action potentials icpp Flashcards
Give 3 properties of action potentials
threshold value must be reached to induce AP
all or nothing principle
propagated without loss of amplitude
Where is an AP initiated?
axon hillock
How do we know Na is important in AP?
when concentration of Na is change outside the size of the AP changes parallel to change in ENa
Explain the changes in ionic permeability in an action potential
Na voltage gates open to influx of Na less negative membrane potential Na gates inactivate and K voltage gates open hyperpolarisation slowly close k gates membrane repolarised
What is the absolute refractory period?
nearly all Na voltage channels are inactivated no AP can be generated
What is the relative refractory period?
Na voltage channels recovery number of channels inactivated as decreased
AP can be induced by greater stimulus required
Why is the refractory period important?
ensure that AP is propagated in one direction only
What is the molecular structure of a sodium voltage gated ion channels?
1 alpha subunit with 4 repeats
S5/6- of each repeat = pore
S4= voltage sensor
S1/S6 of repeats3/4- inactivation particle
How does the voltage sensor in the various ion channels work?
S4 =series of positively charge aa residues in the voltage filed
change in voltage field results in a change in conformation and the opening of the pore
What is the molecular structure of the k voltage ion channel ?
4 x alpha subunits
s4 = voltage sensor
s5/6= pore
Name a local anaesthetic
lidocaine
What are the theories of how local anaesthetics work?
- blocking occurs at closed state- poorly understood
- open channel block pathway, protonated form enters open pore and blocks it strongest block at inactivation state
- hydrophobic block pathway
Explain local current theory.
depolarisation causes local membrane depolarisation.
the further away the point of AP initiation the smaller the depolarisation
any region that depolarisation is above threshold induces an AP, those Na channels closest to initial AP
What is the length constant? What does it depend on?
the distance it takes for the potential to fall to 37% of its original value.
depends on capacitance and membrane resistance
How does conductance velocity change with diameter?
increase with diameter
increase membrane resistance
decrease capacitance
What cells produce myelin?
schwann cells
What are the regions where there is a high concentration of Na voltage channels called in a myelinated axon?
nodes on ranvier
How does a myelin sheath improve conductance velocity?
myelin is a good insulator
local currents depolarise the next node above threshold
AP jumps from node to node
increase membrane resistance
decrease capacitance
increasing length constant
increasing conduction speed
What is they type of conduction seen in a myelinated axon?
saltatory conduction
What is the effect of demyelination?
stop saltatory conduction
reduced insulation and not enough Na channels in myelin covered area to induce AP in next node
AP stopped
Name a condition where myelin is affected in the CNS
multiple sclerosis
Name a condition where myelin is affected in the PNS
Laudry- Guillain-Barre syndrome
How do we know Ca implicated in NT release?
decrease in extracellular Ca causes decrease in end plate potentials
Describe molecular structure of Ca channel?
1 alpha subunit 4 repeats
very similar to Na voltage channel
