PATHWAYS / GRAY MATTER [ MESSENGERS ( THE NERVOUS SYSTEM ) ]

PATHWAYS

Pain signals take rwo tracks on their way to the brain. The express line, like a nonstop train between cities, sends signals through the spinal cord and connects directly to the thalamus. While some pain signals are diverted along the way, those that reach the thalamus are relayed to the cerebral cortex, where they quickly get analyzed.

When you cut your finger while slicing an onion, the quick pathway of pain activates the cortex to figure out how much pain you feel and where you feel it. The brain’s quick recognition of the danger may stop you from bringing down the knife blade again and slicing your finger a second time.

The other, slower pathway travels through slow, narrow nerve fibers with frequent synaptic connections, lumbering like a commuter train that stops at every little burg. These sensations register in the brain stem and hypothalamus, as well as in other deep brain regions, before a portion of them reach the thalamus. Effects include longer-lasting aches as well as emotional reactions to pain, such as the sheepishness of realizing you injured yourself through either clumsiness or negligence (or both). These slow-action pains include the unremitting discomfort of chronic diseases such as cancer.

GRAY MATTER

But not all pain sensations terminate in the thalamus. Many halt at a portion of the brain stem known as the mesencephalic central gray matter. It’s a tiny spot that is difficult to locate. But as a conver gence zone for pain impulses, this area is highly sensitive. When lab animals have their mesencephalic gray matter stimulated by electricity, they can be operated on without painkillers. Yet they maintain their sensitivity to touch, heat, and other sensations in the pain- affected body parts.

CAPTAIN AHAB asked his ship’s carpenter for a special bit of work in the novel Moby-Dick. Ahab, who had lost a leg to the teeth of a white whale, hoped a replacement limb might expunge the feeling of “another leg in the same identical place with … my lost leg.” “Phantom” limbs, such as Ahab’s lost leg, have been reported since ancient times. American neurologist Silas Weir Mitchell cataloged many varieties in the Civil War. About 70 percent of phantom limbs proved excrUCiatingly and chronically painful. How could a missing leg create the illusion of existence, or even pain? The answer lies in the brain.