Slow positive dorsal cord potentials activated by heterosegmental stimuli

Contralaterally induced dorsal root reflex shares common final pathway with ipsilaterally elicited ones in Wistar rats

The dorsal root reflex (DRR) in single fibers, recorded in the severed L5 dorsal root central stump, initiated by sciatic nerve (SN) stimulation (St), was studied in anesthetized rats. Both ipsi- and contralateral Sts elicited DRR, with central delays of 4.6 +/- 0.3 (n = 10) and 27.3 +/- 2.5 (n = 5) ms, respectively. Ipsilaterally induced DRRs showed repetitive firing (mean = 3 +/- 0.5 spikes, n = 5), while contralaterally induced ones were abolished after spinalization (T13). When the time interval between two ipsilateral suprathreshold Sts was less than 12 +/- 2.4 ms (n = 12), the second DRR was abolished. On the other hand, when it between two ipsilateral subthreshold Sts was less than 4.8 +/- 0.3 ms (n = 7), DRR was activated. Contralaterally induced DRR also showed temporal inhibition and facilitation with time intervals similar to that in ipsilateral cases. Furthermore, when the latency between a contralaterally and a ipsilaterally induced DRR was less than 10.8 +/- 1.8 ms (n = 5), the later DRR was blocked by the former, and vice versa. Facilitation can be also shown by a contralateral and a ipsilateral subthreshold St. These results suggested that contralaterally induced DRRs may share part of final common pathway with ipsilaterally induced ones.

Lumbar but not cervical intrathecal DAMGO suppresses extrasegmental nociception in awake rats

he effect of intrathecally administered [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO) on withdrawal latencies evoked by noxious heat applied to either cervical or lumbar dermatomes was studied in awake rats. Administration of DAMGO to the lumbar intrathecal space produces a dose-dependent suppression of withdrawals evoked by noxious thermal stimulation in either lumbar or cervical dermatomes. Administration of the same doses of DAMGO to the cervical spinal cord produces a suppression of withdrawals evoked by stimulation in cervical but not lumbar dermatomes. Control experiments provide evidence that the drugs administered intrathecally to either enlargement do not spread to the other enlargement.

Somatosensory evoked potentials recorded from the posterior pharynx to stimulation of the median nerve and cauda equina

Somatosensory evoked potentials (ppSEPs) in response to stimulation of the median nerve at the wrist and the cauda equina at the epidural space (the L4 level) were recorded from the posterior wall of the pharynx in 15 patients who underwent spinal surgery under general anesthesia, using disc electrodes attached to the endotracheal tube, and compared with segmental spinal cord potentials (seg-SCPs) that were recorded simultaneously from the posterior epidural space (PES). ppSEPs consisted of the initially positive spike (P9) followed by slow positive (P13) and negative (N22) waves. The P13 and N22 of ppSEPs had phase reversal relationship with the P2 and N2 recorded from the PES, respectively. The peak latencies of P9 (9.40 +/- 0.7 ms) (mean +/- SD), P13 (13.1 +/- 0.9 ms), and N22 (22.0 +/- 2.1 ms) of ppSEPs coincided with those of P1, N1 and P2 of seg-SCPs, respectively, ppSEPs were recorded more clearly with a reference electrode on the dorsal surface of the neck than with the reference electrode at the earlobe or back of the hand. The threshold and maximal stimulus intensities were also similar between the ppSEPs and seg-SCPs. Thus, the P9, P13, and N22 components of ppSEPs were thought to have the same origin as the P1, N1 and P2 of seg-SCPs, respectively. Therefore, the P9, P13 and N22 of ppSEPs may reflect incoming volleys through the root, synchronized activities of the interneurons and primary afferent depolarizations (PAD), respectively. ppSEPs in response to cauda equina stimulation showed that the latencies of the two initial components (4.6 +/- 0.4 and 6.4 +/- 0.6 ms) corresponded to those of the SCPs recorded from the PES (4.6 +/- 0.3 and 6.3 +/- 0.5 ms), suggesting that these potentials reflect impulses conducting through the spinal cord, similar to epidurally recorded SCPs.

