Differential Block refers to the gradual and sequential block of the different type of nerve fibres when they are exposed to a local anesthetic.
Small diameter axons are more susceptible to block than large diameter fibres, however myelinated fibres are more sensitive than non myelinated (because local anaesthetics block the fibre at node of Ranvier), Blockage of minimal length (at least 3 successive nodes of Ranvier) is required to produce inhibition of nerve conduction.
Based on fibre diameter the nerve fibres are classified as type A, B and C. A being the thickest and C the thinnest.
Differential Block depends on concentration of the local anesthetic. A drug at lower concentration will produce only sensory block while at higher concentration can produce motor block.
The nerve fibre type also has a significant effect on the local anesthetic. It can be concluded that the factors which determine the sensitivity of nerve fibres to local anaesthetics are fibre diameter and myelination therefore type B fibres (myelinated) are more readily blocked than type C (non myelinated) in spite of being of thinner diameter than B.
Sequence of Differential Block
The sequence of Differential Block is type B – type C – type A and in functional terms it is autonomic (mediated by C and B fibres) — sensory (mediated by C and also A fibres) — motor and the sequence of recovery is motor — sensory — autonomic. Among sensory fibres sequence of blockade is temperature (cold before hot) — pain — touch — deep pressure — proprioception.
This blockade of different types of fibres is called as Differential Block.