nervous system

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1. neural.
2. unduly excitable or easily agitated.
nervous system the organ system that, along with the endocrine system, correlates the adjustments and reactions of an organism to internal and environmental conditions. It is composed of the brain, spinal cord, and nerves, which act together to serve as the communicating and coordinating system of the body, carrying information to the brain and relaying instructions from the brain. The system has two main divisions: the central nervous system, composed of the brain and spinal cord; and the peripheral nervous system, which is subdivided into the voluntary and autonomic nervous systems. (See color plates.)
The Nerve Cell. The basic unit of the nervous system is the nerve cell, or neuron. This highly specialized cell has many fibers extending from it which carry messages in the form of electrical charges and chemical changes. The fibers of some cells are only a centimeter or less long (a fraction of an inch), but those of others (for example, the sciatic nerve) may extend for half a meter to one meter (18 inches to more than 3 feet). These fibers reach into muscles and organs throughout the body and to the ends of the fingers and toes, and they cluster by the thousands in certain areas of the skin no larger than the head of a pin. The nerve fibers come together from the extremities of the body and gather into cables running to and from the brain. Along the length of the spinal cord are a number of junctions where impulses or messages are sorted or relayed to higher centers. The fibers of connecting nerve cells do not touch each other. Impulses are relayed from one to another by chemical means across the gap or synapse between them. In most cases an impulse must cross more than one synapse to cause the desired action.

In a reflex, the impulse is relayed from one nerve to another by a shortcut that produces a reaction without involving the brain. The knee jerk is an example of the simplest sort of reflex reaction. When the knee is tapped, the impulse travels through the sensory nerve that receives the tap, crosses a single synapse, and activates the motor nerve that controls the quadriceps muscle in the thigh, causing the leg to jerk up automatically.

A very different sort of reflex is the conditioned reflex.Conditioning is the process of building links or paths in the nervous system. When an action is done repeatedly the nervous system becomes familiar with the situation and learns to react automatically. A new reflex has been built into the system. Hundreds of daily actions are conditioned reflexes. Walking, running, going up and down stairs, and even buttoning a shirt all involve great numbers of complex muscle coordinations that have become automatic.
Autonomic and Voluntary Systems. The human peripheral nervous system evolved over many millennia, developing the ability to perform more and more complicated functions. It is divided into two specialized subsystems. The autonomic nervous system operates without conscious control as the caretaker of the body. The voluntary nervous system, which includes both motor and sensory nerves, controls the muscles and carries information to the brain.

The autonomic system is further specialized into two subsidiary systems: the sympathetic and the parasympathetic. The control centers of these systems lie in the hypothalamus. The sympathetic and parasympathetic nervous systems are continuously operative, functioning to adjust body processes to external and internal demands. (See also Plates.)

The sympathetic nervous system has in general an excitatory effect, and in response to danger or some other challenge, almost instantly puts body processes into high gear. This is done by the discharge of stimulating secretions at nerve junctions. These secretions, along with epinephrine discharged into the blood by the adrenal medulla, help start muscle action quickly. Glucose is released from the liver into the blood and thus is made available to all the body's muscles as a source of quick energy. The rates of heart and lung action increase, digestive activity slows down, blood vessels constrict, and sweating begins so that the body will be kept cool while under stress. Thus the body is prepared for an extraordinary effort.

The parasympathetic nervous system prevents body processes from accelerating to extremes. Acting more slowly than the sympathetic system, it causes the discharge of secretions that slow the heartbeat and lung action, restore digestive functioning, and limit the constriction of the blood vessels. Generally it acts as a damper, so that unless the challenge demands a prolonged effort, body processes will begin returning to normal.

The voluntary nervous system has nerves of two kinds, sensory and motor. The sensory nerves bring messages to the brain from all parts of the body. They are sorted in the spinal cord and sent on to the brain to be analyzed, acted upon, associated with other information and stored as memory. Messages from the brain, often in response to information received by way of the sensory nerves, are delivered to the muscles by the motor nerves. One motor nerve with its branching fibers may control thousands of muscle fibers.

The different parts of the nervous system are constantly interacting, and are so well coordinated that humans can think, feel, and act on many different levels and without serious confusion, all at the same time. (See also neurologic assessment.)


1. a set or series of interconnected or interdependent parts or entities (objects, organs, or organisms) that act together in a common purpose or produce results impossible by action of one alone.
2. an organized set of principles or ideas. adj., adj systemat´ic, system´ic.

