The central nervous system (CNS) can be likened to a computer processor that is the command center for most if not all of the functions of the body. The peripheral nervous system is like a set of cables that transfers critical data from the CNS to the body and then feeds back information from the body to the CNS. This “computer system” is very sophisticated and is designed to continually make appropriate adjustments to its inputs and outputs in order to allow one to react and adapt to changes in the external and internal environment (sensory systems), to maintain posture, permit locomotion, and use the fine motor control in our hands to create pieces of art (somatomotor system), to maintain homeostasis (autonomic nervous system), to regulate the transitions between sleep and wakefulness (consciousness), and to allow us to recall past events and to communicate with the outside world (higher cortical functions). This section on neurophysiology will describe the fundamental properties and integrative capabilities of neural systems that allow for the exquisite control of this vast array of physiologic functions. Medical fields such as neurology, neurosurgery, and clinical psychology build on the foundation of neurophysiology.
One of the most common reasons that an individual seeks the advice of a clinician is because they are in pain. Severe chronic pain involves the rewiring of neural circuits that can result in an unpleasant sensation from even a simple touch to the skin. Chronic pain is a devastating health problem that is estimated to affect nearly 1 in 10 Americans (more than 25 million people). Within the past decade or so there have been considerable advancements made in understanding how activity is altered in these individuals and in identifying receptors types that are unique to nociceptive pathways. These findings have led to an expanding research effort to develop novel therapies that specifically target synaptic transmission in central nociceptive pathways and in peripheral sensory transduction. This is welcomed by the many individuals who do not get pain relief from nonsteroidal anti-inflammatory drugs or even morphine. These kinds of research breakthroughs would not be possible without a thorough understanding of how the brain and body communicate with each other.
In addition to chronic pain, there are over 600 known neurologic disorders. Nearly 50 million people in the United States alone and an estimated 1 billion people worldwide suffer from the effects of damage to the central or peripheral nervous system. Each year, nearly 7 million people die of a neurologic disorder or its complications. Neurologic disorders include genetic disorders (eg, Huntington disease), demyelinating diseases (eg, multiple sclerosis), developmental disorders (eg, cerebral palsy), degenerative diseases that target specific types of neurons (eg, Parkinson disease and Alzheimer disease), an imbalance of neurotransmitters (eg, depression, anxiety, and eating disorders), trauma (eg, spinal cord and head injury), and convulsive disorders (eg, epilepsy). In addition, there are neurologic complications associated with cerebrovascular problems (eg, stroke) and exposure to neurotoxic chemicals (eg, nerve gases, mushroom poisoning, and pesticides).
Advances in stem cell biology and brain imaging techniques, a greater understanding of the basis for synaptic plasticity of the brain, a wealth of new knowledge about the regulation of receptors and the release of neurotransmitters, and the detection of genetic and molecular defects that lead to neurologic problems have all contributed to advancements in identifying the pathophysiologic basis for neurologic disorders. They have also set the stage for identifying better therapies to prevent, reverse, or stabilize the physiologic deficits that more than 600 neurologic disorders cause.