Drugs Affecting Nerve Excitability

Question 1

local anaesthetic agents

Answer 1

reversibly block conduction of nerve impulses at the axonal membrane

Question 2

aminoesters

Answer 2

• local anaesthetic
• eg procaine
• short acting, hydrolysed by esterases

Question 3

aminoamides

Answer 3

• local anaesthetics
• eg lignocaine, bupivicaine, ropivicaine
• longer acting, hepatic metabolism

Question 4

local anaesthetics selectively bind

Answer 4

Na+ channel; reversibly without damage

will affect all nerves and excitable tissues

Question 5

What are the two mechanisms of action of local anaesthetics?

Answer 5

hydrophobic and hydrophillic

Question 6

hydrophobic local anaesthetics

Answer 6

• eg benzocaine
• can cross the lipid membrane
• fast onset of action
• non use dependent ie blocks Na+ channels regardless of their activity

Question 7

hydrophilic local anaesthetics

Answer 7

• eg aminoesters and aminoamides (lignocaine)
• can’t cross lipid membrane
• slower onset
• use dependent - work better when the nerves are firing rather than when they are quiet

Question 8

How do local anaesthetics affect excitability of axon membranes?

Answer 8

stabilize membrane by preventing depolarization

they do not change the RMP

Question 9

The effect of local anaesthetics is more profound in what medium (acidic or basic)?

Answer 9

• basic; at pH 7.4 they exist in ionised form
• the uncharged species is more active as it can cross the membrane (hydrophobic mechanism)
• inside the cell favours the charged form which can block Na+ channels (hydrophilic mechanism)

Question 10

What are the two mechanisms of general anaesthetics?

Answer 10

• lipid solubility
  
  • potency related to lipid solubility
  • penetrate and expand lipid membranes to compress proteins from functioning
• receptor interaction
  
  • inhibit excitatory (glutamate, NMDA)
  • enhance inhibitory (GABA, glycine)

Question 11

What are the respiratory side effects of general anaesthetics?

Answer 11

increased likelihood of:

• impaired ventilation
• depression of respiratory centre
• obstruction of airways
  
  • retention of secretions

Question 12

What are the cardiovascular side effects of general anaesthetics?

Answer 12

increased likelhood of:

• decreased vasomotor centre function
• depressed contractility
• peripheral vasodilation
• cardiac arrhythmias
• inadequate responses to fall in BP or CO

Question 13

Inhalational general anaesthetics

Answer 13

desflurane, sevoflurane, isoflurane

Question 14

IV general anaesthetics

Answer 14

propofol, thiopentone