The parts of a system can be referred to as its elements or components; the environment of the system is defined as all of the factors that affect the system and are affected by it. A living system is capable of taking in matter, energy, and information from its environment (input), processing them in some way, and returning matter, energy, and information to its environment as output.

An open system is one in which there is an exchange of matter, energy, and information with the environment; in a closed system there is no such exchange. A living system cannot survive without this exchange, but in order to survive it must maintain pattern and organization in the midst of constant change. Control of self-regulation of an open system is achieved by dynamic interactions among its elements or components. The result of self-regulation is referred to as the steady state; that is, a state of equilibrium. homeostasis is an assemblage of organic regulations that act to maintain steady states of a living organism.

A system can be divided hierarchically into subsystems, which can be further subdivided into sub-subsystems and components. A system and its environment could be considered as a unified whole for purposes of study, or a subsystem could be studied as a system. For example, the collection of glands in the endocrine system can be thought of as a system, each endocrine gland could be viewed as a system, or even specific cells of a single gland could be studied as a system. It is also possible to think of the human body as a living system and the endocrine system as a subsystem. The division of a system into a subsystem and its environment is dependent on the perspective chosen by the person studying a particular phenomenon.
Systems, subsystems, and suprasystems. Within the environment there are suprasystems, such as human society, and systems within the suprasystem, such as the educational and industrial systems and the health care delivery system. Within the health care delivery system are subsystems, such as the patient, family members, the nurse, the physician, and allied health care professionals and paraprofessionals.
alimentary system digestive system.
apothecaries' system see apothecaries' system.
autonomic nervous system see autonomic nervous system.
avoirdupois system see avoirdupois system.
behavioral system in the behavioral system model of nursing, the patterned, repetitive, and purposeful behaviors of an individual.
cardiovascular system the heart and blood vessels, by which blood is pumped and circulated through the body; see also circulatory system.
CD system (cluster designation) a system for classifying cell-surface markers expressed by lymphocytes based on a computer analysis of monoclonal antibodies against hla antigens, with antibodies having similar specificity characteristics being grouped together and assigned a number (CD1, CD2, CD3, etc.); these CD numbers are also applied to the specific antigens recognized by the various groups of monoclonal antibodies. See also CD antigen.
centimeter-gram-second system (CGS) (cgs) a system of measurements in which the units are based on the centimeter as the unit of length, the gram as the unit of mass, and the second as the unit of time.
central nervous system see central nervous system.
centrencephalic system the neurons in the central core of the brainstem from the thalamus to the medulla oblongata, connecting the cerebral hemispheres.
circulatory system see circulatory system.
client system in the general systems framework and theory of goal attainment, the composite of physiological, psychological, sociocultural, and developmental variables that make up the total person.
colloid system (colloidal system) colloid (def. 3).
conduction system (conductive system (of heart)) the system of atypical cardiac muscle fibers, comprising the sinoatrial and atrioventricular nodes, internodal tracts, atrioventricular bundle, bundle branch, and terminal ramifications into the Purkinje network.
digestive system see digestive system.
Emergency Medical Services (EMS) system a comprehensive program designed to provide services to the patient in the prehospital setting. The system is activated when a call is made to the EMS operator, who then dispatches an ambulance to the patient. The patient receives critical interventions and is stabilized at the scene. A communication system allows the health care workers at the scene to contact a trauma center for information regarding further treatment and disposition of the patient, followed by transportation of the patient to the most appropriate facility for treatment.
endocrine system the system of ductless glands and other structures that produce internal secretions (hormones) that are released directly into the circulatory system, influencing metabolism and other body processes; see endocrine glands.
environmental control system environmental control unit.
expert system a set of computer programs designed to serve as an aid in decision making.
extrapyramidal system see extrapyramidal system.
gateway system a software interface between an online searcher and one or more search systems, facilitating the use of the system by searchers who are unfamiliar with it, or with online retrieval in general.
genitourinary system the organs concerned with production and excretion of urine, together with the reproductive organs. (See Plates.) Called also urogenital system.
haversian system a haversian canal and its concentrically arranged lamellae, constituting the basic unit of structure in compact bone (osteon).
Haversian system: Structures of compact and spongy bone with the central haversian canal surrounded by the lamellae. From Applegate, 2000.
health care system see health care system.
heterogeneous system a system or structure made up of mechanically separable parts, as an emulsion or suspension.
His-Purkinje system the intraventricular conduction system from the bundle of His to the distal Purkinje fibers, which carries the impulse to the ventricles.
Home Health Care Classification system see home health care classification system.
