White matter

White matter
The formation of the spinal nerve from the dorsal and ventral roots. (White matter labeled at right.)
Latin	substantia alba
Dorlands/Elsevier	s_27/12766685

White matter is one of the three main solid components of the central nervous system designated by color. The other two are gray matter and substantia nigra.

Structure

It is composed of myelinated nerve cell processes, or axons, which connect various gray matter areas (the locations of nerve cell bodies) of the brain to each other and carry nerve impulses between neurons.

Cerebral and spinal white matter do not contain dendrites, which can only be found in gray matter along with neural cell bodies and shorter axons.

Function

The white matter is the tissue through which messages pass between different areas of gray matter within the nervous system. Using a computer network as an analogy, the gray matter can be thought of as the actual computers themselves, whereas the white matter represents the network cables connecting the computers together. The white matter is white because of the fatty substance (myelin) that surrounds the nerve fibers (axons). This myelin is found in almost all long nerve fibers and acts as an electrical insulation. This is important for it allows the messages to pass quickly from place to place.

The brain in general (and especially a child's brain) can adapt to white matter damage by finding alternative routes which bypass the damaged areas of white matter and can therefore maintain good connections between the various areas of gray matter.

Unlike grey matter, which peaks in development in a persons twenties, the white matter continues to develop and peaks in late middle age.

Location

White matter forms the gay of the deep parts of the brain and the superficial parts of the spinal cord. Aggregates of grey matter such as the basal ganglia (caudate nucleus, putamen, globus pallidus, subthalamic nucleus, nucleus accumbens) and brain stem nuclei (red nucleus, substantia nigra, cranial nerve nuclei) are spread within the cerebral white matter.

The cerebellum is structured in a similar manner as the cerebrum, with a superficial mantle of cerebellar cortex, deep cerebellar white matter (called the "arbor vitae") and aggregates of grey matter surrounded by deep cerebellar white matter (dentate nucleus, globose nucleus, emboliform nucleus, and fastigial nucleus). The fluid-filled cerebral ventricles (lateral ventricles, third ventricle, cerebral aqueduct, fourth ventricle) are also located deep within the cerebral white matter.and suppl

Types of astrocytes

In 1983, M. C. Raff et al. discovered that tissue samples originating from rat optic nerve contained two morphologically distinct types of astrocytes.

• So-called "Type 1 astrocytes" had a fibroblast appearance and resided in both gray matter and white matter.
• "Type 2 astrocytes" has a neuron-like appearance and resided in white matter alone (Sherman, Chris).

Clinical relevance

Multiple Sclerosis is one of the most common disease which affects white matter. In MS lesions, the myelin shield around the axons has been destroyed by inflammation.

Changes in white matter known as amyloid plaques are associated with Alzheimer's disease and other neurodegenerative diseases. White matter injuries ("axonal shearing") may be reversible, while gray matter regeneration is less likely.

A recent Canadian study has shown that pedophiles "have less brain white matter". [1]

Notes

External links

• White Matter Atlas
• White+matter at eMedicine Dictionary

Histology: nervous tissue
Neurons (gray matter)	soma, axon (axon hillock, axoplasm, axolemma, neurofibril/neurofilament), dendrite (Nissl body, dendritic spine, apical dendrite, basal dendrite) types (bipolar, pseudounipolar, multipolar, pyramidal, Purkinje, granule)
Afferent nerve/Sensory nerve/Sensory neuron	GSA, GVA, SSA, SVA, fibers (Ia, Ib or Golgi, II or Aβ, III or Aδ or fast pain, IV or C or slow pain)
Efferent nerve/Motor nerve/Motor neuron	GSE, GVE, SVE, Upper motor neuron, Lower motor neuron (α motorneuron, γ motorneuron)
Synapses	neuropil, synaptic vesicle, neuromuscular junction, electrical synapse - Interneuron (Renshaw)
Sensory receptors	Free nerve ending, Meissner's corpuscle, Merkel nerve ending, Muscle spindle, Pacinian corpuscle, Ruffini ending, Olfactory receptor neuron, Photoreceptor cell, Hair cell, Taste bud
Glial cells	astrocyte, oligodendrocyte, ependymal cells, microglia, radial glia
Myelination (white matter)	Schwann cell, oligodendrocyte, nodes of Ranvier, internode, Schmidt-Lanterman incisures, neurolemma
Related connective tissues	epineurium, perineurium, endoneurium, nerve fascicle, meninges

Anatomy of torso (primarily): the spinal cord
Spinal nerve	Dorsal (Root, Ganglion, Ramus) • Ventral (Root, Ramus) • Sympathetic trunk • rami communicantes (Gray, White)
Gray matter/Rexed laminae	Posterior horn (Column of Clarke, Substantia gelatinosa of Rolando, Nucleus proprius) • Lateral horn • Anterior horn • Central canal/Substantia gelatinosa centralis
White matter: somatic/ascending (blue)	Posterior/PCML: Gracilis • Cuneatus Lateral: Spinocerebellar (Dorsal, Ventral) • Spinothalamic (Lateral, Anterior) • Spinotectal • Posterolateral (Lissauer)
White matter: motor/descending (red)	Lateral: Corticospinal (Lateral) • Ep (Rubrospinal, Olivospinal) Anterior: Corticospinal (Anterior) • Ep (Vestibulospinal, Tectospinal, Reticulospinal)
Layers	Epidural space • Dura mater • Subdural space • Arachnoid mater • Subarachnoid space • Pia mater
Other structures	Denticulate ligaments • Conus medullaris • Cauda equina • Filum terminale ��� Cervical enlargement • Lumbar enlargement • Anterior median fissure