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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.)
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.


1. Relating to a nerve or the nerves.
2. Easily excited or agitated; suffering from mental or emotional instability; tense or anxious.
3. Formerly, denoting a temperament characterized by excessive mental and physical alertness, rapid pulse, excitability, often volubility, but not always fixity of purpose.
[L. nervosus]
Farlex Partner Medical Dictionary © Farlex 2012


a. Of or relating to the nerves or nervous system: nervous tissue.
b. Stemming from or affecting the nerves or nervous system: a nervous disorder.
2. Easily agitated or distressed; high-strung or jumpy.
3. Marked by or having a feeling of unease or apprehension: nervous moments before takeoff.
4. Vigorous in style or feeling; spirited: "the nervous thrust of a modern creation" (Henry A. Kissinger).
5. Archaic Strong; sinewy.

nerv′ous·ly adv.
nerv′ous·ness n.
The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.


adjective Related to a nerve; neural.
adjective Related to anxiety; anxious; jittery, excitable.
Segen's Medical Dictionary. © 2012 Farlex, Inc. All rights reserved.


1. Neural–neurology.
2. Excitable–psychology.
McGraw-Hill Concise Dictionary of Modern Medicine. © 2002 by The McGraw-Hill Companies, Inc.


1. Relating to a nerve or the nerves.
2. Easily excited or agitated; suffering from mental or emotional instability; tense or anxious.
3. Formerly, denoting a temperament characterized by excessive mental and physical alertness, rapid pulse, excitability, often volubility, but not always fixity of purpose.
[L. nervosus]
Medical Dictionary for the Health Professions and Nursing © Farlex 2012

Patient discussion about nervous

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
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