Abstract
It has been suggested that primary afferent C-fibres that respond to innocuous tactile stimuli are important in the sensation of pleasurable touch. Although it is known that C-tactile fibres terminate in the substantia gelatinosa (lamina II) of the spinal cord, virtually all of the neurons in this region are interneurons, and currently it is not known how impulses in C-mechanoreceptors are transmitted to higher centres. In the current study, I have tested the quantitative response properties of ‘wide dynamic range’ projection neurons in lamina I of the spinal cord to graded velocity brushing stimuli to identify whether low-threshold mechanoreceptor input to these neurons arises from myelinated or umyelinated nerve fibres. Graded velocity brushing stimuli (6.6–126 cm s−1) were used to characterize the mechanoreceptor inputs to ‘wide dynamic range’ neurons in lamina I of the dorsal horn that had axons that projected to the contralateral parabrachial nucleus. The most effective tactile stimuli for activation of ‘wide dynamic range’ lamina I spinoparabrachial neurons were low velocity brush strokes: peak discharge occurred at a mean velocity of 9.2 cm s−1 (range 6.6–20.4 cm s−1, s.d. 5.0 cm s−1), and declined exponentially as brush velocity increased. The data indicate that C-fibres, but not A-fibres, conveyed low-threshold mechanoreceptor inputs to lamina I projection neurons.
Highlights
It is well recognized that peripheral nerve fibres with unmyelinated (C) axons are associated with nociception and thermoreception, as well as the concomitant sensations of pain and temperature (Ochoa & Torebjork, 1989)
The anatomical sites of terminations of the axons of wide dynamic range lamina I spinoparabrachial neurons were comparable to the population of lamina I spinoparabrachial neurons as a whole (Andrew, 2009): 90% of neurons were activated from the internal lateral subnucleus, 80% from the external lateral subnucleus and 70% from the Kolliker–Fuse nucleus
The main finding in the current study was that low-threshold mechanoreceptor inputs to lamina I spinoparabrachial neurons appear to be transmitted by C-fibre primary afferents, acting via at least one interneuron
Summary
It is well recognized that peripheral nerve fibres with unmyelinated (C) axons are associated with nociception and thermoreception, as well as the concomitant sensations of pain and temperature (Ochoa & Torebjork, 1989). There is little available information regarding the identity of the central neurons activated by C-fibre mechanoreceptors, and the ascending pathway(s) activated by C-fibre mechanoreceptors are largely unknown. It is known from in vivo intra-axonal labelling studies (Sugiura, 1996) that the central terminals of low-threshold C-fibres occupy a characteristic location in inner lamina II of the spinal dorsal horn. The available anatomical and physiological data support the hypothesis that sensory information that
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