Conserved Patterns of Trigeminal Somatosensory System Organization in Mammals

brainstem, there are 3 separate and complete somatotopic barrelette representations, the principal sensory nucleus, the spinal trigeminal subnucleus interpolaris, and the spinal trigeminal subnucleus caudalis. Broadly, the principal trigeminal sensory nucleus is vital to the sense of discriminative touch, whereas the subnucleus caudalis is important for pain sensation, and the subnucleus interpolaris may be related to discounting the role of movement during active touch behavior [Diamond et al., 2008]. Clear somatotopic maps make it easy for researchers to identify the disruption of normal somatosensory function, which is one reason why the rodent trigeminal system continues to be used in developmental studies, particularly in investigations of the neocortex. Previous researchers investigating comparative aspects of the trigeminal somatosensory areas have noted that the presence or absence of cortical barrels across species does not correlate well with whisking behavior or phylogeny [Woolsey et al., 1975; Horton and Adams, 2005]. In my own work, I used anatomical and electrophysiological methods to delineate vibrissal representations in the brainstem trigeminal pathway. I suspected that the brainstem relays would have fewer higherlevel integrative functions than the neocortical stations, and would therefore exhibit less interspecific variation than the neocorAlmost all mammals have distinct tactile hairs that they can use to explore their environment. In particular, laboratory rodents possess about 60 thick, densely innervated facial vibrissae that they sweep in a characteristically rhythmic “whisking” motion with which they gauge the location, shape, texture, and orientation of objects [Fox, 2008]. This mechanosensory information is transmitted to the central nervous system via the trigeminal pathway ( Fig. 1 a). Much of our current understanding of mammalian tactile senses has been gleaned from the body of literature on the rodent whisker system. Given that research efforts focusing on mice and rats are typically justified by the assumption that these animals are representative of all mammals, including humans, I adopted a comparative neuroanatomical approach in order to test how broadly applicable our knowledge of the rodent trigeminal system is to the trigeminal system of other mammals. In laboratory rodents, the punctate array of facial vibrissae is represented in a one-to-one fashion in anatomically distinct units in 3 stations of the ascending somatosensory pathway: the brainstem, thalamus, and neocortex. These discrete circular modules are termed barrelettes, barreloids, and barrels, respectively [Woolsey and Van der Loos, 1970; Van der Loos, 1976, Belford and Killackey, 1979; Ma, 1991] ( Fig. 1 b). Furthermore, within the Published online: March 28, 2017