Even when a person is at rest there is a low level of activity (1-2 action potentials per second) in these nerves (the vasomotor centre is said to be tonically active). Thus arterioles are partially constricted and there is a resting vascular (vasoconstrictor) tone. An increase in activity (up to 10 action potentials per second) increases the level of vasoconstriction and reduces blood flow (2-7 times). A decrease in this resting activity causes relaxation of vascular smooth muscle cells and vasodilatation.