst equation for Na+ (although never reaches this)

Question 4

What are the three membrane conductances that explain the phases of the action potential?

Answer 4

leak conductance
voltage-dependent Na+ conductance
voltage dependent K+ conductance
each try to clamp the membrane potential to their Nernst potential
actual membrane potential is determines by the relative membrane permeabilities to different ions (approaching GHK predictions)

Question 5

Breakdown of action potential

Answer 5

resting potential: determined by leak conductance (ELeak)
stimulation –> depolarisation of the axon - recruits voltage-dependent Na+ conductance
Na+ conductance increases in a positive feedback loop (towards Ena - dominating leak conductance)
Never reaches Ena due to leak conductance
Na+ conductance inactivates
Delayed recruitment of voltage-dependent K+ conductance - rapid repolarisation (towards Ek).
K+ conductance takes time to turn off - undershoot (membrane more permeable to K+ than at rest)
K+ conductance turns off - membrane potential is set by leak conductance again

Question 6

How can the conductances be separated?

Answer 6

axial voltage-clamp (wire inserted along length of axon connected to an electronic feedback circuit - clamp current and record the current required to achieve this)

Pharmacological blockers (tetrodotoxin TTX blocks Na+. tetraethlyammonium ions (TEA+) block K+)

Question 7

what makes K+ current larger and faster to activate?

Answer 7

increasingly large depolarisation

increasing current amplitude due to increase in K+ conductance (gk) and increase in driving force (Vm-Ek)

Question 8

what affects Na+ current?

Answer 8

activates more quickly than K+
rate of activation increases with depolarisation
Na+ conductance increases monotonically as a function of voltage
as membrane potential approaches the Nernst potential for Na+, Na+ current decreases due to decrease in driving force (Vm-Ena)
when voltage steps above Nernst potential for Na+, the current reverses and Na+ ions are driven out of the cell - wouldn’t occur during action potential
following activation Na+ conductance shows rapid inactivation even if the membrane remains depolarised there is no sustained Na+ current
rate of inactivation increases with depolarisation

Question 9

What is the action potential threshold?

Answer 9

not an intrinsic property of the voltage-gate Na+ conductance
point at which the outward currents can no longer counterbalance the inward currents
dynamic
when subthreshold leak increased driving force or depolarisation increases leak conductance so voltage-gated K+ current counterbalances depolarising Na+ current
prevents entry to positive feedback loop

Question 10

What is repolarisation?

Answer 10

if voltage-gated K+ conductance is blocked, the membrane can still generate an action potential and repolarise
Na+ conductance activates and then inactivates
voltage-gated K+ conductance speeds up repolarisation and enables the axon to fire an action potential again with a shorter delay
increases maximum firing rate

Question 11

what is the refractory period?

Answer 11

takes time for the Na+ conductance to de-inactivate and for the voltage-gated K+ conductance to deactivate
membrane is hypoexcitable

absolute refractory period - impossible to evoke another action potential as there is insufficient Na+ to overcome leak/K+ conductance

relative refractory period - sufficient proportion of Na+ conductance recovered so can fire another action potential however threshold is higher than at rest due to incomplete de-inactivation of the Na+ conductance and incomplete de-activation of voltage-gated K+ conductance

ensures that action potentials only propagates in one direction

Question 12

What does the behaviour of single voltage-dependent ion channels tell us?

Answer 12

current flowing through a single channel flips on and off; it’s stochastic
currents look different trial to trial
statistical behaviour of a large number of channels is what shows the stereotypical behaviour of action potentials

Question 13

what does biophysics tell us about conductances?

Answer 13

operate according to Ohm’s law
INa = gNa(Vm – ENa)
INa = gK(Vm – EK)
INa = gL(Vm – EL)

Question 14

how many voltage-dependent gates are needed?

Answer 14

model that’s good fit to data
4 voltage-dependent gates controlling each conductance
Na+: 3 activation gates (m) and 1 inactivation gate (h)
K+: 4 activation gates
describe the probability that a gate would allow current to flow
predicts tetrameric structure of voltage-gated K+ channels mediating action potential repolarisation and the 4 repeat domains of voltage-gated Na+ channels

Question 15

how is information encoded in action potentials?

Answer 15

in rate and timing of action potentials

Question 16

Why is conductance speed important?

Answer 16

type of signal being conveyed
range in velocities <1ms to >100ms
changes in myelination and structure of white matter tracts
continued remodelling throughout life
learning can induce white matter plasticity
distance travelled results in different conduction delays
change in conduction velocity alters synchronisation across multiple inputs, modulating how information is integrated
MS - immune system attacks myelin sheath

Question 17

How does current flow through axons?

Answer 17

across the local membrane
along the intracellular fluid

wider axons with fewer channels - travel inside
smaller axon with lots of channels - leak through membrane

Question 18

what is the time constant??

Answer 18

t = Rm . Cm
time constant = membrane resistance x membrane capacitance
membrane response slows down

Question 19

what is the length constant?

