Okay - here's a quick potted neurolgy primer. If there's any inacurracies - let me know and i'll update it. It's likely you may know a lot/most/all of this stuff, in which case simply skip ahead, this isn't intended to patronise, but rather provide a primer for a layperson with no knowledge.
Brains of different animals have shared structures; the more 'advanced' an animal the more structures it will share with humans. Dolphin & Whale brains are significantly physically larger than our own.
Two parts are in essence 'grey matter' and 'white matter'. This is because the main cells we think of as 'brain cells' Neurons ( also spelled neurones ) have a body and a long tail. Incoming signals to a single neuron will accumulate and once they reach a certain threshold, they will discharge an electrical signal down the tail, which when reached will release whatever transmitter chemical it has. The neurotransmitter chemical is used to bridge the gaps between neurons when sending messages. ( the release of transmitter from a neuron is 'the message' ). The availablility of neurotransmitter and whether it will fire and be recieved depends upon a number of factors relative to the availability of NT (neurotransmitter) and whether any adjacent neurons have available places with which the transmitter can attach.
The key ones are; speed of release of transmitter once attached, how fast the space between them is cleared up of neurotransmitter, how fast either NT or broken down components of NT are made available to the transmitting neurons.
Some chemicals have a shape such that they will attach to NT sites, but will not trigger a response. These are called Blockers Some chemicals promote 'reuptake' of the NT ( recyling the NT : clearing the space, and providing material to the transmitting neuron) Some chemicals can block their action. These are reuptake inhibitors.
The number of sites available on a given neuron can vary; this is often a function of availability of NT that they respond to.
Salts also are of massive importance here as the proportions are necessary to maintain regular function of the NT cycle. Hence why eating vast quantities of salt, or becoming hyperhydrated can kill. Calcium is also essential for this mechanism to work correctly. This is because the salts are able to alter the conductivity of the ion channels at the boundaries of the cells. Some chemicals (such as nicotine) alter the conductivity and ability of the salts to activate the ion channels ( and thus how much if any NT is transmitted ).
The neurons are not the only cells in the brain, in fact they are significantly outnumbered by glial cells; which provide a lot of the structure to the brain itself. They provide the 'white' color of the middle of the brain. The glial cells most commonly referred to are the schwann cells, which wrap the tails of the neurons. These cells form a nonconductive myelin wrapping surrounding the tail, which in turn allows the electrical signal to flow with the minimum of impedence.
Neurological disorders such as Multiple Sclerosis are the effects of de-myelination in the brain; such that the transmission of signals becomes ineffectual at those sites. Hence why the range of aspects and symptoms involved is so vast.
Given the sheer volume of cells involved cross-talk does happen, and this leads to a feature where different structures that are adjacent will often be able to increase activation of their adjacent structures. Seasonal Affective Disorder is a common example of this effect. (Although the pineal receptors do have a certain similarity to the visucortical receptors).
Right. That's that part done.
Structures.
In order for brevity; if you imagine the brain as a tube, the further up and away from the spinal cord you get, the 'newer' the structures tend to be. ( I'm not going to go into the whole evolution thing for those that it either 'doesn't click', or 'upsets your internal value systems' , so lets just agree on it being order during fetal development )
The older the structure the primitive and necessary it is for you. As you would imagine the life support system of the brain is at the lowest part of the brain, where the spinal cord becomes the brain. This is Reticular Activating System/Structure or RAS. This regulates the heart and breathing and other mechanisms necessary for staying alive.
It also is the most widely connected; with connections in every part of the brain, and with the majority of different NeuroTransmitters. It is the suppression or overstimulation of the RAS that is what makes overdoses fatal. I feel i need to point out here that THC one of the key active ingredients in cannabis has NO connections whatsoever to the RAS and as such it is physically impossible to die from an overdose in the normal method; ( i'm sure some pedant will point out that you could gag to death on 1/2 a ton of it or some other bullshit, but you get the picture ).
It is here that the picture of addiction starts to arise, as two key neurotransmitters involved with the RAS are the cholinergic and adrenergic systems, and it is these that I will now describe after I describe the different nervous systems that the RAS regulates.
There are three main components of the autonomic (or involuntary) nervous system, these being the sympathetic nervous system, parasympathetic nervous system and enteric nervous system
The sympathetic nervous system is the mechanism by which the emergency adrenal fight or flight system works. The parasympathetic nervous system is used to for the non-emergency regulation of the bodys glands and organs, with The enteric nervous system being used to regulate digestion (the gastrointestinal system); this last one only interacts with the brain through the other two and in effect is the most independent subsystem that we have, it also acts as a vast resevoir of NT's.
The parasympathetic nervous system requires some discussion as it's the feed'n'breed aspect of ourselves. The key neurotranmitter familty is the cholinergic system; and uses acetylcholine as the primary transmitter. The receptors for this neurotransmitter are the muscarinic and nicotinic cholinergic receptors ( as some are mediated by muscarine and the others nicotine ). These two additional receptors provide a mediating & balancing effect by altering the conductance of the salts involved in transmission; ( a dual action is played by both of them, for which i'd definately have to dig out my old notes for the exact mechanism of interactions, as my memory is failing me a tad here, as it was about 20 years ago i did this stuff ).
Although muscarine receptors affect heart rate and breathing; whereas nicotine affects digestion and metabolism.
As it was given as a comparison to heroin, i'll just point out here that the two ( nicotine also is used in the sympathetic nervous system ( i'll mention in a bit ) ) are seen as addictive, nicotine in itself is not the direct transmitter, but more of an ancillary transmitter in its mechanism, in that it influences how easy it can be for the actual cells to fire. This more complex and subtle mechanism is what gives nicotine it's ability to both stimulate and calm people at the same time, and can make other drugs that work on the same pathways more effective. With respect to the dopaminergic reward system once again it does not directly affect the receptors, but instead indirectly affects how likely they are to be able to be fired. (hence its addictive capability, and why I maintain that it is not as addictive as heroin).
... I've got stuff to do, and there's still a fair bit to cover. ( such as the sympathetic nervous system ( adrenal ), and the dopaminergic system. I can also cover the different structures, and their direct useages, along with how they affect other proximal structures. As well as associated ailments and behviours seen with respect to parts not working. or any other subsystem. ) I'm open to suggestions as to what else to cover. Please note, while I can and would be happy to go into significantly deeper detail, this document is intended as an introductary primary and high level view, so that those things will make more sense later. If demand is sufficient I will go into depth, but not before giving an overview first.