I'd hate to offer an authoritative answer to you and then be wrong, but I'm pretty confident on this one
There is a word "shin" which essentially means the innermost core of something, where it can be literal or figurative, i.e. it can mean either the tough center of a cabbage, or the innermost heart of a person. It can be written with the kanji for "kokoro" (which you see earlier in this thread), or with the same kokoro with "kusaganmuri" on top (i.e. grass radical above). I'm not on a Japanese capable computer right now and anyway I don't know how to incorporate kanji images in these forums yet.
I'd guess that one is more likely to write the character with grass on top if the meaning is purely physical/literal, and just "kokoro" if the meaning is more figurative/spiritual. However in the latter case there is always the danger (from the writer's perspective) that readers will read it as "kokoro" rather than "shin", so I'm not sure how a native speaking writer would choose. Probably it depends
(gee, I just noticed this is a really old thread. I wonder if Paula's even still reading