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The central nervous system is made up of two types of tissue: the grey matter and the white matter.
The grey matter is mainly composed of neuronal cell bodies and unmyelinated axons. Axons are the processes that extend from neuronal cell bodies, carrying signals between those bodies. In the grey matter, these axons are mainly unmyelinated, meaning they are not covered by a whitish-colored, fatty protein called myelin.
The grey matter serves to process information in the brain. Structures within the grey matter process signals generated in the sensory organs or other areas of the grey matter. This tissue directs sensory (motor) stimuli to nerve cells in the central nervous system where synapses induce a response to the stimuli. These signals reach the grey matter through myelinated axons that make up the bulk of the white matter in the cerebrum, cerebellum and spine.
Also found in the grey matter are the glial cells (astroglia and oligodendrocytes) and capillaries. The glial cells transport nutrients and energy to the neurons and may even influence how well the neurons function and communicate. Because axons in the grey matter are mainly unmyelinated, the greyish hue of the neurons and glial cells combine with the red of the capillaries to give this tissue its greyish-pink color (after which it is named).
White matter, on the other hand, is mainly composed of long-range myelinated axons (that transmit signals to the grey matter) and very few neuronal cell bodies. Myelin forms a protective coating around these axons, insulating them and improving their transmission of neuronal signals. This myelin-dense nervous tissue is therefore whitish in color. The grey matter does contain some myelinated axons, but only a few compared to the white matter, which is where the color difference arises.
White matter is found buried in the inner layer of the brain’s cortex, while the grey matter is mainly located on the surface of the brain. The spinal cord is arranged in the opposite way, with grey matter found deep inside its core and the insulating white matter wrapped around the outside. Some grey matter is also found deep inside the cerebellum in the basal ganglia, thalamus and hypothalamus and white matter is also found in the optic nerves and the brainstem.
The cerebrum is where the more complex brain functions occur and in humans and other large vertebrates, this structure has grown to form a convoluted layer of grey matter. The larger the animal, the more convoluted this grey matter is. Small animals such as the marmoset tend to have smooth brains, while in larger mammals such as the whale or elephant the grey matter is highly convoluted. Inside this outer cortex of grey matter is the white matter containing the myelinated nerve fibres.
Further details about where exactly the grey matter is located are given below.