This is a brain cell, it is the carrier of various neurotransmitters. The myelin sheath is a sort of insulation around the axon. This allows some thoughts to be more common than others. It generally strengthens during adolescence which is part of the reason that your upbringing shapes a lot of who you are. The main thing to remember with this is that cells form connections with each other, the stronger and more extensive it is the more likely that a thought or activity will ensue. This goes into a lot of different thought patterns, some people have negative thoughts or patterns to elicit a hormone or neurotransmitter response. Such as thinking angry thoughts make you more awake and observant so in doing this you will be able to pay attention more. This is part of the reason why some people dislike their jobs or day to day activities, they are simply trying to stay awake.