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PATHOPHYSIOLOGY: STUDY OF WHAT GOES WRONG 19of the nervous system and the structure and function of a computer system. Bothreceive input, both process this input, and both produce an output. However, computercircuitry is much simpler than the circuitry of the brain in two importantways.First, computer circuits are hard-wired at the factory. A computer’s circuitrynever changes. In contrast, as we noted previously, brain circuitry is constantlyadapting through a process of pruning and reestablishing connections. This pruningaffords the nervous system a plasticity that enables it to adapt continually to thedemands of an ever-changing environment.Second, computer circuits have only two conditions: OFF and ON. Conversely,a single circuit in the nervous system can have a variety of graded responses.Although there are several explanations for this graduated circuit activation, a principalbasis is that more than one neurotransmitter can be utilized in a single circuit.Most circuits have a single primary neurotransmitter. There are over 100 neurotransmittersubstances that have been identified, and more are yet to be discovered, butthe most common primary neurotransmitters that you’ll read about are serotonin,dopamine, norepinephrine, acetylcholine, glutamate, and GABA, as well as a novelclass of neurotransmitters called neuropeptides. In a given neural pathway, the secondaryneurotransmitters typically act as cotransmitters that modulate the signal ofthe circuit’s primary neurotransmitter.The importance of brain circuitry is that it reminds us that these chemical neurotransmittersare not released willy-nilly throughout the brain. This is why the term“chemical imbalance” is so unsatisfactory. The brain is not a large water bottle intowhich these chemicals are poured and maintained in some precise balance. For thatreason, there is no simple test to check a neurotransmitter level like the dipstick onyour car’s oil reservoir. Instead, your brain is a delicate instrument with predefinedbut ever-adapting pathways of communication.Each of these pathways (or circuits) serves a particular purpose and works inconcert with other circuits. Technology has only recently allowed us a glimpse ofthese circuits in action. Thus, we are just beginning to map out the pathways andfunctions of the myriad brain circuits involved in the regulation of thought andemotion. As you’ll see later, however, understanding brain circuitry can help us toanticipate what the beneficial effects and side effects of a medication may be.2.3 PATHOPHYSIOLOGY: STUDY OF WHAT GOES WRONGBy now, you know that the nervous system is a communications network that servesto control your body. You also know that nerve impulses are the vehicle that carriesinformation around this network. In addition, you know that chemical neurotransmissionis the means by which these signals are passed from one neuron to another.Finally, you know that neurotransmitters, receptors, and enzymes are the key componentsthat make all of these things happen.We have long suspected that abnormalities in the receptors and enzymes thatinteract with a neurotransmitter are one of the major underlying causes of many