deck_582381 Flashcards
What is an action potential?
A change in voltage across a membrane
What does an action potential depend on to be generated?
Ionic gradients and relative permeability
What is required to be reached for an action potential to be generated?
A threshold voltage level
What is meant by the “all or nothing” theory of action potential generation
Action potentials are either generated or they are not, there are no half or double action potentils
What is the basis of the all or nothingsystem?
The positivefeedback loop, whereby the membrane depolarizes, causing more Na+ channels to open, which means more Na+ enters cell which means the membrane depolarizes further leading to more Na+ channels opening - and so on.
Outline the sodium hypothesis
Once the membrane has been depolarised to threshhold voltage, voltage gated Na+ channels open, allowing Na+ influx as ions attempt to move to their equilibrium potential of +61 mv. This influxdeploarises the membrane further, causing more voltage gated Na+ channels to open.
What two events occur to cause repolarisation?
During maintained depolarisation, Na+ channels close by a mechanism called inactivation.Voltage gated K+ channels are opened by depolaristion, causing a K+ ion efflux as K+ attempts to move towards it own equilibrim potential.
What is A?
Na+ channels open
What is happening at B?
Reducing Na in cell makes ENa more negative
What is C?
Na+ channels close due to inactivation
What is D?
K+ channels open
Why can an action potential not be stopped halfway?
Because once threshold voltage reached, positive feedback of voltage gated sodium ion channels guarantees action potential synthesis.
What is the absolute refractory period?
Nearly all Na+ channels in inactivated state. Excitability is 0.
What is relative refractory period?
Na+ channels re recovering from inactivation, the excitbility returns towards normal as the number of channels in the inactive state decreses.
Howare Na+ channels inactivated?
By N-peptide binding
What is accomadation?
The longerastimulus is, the larger the depolarisation necessary to initiate an action potential.This is because Na+ channels become inactivated.
Describe the structure of voltage gated Na+ and Ca2+ channels
1 peptide with 4 homologous repeatsEach repeat consists of 6 transmembranous domains1 domain is able to sense the voltage field across a membraneFunction required 1 subunit
Describe the structure of voltage gated K+ channels
Four non=linked peptide repeatsSix Transmembrane DomainsOne Domain is Voltage SensitiveFunction requires Four subunits
How do local anaesthetics prevent action potential generation?
Bind and block Na+ channels, stopping action potential generation
In what order do local anaesthetics block nerve fibres?
1.Small myelinated axons2.Non-myelinated axons3.Large myelinated axons
Why do local anaethetics effectneurones in this order?
Because they effect sensory neurones before motor
At what point do LA’s block Na+ channels?
When the channel is open
What can extracllular recording of action potentials give information about?
Conduction velocity under various conditions
How can extracellular recording be used to measure conduction velocity?
Electrodes are used to raise the membrane potential to threshold to generate an action potential. Changes in potential between stimulating (Cathode, -‘ve) and recording (Anode,+’ve) electrodes along an axon.
