Pain is a complex phenomenon, and dental pain, a multifactorial. Since dentistry was first practiced, the prime reason relieving pain. So The relief of oral pain, is the highest priority of the profession.
First, we need to understand what is the mechanism of pain. pain is started by pain receptors or free nerve endings that scattered all over the body, that receptors affected by to any stimuli (chemical, mechanical, thermal .. etc.) after that these receptors transmit this stimulus to Central Nervous System through the nerves. تAlso the receptors are mainly 2 types a myelinated A-delta fibers (fast pain) and unmyelinated C-fibers ( 2nd pain)
The dental pulp is a soft connective tissue that is densely innervated and highly vascularized with numerous nerve endings (receptors) mostly of these receptors are myelinated A-delta fibers (fast pain) these many receptors lead to sever pain due to a small stimulus.
Also the pulp contains a numerous sensory nerve fiber that often associated with blood vessels forming neurovascular bundle, these numerous neurovascular bundles enclosed by rigid mineralized dentin, which strongly limits the ability of the tissue to increase in volume during inflammation and decreases the level of immune defense, these limited area resulting in a severe pain even a little stimulus.