Action potentials Flashcards
What had already been established in the early 20thc
Neurons respond to stimulation with ‘all-or-none’ spikes of electrical activity which self-propagate along axons
What did Bernstein propose as the cause of action potentials
Bernstein (1902)- due to transient loss of selective membrane permeability, which would lead to membrane potential to shoot to 0mV
Whydid the mechanisms underlying action potential generation remain obscure for a long time
It had not been possible to directly record the transmembrane potential in axons as they are so thin- electrophysiological studies on axons had relied on extracellular recordings
Who got a Nobel prize for the functions of neurons
1932- Sherrington and Adrian
Who got a Nobel prize for the function of single nerve fibres
1944- Erlanger and Gasser
Who got a Nobel prize for the mechanisms of axonal excitability
1963- Hodgkin and Huxley
Who got a Nobel prize for the study of single ion channels
1991- Neher and Sakmann
How were the first intracelllular recordings of the action potential carried out
Hodgkin and Huxley (1945)-used the giant axon of a squid as it is big enough to enable the insertion of microelectrodes into the axoplasm
What did the first intracellular recordings of action potential reveal
Hodgkin and Huxley (1945)- the membrane potential reversed during action potentials (overshot 0mV) then became transiently hyperpolarised below resting membrane potential following the action potential (after-hyperpolarisation)
What did the first intracellular recordings of action potential reveal about their cause
Hodgkin and Huxley (1945)- Could not be explanied by loss of selective membrane permeability, but rather changes in the permeability to specific ions
What is the sodium hypothesis
Suggested the upstroke of action potential could be explained by increased Na+ permeability- this could cause Na+ to diffuse into the cell, dragging the membrane potential towards the positive Nernst potential for Na+ ions (around 55mV)
Study investigating the sodium hypothesis
Nastuk and Hodgkin (1950)- plotting action potential peak against the log of extracellular Na+ conc reveals a straight line with a slope predicted by Nernst for Na+, though the membrane potential never quite reaches E Na due to leak channels
What increases the membrane permeability to Na+ in the action potential
Voltage-gated Na+ ion channels- if the membrane is depolarised the channels open and Na+ permeability increases, which increases membrane depolarisation, which increases Na+ permeability in a positive feedback loop
What 3 membrane conductances can be multiple phases of the action potential be explained by
Leak conductance, voltage-dependent Na+ conductance, voltage-dependent K+ conductance that all try to clamp the membrane potential at their Nernst potential
Which of the 3 membrae conductances determines the resting membrae potential
The leak conductance
Which of the 3 membrane conductances is recruited whe the membrane is depolarised
Voltage-dependent Na+ conductance- the membrane potential shoots towards E Na as the Na+ conductance increases in a positive feedback loop, dominating leak conductance
What happens to the Na+ conductance when the peak of the action potential is reached
Na+ conductance inactivates, and there is a delayed recruitement of voltage-dependent K+ conductance that drags the membrane potetial towards E K and rapidly repolarises the membrane
Which of the 3 membrane conductances causes the undershoot/after-hyperpolarisation
The K+ conductance takes time to turn off after repolarisation, causing the undershoot as the membrane is more permeable to K+ ions than at rest
When the K+ conductance turns off, the membrane potential is again set by leak conductance
What is it necessary to do in order to determine the properties of the ionic conductances underlying the action potential by manipulating membrane potential
Membrane potential affects membrane permeability which in turn affects membrane potential, so it is necessary to break this feedback loop in experiments
How can the membrane potential/permeability feedback loop be broken
Axial voltage-clamp- a wire is inserted along the length of an axon and connected to an electronic feedback circuit to clamp the axon’s membrane potential and prevent further depolarisation, while recording the current required to achieve this voltage clamp (Hodgkin and Huxley, 1952)
What does the axial voltage clamp allow the measurement of
Measurement of membrane currents as a function of time at any given voltage (Hodgkin and Huxley, 1952)
Hodgkin and Huxley (1952)- what was the effect of stepping the membrane potential to a depolarised level (0mV)
