human biology - Where does vasoconstriction occur in limbs?

Reduced blood flow to a region of the body occurs through 2 principal mechanisms.

1) The smooth muscle fibers in the tunica media layer of the arteries contract and reduce the diameter of the artery, limiting blood flow due to increased resistance (this is the mechanism in @agrimaldi's answer).

2) Circularly oriented smooth muscle fibers at the junction of a metarteriole (the distal end of an arteriole) and a capillary bed form a precapillary sphincter, which serves as a valve and prevents blood flow into a the capillary bed.

So to answer your question, it is both. The narrowing of the arteries would occur rather continuously across the limb but there are "pinch points" where metarterioles joint the capillary beds.

The endocrine and nervous systems are pretty integrated so in most cases it would be the result of the actions of both systems.