Neurophysiology is the study of the functions of the nervous system. Clinical neurophysiology is the study of the functions of the nervous system in the clinical setting, for diagnostics, treatment, and intensive care purposes.
Neurophysiology is a broad field of study because many different levels are involved in the overall functioning of the nervous system and its components. For example, the transmission of a nervous impulse across the synapses, or the cleft that connects nerve cells, involves chemical reactions at the cellular level of organization. Understanding how messages are relayed from the brain to the hand is best explained at the system level. This involves studying the relationship and pathways between the brain and the organs of the body and the nerves that connect them, both sensory, meaning nerves that receive input from sensors, and motor, meaning the nerves that activate muscle. Thus, neurophysiology studies nervous function ranging from individual nerve cells to the complex behaviors of the central nervous system. Additionally, the nervous system not only functions at the cell and system levels of organization, but also at a mechanistic level, that involves the study of the control or regulatory processes that occur.
The neurophysiology of systems
A branch of neurophysiology describes the function of the major system components of the nervous system of the human body at the system level. The overall nervous system of the body consists of the central nervous system (CNS), and the peripheral nervous system (PNS). The neurophysiology of the CNS studies the function of the brain and spinal cord while that of the PNS studies the function of all the nerves that connect the CNS with organs, muscles, blood vessels and glands. The neurophysiology of the PNS further subdivides into the somatic nervous system (SNS) and the autonomic nervous system (ANS), with the ANS being further divided by function into the sympathetic and parasympathetic systems.
Axon—The part of the nerve cell used to carry impulses away from the cell body.
Electroencephalography (EEG)—Recording of electrical impulses that reflect brain function, used to diagnose extensive variety of nervous system disorders.
Electromyography (EMG)—Electrical testing of nerves and muscles, used to diagnose nervous disorders.
Evoked potentials (EP)—Electrical signals of the nerves, spinal cord and brain in response to light stimulation of the eyes, or sound stimulation of the ears or mild electrical stimulation of the nerves in the arms or legs, used to diagnose nervous system disorders, such as multiple sclerosis, hearing loss, and various spinal cord disorders.
Microneurography—Technique used primarily for research purposes that enable recording of electrical activity of a single axon from the peripheral nerves of awake human subjects.
Nerve condition velocity (NCV)—Technique for studying nerve or muscle disorders, measuring the speed at which nerves transmit signals.
Neurophysiology—The study of the functions of the nervous system.
PNS nerves are of two types: the sensory—or afferent—nerves that transmit information from the sensory organs, muscles, joints, internal organs and all other parts of the body to the CNS, and the motor—or efferent— nerves that transmit signals from the CNS to the body, for example, to the muscles or to internal organs.
The neurophysiology of nerve cells
Neurophysiology is also the study of the physiology, structure and function of nerve cells, or neurons, meaning how individual neurons receive and transmit information using chemical and electrical signals. The most important feature of neurons as compared to other cell types in the body is their high degree of electrical excitability. The transmission of nervous signals is based on changes in this electrical excitability, and neurophysiology studies these effects at the cellular level as well as the electrical properties of neurons. It also seeks to understand the differences between the excitability of muscle and nervous tissue. Examples are: the release of neurotransmitters, substances that are activated by the excitation of neurons; the specific chemical features of the various neurotransmitters; the study of the redistribution of charged ions inside and outside nervous cells, including the pumps used to transport them across cell membranes; and the properties of the various special channels used for this transport.
