Now, let's consider the warp-core. We first saw the glowing tower-whatsit in TMP. In TMP, there's no indications whatsoever that the tower was itself the main-reactor. Watch the movie again. It's pretty obvious that the tower is just a conduit. There's a horizontal conduit in TMP too, & the horizontal one doesn't look any different than the vertical one, so the vertical tower doesn't look any more "important" than the horizontal glowing tube. They're just conduits - in TMP, we never actually see the main reactor itself. It's in the secondary hull somewhere, but we never see it.
Fast-forward to TNG. The set designers & art directors for TNG apparently liked the glowing tower-whatsit from the movies (it does look cool), so they put one in engineering on the E-D. Here's where the writing gets sloppy. The TNG-people decided that the glowing tower thing was itself the main reactor, and thus the "warp-core" was born.
Through TOS and the early movies, the Warp Nacelle were where the reactions were happening. The glowing tube in engineering was simply a conduit. When the set was redesigned and repurposed for TNG, that element became the "warp core." This isn't necessarily invalid approach, but my personal feeling is that it was a rationalization to crowbar in a visually cool element for the sake of having something looking cool. From a dramatic point of view, it makes Engineering more interesting if the super powerful and potentially dangerous reactions are happening in that very area. Even if the prior (and IMHO more logical) intent was that those reactions happened out in the nacelles, away from the crew members, and easily ejected should things go amiss. All the more reason I prefer the TMP style conduit.
Hi, this is my first post here.
There actually is an indication that the shaft in TMP is not just a conduit.
First, approximately from the center of the horizontal shaft onwards it is pulsing toward the end like the TNG warp core's matter and antimatter feed lines, and where it splits into the smaller shafts to the nacelles there is also a moving glow going up into the pylons. So the energy seems to go to
Second, the swirling lights inside the shaft don't seem to go anywhere, they're just standing still. If the actual warp core is ... let's say at the bottom of the vertical shaft, shouldn't the plasma flow upwards?
I would say that from the point on where it starts pulsing it is a conduit but before that it is the core. The only problem I see with this is that the plasma also doesn't flow to said point of the shaft, but this may be explained by the pulsing part only being visible while the ship is in drydock, and later, while accelerating to warp maybe the flow in the visible part of the shaft is very slow (like far away from a waterfall).