M&R Session 4 (Lecture 4.2) Flashcards
Give 2 examples of diseases that affect the conduction of the AP in the CNS.
1) Multiple sclerosis (all CNS nerves)
2) Devic’s disease (optic and spinal cord nerves only)
Give 2 examples of diseases that affect the conduction of the AP in the PNS.
1) Landry-Guillain-Barre syndrome
2) Charcot-Marie-Tooth disease
What are the 6 classes of peripheral axons in diameter and conduction velocity order?
1) A alpha (Ia/Ib)
2) A beta (II)
3) A gamma
4) A delta (III)
5) B
6) C (IV)
Give examples of function of A alpha, beta, B and C fibres.
A alpha - sensory fibres from muscle spindles, motor neurones to skeletal muslce.
A beta - sensory fibres from pain and temperature receptors (sharp localised pain)
B - preganglionic neurones of ANS
C - sensory fibres from pain, temperature and itch receptors (diffuse pain)
How can extracellular recording of an AP be done?
Using an anode and cathode placed on the surface of the cell.
Where does the AP start in extracellular recording and why?
At the cathode as the cathode makes the outside membrane potential more negative so it depolarises the inside of the membrane.
What is the difference between diphasic and monophasic recording?
Diphasic is where both electrodes are on an intact piece of electrode.
Monophasic is where the B electrode is on a deliberately damaged section of a nerve.
Describe the local current theory.
Injection of current into an axon will cause the resulting charge to spread along the axon and cause an immediate local change in the membrane potential.
What determines how far a local current spreads in an axon?
The length constant (lambda) which is the distance it takes for the potential to fall to 37% of its original value.
What are the equivalent structures of capicitance and membrane resistance in the cell?
Capicitance = lipid bilayer
Mem resis = No. of ion channels open. (lower resis = more channels open)
What is the relationship between the length constant, resistance and axon radius?
Lambda = Sq rt (Rm.a/2.Ri)
where Rm = membrane resis and Ri = axoplasm resis
Why does the action potential travel in one direction only?
The tissue behind the AP has all of its VGNC closed (therefore in ARP) and are inactivated so no AP can be generated in that tissue. Therefore the AP can only go on the opposite direction i.e. away.
Why is the membrane resistance lower behind an AP?
K+ channels are open so the length constant decreases.
What does myelination do to the velocity of an AP? How?
Increases the velocity by having a high concentration of Na+ channels at the nodes of ranvier. This is where saltatory conduction takes place so APs are generated at the NoR only.
Why is there no point in having a myelinated neurone below 1 micro metre diameter?
Speed would be slower than an unmyelinated one