Reveals a transient inward current of Na+, followed by a sustained outward current of K+
Hodgkin and Huxley (1952)- how could the current for each ion be independently measured WITHOUT using pharmacological blockers
Removing Na+ from the extracellular fluid eliminates the inward current, leaving the delayed and sustained outward current mediated by the voltage-dependent K+ conductance.. subtraction can calculate the current mediated by the Na+ conductance
Hodgkin and Huxley (1952)- how could the current for each ion be independently measured using pharmacological blockers
Tetrodotoxin (TTX) blocks the Na+ conductance, while tetraethylammonium ions (TEA+) block the delayed K+ conductance
Hodgkin and Huxley (1952)- what is the effect of increasingly large voltage steps on K+ conductance
K+ current is larger and faster to activate with increasingly large depolarisation steps due to an increase in K+ conductance and an increase in driving force- once fully activated, the K+ conductance is sustained throughout the voltage step
Hodgkin and Huxley (1952)- how can conductance be calculated from the current over time
Ohm’s law can be used to calculate conductance
Hodgkin and Huxley (1952)- what is the effect in Na+ conductance of a a given voltage step compared to the effect on K+ conductance
Na+ conductance activates more quickly than K+ conductance, and the rate of activatino increases with depolarisation
Hodgkin and Huxley (1952)- what is the size of the effect of voltage increase on Na+ conductance
Na+ conductance increases monotonically as a function of voltage
Hodgkin and Huxley (1952)- what happens to Na+ current as the membrane potential approaches the Nernst potential for Na+
Na+ current decreases due to the decrease in driving force
Hodgkin and Huxley (1952)- what is the effect on Na+ of voltage steps above Na+’s Nernst potential
Na+ current reverses, and Na+ ions are driven out of the cell (situation would not happen during an action potential))
Hodgkin and Huxley (1952)- what happens to Na+ conductance followig activation
It shows rapid inactivation, even if the membrane remains depolarised
The rate of inactivation increases with depolarisation
What is subthreshold Na+ conductance
Weak inputs may be sufficient to active voltage-gated Na+ conductances, without evoking an action potential
How can weak inputs activate voltage-gated Na+ conductances without evoking an action potential
The increases in the K+ leak current (due to increased driving force from depolarisation) and voltage-gated K+ current can counterbalance the depolarising Na+ current, preventing entry into the positive feedback loop
What does action potential threshold reflect
A point at which the outward currents can no longer counterbalance the inward currents- not an intrinsic property of the Na+ conductance, but dynamic
What happens if voltage-gated K+ conductance is blocked during an action potential
The membrane can still repolarise, as the Na+ conductance deactivates, just more slowly
What is the role of the voltage-gated K+ conductance
Speeding up repolarisation enabling the axon to fire again with a shorter delay, maximal firing rate
Why does the refractory period occur
It takes time for the Na+ conductance to de-inactivate, and for the voltage-gated K+ conductance to de-activate- in this time, the membrane is hypoexcitable
What is the absolute refractory period
The time where it is impossible to evoke another action potential no matter how large the stimulus, because the fraction of Na+ conductance available for activation is not sufficient to overcome the leak/K+ conductance
What is the relative refractory period
The time when a higher threshold is required for another action potential to be fired, as only a limited proportion of the Na+ conductance has recovered from inactivation and voltage-gated K+ conductance has not been fully deactivated
What technique allows us to record the activity of single ion channels
Patch-clamp techniques
What is the behaviour of single voltage-dependent ion channels following the onset of a voltage step
The current flowing through single channels flips ‘on’ and ‘off’ in a stochastic manner, with the currents looking different from trial to trial- it does not show a smooth increase
What do the macroscopic currents recorded by Hodgkin and Huxley represent
The statistical behaviour of a large no of channels, which shows stereotypical patterns from trial to trial
How do neurons encode information
In the rate and/or timing of action potentials
Why is fast conductance important for behaviour
