Development of cortical somatosensory evoked potentials in rats.

The development of the contra- and ipsilateral cortical potential evoked by electrical sciatic nerve stimulation was studied in 77 male albino rats aged 5 to 45 days. A contralateral response was already recorded, as double negativity, in the youngest animals, while an ipsilateral evoked potential was not reliably present until the 10th day. At this time, however, both responses started with an inconstant positive wave and their shape was practically the same. During subsequent development the responses differed only in respect to their dominant component: in the contralateral response, the N1 wave had the highest amplitude for most of the time, while in the ipsilateral response the delayed N2 wave was the largest component. The latent periods of contralateral responses were somewhat shorter than those of ipsilateral evoked potentials. During development we noticed a phase of abrupt shortening of the latent period, which took place before the 15th day in the contralateral response and before the 20th day in the ipsilateral response. We also found a difference in the fatigability of the responses, which was greater in immature rats than in adult animals; in the ipsilateral evoked potential it approached adult values more slowly. The development of the ipsilateral response is thus delayed compared with the development of the contralateral response.     

Effect of mitochondrial toxins on evoked somatosensory activity in rats

Mitochondrial toxins represent an interesting group of neurotoxicants related both to causation and modelling of central nervous damage. 3-nitropropionic acid (3NP), a neurotoxin of herbal and microfungal origin, inhibits succinate dehydrogenase leading thereby to various biochemical and morphological alterations in the brain. Experimental animals treated by 3NP are used to model Huntington’s disease. Manganese, often present in occupational settings and as environmental pollutant, inhibits complex II and III of the mitochondria and is known to cause Parkinson-like CNS damage. In this work, rats were administered a single acute dose of Mn (50 mg Mn²⁺/kg body weight) or 3-NP (20 mg/kg b.w.) and the alterations of the somatosensory cortical evoked potential elicited by stimulation of the whisker pad and the tail base were observed, together with the changes of the action potential in the tail nerve. Latency and amplitude of the two cortical responses changed in parallel, while those of the tail nerve response remained more or less unaltered. The two mitochondrial toxins studied seem to exert their action centrally, primarily on synaptic transmission, rather than peripherally. Recording of evoked activity could be used to follow-up the nervous system effects of mitochondrial toxins, but it requires further investigation.     

Single-sweep cortical somatosensory evoked potentials: N20 and evoked bursts in sevoflurane anaesthesia

Cortical evoked responses to median nerve stimulation were recorded from 21 subjects during sevoflurane anaesthesia at the level of burst suppression in EEG. The N20/P22 wave had the typical form of a negative wave postcentrally, and positive precentrally. The amplitude exceeded 4 μV in all patients, making it easily visible without averaging on the low-amplitude suppression. These results show that two kinds of somatosensory evoked potential can be studied without averaging during EEG suppression in deep anaesthesia. One is the localised N20/P22 wave, which is seen regularly during suppression after stimuli with intervals exceeding 1 s. The other is the burst, involving the whole cortex, which is not evoked by every stimulus. We suggest that somatosensory evoked potentials can be monitored during sevoflurane-induced EEG suppression, and often can be evaluated reliably from a couple of single sweeps with stimulation interval exceeding 1 s. The enhancement of early cortical components of SEP, their adaptation to repeated stimuli, and the disappearance of later polysynaptic components during EEG suppression, give new possibilities to study the generators of SEP and the different effects of anaesthetics.     

Lithium effects on the somatosensory cortical evoked response in the rat and cat

The early positive cortical evoked potential to somatosensory stimuli is regularly increased in amplitude in patients receiving lithium carbonate treatment. This may reflect a unique neurochemical effect of lithium since similar changes have not been observed in humans following other drugs. To investigate this finding, cortical evoked potentials to peripheral somatosensory stimulation were obtained in rats and cats with implanted epidural electrodes. In rats, increasing doses of oral lithium chloride, up to 5 meq/kg/d which approached the LD 50, produced no reliable change in the early positive evoked response amplitude. In cats, an increased amplitude of the early positive-negative cortical potential was observed in every instance and the serum lithium levels were within the range used clinically in humans. The increased cortical evoked response amplitude in cats did not directly correlate with serum lithium levels but was delayed 1 to 5 days after serum lithium levels reached their peak. The findings in cats are similar to the human studies. The negative results observed in rats may reflect important species differences regarding lithium.     

Neural generators of early cortical somatosensory evoked potentials in the awake monkey

Controversy continues to exist regarding the generators of the initial cortical components of the somatosensory evoked potential (SEP). This issue was explored by detailed epidural and intracortical mapping of somatosensory evoked activity in Old World monkeys. In depth recordings, 3 complementary procedures were utilized: (1) the intracortical and subcortical distribution of SEPs was determined from approximately 4000 locations; (2) concomitant profiles of multiple unit activity (MUA) were recorded as an estimate of local action potential profiles; (3) 1-dimensional calculations of current source density (CSD) were used to outline the timing and pattern of regional transmembrane current flow. Our analysis confirms the participation of multiple cortical areas, located on either side of the central sulcus, in the generation of the initial cortical SEP components. Earliest activity, P10, was localized to area 3, followed within milliseconds by activation of areas 1, 2 (P12), and 4 (P13). In SI (Brodmann's areas 3, 1 and 2), the initial SEP components reflect the depolarization of lamina 4 stellate cells and the subsequent activation of adjacent pyramidal cells in laminae 3 and 5. The genesis of later cortical components (P20, N45) represents the composite of activity distributed across multiple cortical laminae and the interaction of overlapping excitatory and inhibitory events. These findings have direct implications for the clinical interpretation of SEP waveforms.     

