<Li> PS: vagus nerve </Li> <Li> S: thoracic and lumbar splanchnic nerves </Li> <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>

How does overall sympathetic or parasympathetic nervous system control effect blood pressure
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