The peripheral endings of sensory nerves contain a variety of nerve endings called receptors. They transform different kinds of energy, such as touch, cold, or heat, into neural impulses (also called action potentials). These impulses from receptors responsive to painful stimuli (called nociceptors) carry messages to your pain pathways in the Central Nervous System (CNS).
The axons of sensory neurons differ by size and the degree of myelin on them. (Myelin is a substance that covers and protects nerves.) The largest axons are encased in a myelin sheath, which makes them big and fast. In fact, their impulses can rush forward at speeds of up to 40 miles an hour, faster than you can drive your car in a school zone. These fast and large axons are called A-Beta fibers.
Small axons with some myelin respond to painful stimulation. They’re your warning system for acute pain. For example, they’re fast enough to set off a withdrawal reflex to make you snatch your hand back from a hot burner. These axons are called A-Delta fibers.
The smallest axons, called C fibers, have no myelin, and they conduct information very slowly (about 3 miles an hour). These axons are the most plentiful, and they can reach any tissue. C fibers are responsible for the pain you feel if something touches the cornea of your eye or you have a toothache. Knowing this information, you probably aren't surprised to discover that a lot of chronic pain comes from activation of C fibers.