<P> Messages travel through the sympathetic nervous system in a bi-directional flow . Efferent messages can trigger changes in different parts of the body simultaneously . For example, the sympathetic nervous system can accelerate heart rate; widen bronchial passages; decrease motility (movement) of the large intestine; constrict blood vessels; increase peristalsis in the oesophagus; cause pupillary dilation, piloerection (goose bumps) and perspiration (sweating); and raise blood pressure . One exception is with certain blood vessels such as those in the cerebral and coronary arteries, which dilate (rather than constrict) with an increase in sympathetic tone . This is because of a proportional increase in the presence of β adrenergic receptors rather than α receptors . β receptors promote vessel dilation instead of constriction like α1 receptors . An alternative explanation is that the primary (and direct) effect of sympathetic stimulation on coronary arteries is vasoconstriction followed by a secondary vasodilation caused by the release of vasodilatory metabolites due to the sympathetically increased cardiac inotropy and heart rate . This secondary vasodilation caused by the primary vasoconstriction is termed functional sympatholysis, the overall effect of which on coronary arteries is dilation . </P> <P> The target synapse of the postganglionic neuron is mediated by adrenergic receptors and is activated by either norepinephrine (noradrenaline) or epinephrine (adrenaline). </P> <Table> Examples of sympathetic system action on various organs except where otherwise indicated . <Tr> <Th> Organ </Th> <Th> Effect </Th> </Tr> <Tr> <Td> Eye </Td> <Td> Dilates pupil </Td> </Tr> <Tr> <Td> Heart </Td> <Td> Increases rate and force of contraction </Td> </Tr> <Tr> <Td> Lungs </Td> <Td> Dilates bronchioles via circulating adrenaline </Td> </Tr> <Tr> <Td> Blood vessels </Td> <Td> Dilate in skeletal muscle (in animals). </Td> </Tr> <Tr> <Td> </Td> <Td> Constricts in gastrointestinal organs </Td> </Tr> <Tr> <Td> Sweat glands </Td> <Td> Activates sweat secretion </Td> </Tr> <Tr> <Td> Digestive tract </Td> <Td> Inhibits peristalsis </Td> </Tr> <Tr> <Td> Kidney </Td> <Td> Increases renin secretion </Td> </Tr> <Tr> <Td> Penis </Td> <Td> Inhibits Tumescence </Td> </Tr> <Tr> <Td> Ductus deferens </Td> <Td> Promotes emission prior to ejaculation </Td> </Tr> </Table> <Tr> <Th> Organ </Th> <Th> Effect </Th> </Tr>

A physiological effect of sympathetic nervous stimulation includes
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