what is the functions of local anaesthetics?
LAs stop nerve conduction by blocking the voltage-gated Na+ channels
some nerve axons differ in their susceptibility to be blocked by LA, in what order are the nerves blocked?
- A-delta
- C fibres
- A-beta
- A-alpha
what is the functions of A-alpha fibres?
- sensory (proprioception)
- motor (skeletal muscle)
what is the function of A-beta fibres?
sensory (mechanoreception)
what is the function of A-delta fibres?
- sensory (mechano, thermo, noci & chemo-receptors)
what is the mechanism of action of local anaesthetic?
- LA binds to a site in the Na+ channel
- LA blocks the channel and prevents Na+ influx
- this blocks action potential generation and propagation
what can occur if LA reaches the bloodstream?
it can cause bradycardia and hypotension
what type of molecules are Local Anaesthetics?
organic molecules
what are the three components of the organic local anaesthetic molecules?
- aromatic region
- ester or amide bond
- basic amine side chain
local anaesthetics are partly dissociated, what does this mean?
- active in ionised form
- can cross membrane only in un-ionised form
why are smaller diameter axons more susceptible to LA block?
- less sodium channels in small fibres
- less channels at the axoplasm = less LA needed
what is the AXOPLASM of a nerve?
where the channels are located
where are the Na+ and K+ found in myelinated axons?
sodium and potassium ion channels are concentrated at the nodes of Ranvier
- nodes of Ranvier are exposed to the local anaesthetic
if a local anaesthetic is going to block a myelinated axon, what must it do? why is this?
- to block the AP, the LA needs to act on several nodes of Ranvier along the axon
- if only one node is blocked, the AP current is strong enough to flow past the blocked region & regenerate the AP at the next node of Ranvier
what is an example of an ester local anaesthetic?
benzocaine
what is are examples of amide local anaesthetics?
- lignocaine
- prilocaine
- articaine
- bupivicaine
how do local anaesthetics (by themselves) affect the blood vessels? why is this a problem?
they are vasodilators
- increased blood flow will increase ‘wash out’ of LA
how is the duration of action of Local Anaesthetics increased?
LA preparations also include a vasoconstrictor
what are examples of vasoconstrictors used in LA preparations?
- adrenaline
- felypressin
what are the different types of adrenoreceptors?
- alpha receptors
- beta 2 receptors
- beta 1 receptors (cardiac muscle)
what does stimulation of alpha adrenoreceptors cause?
vasoconstriction
what does activation of beta 2 adrenoreceptors cause?
vasodilation
if beta 1 adrenoreceptors are stimulated, what effect does this have on the cardiac muscle?
- increases heart rate
- increased pumping force
what effect does adrenaline have on adrenoreceptors?
- vasoconstrictor effect (action on alpha receptors)
- lowers TPR (total peripheral resistance)
