The present invention relates to a closed system and method for selectively infusing anesthetics into a peripheral nerve or plexus. Each discrete current limit value establishes a maximum current magnitude for the motor over a plurality of intermediate velocities less than the operational velocity. A light transmissive generally conformal shell is disposed over the encapsulant and has an inner surface spaced apart by an air gap from, and generally conformal with, an outer surface of the encapsulant. The gate voltage of the second transistor is supplied by a feedback circuit which provides that the current through the first and second transistors is substantially equal.