Thinking about someone who has a serious spinal cord injury allows us to evaluate behavior and illuminates the fact that the spinal cord is the main vessel through which information is channelled up to the brain or is received and sent back out to the extremeties as motor output. If the spinal cord is cut, from the damaged point down, the body is disconnected from it's command center, leaving it seemingly lifeless. Although the body is paralyzed, it can still receive environmental stimuli, even if it is not able to send this information to the brain and receive "acknowledgment", and inturn react. The separation of the body and the brain as is the case when the spinal cord is severed, serves as a model for other behaviors. If any of the infinite "boxes" that make up the brain were for some reason disconnected or became unable to respond to incoming information, the chain of action potentials or signals is broken and the behavioral outcome is different. More specifically, for example,depressed individuals have lower levels of some neurotransmitters in the brain. If we assume that these unusually low levels somehow contribute to the depressed behavior then we can integrate the above model. If a smaller than normal amount of neurotransmitter is released at the synapse, then the intended signal may not be passed along- or acknowledged by the adjoining neuron. This leaves the receiving pathway unstimulated, which results in an abnormal behavior, or one that deviates from the norm due to the pathways miscommunication.

Interesting and appropriate line of thought. Add one idea though, that the disconnected things (as in the case of the spinal cord) are not necessarily "lifeless". They may continue to act (foot withdrawing when toe is pinched) but no longer does so in appropriate relation to the actions of other things. "Blues" occur in most people's lives, but can be alleviated by various other factors. Perhaps depression is an active "box", no longer able to be turned off by other inputs? PG