756 HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 



FIG. 13. Short and medium latency response areas of brain stem and thalamus responding to 

 tooth pulp stimulation. Narrow vertical stripes identify the trigeminal lemniscus (medial lemniscus) 

 which has a short latency and a dominantly contralateral projection. Horizontal stripes mark the 

 trigeminobulbothalamic path (spinobulbothalamic), with short latency and bilateral projections. 

 Stippled areas mark the ascending portion of the central tegmental fasciculus (dorsal secondary 

 trigeminal pathway), with medium latency and bilateral projection. Diagonal stripes designate the 

 Eiscending path within the central grey which possesses medium latency and bilateral projections. 

 The reticular formation yields widespread responses characterized by long latency and bilateral 

 character. The relevant structures are indicated on the leftside of each level. Recording sites (except 

 within reticular formation) are shaded on the right side. Abbreviations : BC, brachium conjunctivum; 

 BIC, brachium of inferior colliculus; CE, centralis; CG, central grey; CM, center median; CTF, 

 central tegmental fasciculus; DBC, decussation of brachium conjunctivum; DM, dorsalis medialis; 

 H, habenula; HIP, habenulointerpeduncular tract; IP, interpeduncularis; LG, lateral geniculate, 

 LP, lateralis posterior; MG, medial geniculate; ML, medial lemniscus; MLF, medial longitudinal 

 fasciculus; .\''/?,red nucleus; Pet/., peduncle; PL, parafascicularis; Pul., pulvinar; P]\ periventricular 

 area; Py, pyramidal tract; SBT, spinobulbothalamic tract; .S'C, superior colliculus; .S'.V, substantia 

 nigra; Sbf., subparafascicularis; STh, subthalamicus, TBT, trigeminobulbothalamic tract; TL, 

 trigeminal lemniscus; TO, optic tract; VL, ventralis lateralis; VPL, ventralis postcrolateralis; VP, 

 ventralis posterior; VPM, ventralis posteromedialis; VTT, ventral tegmental nucleus of Tsai; and 

 ^I, zona incerta. [From Kerr et al. (47).] 



particular portions of the reticular pathways may con- 

 vey signals essential to pain perception. 



It is clearly established that, whatever may be con- 

 tributed by upward-streaming sensory-evoked im- 

 pulses, the central nervous system possesses an im- 

 portant downstream sensory control mechanism 

 which also undoubtedly contributes to the perceptual 



content. The ner\ous system possesses some mecha- 

 nism whereby the amplitude of sensory-evoked re- 

 sponses, and hence the number or synchrony of units 

 responding, can be greatly modified. This mechanism 

 exerts an effect within each of the classical sensory 

 pathways, altering the initiation of impulses or their 

 transmission through the entire succession of sensory 