For the CNS to function, signals must be conducted from the periphery, processed, and transmitted back to effector organs over behaviourally relevant timescles (eg sufficient to catch a prey, remove a hand from a hot plate before tissue damage)
What range does the speed of action potential conduction vary over
<1ms/s to over 100m/s
What physical properties of axonal fibres affect their conductance velocity
Diameter and myelination
Study showing that conduction velocity is not a hard-wired property of an axon
There is continued remodelling of myelination throughout life and learning can induce white matter plasticity (Scholz et al, 2009)
Why will changes in conductance velocity alter the synchronisation across multiple inputs
The inputs arriving onto neurons have to travel over a variety of distances so will experience different conduction delays
Changes in conduction velocity will thus alter synchronisation and modulate how info is processed
What may axonal plasticity be an importatnt process during
Brain development and learning
How dos membrane capacitance affect the response of the membrane
It slows it down, as it takes time to charge up the membrane to its new value
What is the speed of response of the membrance characterised by
Time constant T- the amount of time it takes for the change in potential to reach 63% of its final value
What is the effect of capacitance and membrane resistance on the time constant
The lower the capacitance and membrane resistance, the shorter the time constant aka the faster the membrane responds to a stimulus
What is the equation for the time constant
Tau = 𝒓𝒎*𝑪𝒎
rm is membrane resistance, Cm is membrane capacitance
What happens to the speed of response as the signal propagates along the cable
The membrane is leaky, so as the current gradually leaks across the membrane the response decays exponentially
What is the length constant
The distance along a cable over which the steady-state signal decays to 37% of its original value
What is the equation for length constant
(𝝀 =+√(rm/ri ))
rm and ri are membrane and internal resistances
What is the effect of membrane and and internal resistance on length constant
The higher the membrane resistace and the lower the internal resistance, the longer the signal can travel without decaying as much (higher length constant)
What happens to the current flowing across the membrane as the membrane charges up
It will decrease, thus the current flowing down the axon will increase correspondingly
What is λ for cortical axons
A few hundred microns
How does the action potential curve map onto an action potential moving along an axon length
The sharp peak is at the point at which the action potential is being actively generated, with a leading edge of subthreshold depolarisation and a wake of refractory axon showing Na+ channel inactivation/after-hyperpolarisation
What does the refractory wake of the action potential curve ensure
Ensures that the action potential only propagates in one direction
What does conduction velocity relate to in terms of the axon
How fast the active site of generation moves along the axon, aka how quickly the next segment of axon can be depolarised to threshold
How does conduction velocity relate to length constant
Conduction velocity is proportionate to length constant
How does conduction velocity relate to time constant
Conduction velocity is proportional to 1/tau
Why do large diameter fibres have faster conductio velocities
Less internal resistance due to greater space for current to travel among organelles, decreased capacitance
How does increasing axon diameter affect membrane capacitance
Increased membrane diamater increases membrane area which increases linearly with membrane capacitance as there is more capacitive membrane to charge up
How does increasing diameter affect membrane resistance
Increased diameter means higher membrane area which means more channels, so decreased membrane resistance
How does increasing diameter affect internal resistance
The square of the diameter is proportional to decreased internal resistance
Formula showing the relationship between length constant and d
λ is proportional to√𝑑
Why is time constant independent of diameter
Increased diameter increases membrane capacitance (increases time constant) but decreases membrane resistance (decreases time constant), so the 2 cancel each other out
Considering the effect of diameter on time and length constant, how does diameter affect conduction velocity
Conduction velocity increases according to √𝑑, so an 100 fold increase in axon diameter will yield a 10 fold increase in conduction velocity
How does myelination help enable high conduction velocities in relatively small diameter fibres
Oligodendrocytes/Schwann cells provide insulating sheaths around axons to enhance the passive propagation of electrical signals
