White matter · Grey Matter

White matter is the brain's long-range cabling: bundles of axons, most of them wrapped in myelin, that carry signals between areas of grey matter that are centimetres apart.

White matter. White matter is the brain's long-range cabling: bundles of axons, most of them wrapped in myelin, that carry signals between areas of grey matter that are centimetres apart.

Myelin is fat-rich, which is why the tissue looks white, and it lets an impulse jump between gaps in the sheath instead of creeping along the whole membrane, so a myelinated axon conducts many times faster than a bare one. The fibres run in named bundles. Commissural fibres cross between the hemispheres (the corpus callosum is the largest); association fibres link areas within one hemisphere (the arcuate fasciculus joins Wernicke's area and Broca's area); projection fibres run up and down between cortex, thalamus, brainstem and spinal cord, such as the corticospinal tract that carries movement commands.

Damage to the cables can matter as much as damage to the areas. A small lesion in a tract disconnects every area that used it, which is why the reconstruction of Phineas Gage's injury found that about 4 % of the cortex but over 10 % of the white matter had been cut.

White matter is what diffusion MRI traces. Water moves more easily along axons than across them, and tractography follows that preference to draw the bundles in a living brain.

Its speed comes from myelin, whose mechanism (saltatory conduction) belongs to the action potential card.

Questions: What is the difference between grey matter and white matter? Grey matter is where neurons have their cell bodies, dendrites and synapses, so it is where signals are combined and decisions about firing are made. White matter is made of the axons that carry those signals between distant regions, most of them wrapped in myelin, a fatty sheath that gives the tissue its pale colour and speeds conduction. In the brain the grey matter is outside (the cortex) and in deep nuclei, with white matter between; in the spinal cord the layout is reversed, grey in the centre and white around it. Why does myelin make white matter conduct signals so much faster? An action potential on a bare axon has to regenerate itself on every patch of membrane, which is slow (around 0.5 to 2 m/s). Myelin wraps the axon in many layers of insulating membrane, leaving short gaps, the nodes of Ranvier, where the sodium channels are concentrated; current flows passively under the insulation and the spike is regenerated only at the nodes, so it appears to jump from node to node. Large myelinated axons reach tens of metres per second, up to about 120 m/s, which is what lets distant brain regions exchange signals within milliseconds.