Answer 19

λ= √(Rm/Ri)
how response decays along the membrane
membrane is leaky
exponentially as current leaks across membrane
length constant λ is the distance along a cable over which the steady state of signal decays to o 1/e (37%)
ri = internal resistance

Question 20

what happens to current flowing along the axon?

Answer 20

Over increasing distances from the site of current injection, action potential response gets smaller and slower.
passive propagation is not sufficient for reliable transmission of action potentials through the nervous system

Question 21

what is conduction velocity dependent on

Answer 21

conduction velocity is proportional to λ

conduction velocity is proportional to 1/τ

thus larger diameter fibres have faster conduction velocities than small diameter fibres (proportional relationship)

Also:
How far ahead the depolarisation of the action potential spreads
which path - inside axon or across the axonal membrane
Further down current goes, the further ahead of the action potential the membrane will be depolarised and the faster the action potential will propagate

Question 22

Explain what affects Cm, Rm and Ri

Answer 22

Membrane capacitance (Cm) increases linearly with increasing membrane area (more capacitive membrane)
membrane resistance (rm) decreases linearly with membrane area (more channels) 
axial resistance (ri) is proportional to the volume of axoplasm

Membrane area = π d(iameter) l(ength of axon)
Cross-sectional area = π(d/2)^2l

rm decreases in proportion to the increase in diameter
ri decreases in proportion to the square of the diameter
Cm increases in proportion to the increase in diameter

λ is proportional to √d
conduction will increase according to √d so a 100 fold increase in axon diameter will incrase conduction by 10 fold

τ is independent of d

Question 23

what effect does myelination have on rm, ri and Cm

Answer 23

rm increases in proportion to the thickness of the myelin sheath (m)
ri. remains the same
Cm decreases in proportion to the thickness of the myelin sheath

this changes the length constant but leaves τ unaffected
λ is proportional to √m

Therefore, a 100 myelin membranes surrounding the axon would increase rm by 100
fold, decrease Cm by 100 fold, and increase conduction velocity by a factor of 10.

conduction velocity increases proportionally to √m . √d
in myelinated axons conduction velocity increases linearly with total diameter

Question 24

What is the significance of the Nodes of Ranvier?

Answer 24

myelin is not a perfect insulator so the signal still decays during propagation
necessary to repeatedly interrupt the myelin sheath at the Nodes of Ranvier
contains high densities of voltage-gated Na+ channels and regenerate the action potential
saltatory conduction

Question 25

What affects the velocity of saltatory conduction

Answer 25

greater the amount of nodal membrane (length and number) the slower conduction will be
decrease in Cm of the internodal region is sufficient to increase the veolcity of passive conduction
increase in rm and corresponding increase in λ enable longer internodal regions and fewer nodes
increases the velocity of saltatory conduction

Question 26

What happens to Na+ in the generation of an action potential?

Answer 26

gated-sodium channel
e.g in the skin stretch gated
Stimulus → nerve fibres stretched → Na+ channels open and Na+ enters down electrochemical gradient causing depolarisation of the membrane (generator potential), if above threshold → action potential

Question 27

What happens if continuous depolarising current is passed into a neuron?

Answer 27

many action potentials will be generated in succession.
The rate of action potential generation depends on the magnitude of the continuous depolarising current
The firing frequency of action potentials reflects the magnitude of the depolarising current - encoding stimulation intensity.
Limit to max rate of action potential generation due to absolute refractory period (impossible to generate ap) and relative refractory period (elevated threshold).

Question 28

what is the voltage-gated sodium channel?

Answer 28

Single polypeptide with 4 distinct domains each consisting of 6 transmembrane alpha helices
Clump together to form a pore
Pore closes at negative resting potential
Pore opens at threshold depolarisation
Pore loops assembled into selectivity filter making it 12x more permeable to Na+ than K+
Na+ are stripped of most but not all water molecules the remaining shell is necessary for the ion to pass through the selectivity filter
Ion-water complex is selected for
Voltage sensors found in S4 of the molecule - positively charged amino acid residues are regularly spaced along the coils of the helix
The entire segment can be forced to move by changing the membrane potential
Depolarisation twists S4 causing the gate to open

Question 29

What are the key functional properties of the Na channel?

Answer 29

Open with little delay/ rapidly - explains why rising phase occurs so quickly
Stay open for 1msec then close - explains why action potentials are so brief
Cannot be opened again by depolarisation until the mechanism returns to a negative value near threshold - explains the absolute refractory period
Does not open until a critical level of membrane depolarisation is reached explains threshold

Question 30

What is the voltage-gated potassium channel?

Answer 30

Open in response to depolarisation after about 1msec - delay rectifier
Diminish any further depolarisation by allowing K+ to leave membrane
4 separate polypeptide subunits which form a pore
Sensitive to changes in electrical field across the membrane - twist in shape when membrane is depolarised

Question 31

what effects axonal excitability

Answer 31

Axonal size
number of voltage-gated channels

smaller axons require greater depolarisation and are more likely to be blocked