Nociceptive responses of neurons in medial thalamus and their relationship to spinothalamic pathways

Abstract

1. An extracellular study of the cat medial thalamus has revealed four types of somatosensory neurons. These were located primarily in the n. parafascicularis, n. subparafascicularis, and n. centralis lateralis; none were found in the n. centrum medianum. There was no functional segregation of neurons within each nucleus or between nuclei. Each type of neuron had large and often bilateral receptive areas. No somatotopic organization of neurons was found within the medial thalamus. 2. Noxious (N) and noxious-tap (NT) neurons comprising 72% of the sample (78 of 109 total) were considered to be nociceptive. N cells responded exclusively to noxious mechanical stimulation of skin, muscle fascia, tendons, and joints, and to direct stimulation of A-delta- and C-fiber groups in cutaneous, articular, and muscle nerves. NT cells responded to noxious and tap stimulation in a differential manner and to stimulation of the entire spectrum of A- and C-fibers. N and NT cells accurately signaled the duration of noxious mechanical stimulation. Their nociceptive responses were also graded as a function of both noxious stimulus intensity and the number of activated A-delta- and C-fibers. Stimulation of A- and C-fibers evoked, respectively, an inital burst and a late burst of discharges. A brief period of inhibition intervened between the initial and late bursts of NT cells. Prolonged afterdischarge was often observed following noxious natural stimulation or stimulation of A-delta- and C-fibers. The phenomenon of discharge "windup" was observed during iterative stimulation of C-fibers. 3. Tap (T) neurons (10%) responded only to brisk but innocuous taps applied to skin or underlying tissue. These cells were driven only by activation of A-alpha- and A-beta-fibers. The response to such stimulation was seen as an initial burst of discharges followed by an inhibitory period. 4. Inhibited (I) neurons (18%) had resting discharges that were inhibited by noxious stimuli and stimulation of A-beta- and C-fiber groups. 5. The results obtained from monitoring the peripherally evoked responses of nociceptive N and NT neurons before and after selective lesions of the spinal cord strongly suggested that the spinothalamic tracts were the only spinal projections mediating A- and C-fiber input to these cells. Each spinothalamic tract apparently carried information originating from both sides of the body.

Pain and emotion interactions in subregions of the cingulate gyrus

“…Interestingly, these responses reflect in many ways the properties of neurons in the MITN that have projections to cingulate cortex and have large and bilateral receptive fields that are mainly nociceptive but with some tap responses, and a large percentage of nociceptive multimodal responses. 29,30 Acute nociceptive responses in human include those that are mediated by cingulate cortex, which is one of the most frequently activated regions in the pain neuromatrix. 4, 5 Lenz et al 31 used subdural recording electrodes to show that laser-evoked, nociceptive potentials could be evoked directly from the MCC with latencies of 211-242 milliseconds for negative and 325-352 milliseconds for positive potentials.…”

Parallel Processing of Nociceptive A-δ Inputs in SII and Midcingulate Cortex in Humans

“…It is unlikely that cingulate responses may participate in the precise sensory encoding of stimulus features, given the poor intensity coding properties and very large receptive fields of CC neurons (Casey, 1966;Dong et al, 1978;Sikes and Vogt, 1992;Yamamura et al, 1996) and the reported fact that pain cannot be localized despite preserved CC if the lateral somatosensory areas are destroyed (Ploner et al, 1999). Moreover, electrical microelectrode stimulation at pMCC sites where pain-sensitive neurons were recorded does not elicit pain sensations (Hutchinson et al, 1999).…”

Diffuse optical tomography of pain and tactile stimulation: Activation in cortical sensory and emotional systems

“…Such pathways may be direct, such as the spino-thalamic cortical pathway (Dong et al 1978;Burstein et al 1996;Willis and Westlund 1997) and others may be indirect, such as cortico-cortical pathways or cortical-subcortical pathways. 'Bilateral' representation at the level of the cortex may also be due to bilateral receptive fields terminating in the spinal cord or brainstem.…”