homogeneous system a system or structure made up of parts that cannot be mechanically separated, as a solution.
hypophyseoportal system (hypophysioportal system) (hypothalamo-hypophysial portal system) the venules connecting the hypothalamus with the sinusoidal capillaries of the anterior lobe of the pituitary gland; they carry releasing substances to the pituitary.
immune system see immune system.
interpersonal system in the general systems framework and theory of goal attainment, two or more individuals interacting in a given situation.
lay health system a system comprising an informal referral network and sources of treatment outside the formal biomedical sources of health care; it includes individual consultation and information-seeking through significant others and peers concerning health behaviors, symptoms, and evaluation of treatment before, during, and after consultation with health care professionals.
legal system in the omaha system, anything connected with law or its administration; it includes legal aid, attorney, courts, or Child Protective Services (CPS), and many other agencies and officials.
limbic system a system of brain structures common to the brains of all mammals, comprising the phylogenetically old cortex (archipallium and paleopallium) and its primarily related nuclei. It is associated with olfaction, autonomic functions, and certain aspects of emotion and behavior.
lymphatic system see lymphatic system.
lymphoid system the lymphoid tissue of the body, collectively; it consists of primary (or central) lymphoid tissues, the bone marrow, and thymus, and secondary (or peripheral) tissues, the lymph nodes, spleen, and gut-associated lymphoid tissue (tonsils, Peyer's patches).
lymphoreticular system the lymphoid and reticuloendothelial systems considered together; see also lymphoreticular disorders.
metric system see metric system.
mononuclear phagocyte system the group of highly phagocytic cells that have a common origin from stem cells of the bone marrow and develop circulating monocytes and tissue macrophages, which develop from monocytes that have migrated to connective tissue of the liver (kupffer's cells), lung, spleen, and lymph nodes. The term has been proposed to replace reticuloendothelial system, which includes some cells of different origin and does not include all macrophages.
nervous system see nervous system.
nursing system in the self-care model of nursing, all the actions and interactions of nurses and patients in nursing practice situations; nursing systems fall into three categories: wholly compensatory, partly compensatory, and supportive-educative.
Omaha system see omaha system.
oxygen delivery system a device that delivers oxygen through the upper airways to the lungs at concentrations above that of ambient air. There are two general types: the fixed performance or high flow type, which can supply all of the needs of a patient for inspired gas at a given fractional inspired oxygen; and the variable performance or low flow type, which cannot supply all of the patient's needs for oxygen and delivers fractional inspired oxygen that varies with ventilatory demand.
parasympathetic nervous system see parasympathetic nervous system.
peripheral nervous system the portion of the nervous system consisting of the nerves and ganglia outside the brain and spinal cord.
personal system in the general systems framework and theory of goal attainment, the unified self, a complex whole that is rational, conscious, and feeling and that sets goals and decides on the means of achieving them.
pituitary portal system hypothalamo-hypophysial portal system.
portal system an arrangement by which blood collected from one set of capillaries passes through a large vessel or vessels and another set of capillaries before returning to the systemic circulation, as in the pituitary gland (the hypothalamo-hypophysial portal system) or the liver (the hepatic portal circulation).
renin-angiotensin-aldosterone system see renin-angiotensin-aldosterone system.
respiratory system the group of specialized organs whose specific function is to provide for the transfer of oxygen from the air to the blood and of waste carbon dioxide from the blood to the air. The organs of the system include the nose, the pharynx, the larynx, the trachea, the bronchi, and the lungs. See also respiration and Plates 7 and 8.
reticular activating system see reticular activating system.
reticuloendothelial system see reticuloendothelial system.
safety system see safety system.
SI system see SI units.
skeletal system see skeletal system.
social system in the general systems framework and theory of goal attainment, an organized boundary system of social roles, behaviors, and practices developed to maintain balance for growth, development, and performance, which involves an exchange of energy and information between the person and the environment for regulation and control of stressors.
support system in the omaha system, the circle of friends, family, and associates that provide love, care, and need gratification; it may include church, school, workplace, or other groupings.
sympathetic nervous system see sympathetic nervous system.
Unified Medical Language system see unified medical language system.
Unified Nursing Language system see unified nursing language system.
unit dose system a method of delivery of patient medications directly to the patient care unit. Following review by a nurse, a copy of the physician's original order is sent to the pharmacy, where the pharmacist reviews it again. The pharmacist then fills the order and delivers the medication to the patient care unit, usually in a 24-hour supply. Each patient has an individual supply of medications prepared and labeled by the pharmacist.
urinary system the system formed in the body by the kidneys, ureters, urinary bladder, and urethra, the organs concerned in the production and excretion of urine.
urogenital system genitourinary system.
vascular system circulatory system.
vasomotor system the part of the nervous system that controls the caliber of the blood vessels.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.