The neurophysiology of control mechanisms
The control activities of the nervous system are performed through very complex mechanisms and pathways. In the brain and the spinal cord there are complex regulatory pathways for functions like food and water intake, sleep, pain, and muscle control, to name a few. Investigations of such control systems are of central interest in neurophysiology. This includes, for example, the mechanisms involved in the regulation of sleep, pain, breathing, and the cardiac cycle. The understanding of these mechanisms provides a basis for understanding changes in the functions that may result in diseases. Also the treatment of diseases, for instance the use of drugs to correct dysfunctions, requires that the underlying mechanisms of the disorder be understood. One of the most important topics of neurophysiology is the study of feedback systems that constantly monitor and regulate numerous aspects of body function, such as the levels of oxygen and carbon dioxide, nutrients, hormones, and other chemical substances in the blood. Other higher functions, such as language, learning and memory, and emotions, while being mostly studied by neuropsychology, are also affected by neurophysiological mechanisms and these aspects are also included in neurophysiology.
Measurement techniques in neurophysiology
The electrical signals of the nervous system propagate throughout the body to control movement, breathing, heart rate, and the capacity to think and remember. Neurophysiology also includes all the electrical measuring techniques used to provide information on the function of the brain and nerves. These include:
- Electroencephalography (EEG): EEG is a recording of electrical impulses that reflect brain function. It is used to test for a wide variety of disorders of the nervous system, such as tumor growth and infections as well as the development of the brain in babies and children. EEG is also used in the diagnosis of epilepsy and strokes. EEG is often performed during surgery on the arteries of the neck to ensure that the blood flow to the brain is adequate.
- Evoked potentials (EP): This technique evaluates the condition of nerve pathways. EPs are electrical signals of the nerves, spinal cord, and brain in response to light stimulation of the eyes, sound stimulation of the ears, or mild electrical stimulation of the nerves in the arms or legs. EPs are used to diagnose disorders of the nervous system such as multiple sclerosis, hearing loss, and various spinal cord disorders. EPs are also used during neurosurgery to locate brain structures or check on the patient's response to surgery.
- Polysomnography (PSG): This technique monitors brain wave patterns, eye movements, muscle tension, air flow, respiratory effort, oxygen levels, and heart beat during sleep. It is mostly used to diagnose and treat various sleep disorders.
- Electromyography (EMG): EMG refers to electrical testing of nerves and muscles. The technique is used to diagnose nervous disorders such as muscle spasticity and pinched nerves in the back or neck as well as other nerve or muscle disorders.
- Nerve conduction velocity (NCV): NCV is another technique used to study nerve or muscle disorders, it measures the speed at which nerves transmit signals.
- Microneurography (MN): Microneurography is mostly used for research purposes, it is a technique that makes it possible to record the electrical activity of a single axon from the peripheral nerves of awake human subjects.
The spectacular advances in knowledge of the nervous system during the past decades and the promising developments in the treatment of nervous disorders have made neurophysiology one of the most active branches of modern biology and medicine. The understanding that neurophysiology provides about nervous system functions from the level of the cell to the level of the systems also makes it the foundation stone of other clinical fields like neurology and psychiatry. The development of drugs to control and cure disease also requires an understanding of how drugs affect the nervous system, which is only possible if the detailed neurophysiology of the systems they target is well understood. Neurophysiology research fulfills that role, thus creating a strong link to neuropharmacology and general health care practice.
Baddeley, R., Hancock, P. J. B. and P. Foldiak, eds. Information Theory and the Brain. New York: Bantam Doubleday Dell, 1999.
Johnston, D. and Samuel Miao-Sin Wu. Foundations of Cellular Neurophysiology. Cambridge: MIT Press, 1995.
Levin, K. H. and H. O. Luders. Comprehensive Clinical Neurophysiology. Philadelphia: W. B. Saunders Co.,2000.
Nicholls, John G., A. Robert Martin, Bruce G. Wallace, and Paul A. Fuchs. From Neuron to Brain. 4th ed. Sunderland: Sinauer and Associates, 1999.
Sanes, D. H., Reh, T. A. and W. A. Harris, eds. Development of the Nervous System. New York: Academic Press, 2000.
EMG and Nerve Conduction Homepage. <http://www.teleemg.com/>.
Clinical Neurophysiology on the Web. <http://www.neurophys.com/>.
Monique Laberge, Ph.D.