Most blood vessels, including those in the epidermis, digestive tract, and kidneys, constrict when the sympathetic nervous system is stimulated. This occurs as a result of norepinephrine produced by post-ganglionic sympathetic neurons activating alpha-1 adrenergic receptors. The opposite response - dilation of blood vessels - occurs with stimulation of the parasympathetic nervous system. Vasodilation results from acetylcholine released by parasympathetic nerves binding to muscarinic receptors on vascular smooth muscle cells.
Sympathetic nerve activity increases during exercise to provide more blood to working muscles. This causes peripheral arterial resistance to decrease. Decreased resistance allows more blood to flow through the heart, which sends more oxygen-rich blood to the exercising muscles. The increased blood flow to the muscles during exercise is called the "exercise hyperemia" effect.
Exercise also activates the autonomic nervous system, causing the body to change how it functions. For example, adrenaline levels increase during exercise, causing the heart to beat faster and harder. This helps deliver more oxygen-rich blood to the muscles during times of stress or exertion. Autonomic changes are also seen during sleep. For example, when you wake up feeling tired, it's because your body has been using up its stores of energy for several days. To make more energy available, the brain stimulates the heart to pump more strongly and the lungs to expand more fully.
Most blood vessels, including many in the epidermis, digestive tract, and kidneys, contract when the sympathetic nervous system is stimulated... Function.
|Heart||Increases rate and force of contraction|
|Lungs||Dilates bronchioles via circulating adrenaline|
|Blood vessels||Dilate in skeletal muscle|
Sympathetic Nervous System When stressed, sympathetic nerve activity rises, resulting in direct vasoconstriction of afferent arterioles (norepinephrine action) and stimulation of the adrenal medulla. This response increases blood pressure by reducing renal plasma flow and glomerular filtration rate.
Afferent arteriole constriction leads to reduced inflow into the glomerulus and thus decreased sodium reabsorption from the filtrate. This mechanism helps to conserve sodium and water during times of increased need for these substances (e.g., when facing environmental dehydration or after a large meal). The net effect is a reduction in blood volume and blood pressure.
The efferent arteriole also constricts in response to stress, leading to reduced outflow from the kidney and thus decreased sodium delivery to body tissues. This mechanism acts to restore blood pressure to normal levels.
Sympathetic nervous system activation can occur in situations where there is no actual threat to life; rather, it is believed that this response serves to prepare the body for action or reaction to a future challenge. For example, when preparing for a fight or fleeing from danger, the sympathetic nervous system is activated, causing the body to tense up and pump more blood through the vessels.
Cutaneous vasoconstriction is primarily regulated by the sympathetic nervous system of the autonomic nervous system. The majority of sympathetic activity causes vasoconstriction. However, some evidence suggests that certain branches of the sympathetic nervous system may cause vasodilation instead. Nervous system disorders that can cause vasodilation include: neuropathy (nerve damage) where sympathetic nerves no longer function properly; sympathectomy (removal of part of the sympathetic nervous system) in which case adrenergic receptors become more abundant because there are no longer any sympathetic hormones to compete with; and parasympathectomy (removal of part of the parasympathetic nervous system) where cholinergic receptors become more abundant because there are now no longer any parasympathetic hormones to compete with.
Vasodilation can also be caused by sensory nervous system disorders. These include: mononeuropathies (damage to one nerve causing loss of muscle movement control on that side of the body) where sympathetic fibers from the damaged nerve stimulate blood vessel constriction; and polyneuropathies (multiple nerve damage) where both sympathetic and parasympathetic fibers are affected resulting in loss of muscle tone along with vasodilation.
Finally, vasodilation can be caused by hormonal disorders.
Parasympathetic nerves, on the other hand, innervate salivary glands, gastrointestinal glands, and vaginal erectile tissue, causing vasodilation. Sympathetic activation raises cardiac output, systemic vascular resistance (including arteries and veins), and arterial blood pressure. It also causes dilation of the venous sinuses in the brain and spinal cord, which can lead to increased intracranial and intradural pressure.
Blood vessels are controlled by both sympathetic and parasympathetic nerves. If the sympathic nervous system is activated, then the blood vessels constrict to cause pain or to make room for blood if there is a cut or some other damage to the skin. If the parasympathic nervous system is activated, then the muscles around the blood vessel relax so that more blood flows through the vessel.
So, basically, blood pressure is controlled by two systems: the sympathetic nervous system and the parasympathetic nervous system. The type of control depends on what part of our body needs blood flow or not. For example, if we need more blood flow into an injured part of the body, then we activate the sympathetic nervous system. This causes muscles all over our body to tighten up, including the muscle walls of our blood vessels. This makes it harder for blood to reach those parts of our body that need it most - the muscles repair themselves faster now that there's less bleeding.
Sympathetic stimulation causes: Sympathetic stimulation of the kidney promotes afferent arteriole constriction and an increase in the surface area of the glomerulus. Constriction of afferent arterioles and reduction in glomerulus surface area lead to decreased filtration rate and water retention by the kidney. These effects are important mechanisms by which the body tries to conserve sodium and water during times of increased activity of the sympathoadrenal system.
Vasoconstriction occurs not only in arteries, but also in the venous vasculature, more in big venules than in tiny venules, and especially in the splanchnic region, and is mostly mediated by increased sympathetic nervous system activity and vasopressin production. Vasoconstriction in both arteries and veins can have negative effects on blood flow and therefore tissue oxygenation.
Vasoconstriction can occur anywhere within the vascular system, but it most commonly affects small arteries near the surface of the skin (cutaneous arterioles) and large veins back into which they drain (venular valves). Arteries constrict to limit blood flow to tissues that don't need it and allow more rapid movement of blood through areas with higher metabolic demands. Veins constrict to reduce the volume of blood flowing into them, which would otherwise fill up with blood and cause them to swell beyond their normal size.
The splanchnic region is a term used to describe the organs inside the abdominal cavity: liver, pancreas, stomach, intestines, and spleen. It should be noted that the splanchnic region includes both arteries and veins; however, for simplicity's sake we will refer only to arteries in this context.
The splanchnic region is made up of large and small vessels that supply the muscles, glands, and other tissues in this area.
The sympathetic nervous system, like the rest of the nervous system, works through a network of linked neurons. Postganglionic neurons mostly release noradrenaline in response to this stimuli (norepinephrine). Prolonged stimulation might cause the adrenal medulla to produce adrenaline. The release of these hormones into the blood causes specific changes to occur in most organ systems of the body. For example, the heart beats faster, the lungs breathe more deeply, and the liver releases glucose into the blood stream for use by other tissues.
Sympathetic nerves arise from two primary sources: the cranial nerves and the spinal cord. The cranial nerves are numbered according to which part of the head they control. The twelve major cranial nerves all contain sympathetic fibers that go to different organs including the tongue, lips, ears, nose, eyes, brain, and neck muscles. The fibers from each nerve communicate with the central nervous system (CNS) via one of three pathways: anterior, posterior, or lateral. Anterior ganglia connect directly with the CNS via the anterior horn cells of the spinal cord. Posterior ganglia receive input only from neighboring ganglia using synapses in the dorsal root entry zones of the spinal cord. Lateral ganglia receive input only from adjacent ganglia using synapses in the ventrolateral funiculi of the spinal cord.
Spinal sympathetic nerves originate in the brain and spine.