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=                            Grey matter                             =
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                            Introduction
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Grey matter (or gray matter) is a major component of the central
nervous system, consisting of neuronal cell bodies, neuropil
(dendrites and myelinated as well as unmyelinated axons), glial cells
(astrocytes and oligodendrocytes), synapses, and capillaries. Grey
matter is distinguished from white matter in that it contains numerous
cell bodies and relatively few myelinated axons, while white matter
contains relatively few cell bodies and is composed chiefly of
long-range myelinated axons The colour difference arises mainly from
the whiteness of myelin. In living tissue, grey matter actually has a
very light grey colour with yellowish or pinkish hues, which come from
capillary blood vessels and neuronal cell bodies.


                             Structure
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Grey matter refers to unmyelinated neurons and other cells of the
central nervous system. It is present in the brain, brainstem and
cerebellum, and present throughout the spinal cord.

Grey matter is distributed at the surface of the cerebral hemispheres
(cerebral cortex) and of the cerebellum (cerebellar cortex), as well
as in the depths of the cerebrum (thalamus; hypothalamus; subthalamus,
basal ganglia - putamen, globus pallidus, nucleus accumbens; septal
nuclei), cerebellar (deep cerebellar nuclei - dentate nucleus, globose
nucleus, emboliform nucleus, fastigial nucleus), brainstem (substantia
nigra, red nucleus, olivary nuclei, cranial nerve nuclei).

Grey matter in the spinal cord is known as the grey column which
travels down the spinal cord distributed in three grey columns that
are presented in an "H" shape. The forward-facing column is the
anterior grey column, the rear-facing one is the posterior grey column
and the interlinking one is the lateral grey column. The grey matter
on the left and right side is connected by the grey commissure. The
grey matter in the spinal cord consists of interneurons, as well as
the cell bodies of projection neurons.


File:Cervical_vertebra_english.png|Cross-section of a spinal vertebra
with the spinal cord in the centre (and grey matter labelled).
File:Medulla spinalis - Section - English.svg|Cross-section of spinal
cord with the grey matter labelled.


Grey matter undergoes development and growth throughout childhood and
adolescence. Recent studies using cross-sectional neuroimaging have
shown that by around the age of 8 the volume of grey matter begins to
decrease. However, the density of grey matter appears to increase as a
child develops into early adulthood. Males tend to exhibit grey matter
of increased volume but lower density than that of females.


                              Function
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Grey matter contains most of the brain's neuronal cell bodies. The
grey matter includes regions of the brain involved in muscle control,
and sensory perception such as seeing and hearing, memory, emotions,
speech, decision making, and self-control.

The grey matter in the spinal cord is split into three grey columns:
* The anterior grey column contains motor neurons. These synapse with
interneurons and the axons of cells that have travelled down the
pyramidal tract. These cells are responsible for the movement of
muscles.
* The posterior grey column contains the points where sensory neurons
synapse. These receive sensory information from the body, including
fine touch, proprioception, and vibration. This information is sent
from receptors of the skin, bones, and joints through sensory neurons
whose cell bodies lie in the dorsal root ganglion. This information is
then transmitted in axons up the spinal cord in spinal tracts,
including the dorsal column-medial lemniscus tract and the
spinothalamic tract.
* The lateral grey column is the third column of the spinal cord.

The grey matter of the spinal cord can be divided into different
layers, called Rexed laminae. These describe, in general, the purpose
of the cells within the grey matter of the spinal cord at a particular
location.


File:Anatomy and physiology of animals A reflex arc.jpg|Interneurons
present in the grey matter of the spinal cord
File:Medulla spinalis - Substantia grisea - English.svg|Rexed laminae
groups the grey matter in the spinal cord according to its function.


                       Clinical significance
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High alcohol consumption has been correlated with significant
reductions in grey matter volume. Short-term cannabis use (30 days) is
not correlated with changes in white or grey matter. However, several
cross-sectional studies have shown that repeated long-term cannabis
use is associated with smaller grey matter volumes in the hippocampus,
amygdala, medial temporal cortex, and prefrontal cortex, with
increased grey matter volume in the cerebellum. Long-term cannabis use
is also associated with alterations in white matter integrity in an
age-dependent manner, with heavy cannabis use during adolescence and
early adulthood associated with the greatest amount of change.

Meditation has been shown to change grey matter structure.

Habitual playing of action video games has been reported to promote a
reduction of grey matter in the hippocampus while 3D platformer games
have been reported to increase grey matter in the hippocampus.

Women and men with equivalent IQ scores have differing proportions of
grey to white matter in cortical brain regions associated with
intelligence.

Pregnancy renders substantial changes in brain structure, primarily
reductions in gray matter volume in regions subserving social
cognition. The gray matter reductions endured for at least 2 years
post-pregnancy. The profile of brain changes is comparable to that
taking place during adolescence, another hormonally similar
transitional period of life.


Etymology
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In the current edition of the official Latin nomenclature,
'Terminologia Anatomica', 'substantia grisea' is used for English
'grey matter'. The adjective 'grisea' for 'grey' is however not
attested in classical Latin. The adjective 'grisea' is derived from
the French word for grey, 'gris'. Alternative designations like
'substantia cana'  and 'substantia cinerea' are being used
alternatively. The adjective 'cana', attested in classical Latin, can
mean 'grey', or 'greyish white'. The classical Latin 'cinerea' means
'ash-coloured'.


                         Additional images
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Image:Human brain right dissected lateral view description.JPG |Human
brain right dissected lateral view
Image:Gray678.png|Schematic representation of the chief ganglionic
categories (I to V).


                              See also
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* Grey matter heterotopia


                           External links
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* [http://www.livescience.com/32605-why-is-gray-matter-gray.html Why
Gray Matter is Gray]


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