Session 4 Flashcards
Define action potential
The change in voltage across a membrane
What are the properties of action potentials?
Depends on ionic gradients and relative permeability, need to reach threshold for it to occur, all or nothing, propagated without loss of amplitudes
How are action potentials generated?
Increased permeability to Na+, bringing the membrane closer to the Na+ equilibrium potential
Describe the structure of Na+ channels
Main pore is one peptide consisting of 4 peptide chains. Each repeat consists of 6 transmembrane spanning domains
Give an example of a local anaesthetic and how it works
Procaine. It works by binding to and blocking Na+ channels, thereby stopping generation of action potentials
How are nerve fibres blocked by anaesthetics?
Small, myelinated axons
Non-myelinated axons
Large, myelinated axons
What is accommodation?
The longer the stimulus, the larger the depolarisation necessary to initiate an action potential therefore the threshold becomes more positive. Causes an accumulation of Na+ channels in the inactive state.
Define absolute refractory period
When nearly all Na+ channels are in the inactivated state
Define relative refractory period
When Na+ channels are recovering from inactivation, excitability returns to normal as the number of channels in the inactivated state decreases
How can you record electrical stimulation of a cell?
Stimulate them under a cathode, excitability is reduced under the anode
Why would we need an extracellular recording of an action potential?
It can give information about conduction velocity under various conditions
What is conduction velocity?
Nerve conduction velocity is the speed at which an electrochemical signal propagates down a neural pathway.
How do we calculate conduction velocity?
Distance between electrodes/time taken
How is an action potential conducted along an axon?
Change in membrane potential spreads down the axon due to local current spread
Conduction velocity is determined by how far along the axon these local currents spread
If depolarisation occurs –> ap is initiated.
Which properties give rise to a high conduction velocity?
High membrane resistance
Low membrane capacitance
Large axon diameter (leading to low cytoplasmic resistance)
Define capacitance
Ability to store charge.
What does a high capacitance mean?
It takes more current to charge (or a longer time) and can cause a decrease in spread of local current, especially with brief current pulses
What does a lower resistance mean for ion channels?
More ion channels are open, meaning that there is more loss of the local current effect
What is the purpose of the refractory period?
Allows unidirectionality because Na+ channels are inactive for this period of time
What effect does myelination have on the conduction velocity?
Conduction velocity is increased considerably by myelination of axons
What effect does myelination have on capacitance?
Decreases capacitance
What effect does myelination have on resistance?
Increases resistance
What is myelin formed by?
Schwann cells in peripheral axons
Oligodendrocytes in axons in the CNS
Describe the relationship between velocity and diameter in myelinated axons
Velocity is proportional to the diameter
Describe the relationship between velocity and diameter in non-myelinated axons
Velocity is proportional to the square root of the diameter
What is the cause of multiple sclerosis?
Autoimmune disease in which myelin is destroyed in certain areas of the CNS, leading to decreased conduction velocity, complete block or partial transmission of action potentials.
What does activation mean?
What does inactivation mean?
Activation means that the voltage favours the opening of the channel and inactivation means the voltage is not sufficient for it to be active (open)
Define accommodation when applied to nerves
Over a period of time there are too many inactivated Na channels and thus even if the threshold is exceeded there will be no action potential.
What is the function of myelin?
Reduces capacitance and increases the resistance of the axons
