There are two often-confused words: analgesia and anaesthesia.
Analgesia is ‘the removal of pain sensation’ whereas anaesthesia is ‘the loss of
sensation in general’ (including pain).
This difference in meaning is often missing, with many people as well as publications using the terminology ‘local anaesthesia’ when ‘local analgesia’ is what is meant.
Local analgesia is the effect that a dentist try to achieve in normal clinical activity; however, ‘local anaesthesia’ is the term commonly used in medicine and dentistry.
Local anaesthetics inhibit the generation of electrical impulses and their conduction along the neuronal axon membrane, by reversible blockade of sodium ion channels.
There is a continuous imbalance of sodium and potassium ions between the cytoplasm of neurons and the intercellular fluid.
This is maintained at about 25 times more potassium intracellularly and 15 times more sodium in interstitial space.
This imbalance is upheld through the sodium pump protein in the cell membrane,
which maintains the resting membrane potential of -60 to -90 mV.
Action potentials are propagated along a neuron process through the rapid flux of ions against the resting gradient.
This results in a positive spike in the membrane potential that is dependent on the sodium pump.
Local anaesthetic drugs affect the movement of sodium ions across the membrane by blocking sodium channels.
However, this blockade can only happen from the intracellular side of the channel and thus a local anaesthetic drug is required to move through the cell membrane of the neuron before being able to block action propagation.
The progression of local anaesthetic block is dependant on nerve fiber diameter, myelination and conduction velocity.
Ref: Therapeutic guidelines 2008