The autonomic nervous system controls automatic functions such as circulation, blood pressure, and heart rate, temperature regulation and perspiration, breathing, cognition, and short-term memory, digestion and excretion, adapting to an upright posture, interpretation of external sensory stimuli, processing of pain, ability to sleep and wake, tears, and many other functions. The autonomic nervous system is comprised of two parts: The Sympathetic and the Parasympathetic.
Sympathetic Nervous System
The Sympathetic Nervous System, a portion of the autonomic nervous system, is responsible for responding to stresses of daily living (see handout on stress), to provide stimulation to vital organs and glands by triggering actions such as increasing heart rate, increasing oxygenation, and in producing perspiration and pupillary dilation. It is the system, which is responsible for the fight or flight response. Sometimes it is described as the gas pedal in a car increasing the ability to work harder to run away from an aggressor, or attack.
Parasympathetic Nervous System
The Parasympathetic Nervous System is responsible for the restorative needs of the body, including rest, relaxation, and digestion. It has been described as the break-in an automobile and is most active when the body is at rest and is in need of restoration. When the balance is off between the break and the gas this adversely affects the function of many and/or all organs systems of the body.
During the stress response, one half of the autonomic nervous system is activated in response to stress, one half is suppressed. Half of the autonomic nervous system that is turned on is called the SYMPATHETIC NERVOUS SYSTEM. The nerve endings of this system release adrenaline (the British name for epinephrine- the American term). Sympathetic nerve endings also release the closely related substance norepinephrine. Epinephrine is secreted as a result of the actions of the sympathetic nerve endings in your adrenal glands (located just above your kidneys); norepinephrine is secreted by all the other sympathetic nerve endings throughout the body. The other half of the autonomic nervous system, the PARASYMPATHETIC NERVOUS SYSTEM, plays an opposing role. This parasympathetic component mediates calm, vegetative activities. It promotes growth, energy storage, and other optimistic processes.
The neural route represented by the sympathetic system is a first and immediate means by which the brain can mobilize waves of activity in response to a stressor. There is another way as well— through the secretion of hormones. When the sympathetic nerve endings in your heart secrete norepinephrine, which causes the heart muscle to contract more vigorously and more rapidly, norepinephrine is playing a neurotransmitter role. If a neuron (or any cell) secretes a messenger that, instead, percolates into the bloodstream and affects events far and wide, that messenger is a Hormone. A small gland in the brain, the pituitary, contains a whole array of hormones that run the show throughout the rest of the body; it is the pituitary that actually knows the game plan and regulates what all the other glands do. Initially, it was thought that the pituitary was the master gland, but later it was determined that the brain controls certain pituitary hormones by stimulating their release and controls others by inhibiting them. It is now recognized that in the base of the brain, the hypothalamus, contains a huge array of those releasing and inhibiting hormones, which instruct the pituitary, which in turn regulates the secretions of the peripheral glands.
Two hormones vital to the stress-response, as already noted, are epinephrine and norepinephrine, released by the sympathetic nervous system. Another important class of hormones in response to stress are called glucocorticoids. Glucocorticoids are steroid hormones. (Steroid is used to describe the general chemical structure of five classes of hormones: Androgens —the famed “anabolic” steroids like testosterone that get you thrown out of the Olympics—Estrogens, Progestins, Mineralocorticoids, and Glucocorticoids). Secreted by the adrenal gland, they often act, in ways similar to epinephrine. Epinephrine acts within seconds; glucocorticoids back this activity up over the course of minutes or hours. A general outline of their release during stress is- a stressor is sensed or anticipated in the brain, triggering the release of CRH (Corticotropin-Releasing Hormone and related hormones such as Beta-endorphin) by the hypothalamus. These hormones enter the private circulatory system linking the hypothalamus and the anterior pituitary, causing the release of ACTH (Adrenocorticotropic Hormone) by the anterior pituitary. ACTH enters the general circulation and triggers the release of glucocorticoids by the adrenal gland. After ACTH is released into the bloodstream, it reaches the adrenal gland and, within a few minutes, triggers glucocorticoid release. Together, glucocorticoids and the secretions of the sympathetic nervous system (epinephrine and norepinephrine) account for a large percentage of what happens in your body during stress.
The patient with dysautonomia will often complain of difficulty standing still, fatigue, lightheadedness, nausea, and other GI symptoms, brain fog or mental clouding, palpitations, or chest discomfort, shortness of breath or difficulty breathing. A major way dysautonomia does cause problems is by producing orthostatic intolerance. Patients with orthostatic intolerance cannot tolerate prolonged standing. Orthostatic intolerance is a state of being that involves many symptoms including dizziness or lightheadedness while standing. There is a spectrum of orthostatic intolerance and may be divided into 3 types including episodic, chronic and neurocirculatory failure.
Episodic orthostatic intolerance means the difficulty in standing does not occur consistently enough to warrant ongoing clinical management. Dehydration, stress, prolonged bed rest, or illnesses may cause this occasional faintness, or drop in blood pressure. This occasional problem is a normal reaction to the abnormal flow of blood in a person with the help of the autonomic nervous system. This is must be differentiated for more serious conditions.
Chronic orthostatic intolerance is the category into which most dysautonomia patient’s fall. The severity of symptoms may fluctuate over time, but the patient consistently experiences symptoms and demonstrates abnormal drops in blood pressure (neurally mediated hypotension/neurogenic orthostatic hypotension) or abnormal increases in heart rate (POTS), or both upon standing. In these chronic cases, even though the person’s blood pressure is maintained for a period of time during standing, the person still feels dizzy or lightheaded and often experiences additional symptoms. The severity of the symptoms may change and worsen over time with external stress, emotional stress, physical, stress, or pain. Common dysautonomias involving chronic orthostatic intolerance are postural orthostatic tachycardia syndrome (POTS), and autonomically mediated syncope.
In Neurocirculatory Failure, orthostatic intolerance is a failure of the sympathetic nervous system to correctly regulate the heart and blood vessels. The patient consistently has a fall in blood pressure during standing. Some conditions involving neurocirculatory failure include Parkinson’s disease with neurogenic orthostatic hypotension, pure autonomic failure, and immune autonomic ganglionopathy. Most cases of the neurocirculatory failure are neurodegenerative diseases and hence are not treated by physical therapy.
Dysautonomias can occur at any age in the lifecycle. In infants and children dysautonomia, often reflect problems in the development of the autonomic nervous system. Where frequently the cause is a genetic change or mutation. In teens and adults, dysautonomia usually reflects functional changes in a generally intact autonomic nervous system. When a person suffers from frequent episodes of fainting or near fainting, it may be the result of POTS. Neurally mediated syncope, vasovagal syncope, or neurocardiogenic syncope.
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