Surgical procedures on the brain pose a special problem in that usually the intended target is not visible, but buried beneath other structures which cannot be disturbed without causing serious damage to the organism. Since the target cannot be located visually an alternative method is necessary.
Stereotaxic Surgery is a technique used to locate and manipulate discrete areas of the brain using a coordinate system. Brain areas are located by knowing their spatial relationship to visible landmarks. The spatial relationship is expressed using 3 coordinates: Anterior-Posterior (AP),Medial-Lateral (ML),and Dorsal-Ventral DV. The coordinates specify the distance (in mm) of the target from a specified landmark at a given orientation of the head.
The coordinates are usually obtain from a stereotaxic atlas.A stereotaxic atlas is a pictorial 3-D reconstuction of the brain, compiled using serial sections. A brain is mounted at a specific angle which can be easily reproduced. Serial sections are taken, mounted on slides, stained photographed and traced. As you flip through the atlas, the 3-D structure of the brain is revealed. In addition to providing the coordinates, a stereotaxic atlas will provide you with information about the landmarks used as reference points and the orientation of the head. The most common landmarks used in stereotaxic surgery are Bregma and Lambda. Another landmark that is occaisionally used is the Intra Aural Line (IAL). This reference point is located on the stereotaxic instrument,rather than the rat. Bregma is by far the most commonly used reference point. The two most common head orientations used in stereotaxic surgery are Skull Flat and the plane of deGroot. The location of the IAL can be seen in the illustration showing the plane of deGroot.
A stereotaxic atlas is compiled using rats of a specific size, sex and strain. The coordinates in the atlas are appropriate for rats of the specified size sex and strain. As the rat you are using deviates from the "atlas" rat, accuracy will suffer. Coordinates from an atlas compiled using 350 g rats will not be appropriate for surgery on a 40 g rat pup.
Another source of Stereotaxic coordinates is journal articles. The information from journal articles does not always provide all the information necessary. Usually, if unstated, the reference point is bregma and the head orientation is skull flat, but this is not always the case. Double checking with a stereotaxic atlas is prudent.
The principal tool used in stereotaxic surgery is the Stereotaxic Instrument. The stereotaxic instrument holds the rat's head immobile at the appropriate angle and allows the experimenter to position an electode accurately using 3 precision drives which control the movement of the electode holder in the three stereotaxic planes: AP, ML, and DV. The illustration shows a simple steretaxic instrument designed for positioning a single electrode. More elaborate stereotaxic instruments which can accommodate several electrode carriers are available.