Anxiety disorders are the most prevalent mental disorders in thegeneral population [1]. They are a family of related but distinctdisorders. Anxiety disorders are complex disorders and may bedifficult to recognize and treat, especially if there are comorbidproblems such as depression and substance abuse. There areno distinctively characteristic markers to make a presumptivediagnosis of an anxiety disorder despite an abundance of physiologicsymptoms [1]. Psychophysiology, a noninvasive tool dealingwith the correlation of the mind and body, historically hasrelied on electrical signals generated by the body and recordedby way of electrodes on the scalp, hands, and face. It is used toquantify normal and abnormal physiological activity and reactivity[2]. In addition to measures such as palmar sweating, respiration,heart rate, blood pressure, and reflexes, psychophysiologyhas incorporated neuroendocrine physiology and brain imagingsuch as positron emission tomography (PET) [2]. It is interestingto note that despite physiological symptoms playing a crucialrole in the diagnostic profile of anxiety disorders, the evaluationof symptoms and the formal diagnosis are based primarily onverbal self-reports. Physiological measures have the potential tohelp objectively assess and better characterize anxiety symptomsand identify psychological and neurobiological dysfunctions [2].Psychophysiology essentially remains a research tool, however,regarding the causes and nature of anxiety.

NEUROTRANSMITTERS
To improve one’s grasp of basic psychopharmacology and how effective psychiatric medications exert their beneficial as well as side effects, this chapter provides an overview of neurotransmitters (chemical messengers), receptors, and other biological information. The information includes general principles and definitions that can facilitate a better understanding of anxiety disorders and their comorbidities. The basic definition of a neurotransmitter is a substance that excites or inhibits a target cell [1]. A more detailed and technical classic definition of a neurotransmitter includes the following:

  • Synthesized and released from neurons
  • Released from nerve terminals in an identifiable form (chemical or pharmacological)
  • Interacts with a postsynaptic receptor with the same effects seen with presynaptic neuron stimulation
  • Interacts with a postsynaptic receptor with a specific pharmacology
  • Terminates actions by an active process

Under certain circumstances, some neurotransmitters, neuromodulators, and neurohormones do not meet all the criteria [6], but are still referred to as such.
Compounds that are active in the central nervous system (CNS) are either made locally in the brain or cross the blood–brain barrier. Dozens of substances within the CNS function as neurotransmitters. Different chemicals can convey different information within the brain. It is interesting to note that a single neuron can distinguish information from many different incoming nerve terminals by possessing different receptor sites. The same chemical can convey different messages as well, depending on its action on receptor subtypes [6]. Other important ways of conveying information in the brain include different neuronal firing patterns andspatial coding [6].