White matter keeps maturing until middle age, long after grey matter peaks
White matter was long dismissed as passive packing, yet its fatty, insulated fibres shape learning by relaying and coordinating signals between brain regions. One small study measured 176,000 km of myelinated fibre in 20-year-old men and 149,000 km in women, with the total shrinking roughly 10 percent each decade.
The tissue consists mainly of myelinated axons bundled into tracts that link areas of grey matter, where nerve cell bodies sit. Myelin is an insulator: it lets electrical signals leap along an axon instead of creeping through it, which speeds every message. It looks white in preserved specimens because of formaldehyde; fresh, it is pinkish-white, since the fatty myelin is threaded with capillaries. Blood vessels make up 1.7 to 3.6 percent of it in adults who are not elderly.
It fills the deep interior of the brain and the outer layers of the spinal cord, with clusters of grey matter such as the basal ganglia embedded inside. The corpus callosum, which joins the two hemispheres, is the largest white structure. Within a hemisphere, long-range fibres are only 2 percent of connections between cortical areas, and as a rough rule, the longer a class of fibre, the fewer of them exist. In the cerebellum the deep white matter branches in a pattern called the arbor vitae.
By age 80, the fibre-length study found, men were down to 97,200 km and women to 82,000 km, mostly through loss of thinner fibres, though the sex gap may simply reflect men's larger brains. Grey matter peaks in a person's twenties, whereas white matter keeps developing until middle age. Regular aerobic exercise appears to delay its ageing or even strengthen its integrity over time.
Damage has many sources. Multiple sclerosis, the most common inflammatory demyelinating disease, strips myelin from axons; alcohol use disorders are linked to reduced volume; and higher nitrogen dioxide exposure in the first four years of life has been associated with weaker white matter integrity. Lesions visible on MRI scans are linked to cognitive impairment and depression.
Source: White matter
