Quantitative responses of spinothalamic lamina I neurones to graded mechanical stimulation in the cat

• Published in: The Journal of physiology Vol. 545; no. 3; pp. 913 - 931
• Main Authors: Andrew, David, Craig, A. D. (Bud)
• Format: Journal Article
• Language: English
• Published: Oxford, UK The Physiological Society 15-12-2002; Blackwell Publishing Ltd; Blackwell Science Inc
• Subjects: Animals; Cats; Cold Temperature; Differential Threshold; Electrophysiology; Female; Hot Temperature; Male; Neurons - classification; Neurons - physiology; Nociceptors - physiology; Original; Physical Stimulation - methods; Reproducibility of Results; Spinothalamic Tracts - physiology
• ISSN: 0022-3751; 1469-7793
• DOI: 10.1113/jphysiol.2002.027342

Nociceptive spinothalamic tract (STT) neurones in lamina I of the lumbosacral spinal cord of anaesthetized cats were characterized
by recording their responses to graded mechanical stimulation with controlled forces of 10-120 g and probes of 5.0, 0.5 and
0.1 mm 2 contact area. Neurones were identified by antidromic activation from the contralateral thalamus, and cells that responded
to noxious stimulation were categorized as either nociceptive specific (NS, n = 20) or as polymodal nociceptive (HPC, responsive to h eat, p inch and c old, n = 19) based on their responses to quantitative thermal stimuli. The mean responses of the 39 units increased linearly as
stimulus intensity increased, and the population stimulus-response curves evoked by each of the three probes were all significantly
different from each other. Thresholds were 45 g for the 5.0 mm 2 probe, 30 g for the 0.5 mm 2 probe and 20 g for the 0.1 mm 2 probe. Further analysis showed that the NS neurones encoded both stimulus intensity and area (probe size) significantly better
than HPC neurones in terms of their thresholds to individual probes, their peak discharge rates, their suprathreshold responsiveness
and their ability to discriminate the three different probe sizes. These differences are consistent with the known differences
between the mechanical encoding properties of A-fibre nociceptors, which provide the dominant inputs to NS neurones, and C-fibre
nociceptors, which are the dominant inputs to HPC cells. Comparison of the stimulus-response curves of NS and HPC neurones
indicated that the discharge of NS neurones better match the psychophysics of mechanical pain sensations in humans than the
discharge of the HPC neurones do. Our findings support the view that NS neurones have a prominent role in mechanical pain
and sharpness, and they corroborate the concept that the lamina I STT projection comprises several discrete channels that
are integrated in the forebrain to generate qualitatively distinct sensations.