How does myelination affect membrane resistace
Membrane resistance increases in proportion to thickness of the myelin sheath, meaning less leakage of current of current across the membrane and faster passive propagation
How does myelination affect membrane capacitance
Membrane capacitance decreases in proportion to thickness of the myelin sheath as increased space between the 2 conducting solutions either side of the membrane decreases the capacitance
How does myelination affect the length and time constant
The length constant is increase proportional to √m, while tau is unaffected
Why are the Nodes of Ranvier necessary
Myelin is not a perfect insulator, so as the signal propagates passively along the axon it decays- the Nodes of Ranvier are necessary to regenerate the action potential
What are the Nodes of Ranvier
Regular interruptions of the myelin sheath, contain high densities of voltage-gated Na+ channels to regenerate the action potential
What term describes the way the action potential jumps from node to ndoe
Salutatory conduction
How does the amount of nodal membrane (length and membrane) affect rate of conduction
The greater the amount of nodal membrane (length and number) the slower conduction will be, as propagation in the ndoel region operates the same way as in unmyelinated axons
What enables longer internodal regions (and fewer nodes) to increase salutaotry conduction speed
Higher velocity of passive conductance aka increased length constant
What effect will increased total axon diameter of a myelinated axon have on myelin thickness
Increased axon AND myelin sheath thickness (as there is a fixed ratio between axon diameter and that of the total nerve)
What is the effect of increased diameter increasing both axon and myelin thickness
Conductino velocity increases approximately linearly with total nerve diameter, as increased axon diameter and increased myelination both boost velocity
What would be the effect of stripping away the segments of myelin sheath
Would leave a bare section of axon with low expression of voltage-gated Na+ channels- conduction can still occur even if 2-3 myelin segments are removed, but increased conduction delay eventually causes current to be shunted by the exposed membrane, causing conduction failure
What is the difference in channels between demyelinated axons and unmyelinated axons
Unmyelinated axons have high densities of voltage-gated Na+ channels along their length (are meant to be unmyelinated)
Examples of unmyelinated fibres
Slow conduction fibres, C fibres in periphery, local projections within grey matter
What are unmyelinated fibres more efficient for and why
Myelination costs space and energy, so unmyelinated fibres are more efficient for signals that don’t need to be propagated quickly
What diamater of unmyelinated axons are likely to conduct faster than myelinated fibres
Total diameters below 1um, as myelin takes up space
What is an action potential
An all or nothing signal that conveys information by its rate and duration of firing over distances in the nervous system
What does it mean that action potentials are all or nothing
They can only be generated if the cell becomes depolarised enough to reach the threshold, and always follow the same shape/duration/reaches the same peak voltage’sa
What is an osscilloscope
A special type of voltmeter that can be used to study action potentials by recording the voltage across the membrane as it changes over time
What identifiable parts does each action potential have
Threshold, rising phase where membrane is depolarised, overshoot, falling phase where rapidly repolarisation occurs, then the undershoot/after-hyperpolarisation, then a gradual restoration of the resting potential
How long do action potentials last
Around 2msec
How is the action potential triggered
Na+ voltage-gated sodium channels open at the axon hillock in response to a stimulus eg membrane stretching or the binding of neurotransmitters
How do we generate multiple successive action potentials
Passing continuous current into a neuron with a microeelctrode
What does the rate of action potential generation depend on (current)
The magnitude of the continuous depolarising current- if threshold is just reached the cell may generate 1 action potential per second (1Hz), but if current is increased the rate may increase to 50Hz
How can stimulation intensity be encoding by action potential firing
A higher depolarising current means a higher rate of action potential firing
What is the maximum firing frequency for a neuron and why
About 1000Hz, as the absolute refractory period is about 1msec
