Question

The sympathetic nervous system will cause vasodilation in the arteries of many blood vessels, but vasoconstriction in the brain. Why does this occur?

Answer 1

The sympathetic nervous system exerts its effect on blood vessels by releasing adrenaline and noradrenaline.
These two catecholamines binds to the adrenergic receptors present in the blood vessels. Blood vessels contains alpha as well as beta adrenergic receptors. Binding of adrenaline to alpha adrenergic receptors results in vasoconstriction while binding to beta adrenergic receptors cause vasodilatation. If a blood vessel contains more of beta adrenergic receptors it will vasodilate to the action of sympathetic nervous system and if a blood vessel contains more of alpha adrenergic receptors it will vasoconstrict.
The vasoconstriction of blood vessels of brain during sympathetic stimulation helps limit excess blood flow to the brain and thus regulates the peripheral resistance and blood pressure in brain. Excess blood flow yo brain causes disruption of blood brain barrier.