An assembly that consist of a cannula an electrode and a syringe. The cannula has a hub that defines a set of adjacent bores. A first proximal bore receives the head of the electrode. A second bore receives the head of the syringe. The cannula has a stop that limits the extent to which the electrode can be inserted in the proximal bore. When the syringe is inserted in the cannula hub, the syringe is inserted beyond the stop. The syringe presses against an adjacent surface that defines the second bore to form a seal between the cannula and the syringe.
