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The Autonomic Nervous System (ANS) is divided into two functionally distinct divisions, the Sympathetic and the Parasympathetic Divisions. Each consists of nerves that control the activity of the heart, blood vessels, glands, and other organs containing smooth muscles. The ANS controls numerous body functions that we don't normally think about, such as heart rate, blood pressure, digestion, and perspiration.
The two divisions of the ANS act in opposition and balance with each other; excitation of one division results in a balanced inhibition of the other division. For example, the sympathetic division increases heart rate; the parasympathetic division decreases it. The relative activation of each division is dependent on demands posed by the environment. The parasympathetic division increases digestive activity; the sympathetic division decreases it. The Sympathetic Division of the ANS is responsible for mobilizing the body in response to situations that are threatening or otherwise exciting. It activates functions needed to energize what is called the fight-or-flight response, the preparation to attack or run from an enemy. We experience the sympathetic response with intense emotions such as fear, anger, and even ecstasy. The Parasympathetic Division of the ANS conserves the body's energy during times of relaxation and supports internal body functions, such as the digestion of food. Finally, it is important to note that the relative activation of each division is dependent on the individual's perception of an event. The sympathetic division will be activated when we perceive an event to be particularly threatening or exciting. The responses are therefore subject to differences across individual people.
A common example of the balancing act between the sympathetic and parasympathetic divisions of the autonomic nervous system occurs on the roadway. Imagine yourself driving calmly on the expressway on your way to work. Another car cuts in front of you without signaling and almost causes an accident. You notice that a physical reaction to this threatening situation occurs almost immediately. Your heart rate accelerates, heavy breathing sets in, you feel hot and sweaty, and your stomach is upset. You feel an intense mixture of fear and anger, and perhaps express the latter through gestures! This is your sympathetic activity energizing you to either fight or get out of the way. Settling back to the relaxed state you were in before this near-mishap takes some time, and you are still rattled by the experience by the time you get to work. This prolonged state of duress reflects the slower response of the parasympathetic division.
Figure 5 summarizes some of the important body functions controlled by each division of the ANS.
The sympathetic division consists of two paired chains of collections of nerve cell bodies called ganglia. These ganglia lie just to the left and right of the spinal cord in the centrally located thoracic and lumbar regions. Axons extend from the ganglion to the body's organs. Because the ganglia of the sympathetic division are linked, they tend to work as a group; or "in sympathy" with each other. The sweat glands, adrenal glands, muscles that constrict blood vessels, and muscles that erect skin hairs are under control of the sympathetic division only.
Para- means beside or related to; and indeed the functions of the parasympathetic division work in conjunction with the sympathetic division by opposing its effects. Although the two divisions of the ANS work in opposition to one another, they are normally both active simultaneously. The parasympathetic division is sometimes known as the craniosacral system because its nerves derive from the cranial nerves at the base of the brain and the sacral spinal cord. Unlike the sympathetic division, the nerve cell collections or ganglia of the parasympathetic division are located close to the organ and not along the spinal cord. In addition, the parasympathetic ganglia are not connected like those of the sympathetic division. They are, therefore, capable of acting independently on the organs they control.
Most of the final synapses of the sympathetic division use the neurotransmitter norepinephrine, whereas the final synapses of the parasympathetic division use the neurotransmitter acetylcholine. This difference in neurochemical activity makes selective stimulation of each division possible with certain drugs. Non-prescription cold medications act by either decreasing parasympathetic activity or increasing sympathetic activity. Side effects of cold remedies that decrease parasympathetic activity, such as having a dry mouth (decreased salivation) and increased heart rate, stem from their tendency to increase sympathetic activity indirectly.
Individuals differ in the reactivity of their sympathetic division to life events. These differences are related to differences in personality. People with a stronger and more rapid sympathetic response tend to respond in this way to a wider range of life events. They are generally more emotionally expressive, more active, more impulsive, more distractible, and less patient. These individuals may be more prone to develop psychosomatic illnesses such as heart disease and ulcer formation.
Prolonged stress has a negative effect on the immune system's ability to defend against intruders such as bacteria, viruses, and cancerous cells. The sympathetic division's effect on the immune response occurs through the activation of a sequence of physiological events mediated through the hypothalamic / pituitary gland / adrenal gland axis. The hypothalamus induces the pituitary to secrete adrenocorticotrophic hormone (ACTH), which stimulates the adrenal release of cortisol and other hormones. Cortisol shifts the metabolic pathways away from protein synthesis toward glucose synthesis (needed as an immediate source of energy). Hence, the production of leukocytes is reduced. B cells produce antibodies to attack specific antigens, and T cells attack intruder cells such as viruses and cancer cells. The field that has emerged to study these phenomena is called Psychoneuroimmunology. Behavioral Medicine is a psychological sub-field that applies the knowledge of psychological effects on health to the diagnosis and treatment of medical disorders. Whereas, Health Psychology is a sub-field that has emerged to apply these same principles toward the prevention of illness.
Autonomic arousal is measured objectively by a sub-field of psychology called Psychophysiology. Electrodes, usually applied to the surface of the body, are used to detect and quantify the electrical activity reflecting activation of the sympathetic response. Multiple readings are usually obtained simultaneously. These measures include the electrocardiogram for heart rate, plethysmography for blood pressure, galvanic skin response for sweating, and electroencephalography for brain wave activity. Acid-base electrodes lowered into the stomach can be used to quantify the secretion of gastric juices. The sub-field of Biofeedback uses these same measures to help people learn how to control these normally automatic processes. This technique has proven useful in helping the reactive person to reduce the sympathetic response leading to illness.
|Suggestions for further study|
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(Weight Control and Obesity)
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