Here's a simple method that you can try if you're just trying to get more shadow/highlight detail (or contrast) out of your scans using Photoshop, and curves alone aren't doing it. I'll first say that Aperture (and presumably Lightroom) work faster and better. But if you don't have one of those apps (or one of the others suggested), this works fairly well.
Open your document in Photoshop and have your Layers, and Channels windows open, with the Channels window visible.
Hold down the Shift and Command keys then click on the RGB Channel. That action selects the pixels in the highlight range of the image. If I understand it correctly, you have selected all of the pixels with an rgb value of 128 higher.
* If you click it again (still holding the shift/command keys) you'll add to the selection the next 50% of the upper range of remaining pixels. Repeat as necessary.
* You can invert the selection to automatically have the lower range of pixels (shadows) mapped.
* Map the lower range of pixels as described, and create a layer/shadow mask. Then make a new layer of just the highlight range, as described above. Select the shadow mask, then add to the selection, the selection of the highlight mask. If you're still with me, you now have a selection that includes the shadows and highlights, but doesn't include the midrange. Invert that selection to create a midrange layer/mask.
Now you're free to make adjustments to each of the three tonal ranges.