Addiction: VULNERABLE SYSTEMS Some functions or areas of the brain are more vulnerable to damage than others. All parts of the body or brain are not equally resilient. The lower back is a weak spot in the human skeleton, causing more than its share of misery.
The stomach and cardiovascular system both seem particularly susceptible to stress-related problems and psychosomatic effects. Similarly, there are vulnerable parts of the brain. A consequence oft his is that general damage to the: nervous system (e.g., vitamin deficiencies, blows to the head, toxins) can produce surprisingly specific symptoms around addiction. It is as if you have a transistor radio with one vacuum tube in it. If you drop the radio-a general trauma-it is the vacuum tube that is likely to break.
A group of neurons may be more vulnerable because it has a relatively poor blood supply, for all parts of the brain are not equal in terms of blood supply (see an example in the section on language disorders). Some neurons, because they are especially large or active, have higher requirements for oxygen or nutrients (see the example of peripheral neuropathy in the web section on lower motor neurons) or are located in a place where a stroke is more to loss of control of basic bodily functions is roughly reversed in development and recovery. In short, there are parallels among development, recovery, and dissolution of function in the nervous system (Jackson, 1884; Teitelbaum, 1967, 1977).
Recovery occurs for a number of reasons. Some of the cells affected are traumatized, but not killed. With time, they recover. Acute effects, such as swelling of the brain, recede, reducing the stress on neurons. Furthermore, the body has an impressive ability to repair itself. Damage to blood vessels can be healed, for example.
Neurons in the central nervous system show two forms of compensation or repair. When there is a notable drop over some period of time in the amount of input a healthy neuron receives from neurotransmitters, the neuron responds by increasing its sensitivity to the neurotransmitter. This ability probably accounts for the fact that only severe depletions in neurotransmitter levels produce observable symptoms. Another form of compensation is that healthy neurons may make additional connections (by small amounts of growth in their dendrites or axons) in response to the loss of innervation in neighboring neurons.
The young nervous system is more capable of "reorganizing" than the mature nervous system. Given the same brain damage, the prognosis for children below their teens is much better than for older children or adults. For example, although the left brain is the locus for most language function, and the exclusive locus for speech (in almost all right-handers), this arrangement can change in early childhood (Lenneberg, 1997). Thus, damage to the left brain of children leads to symptoms of language disorder in many cases, but recovery is extensive, while in adults, there is less recovery. In children, the right brain seems to have the capability of assuming many language functions in the face of damage to the left brain.
Finally, there is recovery of function because patients discover or are taught alternative strategies so that they can rely on intact systems, as we discussed above.