Structure and function of the terminal branches of first-order tactile neurons

Manipulating objects to complete tasks in everyday life relies heavily on tactile processing. A key aspect of object manipulation is the ability to discern shape and form through tactile cues, such as the orientation of an object relative to the fingertips. Historically, feature extraction has been considered a hallmark of cortical processing. However, recent research has shown that tactile processing and integration begin subcortically, as early as the first-order tactile neurons innervating the glabrous skin of the hand. This thesis expands our understanding of the complexities of these first-order tactile neurons and their role in edge orientation discrimination, edge perception, and the anatomical basis of tactile processing in the periphery. In Chapter 2, we present results from microneurography studies conducted on awake humans. First-order tactile neurons (FA-1 and SA-1) signal edge orientation differences as fine as 5° across a broad range of stimulation speeds (15-180 mm/s) in their intensity and spatiotemporal pattern within each speed. Neurons signal better at slower (2.5-10 mm/s) than average speeds that people use in similar tasks (20-30 mm/s), particularly for fine edge differences (5°), and fall off quickly at faster speeds. Both FA-1 and SA-1 neurons signal edge differences in a speed-invariant manner in the sequential structure of the evoked spike trains when represented in the spatial domain. Building on this, in Chapter 3, we investigated whether human edge orientation perception capacity improves with slower speeds in a behavioral study. Our participants moved their finger over two pairs of edges, one pair parallel and the other nonparallel to varying degrees (4-20°), and were asked to identify which of the two pairs was nonparallel. Consistent with the neural data in Chapter 2, participants performed better at speeds slower than their natural speeds, particularly for the smallest differences (4°). The first-order tactile neurons branch extensively before innervating multiple mechanoreceptors superficially in the glabrous skin of the hand. Our overarching hypothesis relates this anatomical branching to the signaling capacity of first-order tactile neurons. In Chapter 4, we describe the morphometrics of Meissner corpuscles, which are crucial for fine detail processing, and the first-order tactile Aꞵ neurons (FA-1) innervating them. We provide an account of the complex branching patterns seen in these neurons, as the putative anatomical substrate of the tactile processing machinery in the periphery. Overall, these studies enhance our understanding of tactile processing and highlight the complexities of the tactile periphery.