Tutorial:
http://www.adobe.com/print/events/pdf/Brill_Handout.pdf#search=%22tutorial%20pdf%20photoshop%20%20brain%20%22
SEE page 2 for the brain example!
http://graphicssoft.about.com/od/photoshoptutorials/
http://www.grafx-design.com/phototut.html
Here you can see an example (but with copyright!) :
This image began life as a line drawing of the human brain. I then added color and texture via Photoshop. Final image is the brain in lateral aspect from the left. I added special emphasis to the contours and lines of the brain's surface.
http://www.science-art.com/image.asp?id=567&search=1
Here you will find cartoons :
http://712designs.com/illustrations/AARP/gettingToKnow.html
