Gray Matter/White Matter

Brady

I don't understand how specific areas of the brain can contain gray or white matter. The definition of gray matter is a category of nervous tissue with many nerve cell bodies and few myelinated axons, but how is this possible? Aren't nerve cells always complete? In other words, my line of logic says: Nerve cell bodies are part of nerve cells. They are not separate entities. If there are a lot of nerve cell bodies, then there are a lot of nerve cells. Therefore, there are a lot of axons as well.

How is gray/white matter distinguised if everything's part of the neuron?

Thanks for the help.

neurocomp2003

the distinguished effect is myelination on axons..the more myelination the more it goes from gray to white. Not all axons are myelinated...
if i remember correctly in development its myelination from birth(or yr2(terrible 2s)) to year5 and pruning from yr5-teens.

honestrosewater

A neuron (nerve cell) consists of a cell body and processes (axons and dendrites). The processes branch off from the cell body. Just look at this diagram. Imagine that a bunch of those neurons are all lined up in the same way, with cell bodies and dendrites all next to each other and the axons all next to each other. Your diagram would then have a big blotch of yellow (cell bodies and dendrites) and a blotch of purple (myelinated axons).

DocToxyn

Just to bring a little more to the discussion. In the brain, position, structure and order is the key to everything. Nerve cell bodies are typically organized into specific units within the brain, usually collectively called nuclei. In many cases the nuclei are sending axons to similar areas or at least in a similar direction, i.e., caudal to rostral, left hemisphere to right. So the best way to make this happen is to bunch all the axons together into a fiber bundle, like a bunch of wires, and route them this way. Thus these wires, each covered in the insulating myelin, make up the white matter because in those areas they are the predominant structure. Regions like the anterior commisure, medial forebrain bundle, corpus collosum and optic chiasm are some white matter regions that come to mind. Regions where you have a majority of cell bodies/dendrites would be more gray areas, but this does not mean there aren't any axons there, they just aren't as densely packed as they are in white matter areas.

Brady

Thanks a lot for the responses, I think I understand better now. So everything's in terms of relative concentration?

DocToxyn

At the most basic level, that is pretty much correct.