The interaction of the somatosensory evoked potentials to simultaneous finger stimuli in the human central nervous system. A study using direct recordings

In order to investigate the interaction of sensory electrophysiologic fields arising from the adjacent second (II) and third (III) fingers and the distant second and fifth (V) fingers, direct recordings of somatosensory evoked potentials (SEPs) were performed from the sensory and motor cortices, the sensory thalamic nucleus (nucleus ventralis caudalis, VC) and the cuneate nucleus in humans during neurosurgical operations. Electrical stimulation was given to the II, III or V fingers individually, and also to pairs of either the II and III fingers or the II and V fingers simultaneously. The interaction ratio (IR) was devised as the ratio of amplitude attenuation caused by the simultaneous stimulation to two fingers compared with the amplitude of the arithmetically summed SEPs to the individual stimulation of two fingers. The IRs were calculated on N20 and P25 from the sensory cortex, P22 from the motor cortex, P17thal from the VC, and N16cune and P35cune from the cuneate nucleus. With both stimulations to the II and III fingers and the II and V fingers, P25 showed the greatest IR, followed by P22, then by P17thal, with N16cune exhibited the smallest IR. N20 and P35cune showed similar IRs and significantly greater IRs with II and III finger stimulation compared with II and V finger stimulation. These results thus indicate that the interaction of somatosensory impulses occurs in several structures along the sensory pathway in CNS, including the cuneate nucleus, the sensory thalamic nucleus, as well as sensory and motor cortices, with the greatest IRs in the cerebral cortices and the weakest ones in the brain-stem.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms underlying antinociception provoked by heterosegmental noxious stimulation in the rat tail-flick test

Physiological studies were conducted to examine the effects of noxious stimulation of one hindpaw or one forepaw on the latency of the withdrawal reflex in the tail-flick test in lightly anesthetized spinally intact or transected rats. Male Sprague-Dawley rats were anesthetized with an intraperitoneal injection of a mixture of Na-pentobarbital (20 mg/kg) and chloral hydrate (120 mg/kg). After baseline readings were taken in the tail-flick test, the effects of various noxious stimuli applied to remote body regions were assessed. The noxious stimuli included unilateral or bilateral hindpaw or unilateral forepaw thermal (immersion in water at 55 degrees C for 90 s), unilateral or bilateral chemical (subcutaneous hindpaw injection of 50 microliters of 5% formalin) and unilateral or bilateral mechanical (pinch with clamp exerting a force of 14.75 or 27 N) stimulation. Bilateral chemical and thermal, and unilateral thermal stimulation induced an antinociceptive response, consisting of an increase in tail-flick latency, peaking at 30 s after stimulation. Recovery to baseline levels occurred over the next 3-6 min. The antinociceptive effect of noxious thermal stimulation was attenuated or absent in chronically spinalized animals (T6/7) following hindpaw or forepaw immersion, respectively. Noxious mechanical stimulation had no effect on tail-flick latency. The data provide evidence that a noxious thermal or chemical stimulus produces a heterosegmental antinociceptive effect which is mediated in part via a supraspinal mechanism and in part via a local spinal mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of dorsal root entry zone lesion on spinal cord potentials evoked by segmental, ascending and descending volleys

The spinal cord potentials (SCPs) were recorded from the dorsal root entry zone (DREZ) and posterior epidural space in patients before and after dorsal root entry zone lesion (DREZL) during general anaesthesia. The SCPs from the DREZ activated by segmental, ascending and descending volleys were basically the same in fundamental waveform as those recorded from the posterior epidural space. Segmentally activated slow negative (N1) wave, reflecting synchronized activities of dorsal horn neurones, and positive (P2) wave, thought to indicate primary afferent depolarization, were affected by DREZL in all 4 subjects tested, even by contralateral stimulation, suggesting that these components of the segmental SCPs in man partly reflect the activities of the contralateral dorsal horn. The spike-like potentials activated by ascending volleys were not affected by DREZL, while the subsequent slow components were decreased in the lesioned level. This may indicate that ascending spinal cord tracts are not affected by the operation, and suggests that the origin of the slow components by ascending volleys lies at least in part in the segmental dorsal horn. The slow negative and positive components, recorded at a remote segment from DREZL, in response to the descending volleys, were augmented after DREZL, suggesting that activation of ascending or descending inhibition through a feedback loop via the supraspinal structures might occur at least transiently following DREZL. All components of the SCPs activated by descending volleys were decreased or disappeared in recording from the lesioned level, as expected. Thus, intra-operative recording of the SCPs during DREZL might be beneficial for monitoring and studying human spinal cord function.