Decoding the mechanisms by which the individual qualities of a sensory stimulus are extracted from the periphery, and conveyed to, integrated, and processed within the central nervous system requires visualization of the organization of primary sensory neuron projections. Here, we have used a combination of molecular-genetic labeling and somatotopic retrograde tracing approaches to visualize the organization of peripheral and central axonal endings of the physiologically distinct LTMR subtypes that mediate the sense of touch. Our findings support a model in which individual features of a complex tactile stimulus are extracted by three functionally distinct hair follicle types and conveyed via the activities of unique combinations of Aβ-, Aδ- and C-LTMRs to dorsal horn LTMR columns, where these features are represented, integrated, and processed for output to the brain.
Similar to primary sensory neurons of other sensory systems, such as olfactory receptor neurons of the olfactory system and auditory receptors of the auditory system, cutaneous LTMRs of the somatosensory system are poised to extract individual qualities of complex stimuli from the external world. Yet, despite their intense study for decades, the unique functions and the relative organization of LTMR subtype peripheral endings, especially in hairy skin, have remained largely unclear. We found that the peripheral endings of each LTMR subtype associate with either one or two of the three types of trunk and proximal limb hairy skin hair follicles. Furthermore, axons of select LTMR subtypes are intimately associated with one another, having entwined projections and interdigitated lanceolate endings that innervate the same hair follicle. Conversely, each of the three hair follicle types receives a unique and invariant combination of LTMR endings. Indeed, guard hair follicles are uniquely associated with a combination of Aβ RA-LTMR and Aβ SA1-LTMRs; Awl/auchene hairs are triply innervated by Aβ RA-LTMRs, Aδ-LTMRs, and C-LTMRs; and Zigzag hair follicles are innervated by both C-LTMRs and Aδ-LTMRs. These findings are consistent with classic neurophysiological measurements in the cat and rabbit indicating that Aβ RA-LTMRs and Aδ-LTMRs can be differentially activated by deflection of distinct hair follicle types (Brown and Iggo, 1967
; Burgess et al., 1968
). In addition, because the three hair follicle types exhibit different shapes, sizes, and cellular compositions, they are likely to have distinct deflectional or vibrational tuning properties. Thus, our findings indicate that guard, awl/auchene, and zigzag hairs are physiologically distinct mechanosensory end organs. We suggest that it is the combination of: 1) the relative numbers, unique spatial distributions, and distinct morphological and deflectional properties of the three types of hair follicles, 2) the unique combinations of LTMR subtype endings associated with each of the three hair follicle types, and 3) distinct sensitivities, conduction velocities, spike train patterns, and adaptation properties of the four main classes of hair-follicle-associated LTMRs that enables the hairy skin mechanosensory system to extract and convey to the CNS the complex combinations of qualities that define a touch.
The remarkable organization of peripheral LTMR endings reveals a fundamental feature of the somatosensory system and supports an integrative model of mechanosensation. This integrative model posits that individual mechanical properties of a complex tactile stimulus engage distinct combinations of the three hair follicle types and thus differentially activate the unique combinations of LTMRs with which these follicles associate. We suggest that certain mechanical stimuli, such as the flutter of an insect's wings, raindrops, or light contact with folliage, may preferentially stimulate long guard hairs and thus Aβ RA-LTMRs. On the other hand, stroking of the coat by a nurturing mother may preferentially activate Aδ-LTMRs and C-LTMRs associated with the small, abundant zigzag hair follicles. Indentation of a patch of hairy skin with a blunt object may activate Aβ SA1-LTMRs as well as all other LTMRs associated with hair follicles of the indented and surrounding skin region. Thus, the key feature of this integrative model is that a large number of potential combinations of deflections or vibrations of the three hair follicle types, with or without concomitant skin indentation, endows the somatosensory system with a vast array of potential ensembles of LTMR activities that could encode the properties or qualities that define a particular tactile stimulus.
How are individual properties or qualities of a touch represented and processed within the central nervous system to generate its unique percept? We observed that the central projections of the Aβ-LTMRs, Aδ-LTMRs, and C-LTMRs that innervate a common peripheral LTMR unit align in a columnar manner in the spinal cord dorsal horn. We suggest that dorsal horn LTMR columns are the initial and perhaps principal CNS sites of integration and processing of neural inputs that represent skin indentation and the relative movements of each hair follicle type in a small skin region. It is noteworthy that a distinguishing feature of Aβ-, Aδ- and C-LTMRs is their highly divergent action potential conduction velocities, which in the mouse range from greater than 10 m/sec for Aβ RA- and SA-LTMRs to less than 1 m/sec for C-LTMRs. Thus, a tactile stimulus is likely to be encoded by temporally coordinated ensembles of Aβ RA-, Aβ SA-, Aδ- and C-LTMR activities that converge onto dorsal horn LTMR columns, where these activities are integrated and processed. The outputs of LTMR columns are conveyed via dorsal horn projection neurons that comprise the postsynaptic dorsal column pathway and spinocervical tract to the brain, presumably undergoing further integration with mechanosensory information carried by the direct dorsal column pathway and other sensory inputs (Brown, 1981b
; Brown and Franz, 1969
; Brown et al., 1987
; Cliffer and Giesler, 1989
; Giesler and Cliffer, 1985
; Giesler et al., 1984
). The development of the exquisite organization of peripheral LTMR units and dorsal horn LTMR columns as well as the extent, mechanisms, and functions of integration and processing of Aβ RA-LTMR, Aβ SA-LTMR, Aδ-LTMR, and C-LTMR inputs within LTMR columns are intriguing topics for future studies.