Lecture 9 - Action Potentials Flashcards
What is an action potential?
The rapid depolarisation followed by repolarisation in a characteristic pattern
What does an action potential depend on to cause the changes in potential difference?
Ionic gradients + selective permeability of the membrane to ions
What needs to be reached in order for an action potential to be fired?
Threshold potential
Why is an action potential described as being all or nothing?
If threshold potential is not reached then NO action potential
If threshold potential is reached there will always be an action potential that reaches the set membrane potential
What is meant by action potentials are propagated without loss of amplitude?
They do not lose their height/maximum point of depolarisation
Height of the depolarisation stays the same
What are the 5 properties of an action potential?
Rapid change in voltage across a membrane
Depends on ionic gradients and the selective permeability of the membrane to ions
Only happens if threshold level reached
All or nothing
Propagated without loss of amplitude
How does the length of action potentials in the heart/SAN compare to axons and skeletal muscle?
Skeletal muscle = short (0.5ms)
SAN/Cardiac ventricle = LONGER (100ms)
How does membrane selectivity (conductance) for an ion affect the membrane potential for the cell?
Higher the membrane selectivity/conductance for that ion the closer the membrane potential gets to the Equilibrium potential for that ion (Eion)
What effects membrane selectivity/conductance to an ion?
Number of the ion channels open to that ion
What causes the rapid upstroke in an action potential?
Permeability/conduction to what ion increases?
Which direction do these ions move and why?
What ion channel is affected?
Na+
Na+ influx down its electrochemical gradient
Voltage gated sodium channels (VGSC)
How can the upstroke of action potential be referred to as an example of positive feedback?
Once threshold potential is reached from the influx of Na+ ions, more and more Na+ voltage gated ion channels (VGSC) open allowing influx of more and more Na+
When do the voltage gated sodium ion channels inactivate?
The peak of the depolarisation
When do the Potassium voltage gated ion channels open in an action potential?
Following the inactivation of the Na+ voltage gated ion channels at the peak of depolarisation
In the action potential why is the Na+/K+ ATPase NOT involved in the repolarisation of the action potential?
It is working all the time trying to maintain the gradient of Na+ and K+
What are the 2 stages to recovery after an action potential?
ARP = Absolute refractory period
RRP = Relative refractory period
What is important about the Absolute Refractory Period and why does this happen?
Another action potential can not be propagated in this period
All/most Na+ Voltage gated ion channels are INACTIVATED
What is the Relative Refractory period (RRP)?
Starts hen VGSC no longer inactivated
VGSC recovering from inactivation
Number of VGSC in inactivated state decreases as they become closed
What is different about the Absolute refractory period compared to the relative refractory period?
ARP an action potential can never be propagated
RRP it is possible for another AP its just difficult
What are the 3 states a voltage gated sodium ion channel can be in?
Closed
Open
Inactivated
What is the difference of a VGSC being closed or inactivated?
When closed, depolarisation causes it to open
When inactivated, no amount of depolarisation will cause it to open
What changes a VGSC from inactivated to closed?
Hyperpolarisation
How many subunits make up a voltage gated Na+ channel?
4
What is the significance of charged amino acids being present on the repeats I and IV of a VGSC?
Found in the membrane space
They react to changes in membrane potential causing the conformational change opening or closing the channel
What is the inactivation particle of a VGSC?
Structure that blocks the pore of the VGSC inactivating it
What 2 repeat units of the VGSC does the inactivation particle connect between?
III and IV
How many alpha subunits make up a VGSC?
1
How does the size of a Voltage Gated Potassium channel relate to a VGSC?
VGPC SMALLER than VGSC
How does the size of a Voltage Gated Potassium channel relate to a VGSC?
VGPC SMALLER than VGSC
What is a significant difference in the structure of a voltage gated potassium channel compared to a VGSC?
VGSC have an INACTIVATING PARTICLE so can be INACTIVATED
VGPC do NOT have an inactivating particle
What is the significance of the VGPC not having an inactivating particle?
They slowly close which is what causes the excess efflux/trickling of K+ out of the cell causing HYPERPOLARISATION
How many alpha subunits make up a VGPC?
4
What is an anaesthetic?
Drug which causes numbness/inability to feel pain (analgesia)
What is Lidocaine?
Anaesthetic
What are they 2 forms of local anaesthetics?
Protonated
Unprotonated
Which form of local anaesthetic is membrane soluble?
Unprotonated/uncharged
What channels do local anaesthetics block?
Na+ channels
Which form of the local anaesthetic enters the membrane and then which form causes the block affect?
Uncharged/Unprotonated enters the membrane
Charged/protonated form causes the block
What are the 2 blocking pathways of the Na+ that local anaesthetics act by?
Hydrophilic pathway
Hydrophobic pathway
What is the hydrophilic pathway of local anaesthetic block?
Blocks open channel/pore
(Prevents Na+ moving in)
What is the hydrophobic pathway of anaesthetic block?
Blocks pore of inactivated Na+ channel
(Prevents reactivation)
Why is important that VGSC get inactivated in the Absolute Refractory Period?
Prevents excess influx of Na+ preventing cell becoming to depolarised and therefore slowing down action potentials due to repolarisation being slower
Ensures unidirectional propagation of action potentials
How does diameter of an axon relate to conduction velocity?
Larger diameter = Faster Conduction velocity
What is local current theory?
If a current/action potential is injected into an axon, the resulting charge/depolarisation will spread/diffuse along the axon causing an immediate local change
What is Length constant (λ)?
The distance taken for the membrane potential to fall to its original value
How does length constant relate to speed of action potential conductance?
LARGER length constant = faster conductance velocity
What is Capacitance?
The ability to store charge
Its a property of the lipid bilayer (doesn’t change)
What is the effect of a high capacitance?
Voltage changes more slowly in response to current injection
What does membrane resistance depend on?
The number of ion channels open
What does the spread of a local current depend on?
Membrane resistance
Capacitance
How does a lower resistance effect length constant?
Longer/larger length constant
What happens when local depolarisation/local current injected into an axon spreads down the axon?
If depolarisation reaches threshold, AP propagated
What is important about the refractory period?
Ensures unilateral propagation of action potentials
What cells myelinate axons of the PNS?
Schwann cells
What cells myelinate axons of the CNS?
Oligodendrocyte
What are the regions along a myelinated axon called that have no myelination called?
Nodes of Ranvier
What is resistance and capacitance like under the myelinated region of a myelinated neurone compared to the nodes of Ranvier?
Higher Resistance
Decreased capacitance
How does myelination increase conduction velocity?
Inc membrane resistance
Dec membrane capacitance
Both cause INC in length constant
What is saltatory conduction?
Reduced capacitance under myelin sheath means the local current spreads further down the axon to delpolarise the next node without firing an AP under the myelin sheath area
The way the action potential quickly jumps from node to node in a myelinated axon
Length constant is increase so local circuit current depolarise the next node above threshold
What is Multiple sclerosis?
Myelin sheath of all CNS nerves attacked
What is Devic’s disease?
Optic and spinal cord nerves demyelinated
What nerves are demyelinated in Landry-Guillain-Barre syndrome and Charcot-Marie-Tooth disease?
PNS nerves
Why does demyelination of axons cause problems?
Local current May not be able to spread enough to cause depolarisation to reach threshold at the next node of Ranvier
