These are some very schematic drawing, which I use to teach medical students. I'm no neuro-scientist but, if you understand the major mechanisms, it will help you to understand the other factors involved.

This is the afferent neuron. Afferent means it's coming into the brain. There are billions and billions of these. They look like a cobweb. Over here is the efferent neuron. It is going out of the brain. The impulse is not a solid core nerve transmission. It goes through to an extra cellular fluid space, called the synaptic cleft. You'll notice Tyrosine on the left, that is an amino acid and, when it's broken down by an enzyme, it forms little vesicles of a neurotransmitter called Dopamine. Dopamine (DA) is the chemical boat that's going to carry the impulse. It is basically a stimulant neurotransmitter that gives us a pleasure pay-off.

There are many other neurotransmitters but this is the one we really want to understand most fully because of it's pay-off role.

So, now these little vesicles of dopamine are formed and ready to go. In comes a normal impulse; the dopamine goes out into that little fluid space; goes to those little hair follicles, which are called receptor sites; then it turns around and comes back over these re-uptake sites, where the dopamine is reabsorbed and put back into a vesicle, where it is ready to go again.

This system will work in perpetuity, if it's not interrupted by age, disease, drugs or death. In drugs, alcohol and compulsive behaviours this is where the problem lies.