ner·vous sys·tem

the entire nerve apparatus, composed of a central part (the brain and spinal cord) and a peripheral part (the cranial and spinal nerves, autonomic ganglia, plexuses and peripheral nerves).
Synonym(s): systema nervosum [TA]
Farlex Partner Medical Dictionary © Farlex 2012

nervous system

The system of cells, tissues, and organs that regulates the body's responses to internal and external stimuli. In vertebrates it consists of the brain, spinal cord, nerves, ganglia, and parts of the receptor and effector organs.
The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.

ner·vous sys·tem

(nĕr'vŭs sis'tĕm) [TA]
The entire nerve apparatus, composed of a central part, the brain and spinal cord, and a peripheral part, the cranial and spinal nerves, autonomic ganglia, and plexuses.
Medical Dictionary for the Health Professions and Nursing © Farlex 2012

nervous system

The controlling, integrating, recording and effecting structure of the body. The nervous system is also the seat of consciousness, of the intellect, of the emotions and of all bodily satisfaction. The central nervous system consists of the brain and spinal cord. The peripheral nervous system consists of the massive ramification of nerves running to every part of the body outside the brain and cord.
Collins Dictionary of Medicine © Robert M. Youngson 2004, 2005

nervous system

the main means by which METAZOAN animals coordinate their activities, through quick, short-lasting reactions (the ENDOCRINE GLAND system also has a coordinating function but this is slower and longer lasting). Receptors receive stimuli and pass them on to effector organs by means of nerve cells. Nerve cells are linked by SYNAPSES, and may connect with other nerve cells, receptors or effectors. Complex connections occur which give rise to complex reactions, and coordination takes place in the CENTRAL NERVOUS SYSTEM (CNS) where synapses are present in profusion. Coordination is brought about by the transmission of NERVE IMPULSES. See AUTONOMIC NERVOUS SYSTEM, SYMPATHETIC NERVOUS SYSTEM, PARASYMPATHETIC NERVOUS SYSTEM, PERIPHERAL NERVOUS SYSTEM.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005

Nervous system

The system that transmits information, in the form of electrochemical impulses, throughout the body. It comprises the brain, spinal cord, and nerves.
Gale Encyclopedia of Medicine. Copyright 2008 The Gale Group, Inc. All rights reserved.

ner·vous sys·tem

(nĕr'vŭs sis'tĕm) [TA]
The entire neural apparatus, composed of a central part, the brain and spinal cord, and a peripheral part, the cranial and spinal nerves, autonomic ganglia, and plexuses.
Medical Dictionary for the Dental Professions © Farlex 2012

Patient discussion about nervous system

Q. Is fibromyalgia related to Central Nervous System? Is fibromyalgia related to Central Nervous System? Among men and women who is more prone to the symptoms of fibromyalgia?

A. here is a quote from the National Fibromyalgia Association site:

"Little research has been conducted that measures the prevalence of fibromyalgia, and estimates vary widely as to the proportion of male versus female patients. A 1999 epidemiology study conducted in London found a female to male ratio of roughly three to one. However, a 2001 review of the research literature in Current Rheumatology Reports stated the ratio was nine to one."

Q. What is dysautonomia? My friend has dysautonomia. What does it mean? What are the symptoms? Is it curable?

A. Dysautonomia is any disease or malfunction of the autonomic nervous system. The symptoms of dysautonomia conditions are usually “invisible” to the untrained eye. The child can appear to be as healthy as other children. The manifestations are occurring internally, and although the symptoms are often are not visible on the outside. Symptoms can be unpredictable, may come and go, appear in any combination, and may vary in severity).There is no cure for dysautonomia. There are medications to assist in stabilization, but are often needed on a long-term basis.

More discussions about nervous system
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References in periodicals archive ?
- US-based neuromodulation company PathMaker Neurosystems Inc.'s MyoRegulator PM-2200 system has received Expedited Access Pathway designation and priority processing status from the US Food and Drug Administration, the company said.
RESET Therapy uses specific therapy techniques combined with acoustical neuromodulation, provided by a device from Insight NeuroSystems, to "reset" fear circuits in the brain back to pre-trauma levels.
Reducing the number of parameters used in the input vectors given to the input of the MPI neural network through applying analysis of input relevance (User's Manual--Siemens AG, I&S Neurosystems) also works on the reduction of the total number of neurons in all layers thus reducing the TNN.