2. unduly excitable or easily agitated.
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.
an organized set of principles or ideas. adj., adj
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.
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.
in the behavioral system model
of nursing, the patterned, repetitive, and purposeful behaviors of an individual.
the heart and blood vessels, by which blood is pumped and circulated through the body; see also circulatory system
) 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
(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
the neurons in the central core of the brainstem
from the thalamus to the medulla oblongata, connecting the cerebral hemispheres
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.
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.
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
expert system a set of computer programs designed to serve as an aid in decision making.
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.
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.
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.
) (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.
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.
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.
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).
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.
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.
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.
peripheral nervous system
the portion of the nervous system
consisting of the nerves and ganglia outside the brain and spinal cord.
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).
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.
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.
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.
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.
the system formed in the body by the kidneys
, urinary bladder
, and urethra
, the organs concerned in the production and excretion of urine
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.