Nerves 4 Flashcards
What is the threshold potential?
-55mV
What happens once the threshold potential has been reached?
Sudden massive depolarisation and overshoot to +30mV
In AP what causes depolarisation?
Voltage gated Na channels allowing Na to flood into the cell
What happens immediately after threshold is reached?
- Na floods in and depolarises the cell
- Massive increase in permeability causes decrease in resistance
What is an example of positive feedback?
Opening of Na channels once threshold is reached
What increases K permeability?
The opening of more K channels
What does the opening of K channels cause?
Hyperpolarisation then repolarisation
What is the refractory period?
Time in which the AP cannot be fired immediately after channels close
What can block voltage dependent Na channels?
- Local anaesthetics (procain/ Iidocaine)
- Puffer fish toxin = tetrodotoxin
- Shellfish toxin - saxitoxin
What are the properties of AP?
- Have a threshold
- All or none
- Cannot encode stimulus intensity in amplitude, only frequency
- Self-propagate
- Voltage-gated channels
- Travel slowly
- Refractory period
What does increasing the stimulus of an AP do?
Increases the time the AP are fired for
In AP what causes hyperpolarisation?
Voltage gated K channels allowing K to flow out of the cell
What ensures that AP only travel in 1 direction?
Refractory period
What does self-propagation of AP do?
Depolarises the next bit of the axon and opens the voltage dependent Na channels there
Why do AP travel slowly?
Delay in the opening of the Na channels
What can improve conduction velocity?
- Big axons
- Myelination
Why does large axons increase conduction velocity?
- Bigger axons = lower axial resistance
- Depolarisation can spread further
- Na channels can be spread out further
- Less opening of voltage gated channels
What is myelin?
Folds of membrane from
- Schwann cells (PNS)
- Oliodendrocytes (CNS)
What are the gaps between myelin sheaths called?
Nodes of Raniver
How does AP spread with little decrement in a myelinated axon?
- Na channels are present in the nodes.
- Myelin increases resistance of membrane and allows AP to spread like a local current
What kind of conduction takes place in myelinated axons?
Saltatory conduction
What are the advantages of myelination?
- Speeds up conduction
- Metabolically good = ions only cross membrane at nodes
- Saves space
What do Schwann cells do?
- Prevent leaking
- Increases membrane resistance
- Decrease membrane capacitance
Why does the current in a myelinated axon spread further than in a non-myelinated axon?
There is less current wasted leaking out of membrane or charging up the capacitance
What conditions involve de-myelination?
- Multiple sclerosis
- Guillain-Barre syndrome
What is the effect of de-myelination?
- Big local current decays quicker
- Does not depolarise to next node to threshold
- And conduction fails
- It decays too quickly and there is not sufficient depolarisation to reach the threshold at the next node
Why are compound action potentials evoked?
- Mammals have small and large myelinated and unmyelinated axons.
- Therefore extracellular recording from a bundle of axons (a nerve trunk) evokes a compound AP.
What fibres are most sensitive to anoxia?
Big ones
What fibres are most sensitive to local anaesthetics?
Small ones
What are the classes of fibres and what are they responsible for?
- Aa: proprioception, motor neurones
- AB: touch, pressure
- Ay: motor neurones of muscle spindles
- Ao: touch, cold, fast pain
- B: preganglonic autonomic fibres
- C: heat, slow pain
