Session 4

Question 0

Describe the basis of the all-or-nothing law and refractoriness in terms of these changes in permeability

Answer 0

All-or-nothing law - action potential is either generated or not, threshold value must be reached
Refractoriness - after an AP most of the Na+ channels will be inactive, they need time to recover (pore blocked by an N terminus ball protein)
ARP - nearly all Na+ channels are in inactive state - AP cannot be stimulated
RRP - Na+ channels are recovering from inactivation - takes more effort to stimulate AP

Question 1

Describe the properties of the action potential, its ionic basis and the associated changes in membrane ionic permeability

Answer 1

Action potentials change in voltage across membranes, depend on ionic gradients and relative permeabilities, only occur if a threshold value is reached, are all or nothing, are propagated without loss of amplitude
Na+ channels open, Na+ channels close/inactivated, K+ channels open

Question 2

Describe the key molecular properties of ion channels

Answer 2

Na+, Ca2+ - main pore forming subunit is one peptide consisting of 4 homologous repeats. Each repeat consists of 6 transmembrane spanning domains, one of these domains can sense voltage field across the membrane
K+ - each repeat is a separate subunit

Question 3

Outline the action of local anaesthetics

Answer 3

Procaine - bind to and block Na+ channels –> stop AP generation. Block Na+ channels easier when the channel is open, have a higher affinity when channel is in inactivated state.
Block conduction in nerve fibres in following order: small myelinated axons, unmyelinated axons, large myelinated axons
Local anaesthetics are weak bases and cross the membrane in their unionised form.

Question 4

Describe the results of extracellular recording and how this can be used to measure condition velocity

Answer 4

Use electrodes to stimulate AP.
Diphasic or monophasic.
Conduction velocity is calculated by measuring distance between the stimulating electrode and the recording electrode and the time gap between the stimulus and the AP being registered by the recording electrode
Conduction velocity = distance/time

Question 5

Explain how axons are raised to threshold

Answer 5

Na+ channels open (–> positive feedback)

Question 6

Explain the local circuit theory of propagation

Answer 6

A change in membrane potential in one part can spread to adjacent areas of the axon. When local current spread causes depolarisation of part of the axon to threshold then an AP is initiated in that location. The further the local current spreads down the axon the faster the conduction velocity of that axon will be.

Question 7

Explain how conduction velocity is linked to fibre diameter

Answer 7

High conduction velocity = large axon diameter (low cytoplasmic resistance)
Myelination - velocity proportional to diameter
Unmyelination - velocity proportional to root of diameter

Question 8

Explain the implications of myelination for conduction

Answer 8

Conduction is increased significantly by myelination of axons.
Myelination reduces capacitance and increases resistance.
In the CNS it is formed by oligodendrocytes, in the PNS it is formed by Schwann cells.
Myelinated axons - max. 120ms-1
Unmyelinated axons - max. 20ms-1

Question 9

Describe certain consequences of demyelination

Answer 9

Some axons can lose their myelin sheath. It may be destroyed e.g. Multiple Sclerosis. Current leaks and this can lead to decreased conduction velocity, complete block, or cases where only some APs are transmitted.