So, I've read many times that: "The major neuropathologic feature of HSP is axonal degeneration that is maximal in the terminal portions of the longest descending and ascending tracts. These include the crossed and uncrossed corticospinal tracts to the legs and fasciculus gracilis. The spinocerebellar tract is involved to a lesser extent". (from Wikipedia), and various references on many pages to Pyramidal Tracts.
I was wondering what/where these parts are, and what else is 'nearby' - i.e. some kind of wiring diagram for the nervous system. The BBC website had a nice diagram: http://www.bbc.co.uk/science/humanbody/body/factfiles/nervous_anatomy.shtml and there's a much more complicated picture in the enormous post on Wikipedia: http://en.wikipedia.org/wiki/Nervous_system.
terminal portions of the longest descending and ascending tracts: The descending tracts are muscle control (i.e. signals travelling down from the brain) and the ascending tracts are perception and touch information (i.e. signals travelling up to the brain). The longest tracts are those that serve the lower part of the body, and the terminal portion of the spinal column is where the longest tracts stop and nerves descend further. This page shows what happens at different sections of the spine. http://en.wikipedia.org/wiki/Spinal_cord.
Then, given that the nerves 'pop out' at the section nearest the organ/muscles that they control, the longest tracts would appear to be those that are "Lumbar" - control of leg muscles and "Sacral" - bowel, bladder and sexual function. http://www.spinalinjury.net/html/_anatomy_of_the_spinal_cord_co.html. There are descending and ascending versions of these.
corticospinal tracts: The lateral corticospinal tract carries messages from the brain to control the muscles. There are two tracts on opposite sides of the spine, and there are also a number of other tracts which control muscles (Extrapyramidal tracts and the anterior corticospinal tract). These are also known as the pyramidal tracts. I haven't really understood the difference between crossed and uncrossed.
fasciculus gracilis: The " fasciculus gracilis" appears to be part of the Dorsal column, alternatively known as the "Gracile fasciculus" and next to this the "Cuneate fasciculus". The dorsal column is in the centre of the spine and sends messages from the skin and positional information back up to the brain, and there are a number of other tracts which carry similar information up (spinocerebellar tracts, spinothalamic tracts and Spino-olivary fibers).
This page is another goes some way to explain the function of each name mentioned: http://www.becomehealthynow.com/article/bodynervousadvanced/820/.
I cant quite work out if HSP affects the spinal column itself or the nerves which connect to the column.