Electrical excitability

Question 1

Describe the properties of voltage gated ion channels

Answer 1

Na and Ca have one protein, comprising of 4 subunits, with each subunit made up of 6 transmembrane domains,
Domain 4 is voltage sensitive
Domain 6 opens and closes the channel

Question 2

Describe the permeability changes associated with action potentials

Answer 2

Sodium has a high extracellular concentration. Sodium channels open, depolarising the membrane. This depolarisation opens voltage gated sodium channels and allows threshold to be reached. Na channels then become inactive and K channels open. Repolarisation then occurs.

Question 3

Discuss factors affecting velocity in nerves and salutatory conduction

Answer 3

Saltatory conduction occurs due to myelination. Movement of Na ions into the cell occurs only at Nodes of Ranvier, markedly reducing the membrane capacitance. This causes the action potential to ‘jump’ between nodes, speeding up conduction velocity.

Question 4

Explain the general mechanisms of action potentials

Answer 4

• Change in voltage across the membrane
• Depends on ion gradients relative to permeability
• Occurs only if threshold has been reached
• Propagated without loss of amplitude

Question 5

Describe the all or nothing law and refractoriness in terms of permeability

Answer 5

• Enough Na channels need to open to reach threshold
• Absolute Refractory Period: All Na channels inactive
• Relative Refractory Period: Some Na channels opening

Question 6

Outline the action of local anaesthesia

Answer 6

They act by binding to and blocking Na channels, thereby stopping action potential generation and the perception of pain.

Question 7

Explain the Nernst equation

Answer 7

61/ valency x log10 {outside conc/ inside conc]

Question 8

Explain the local circuit theory of propagation

Answer 8

A change in local membrane potential can spread to adjacent areas of an axon, due to local current spread. It can only travel forwards due to the inactivation of Na channels.

Question 9

Explain how conduction is linked to fibre diameter

Answer 9

Large axon diameter leads to low cytoplasmic resistance and therefore a higher conduction velocity

Question 10

Describe consequences of demyelination

Answer 10

Demyelination can occur for a number of reasons. It causes decreased conduction velocity, complete block or cases where only some action potentials are transmitted.

Question 11

Explain capacitance

Answer 11

Capacitance is the ability to store charge. A high capacitance needs more charge to depolarise it, therefore can hinder the local spread of a current.