How are the voltage-gated Na+ channels selective for Na+
They have pore loops assembeld into a selectivity filter, making the channel 12x more permeable to Na+ than K+
What mechanism underlies the voltage-gating of the Na+ channel
A voltage sensor in segment s4 of the molecule that contains +ve amino acid residues regularly spaced along the helix coils, meaning the entire S4 coil can be twisted by a change in membrane potential, opening the gate
What is the threshold for neurons
-40mV
How long do the Na+ channels stay open for following threshold
Stay open for 1msec then inactivate when the membrane is depolarised
Disorder demonstrating the importance of the Na+ channels closing quickly
Single amino acid mutations in the extracellular region of one sodium channel cause generalised epilepsy with febrile seizures which involves explosive electrical activity in the brain- the mutation slows the inactivation of Na+ channels, prolonging the action potential
What is a channelopathy
A human genetic disease caused by alterations in the structure and function of ion channels
Effect of TTX on voltage-gated sodium chael
Tetrodotoxin is a pufferfish toxin that is fatal if ingested- binds tightly to a site outside the channel blocking the pore and blocking all sodium-dependent action potentials
Effect of frog toxin on voltage-gated sodium channel
Batrachotoxin causes sodium channels to open at more negative potentials and stay open much longer than usual, scrambling the info encoded by the action potentials
What do voltage-gated potassium channels oepn in response to
Depolarisation of the membrane- there is a 1msec delay in them opening
Why did Hodgkin and Huxley call the voltage-gated K+ conductance the delayed rectifier
There is a 1msec delay, and the K+ conductance out the cell due to the electrical driving force resets the membrane potential
What voltage does the membrane potential rise to in an action potential
Around 40mV, around E Na
How is the action potential actively propagated along the axon
The influx of positive charge at one region of the axon spreads inside the axon to depolarise the adjacent segment of membrane and continue the propogation
What is orthodromic conduction
Action potential conduction from soma to axon terminal
What is antidromic conduction
Action potential propagating backwards, elicited experimentally
What is the typical rate of action potential velocity
10m/sec
How does Lidocaine (local anaesthetic work)
Prevents action potentials by binding to the voltage-gated sodium channels, preventing the flow of Na+ that usually depolarises the channel
Why are smaller axons affected by local anaesthetics before larger axons
In smaller axons, more of the voltage-gated sodium are required to work to ensure the action potential doesn’t fizzle out as it conducts down the axons
Demyelinatnig disease thats not MS
Guillain-Barre syndrome- attacks the myelin of peripheral nerves that innervate muscle and skin, causing slowed/failed action potential firing in the axons innervatnig the muscles, causing profoundly slowed response times
What is the axon hillock often called
The spike-initiation zone
How does the spike initiation zone in sensory neurons differ fro typical neurons in the brain/spinal cord
In most sensory neurons, the spike initiation zone occurs near the sensory nerve endings, where depolarisation caused by sensory stimulation leads to the geenration of action potentials that propagate along the sensory nerves
How does synaptic input cause generation of action potentials in typical neurons in the brain/spinal cord
Depolarisation of the dendrites and some caused by synaptic input leads to the generation of action potentials if the membrane of the axon hillock is depolarised beyond threshold
How can the gating of voltage-sensitive channels be influenced by intracellular ion concentration eg Ca2+
In some cels, increased intracellular Ca2+ conc increases the probability that calcium-sensitive K+ channels open, but can also inactivate Ca2+ channels, so can have opposing effects
How can excitability propertirs vary among differenr neurons
Some cellsshow a constant firing frequency in response to constant excitatory input, while some show decelerating/accelerating trains
Some neuron firing rates increase with small changes in input, while some require large input changes
How does the spatial distribution of channel types in different regions of neurons vary with a direct bearing on function
eg dendrites/cell body/axon hillock/nerver terminal contain a greater variety of channels than the axon, because the input and output zones actively transform the signals they receive, while the axon is a simple relay line between these zones
