ICPP 5 Action Potential + Properties + Propagation Flashcards
What is an action potential?
Change in voltage across the membrane
What are the properties of action potentials?
- depend on ionic gradients and relative permeability
- only occur if a threshold voltage is reached
- are propagated without loss of amplitude
What does the conductance of a membrane to an ion dependent on?
Number of open channels for that ion
What happens to the membrane potential if conductance of an ion increases?
Moves close to equilibrium potential for that ion
What is the absolute refractory period and how long does it last?
Time between initally opening and initially closing Na+ channels
1ms
What is the relative refractory period and how long does it last?
Time for Na+ channels to recover back to resting potential
4ms
Describe the sodium hypothesis of action potential
1- threshold potential is reached
2- Na+ channels open > Na+ influx > depolarisation
3- Na+ channels inactive + K+ channels open > K+ efflux repolarisation
What is the threshold potential?
The level the membrane potential must be depolarised to to initiate an action potential
What happens during repolarisation?
K+ channels open > K+ efflux
Na+ channel inactive
Outline the action of local anaesthetics
Bind to and block open + inactive Na+ channels»_space; stop AP generation
In what order do local anaesthesia block conduction in nerve fibres?
Small myelinated axons
Non- myelinated axons
Large myelinated axons
What are the 3 axon properties that lead to high conduction velocity?
- high membrane resistance
- low membrane capacitance
- large axon diameter
What is capacitance?
Ability of a lipid bilayer to store charge
Why does low capacitance lead to faster conduction velocity?
Decrease in local current spread
Low capacitance takes less current to charge
Conduction velocity equation
CV = distance / time
How is an action potential conducted along an axon?
1- change in membrane potential spreads to adjacent areas by local currents
2- local current spread causes depolarisation to threshold potential
3- action potential is initiated
What is electrical stimulation?
Used to stimulate an axon to threshold potential
Initiates an AP
How is electrical stimulation done?
Under a cathode (negatively charged)
Describe Na+ channels in absolute refractory period
Mostly in inactivated state
Describe Na+ channels in the relative refractory period
Recovering from inactivation
What is membrane resistance?
Function of the number of open ion channels
How does a low membrane resistance lead to a slower conduction velocity
Lower the resistance > the more ion channels are open > the more loss of local current > slower conduction velocity
Why do action potentials not move backwards?
The area of axon that has just fired an action potential is refractory
Cannot fire another AP until it has recovered from being refractory
Explain how the myelin sheath affects conduction velocity?
Reduces capacitance +increases resistance
Increases overall conduction velocity
What cells form myelin in the CNS?
Oligodendrocytes
What cells form myelin in PNS?
Schwann cells
Describe saltatory conduction
Propagation of AP along myelinated axons between nodes of Ranvier
Increases conduction velocity
What is the relationship between fibre diameter and conductance velocity in myelinated fibres?
Velocity is proportional to diameter
What is the relationship between fibre diameter and conductance velocity in unmyelinated fibres?
Velocity proportional to square root of diameter
Outline multiple sclerosis
Autoimmune disease
Myelin is destroyed in the CNS
Decreased conduction velocity
OR complete block of AP
