Answer
Usually, opposite ANS effects are produced in structure/organs by antagonistic or modifying activities of the sympathetic innervation and parasympathetic innervation. But dual ANS innervation is not essential for modifying or inhibiting an ANS-regulated process. For instance, the degree of constriction of blood vessels determines blood pressure. Vascular constriction depends on the level of contraction of the vascular smooth muscles, and this is dependent on the degree of stimulation by the sympathetic system--alone. No parasympathetic input is applied to regulate vasomotor tone.
Work Step by Step
The extant pressure in a blood vessel is partly due to the level of sympathetic stimulation. If the frequency of sympathetic firing decreases, the vascular smooth muscles relax ; the blood hydrostatic pressure then pushes against the blood vessel walls which relax as resistance falls. Consequently, blood pressure fall-- without the the opposing activity of an absent parasympathetic innervation.
The ANS can also regulate blood flow between alternative deep and superficial routes through control of vasomotor tone by sympathetic innervation, alone.
