M&R Session 4- Conduction Of The Nerve Impulse Flashcards
Describe the local current theory.
That depolarisation in one region will cause transmembrane currents in neighbouring regions on both sides of the depolarised region.
This is because the opening of voltage gated sodium channels in the first region causes other to open in regions neighbouring it.
This propagates the AP.
Describe extracellular recording of action potentials.
Electrodes are used to raise the membrane potential to the threshold in order to generate an AP.
To calculate the conduction velocity of the AP you records the distance between the stimulating electrode and the recording electrode, and the time taken for the AP to travel between them.
Then use the equation CV= distance/time.
What does further spread of an AP indicate?
A faster conduction velocity.
What three factors cause a high conduction velocity and why?
- high membrane resistance- because due to ohms law, if R is high, the potential difference across the membrane will be greater, therefore more sodium channels will be one and a threshold is easier to reach.
- large axon diameter- because due to ohms law again, low cytoplasmic resistance will mean a larger current, therefore the AP travels further.
- low membrane capacitance- the membrane takes less time to charge.
Why are action potentials unidirectional?
An area of an axon that has just fired an AP will be refractory and therefore can’t fire another AP until it has recovered.
What effect does myelination have on the conduction velocity and why?
It increases it because myelination increases membrane resistance and decreases capacitance.
What is saltatory conduction? How is it brought about?
It is the jumping of conduction between nodes of Ranvier.
Reduced capacitance in the internodal regions means that the local current induced by an AP at the node will move further down the axon to depolarise the next node, without firing an AP in the internodal region. Local current spread is faster than AP spread, therefore is saltatory.
What is the relationship between conduction velocity and fibre diameter in myelinated axons?
Direct proportion.
What is the relationship between conduction velocity and fibre diameter in unmyelinated axons?
CV is proportional to the square root of the diameter.
What effect can demyelination have on conduction velocity? (3)
Can reduce it, block it or only allow some APs to be transmitted.
What disease can cause demyelination?
Multiple sclerosis
Which cells myelinated peripheral axons?
Schwann cells
Which cells myelinated CNS axons?
Oligodendrocytes
