Proceedings of The Physiological Society
University of Cambridge (2008) Proc Physiol Soc 11, C74
Quantitative characterization of low-threshold mechanoreceptor inputs to wide dynamic range lamina I spinoparabrachial neurons
1. Oral & Maxillofacial Surgery, University of Sheffield School of Clinical Dentistry, Sheffield, United Kingdom.
The cell types that comprise ascending projections from the superficial dorsal horn of the spinal cord are typically modality specific e.g. nociceptive-specific, polymodal nociceptive, thermoreceptive (Craig et al. 2001). However, in monkey and rat a small proportion of projection neurons receive convergent inputs from both low-threshold mechanoreceptors and nociceptors (Bester et al. 2000, Ferrington et al. 1987). These ‘wide dynamic range’ neurons comprise the majority of neurons in pathways from spinal laminae IV and V, but they are rare (ca. 10%) in ascending projections from lamina I. It has been assumed that rapidly-conducting A-fibre (myelinated) afferents provide the low-threshold inputs to wide dynamic range neurons, as these cells typically receive inputs from primary afferents with Aβ conduction velocities. However, myelinated low-threshold mechanoreceptors do not terminate in lamina I (Brown, 1981). One potential source of low-threshold mechanoreceptor input to lamina I projection neurons is C-fibre mechanoreceptors. These fibres terminate heavily in the superficial dorsal horn, and they are preferentially activated by slowly moving stimuli (Valbo et al. 1999), a feature that distinguishes them from myelinated mechanoreceptors. The aim of the present study was to quantitatively characterize the low-threshold inputs to wide dynamic range projection neurons in lamina I of the spinal cord to investigate their source. Experiments were performed on male Sprague-Dawley rats that were anaesthetized with Urethane (1.2g/kg I.P.) and neuromuscularly blocked with Tubocurarine (150μg I.V.). During neuromuscular blockade, anaesthetic depth was considered sufficient if blood pressure and heart rate were stable during noxious stimulation. The activity of single, antidromically identified lamina I spinoparabrachial neurons with hindlimb receptive fields was recorded extracellularly. Wide dynamic range neurons were characterized with graded velocity brushing (6.6 - 126 cm/s) stimuli. Cells were also tested with graded thermal and mechanical stimulation as well as electrical stimulation of peripheral nerve fibres. Nine of 95 lamina I spinoparabrachial neurons were responsive to low- and high-threshold stimuli. All of the neurons had receptive fields that included both glabrous and hairy skin, but low-threshold responses were only evoked from hairy skin. Most neurons showed decreasing responses to increasing brush velocity (n=7), one cell had a flat stimulus-response curve and another showed a U-shaped relationship between discharge and brush velocity. None of the cells studied received inputs from Aβ axons. The present findings suggest that C-fibre mechanoreceptors provide the dominant low-threshold inputs to wide dynamic range neurons in the lamina I spinoparabrachial pathway. As C-fibre mechanoreceptors are thought to be important in affective touch, the lamina I spinoparabrachial pathway may be the projection that relays this information to the brain.
Where applicable, experiments conform with Society ethical requirements