382 W. MAKOUS AND OTHERS 



The procedure employed was essentially that used by Halpern and 

 Erickson. In both the rat and cat preparations, sodium pentobarbital 

 anesthesia was induced, the head was fixed firmly by a head holder, and 

 the cerebellum removed to expose the floor of the IVth ventricle so that it 

 could be explored systematically with fine insulated wire or microelectrodes 

 in a three-dimensional manipulator. 



MULTIUNIT ACTIVITY IN RAT MEDULLA 



A 30// enamelled wire yielded sufficiently localized multiunit activity to 

 permit focal mapping of the mouth, head, and body surface as projected 

 on the medulla. Figurine charts of the rat were constructed to show the 

 areas from which stimulation elicited a response as observed on the CRO 

 and audio monitor. Regions yielding responses greater than 100 //V were 

 indicated by sohd coloring, between 25-100//V by crosshatching, and less 

 than 25//V by gray shading. Figure 1 shows the relation between the 

 gustatory responsive areas and temperature and tactile responsivity of 

 the mouth and other body parts at a plane 2- 9mm anterior to the obex. 

 The figurine plot was constructed directly from the protocol descriptions 

 at each electrode placement; the anatomical insert was reconstructed from 

 photomicrographs after the brains had been fixed, sectioned, and stained 

 for histological study. The figure shows that reactivity to gustatory stimuli 

 can appear in isolation, i.e. with no tactile or temperature responses, even 

 with multiunit recording. This can be seen in the figure in the most dorsal 

 point of the second penetration from the midline, second from right in 

 the figure. Only the application of taste stimuli to the anterior tongue 

 surface elicited neural activity at this point. As the electrode advanced, 

 cold water on the tongue became an effective stimulus in addition to taste. 

 Still deeper in this penetration, tactile stimulation of the lower teeth and 

 chin became effective in addition to taste and temperature. Taste and 

 tongue temperature sensitivity then dropped out so that only tactile 

 responses to stimulating the lower teeth and chin remained. Finally, all 

 tactile sensitivity dropped out. The electrode placements on either side 

 yielded taste responses, but these occurred in conjunction with tactile 

 sensitivity of lower teeth or chin and, in one instance in this figure, with 

 tongue tactile sensitivity. In the fourth column from the midline, tongue 

 tactile sensitivity was observed with no gustatory responses. Still more 

 laterally, tooth, lip, and vibrissae were found. Most of the responses were 

 evoked by homolateral stimulation but tongue tactile sensitivity in this 

 figure was bilateral. 



Figure 2 shows a similar mapping at a more caudal plane, 1 • 5mm rostral 

 to the obex. No taste or tongue tactile sensitivity is represented here 

 although the teeth and homolateral mucosa of the mouth are well repre- 



