Lecture 2 Flashcards
Outline the key features of action potentials
All or nothing signals
Rapid and transient change of membrane potential
There is an upper limit to action potentials
Self-propagating
What is excitability?
Measure of how easy it is to fire an action potential
What is refractoriness?
Reduction in membrane excitability following an action potential
How could you demonstrate that refractory periods exist?
Administer 2 current stimuli in succession with varying timescales and measure the threshold of the amplitude of the 2nd stimulus
Outline local current circuit theory
The local currents spread depolarisation and are dependent upon passive flow of current along electrical gradients.
What effect would an increase in the length constant have on AP conduction velocity?
It would increase
Give evidence for local circuit theory
Hodgkin 1937 - AP was blocked in a 3mm mid section by cooling, this slows active processes causing the AP to fail in this region. Small voltage transients were still recorded which decreased with distance and showed AP was dependent on passive conduction
Threaded fine silver wire through the jellyfish giant axon which decreased Ra therefore increased conduction velocity.
Bathed nerves in paraffin increasing extracellular resistance decreasing conduction velocity
What is salutatory conduction?
Leaping conduction as occurs in myelinated axons
What features connect the axon to the ECF when an axon is surrounded by support cells?
Mesaxons
What support cells are in the CNS?
Oligodendrocytes
What support cells are in the Periphery?
Schwann cells
How does myelination increase electrotonic conduction?
Many layers of cell membrane electrically isolate the axon increasing membrane resistance and decreasing membrane capacitance which decreases the time constant
Describe the basic structural features of a Node of Ranvier
Na channels located at node
K channels located at periphery
Provide evidence that current flow only occurs at the Node of Ranvier
Current stimuli required to trigger AP is less at the nodes than at the internodes.
Local anaesthetic to block conduction is more effective at the nodes than the internodes
Bathe in three electrically isolated compartments of Ringers solution - when the node is in the solution AP detected when node is out of solution no AP detected
What is the relationship between the conduction velocity of an unmyelinated axon and diameter?
v is proportional to diameter
What is the relationship between the conduction velocity of an myelinated axon and diameter?
v is proportional to the square root of diameter
