Image of a human brain section showing the regional distribution of a specific subtype of receptors for a neurotransmitter called serotonin (the subtype is called 5-HT₂). From Waeber and Palacios, 1994.

The darker the area, the higher the concentration of receptors. The actual size of the image is about 1x2 inches.The dark area in the upper left corner is a small part of the cortex, the rest of the section (the one with the dark patches) is called caudate/putamen. It is thought to be involved in the control of movement, but also in cognitive functions.

For those of you interested in the technical aspects of this image, here is how it was done:

Blocks of human brain were obtained at autopsy, frozen, sectioned at 10 micron thickness (0.0004 inches) and mounted on microscope slides. The slides were then placed in a solution containing a radioactive drug that binds with high affinity to 5-HT₂ receptors. The slides were then washed in ice-cold solution (without radioactive drug), dried and placed on a special sheet of film (similar to X-ray film). The radiation from the drug bound to the receptors exposed the film (in this case for about 3 weeks), which was then processed in Kodak D-19.