Electrical Excitabilty (the AP And Properties) Flashcards
What are features of an Action Potential?
A change in the voltage across membrane Depend on ionic gradients and relative permeability of membrane Only occur when threshold is reached All or nothing Propagated without loss of amplitude
How does depolarisation imitate an action potential?
Single Action potential at each synapse releases nertotransmittter which binds to the axon hillock and acts to depolarise it.
AP’s from separate synapses all release neurotransmitter so has summarise effect that allows threshold to be reached.
This hen opens voltage gated ion channels in axon.
What happens if conductance (membrane permeability) to an io is increased?
The membrane potential will move closer t the equilibrium potential for that ion.
What is membrane permeability to an ion dependant on?
The number of channels for that ion that are open
How do we know which direction an ion will travel in?
It will travel dependant on the direction and size of its electrochemical gradient.
Na+ moves in (to depolarise)
K+ moves it (to hyperpolarise)
How can it be shown experimentally that Na+ is responsible for AP depolarisation?
As external (Na+) was reduced the E(Na+) reduced in a straight line.
The peak in action potential changed in a manner parallel to the E(Na+) change.
Supports idea that upstroke in action potential is due to a large increase in permeability to Na+ ions
How much ion movement is needed to fire an AP?
A very small amount, a few mM.
Even lower in wider axons
What is not involved in the repolarization of the AP?
The sodium potassium pump
Where is there an example of positive feedback in an AP?
The Na influx (it causes the membrane to depolarise which causes more Na channels o open and more Na to influx)
It’s switched of when the k channels often and the Na channels close
How does the membrane repolarised after an action potential?
Na influx stops and channels are inactivated
K+ efflux begins
What is an absolute refractory period?
When nearly all Na+ channels are in the inactivated state.
Relative excitability is 0
What is a relative refractory period?
Na+ channels are recovering from inactivation, xcitabilty returns towards normal as number of inactive channels decreases and the number of K close.
A very strong stimulus is required to make an AP
What is the basic structure of a voltage-gated Na+ channel?
One long peptide chain with 4 subunits
Pore region
An inactivation particle that can plug the pore
Has a voltage sensor (S4)
What is the role of the VOltage sensor (S4) in a voltage-gated Na+ channel?
It is positively charged AA’s in the voltage field of the channel.
When the voltage of the membrane changes their charge changes, this causes a conformational change to the protein opening its pore to allow Na+ through into the cell.
What is the basic structure of a voltage-gated K+ ion channel?
4 individual subunit peptide chains
Pore region contributes to selectivity
Voltage sensing S4 region
How does the opening of channels allow us to see membrane potential change?
Channels open and close in a random manner
The potential is an average of the channels
Give an example of a local anaesthetic and explain it two forms and their relevance.
Lidocaine is an anaesthetic (also treat arrhythmia)
It has an in-protonated form and a protonated form. The unprotonated form is membrane permeable to can get into cells. The protonated form is membrane impermeable,
At the body’s pH of 7.2 most molecules are in protonated (insoluble) form.
How do local anaesthetics work?
The work in 2 ways depending if in un-protonated or protonated form.
Un-protonated=soluble, moves through membrane and blocks a closed channel by filling the pore from the other side. This can block the channel in an inactivated state. This is most effective.
Protonated=can block an open channel by using pore.
Both blocks stop Na+ getting in so stop AP’s being created so stop pain sensing fibres receiving info
In what order to local anaesthetics block axons?
First= small myelinated axons Then= in-myelinated axons Last= large myelinated axons
Why do diff nerves have diff conductance velocity?
As large (diameter) axons conduct faster. Small (diameter) axons conduct slower.
Explain the local current theory.
Injection of a current into an axon wil cause the resulting potential to spread along the axon in either direction and cause an immediate local change in membrane potential.
This doesn’t use channels only local charges
What is capacitance?
The ability to store charge
If capacitance is high, the change in voltage spreads further along the axon so more channels open
What does membrane resistance depend on?
Depends on the number of ion channels open. The more channels open the lower the resistance.
If resistance is high the collage changes more slowly in response to current injection and doesn’t travel as far
How can we make AP’s travel further along the axon?
By opening more channels (increase resistance)
By increasing the diameter of the axon
If the voltage travels further, more of the axon is initiated per AP, so conduction velocity is increased
How does refractory period effect the direction of an impulse?
It prevents it travelling backward along the axon after current injection.
It’s sped up by hyperpolarisation
Hw does myelination effect a nerve?
There are nodes of Ranvier between myelin sheaths. This is where all the Na+ channels are concentrated into.
It’s acts as a good insulation so local circuit currents depolarise the next node above threshold and create an action poteniton.
In this way AP’s jump between nodes in Saltatory conduction. This is faster than it passing along an axon.
State some diseases of the central nervous system. What cases them?
Multiple SClerosis - all CNS nerves
d=Devic’s disease- optic + spinal nerve only
Result from breakdown/damage to myelin sheaths.
Symptoms depend on where myelin is damaged
State some disease of the peripheral nervous system
Landry-Gillian-Barre syndrome
Charcot-Marie-Tooth disease
What happens in demyelination
In regions of demyelination the density of AP is reduced because of resistive and capacitive shunting. This means the next node of ranvier does not reach threshold value and AP stops
