Targeting the axons responsible for chronic pain

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.

Responding to pain

A few more aspects of pain pathways can lead to chronic pain or effects of chronic pain. Axons of the spinal cells that receive pain signals branch out through the CNS where they perform different actions. Some actions are important to understanding pain:

·     Some branches go to the brain stem located between the spinal cord and the brain (also called the cerebrum). Nuclei in the brain stem regulate sleep and wakefulness. Input to these regions arouses you and can prevent you from sleeping. Loss of sleep can be a major problem for people with chronic pain.

·      The brain stem is a major player in controlling your muscular tone and coordinating reflexes that contribute to all your movements. For example, the brain stem coordinates your withdrawal from a painful stimulus in a way that prevents you from falling over. It also governs your reflexes and can inhibit them; for example, it keeps withdrawal reflexes from going off time and time again if the pain doesn’t stop. Unlike the alarm on a timer, which doesn’t stop until someone turns it off, your brain is smart and turns the withdrawal reflex off after awhile.

·     The brain stem inhibits reflexes with axons in pathways that descend to your spinal cord. There is some spillover of inhibition to spinothalamic cells in the spinal cord. Therefore, some scientists think the brain stem may play a role in regulating pain.

·    Systems within the brain regulate stress reactions. One form of stress, called psychological stress, activates both the hypothalamus and the pituitary gland, which in turn leads to activation of the sympathetic nervous system. The result can be increased pain.