In scuba diving, are nitrogen narcosis and high pressure nervous syndrome the same thing?
These are different things. 

Nitrogen Narcosis or more commonly Gas Narcosis is the narcotic effect of gasses like Oxygen and Nitrogen under pressure. The effects are most prominent under 30m and commercial/technical dives doing dives to 40/50m on air tend to experience the effects most. (Narcosis is managed by adding an inert gas - in most gases Helium - to air to form a mixture referred to as Trimix (Helium/Oxygen/Nitrogen) this then produces an effect as if the diver is diving to a shallower depth and is referred to as Equivalent Narcotic Depth, in some cases Heliox (Helium &amp; Oxygen) was also used)

High Pressure Nervous Syndrome is an interesting aspect of deep diving and used to be called Helium Jitters/Tremors as the accepted standard was that it was caused by breathing Helium mixtures under 150m. Some people hold this to be true and others claim that it has nothing to do with Helium but rather the pressure disrupting the flow of electric signals thru the nervous system.

What is interesting is that they found that if you used Trimix (Helium/Nitrogen/Oxygen) rather than Heliox (Helium/Oxygen) the onset happened later or not as severe. And the slower your descent is the slower the onset of the symptoms.

Some Docs to Read:

Narcosis - Relatively simple document on Narcosis

Narcosis - "Narcosis is not unique to nitrogen; however, it can occur with many of the so-called “noble” or inert gases, with the exception of helium. Add to this the fact that other inert gases each have their own brand of narcotic effects at depth, and you have a complicated picture for technical and commercial divers. One of these rare gases, argon, for example, has about twice the narcotic potency of nitrogen, but helium has very weak narcotic properties and is less soluble than nitrogen in body tissues."

HPNS - Relatively Simple doc on HPNS

HPNS - Very technical discussion on gasses, from a HPNS and Narcosis view point.

References:

Bennett, P.B. 1982b. The high pressure nervous syndrome in man. In: The Physiology and Medicine of Diving and Compressed Air Work. (P.B. Bennett and D.H. Elliot, eds), Balliere-Tindall, London. pp. 262-296.

Bennett, P.B. 1990. Inert gas narcosis and HPNS. In: Diving Medicine, Second Edition (A.A. Bove and J.C. Davis, eds.).W.B. Saunders Company, Philadelphia. pp. 69-81.

Bennett, P.B, R. Coggin, and J. Roby. 1981. Control of HPNS in humans during rapid compression with trimix to 650 m (2132 ft). Undersea Biomed. Res., 8(2): 85-100.