A Tale of Two Touches
The skin senses are a truly multisensory modality with specific populations of cutaneous receptors & afferent nerve fibres coding for touch, temperature, pain and itch. It has been known for some time that mammalian skin is innervated by a population of low threshold mechanosensory C-fibres which had been thought to have ‘evolved-out’ in primates. However, in the early 1990’s Ake Vallbo and his colleagues in Sweden, using the technique of microneurography they had developed, identified low threshold mechanosensory c-fibres in human hairy skin (called C-tactile afferents CT). Subsequent efforts over the ensuing decades has characterised the response properties of CTs by coupling microneurography with psychophysics and fMRI approaches demonstrating that CT afferents show velocity and temperature dependent spiking in response to gentle stroking touch, leading to the hypothesis that these slowly conducting afferents encode the rewarding and affiliative properties of close physical touch. With the 1st order neurone characterised and its cortical project site identified we are now focussing on the 2nd order pathway, via the dorsal horn. The CT ascending spinal projection pathways in humans are currently unknown. To address this we tested for alterations in pleasant touch following therapeutic spinothalamic tract ablation (cordotomy) by gently stroking at CT optimal (3cm/s) and sub-optimal (0.3 and 30cm/s) velocities the lesioned and non-lesioned forearm. Patients were asked to rate the pleasantness of touch. Absolute pleasantness ratings and the CT preference index ([rating for 3cm/s x 2 - ratings for 0.3cm/s + 30cm/s] / 2 ) were calculated. Discriminative touch was assessed using two-point discrimination, tactile detection thresholds and graphesthesia. Findings and next steps will be presented.