1. The White Matter is composed of axons whose function is to conduct electrical signals rapidly from one site to another in the CNS.
2. Gross examination of sections of the brain display white matter
3. Within each section of the white matter there are thousands if not millions of nerve fibres and gross examination does not provide information about their origin or destination (target).
4. However gross visual examination identifies certain areas that have important functions.
5. One such structure is the corpus callosum which carries axons from one hemisphere to the other.
6. Another is the Internal Capsule, a narrow area where axons communicating between the cortex and the brainstem or spinal cord pass between the caudate nucleus and the putamen, globus pallidus and thalamus
7. The internal capsule is of particular importance because it is supplied by the middle cerebral artery, which can become occluded and cause stroke.

Artist's Impression of the white matter of the brain (http://johnjchampionseatacarticle.org/)

Five Mysteries of the brain

Touch and Vibration.

Information about light touch and vibration pass through the dorsal columns to reach the dorsal column nuclei on the same side of the medulla. Here there is a relay and the second order neurones cross over to the opposite side of the neuraxis in the medulla to reach the thalamus. This is sometimes called the 'sensory decussation'.

Pain and Temperature.

Information about injurious stimuli and temperature are relayed in the dorsal horn of the same side of the body. Within a few segments, the axons of these neurones then pass to the opposite lateral columns and project to the thalamus in the spino thalamic tract, which is the 'classical' pain pathway. Other ascending tracts carrying information about nociception pass rostrally in the antero-lateral system and other smaller pathways.

The dorsal columns contain sensory axon collaterals that project rostrally to the dorsal column nuclei at the caudal end of the medulla. They carry information concernted with somatic sensations such as touch and proprioception.

In lower segments of the cord the dorsal columns are thin, and these fibres remain in a medial position.

Axons from higher segments of the cord are added to the lateral edge of the dorsal columns. In the cervical region there are two distinct components of the doral columns - the fasciculus gracilis (medial) and fasciculus cuneatus (lateral).

In some segments the dorsal columns contain proprioceptive afferents that project to the cerebellum, e.g in the lower segments of the cord, where these afferents project to Clarke's Column. In the cervical cord, they project to the accessory cuneate nucleus.