Local anesthetics block generation and conduction of nerve impulse (action potential). Myelinated fibers are more susceptible than nonmyelinated fibers and small fibers are more sensitive than large fibers. Addition of vasconstrictor decreases systemic absorption and toxicity. All modalities of sensation are not blocked at the same time because different sensory fibers have different sensitivity.Order of block of sensory modalities :- Temperature > Pain > touch > deep pressure > Proprioception.