White matter of telencephalon

At a glance

English

White matter of telencephalon

Latin (TA2)

Substantia alba telencephali

System

Brain and spinal cord

Region

Head

Side

Paired, left and right

About the white matter of telencephalon

The white matter of the telencephalon, the cerebral white matter, fills most of each hemisphere beneath the folded sheet of grey cortex. It is made of nerve fibres (axons), most of them wrapped in myelin, together with glial cells and blood vessels; it contains no nerve cell bodies and no dendrites of note. Myelin is rich in lipid, which gives the tissue its pale colour. A fresh brain looks pinkish-white because of its blood, and the white contrast is sharpest after fixation. Embedded in it are the deep grey masses of the basal nuclei, such as the caudate, putamen and globus pallidus, and the lateral ventricles lie within it.

Its fibres fall into three classes by where they go. Association fibres stay within one hemisphere: short U-fibres join neighbouring gyri, and long bundles such as the superior longitudinal (arcuate), inferior longitudinal, uncinate and inferior fronto-occipital fasciculi and the cingulum join distant lobes. Commissural fibres cross the midline to the other hemisphere, chiefly in the corpus callosum, with the smaller anterior and hippocampal commissures. Projection fibres connect the cortex with the thalamus, basal nuclei, brainstem and spinal cord; they fan out under the cortex as the corona radiata and gather into the internal capsule between the thalamus and the lentiform nucleus. The centrum semiovale is the large oval of white matter seen above the ventricles in a horizontal slice.

The myelin sheath, formed by oligodendrocytes, insulates the axon and lets impulses jump from node to node, so signals travel much faster than in unmyelinated fibres. White matter is not merely passive cabling; its myelination continues to change with learning, as shown by imaging after people practise a new motor skill, and it keeps maturing into middle age, later than the cortex. Diffusion tensor imaging can trace its main bundles in the living brain.

Many diseases strike it. Multiple sclerosis causes patches of inflammatory demyelination. Small-vessel disease from high blood pressure and ageing produces diffuse changes, seen as bright areas on MRI (leukoaraiosis), linked with slowing of thought, depression and vascular dementia. Strokes in the deep branches of the middle cerebral artery damage the internal capsule and cause weakness of the opposite side. Inherited leukodystrophies, head injury with shearing of axons, and heavy alcohol use also reduce or damage it.

Function and supply

Function

Carries signals between cortical areas of one hemisphere, between the two hemispheres, and between the cortex and deeper grey matter, brainstem and spinal cord.

Blood supply

Long medullary branches from the cortical branches of the anterior, middle and posterior cerebral arteries, and deep perforating (lenticulostriate and choroidal) arteries; the deep zone is an end-arterial border area prone to ischaemia.