Why do local anesthetics block pain signals before other sensations?

Local anesthetics block pain signals before other sensations primarily due to the physical characteristics of the nerve fibres involved. The ability of these drugs to inhibit nerve impulses depends on the diameter of the nerve fibre and the length of the fibre that must be affected to stop conduction.1

In general, thinner fibres are more susceptible to the effects of local anesthetics and are blocked first. Fortunately, the fibres responsible for conveying the sensation of dull aching pain are among the thinnest in the body. Consequently, when a standard dosage is administered, pain is the first sensation to disappear, followed in order by sensations of cold, warmth, touch, and deep pressure.1

By penetrating the fatty membrane of these fibres, the anesthetic molecules block ion channels from the inside, preventing the movement of sodium ions necessary for transmitting electrical signals.1