Overcoming Internet Addiction For Dummies. David N. Greenfield

The limbic part of the brain is much older than the neocortex. Its development is often associated with emotion and more primitive survival functions, and it’s sometimes referred to as the mammalian brain. It has a long evolutionary history, having evolved over millions of years in mammals, far predating human evolution. Much of the limbic system’s job involves supporting various essential activities that developed to ensure species survival. By comparison, the neocortex is probably only about 200,000 years old, and as such, it’s a relatively new arrival on the block when it comes to brain development.

      So, what does the limbic area of the brain have to do with addiction? The answer is plenty, and it makes a lot of sense when looking at the relative utility of all biological structures and functions throughout the history of our species and understanding that nothing occurs by accident. This all becomes clear when you examine these functions throughout our evolutionary development; the following sections walk you through the brain’s evolution when it comes to addiction.

      Discovering humans’ original addictions

      Many brain structures involved in our survival have to do with pleasure, such as the ventral tegmental area, substantia nigra, amygdala, anterior cingulate, prefrontal cortex, hippocampus, and nucleus accumbens (Chapter 3 has more details). However, the nucleus accumbens (NA), located in the brain’s limbic system, is the major source of the experience of pleasure.

      

The nucleus accumbens is a dense collection of cells that are specifically receptive to dopamine. Dopamine is one of the major excitatory neurotransmitters responsible for pleasure and movement. So, why would our survival be linked to an excitatory pleasure neurotransmitter? Well, as nature would have it, a mammal’s survival is at least minimally based on the ability to engage in two behaviors on a predictable, consistent, and efficient basis. In other words, nature essentially needs a strong guarantee (from a genetic survival perspective) that can increase the odds of survival. These two behaviors (not surprisingly) are sustenance (eating, and all the behaviors associated with obtaining and consuming nutrition) and procreation (mating). Obviously, adequate nutrition is necessary for effective procreation as well.

The interesting thing here is that food — or more specifically, eating — is highly pleasurable. A strong biological pleasure drive (not just hunger) is linked to consuming food, and this is nature’s way of ensuring that food is consumed. So, when we eat, we experience an associated strong elevation of post-synaptic dopamine in the nucleus accumbens; this flood of dopamine is experienced as pleasure. Just think about your last great meal or the intensity of pleasure your dog seems to experience when eating. Food is extremely pleasurable and satisfying to consume for all mammals. Even without language or consciousness to understand this connection, on a strictly biological level, this pleasure increases the likelihood of a behavior being repeated reliably and consistently — thus ensuring survival.

      The same applies to procreation and sex. Obviously, it would make sense to make procreation pleasurable on some level. Not just humans, but all mammals have dopamine linked to their mating behavior. This is nature’s hedge to increase the odds of fulfilling the biological imperative — to get genes transferred into the next generation. This is the basic survival instinct that accounts for at least part of human behavior — the circle of life, if you will.

      Distinguishing the new brain from the old brain

      Sometimes we hear references to new brain and old brain. However, that is not entirely accurate. When we refer to new brain or old brain, what we’re really talking about is how the different parts of the brain developed from an evolutionary, biological perspective. As mammals evolved over eons, the brain developed in increasing complexity from the bottom up; this means that as evolution moved forward, it progressed from the lower brain, including the upper brain stem, to the midbrain or limbic system (where the reward centers are), and finally to the latest development in our neurobiological evolution, the neocortex, or new brain.

      By the way, it’s the prefrontal part of the cortex that helps inhibit the more animalistic survival drives that emerge from the more primitive limbic structures of the brain. And addiction is one of those hijacked drives piggybacked onto the limbic reward center gone awry.

Identifying the Parts of the Brain Involved in Addiction

      Many areas of the brain are involved in addiction, and most of them are found in the limbic system. The interesting thing about addiction is that it isn’t simply about dopamine and the pleasurable feeling it provides. Numerous other systems are involved with addiction, including the endocrine system and certain hormones, as well as other neurotransmitters. The major areas of the brain involved in addiction include the ventral tegmental area, substantia nigra, amygdala, anterior cingulate, prefrontal cortex, hippocampus, and nucleus accumbens (also discussed in Chapter 3).

The nucleus accumbens is the major area in the limbic region of the brain; it’s a dense bundle of cells where a very large number of dopamine receptors reside. Dopamine is one of the major excitatory neurotransmitters responsible for pleasure and movement, and along with serotonin and norepinephrine, it has a lot to do with mood, reward, motivation, pleasure, and compulsion. (Neurotransmitters are chemicals that enable nerve cells to communicate with one another.) The nucleus accumbens is the major traffic center for pleasure for most of the things we do, and as mentioned earlier, the brain is wired to make sure that certain behaviors associated with survival and thriving are pleasurable. It makes sense that nature would do this, as it increases our survival potential. Figure 2-1 shows the reward centers of the brain.

      © John Wiley & Sons, Inc.

      FIGURE 2-1: The reward centers of the brain.

      

As you read the following sections, keep in mind that addiction involves borrowing those pleasure pathways meant for survival, so in an odd way, addiction is really a natural phenomenon gone awry — much like many medical conditions. It’s also important to note that the human brain and nervous system are always about checks and balances — where excitatory neurotransmitters are held in check by other inhibitory neurotransmitters such as GABA (gamma-aminobutyric acid).

      Examining the role of neurons and neurotransmitters

Neurons are essentially the central part of the electrochemical connection process that makes up our nervous system; most of what we’re talking about with addiction involves the central nervous system, where neurons do their job. The central nervous system includes the brain and spinal cord. A neuron is made up of three major parts, as shown in Figure 2-2:

      © John Wiley & Sons, Inc.

      FIGURE 2-2: The major parts of a neuron.

       Soma, or cell body

       Dendrites, which are the connecting strands that allow all neurons to interconnect

       Axons, which are the fibers that act like electrical cables, connecting cell bodies to their dendritic terminals