There are at least five problems with the chemical imbalance model of mental disease:
1) the model ignores the reality that there has never been a consistently reproducible biological marker, to substantiate the levels of normal or abnormal neurotransmitters in the human nervous system
2) the model fails to respect the enormous complexity of neurotransmission in the human brain:
a) there are over five kinds of dopamine receptors which have been characterized to date, and even the best researchers know nothing about the D5 subtype
b) there are five separate kinds of cholinergic receptors
c) there are fifteen different kinds of serotonin receptors
d) neuroscientists do not yet understand the relationship between neuroreceptor density, sensitivity, or neurotransmitter turnover
3) the model fails to consider the fact that many of the neurochemicals which are presumed to be the basis of "mental disease" are, in fact, broadly distributed throughout the body. This fact casts doubt about our conceptualization of "brain tissue" (perhaps it is not limited to the cranial vault) and also raises questions about the reliability of serum or urine tests, as those assays may be capturing levels which reflect non-brain locations of neurotransmitter activity:
a) over 90% of the serotonin in the human body is made by the enterochromaffin cells of the stomach and small intestine, rather than the raphe nucleus of the midbrain and pons
b) a broad variety of cells in the human body possess receptors for many of the neurotransmitters, including white blood cells and platelets
4) the model fails to acknowledge the impossibility of measuring discrete events in the human brain, due to the speed of neurotransmission; and due to the relative bulk of our measuring devices, relative to the size and complexity of each synapse
5) the model fails to acknowledge the impossibility of explaining the brain in reductionistic terms. That is to say, the organic whole may so far exceed the sum of the component parts, that science will never be able to fully explain the workings of this magnificent system. Part of the problem here is that the brain is never capable of being studied in a vacuum - the system is forever open, due to the conscious, and unconscious, processes of the subject who is being observed. Part of the problem, too, arises from the phenomenon of diaschisis, or non-local effects, through which changes in one part of the brain reflect, and then precipitate, complex cascades of events in multiple locations throughout the nervous system. Thus, it is impossible to speak of serotonin or dopamine without analyzing the interactions of all complex chemicals, peptides, and amino acids upon each other, but far too little research has occurred to study the gestalt of these intercommunications.
