The sympathetic nervous system can cause perspiration (sweating), widen blood vessels (vasodilation), and constrict blood vessels (vasoconstriction).
The sympathetic nervous system (SNS) aids in the control of most of the body’s internal organs. It is responsible for regulating many homeostatic mechanisms in living organisms, including the skin. The SNS is perhaps best known for mediating the neuronal and hormonal stress response commonly known as the fight-or-flight response, also known as sympatho-adrenal response of the body. This occurs as the preganglionic sympathetic fibers that end in the adrenal medulla secrete acetylcholine, which activates the secretion of adrenaline (epinephrine), and to a lesser extent noradrenaline (norepinephrine).
This response is mediated directly via impulses transmitted through the sympathetic nervous system, and also indirectly via catecholamines that are secreted from the adrenal medulla. Messages travel through the SNS in a bidirectional flow. Efferent messages can trigger simultaneous changes in different parts of the body. For example, the sympathetic nervous system can cause perspiration (sweating), widen blood vessels (vasodilation), and constrict blood vessels (vasoconstriction).
Perspiration, or sweating, is the production of fluids secreted by the sweat glands in the skin of mammals. Two types of sweat glands can be found in humans: eccrine glands and apocrine glands. Eccrine glands are the major sweat glands of the human body, found in virtually all skin. They produce a clear, odorless substance consisting primarily of water and NaCl (note that the odor from sweat is due to bacterial activity on the secretions of the apocrine glands). NaCl is reabsorbed in the duct to reduce salt loss. Eccrine glands are active in thermoregulation and are stimulated by the sympathetic nervous system. This involuntary increase in sweating increases skin conductivity, which is an indication of psychological and physiological arousal.
Apocrine sweat glands are inactive until they are stimulated by hormonal changes in puberty. Apocrine sweat glands are mainly thought to function as olfactory pheromones, chemicals important in attracting a potential mate. The stimulus for the secretion of apocrine sweat glands is adrenaline, which is a hormone carried in the blood.
The secretion of medullary epinephrine and norepinephrine is controlled by a neural pathway that originates from the hypothalamus in response to danger or stress (the SAM pathway). Both epinephrine and norepinephrine signal the liver and skeletal muscle cells to convert glycogen into glucose, resulting in increased blood glucose levels. These hormones increase the heart rate, pulse, and blood pressure to prepare the body to fight the perceived threat or flee from it. In addition, the pathway dilates the airways, raising blood oxygen levels. It also prompts vasodilation, further increasing the oxygenation of important organs such as the lungs, brain, heart, and skeletal muscle. At the same time, it triggers vasoconstriction to blood vessels serving less essential organs such as the gastrointestinal tract, kidneys, and skin, and downregulates some components of the immune system. Other effects include a dry mouth, loss of appetite, pupil dilation, and a loss of peripheral vision.
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Key Points
• Eccrine glands, the major sweat glands of the human body, produce a clear, odorless substance, consisting primarily of water and NaCl. NaCl is reabsorbed in the duct to reduce salt loss.
• Apocrine sweat glands are found only in certain locations of the body: the axillae (armpits), areola and nipples of the breast, ear canal, perianal region, and some parts of the external genitalia.
• Increased adrenaline stimulates the apocrine glands for sweating.
• The hormone epinephrine can cause both vasoconstriction and vasodilation.
Key Terms
Perspiration (sweating): The production of fluids secreted by the sweat glands in the skin of mammals
Apocrine: Secretion in the form of membrane-bound vesicles.
Vasodilation: The dilatation of blood vessels, which decreases blood pressure.
Vasoconstriction: The narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels.
Sympathetic nervous system (SNS): Part of the autonomic nervous system (ANS). The sympathetic nervous system activates what is often termed the fight or flight response.
Adrenaline (epinephrine): A hormone secreted by the adrenal glands, especially in conditions of stress, increasing rates of blood circulation, breathing, and carbohydrate metabolism and preparing muscles for exertion.
Noradrenaline (norepinephrine): A substance that is released predominantly from the ends of sympathetic nerve fibres and that acts to increase the force of skeletal muscle contraction and the rate and force of contraction of the heart.