In our somatosensory cortex our fingertips have representations that are disproportionally large compared to the skin surface area that is not clearly explained by innervation patterns[1, 2]. The large representations of our fingertips is correlated with higher acuity touch in these regions. Disproportionate cortical representation of sensory surfaces is sometimes termed cortical magnification, but the relative contribution of cortical and subcortical somatosensory areas to this phenomenon has not been addressed. The subcortical specializations of representing these high acuity areas may be underappreciated. This study investigates the subcortical representations of high acuity touch using the star of the star-nosed mole. This animal is the ideal animal in which to study fundamental questions in cortical magnification. The star of the star-nosed mole has many features in common with the hands of primates, such as being a glabrous skin surface, varying in acuity across the skin surface, and cortical over-representation of the highest acuity "fovea" area. The star also has many features in common with the well-studied rodent whisker system, such as being a trigeminal somatosensory specialization and being represented by segmented somatotopic maps in the somatosensory cortex. In rodents these maps are also found in the ascending trigeminal somatosensory pathway in the thalamus and the brainstem. I hypothesize, and have pilot data suggesting, that there will be analogues maps of the star in these nuclei. Once found, the volumes of ray representations can be measured. This study proposes to use electrophysiology, histochemistry, and neural tract tracing to describe and measure the subcortical star representations in this pathway, specifically in the spinal trigeminal complex and the ventral posterior medial nucleus of the thalamus. This will allow us to ask which somatosensory areas contribute to the observed cortical magnification, which may give us insights into the roles of these areas in touch processing. I will also use the new technology of serial block-face electron microscopy to investigate patterns of sensory neuron arborization at the star surface. A better understanding of the fundamental organization of the somatosensory system is vital to the development of treatments for somatosensory disorders and sensory loss.
I will use the mammal with the finest touch acuity of any mammal, the star-nosed mole, to address the unsolved question of why and how some body regions come to have more cortical tissue devoted to them than others. This will provide insight into the fundamental organization of the touch system, specifically the areas with high acuity capabilities like human fingers. Fundamental knowledge about how touch is represented in the somatosensory system will be useful to numerous disorders of the touch system.
|Sawyer, Eva K; Leitch, Duncan B; Catania, Kenneth C (2014) Organization of the spinal trigeminal nucleus in star-nosed moles. J Comp Neurol 522:3335-50|