A comparison of forepaw and vibrissae somatosensory cortical evoked potentials in the rat

Somatosensory evoked potentials were elicited in anesthetized rats by electrical stimulation of the forepaw (F-SEP) or the vibrissae (V-SEP) and were compared in order to study which of these is more valid animal model for studying the physiology and pathophysiology of somatosensory evoked potentials (SEPs) that are often recorded in man in a clinical setting. Intensity and rate functions were measured for the two potentials. The V-SEPs had larger amplitudes than the F-SEPs at high stimulus intensity and low stimulus rate. Furthermore, the ratios of the maximal amplitude of the F-SEP to that of the V-SEP (0.66) and of the areas under the curves of the two responses (0.75) reflected the smaller representation of the forepaw in the primary somatosensory cortex of the rat, compared to the vibrissae (ratio of cortical areas about 0.79). The differences should be taken into account when using median nerve SEP in the rat as a model of the human SEP. Study of V-SEPs in rat may provide insight into trigeminal nerve SEPs in man, which are also occasionally used for neurological evaluation.     

Serial changes in somatosensory evoked potentials following cerebral infarction

The relationship between somatosensory evoked potentials (SEPs) and recovery from stroke was investigated in 12 patients. All had suffered recent cerebral infarction. SEPs were performed within the first week, 6 weeks, 3 months and 6 months after stroke onset. Improvement of initially abnormal SEPs was maximal in the first 6 weeks and this correlated closely with the period of maximum clinical improvement. The results of this study suggest that the major effect of stroke on on SEPs occurs acutely and is little affected by secondary degenerative processes.     

Stimulus frequency dependence of the central and peripheral somatosensory evoked activity in rats treated with various pesticides

Rats were treated with a combination of insecticide agents in different timing schemes. In acute administration, 1/5 LD50 of the three insecticides: dimethoate, propoxur and cypermethrin, or their combination, was given once by gavage. In the developmental model, female rats received oral doses of 1/25 LD50 of the above insecticides in combination in three timing schemes including pregnancy and lactation. Responses in the somatosensory cortex and in the tail nerve, evoked by peripheral electric stimulation, were recorded in acute preparation under urethane anesthesia. It was tested whether the parameters of the cortical and peripheral evoked response are dependent on the frequency and whether this dependence is different in control and treated animals. The latency increase of the cortical responses with increasing stimulation frequency was significantly stronger in rats treated acutely with cypermethrin and the combination, and in rats receiving the combination during both intra- and extrauterine development. On the duration, the effects were less clear. Frequency dependent increase of the tail nerve action potential latency was significantly intensified by cypermethrin, and the amplitude decrease, by cypermethrin and dimethoate. Fatigue of this response during a stimulation series was also altered by the insecticides. Frequency dependence and fatigue possibly reflect the actual state of the nervous system and may have the potency to be developed to functional biomarkers.     

Time-frequency component analysis of somatosensory evoked potentials in rats

Background  

Somatosensory evoked potential (SEP) signal usually contains a set of detailed temporal components measured and identified in a time domain, giving meaningful information on physiological mechanisms of the nervous system. The purpose of this study is to measure and identify detailed time-frequency components in normal SEP using time-frequency analysis (TFA) methods and to obtain their distribution pattern in the time-frequency domain.     

The background and evoked neuronal impulse activity of embryonic neurotransplant of somatosensory brain cortex in rats

The investigation was intended for studying the character of the background and evoked impulse activity of embryonic neurotransplant neurons 4 months after homotopical allotransplantation into the barrel field of somatosensory recipient's brain cortex of the rat. It is established, that the current average frequency of background impulse activity of transplant neurons is reduced in comparison with one of the control rats. It is shown that the evoked impulse activity of neurotransplant develops with the long latency than in somatosensory cortex of the control animals. Thus in patterns of the evoked activity of neurotransplant cells reactions, characteristic for the neurons of barrel field somatosensory recipient's cortex of control rats are registered: an increase of frequency of pulses' generating, or alternating of the activation and reduction periods of impulse frequency with its subsequent regeneration up to a pristine level.     

Neurometric evaluation of the cortical somatosensory evoked potential in acute incomplete spinal cord injuries

The clinical application of evoked potentials has at times been criticized for its failure to provide an objective quantifiable assessment of the processing of sensory information and a reflection of nervous system integrity at least comparable in accuracy to other methods of assessment. The challenge is to quantify the SEP in a manner that accurately reflects the function of the somatosensory system.Therefore, neurometrics, an approach which emphasizes the transformation of the neurophysiological data to a common metric of relative probability by reference to normative standards and the classification of critical features of data by multivariate statistics, was employed.The 3-part study involved the evaluation of the diagnostic, prognostic and localization functions of the cortical tibial SEP in patients with incomplete spinal cord injuries. Two neurometric indices which correlated well with concurrent and future neurological status were developed. The distributions of influence of dorsal columns and spino-thalamic tracts on the SEP were established.     

Age-dependent changes of short-latency somatosensory evoked potentials in healthy adults

Age-dependent changes of short-latency somatosensory evoked potentials following median nerve stimulation in humans were investigated in two groups of healthy adults aged 20-30 and 50-60 years. Normative values for both age groups are given. Compared to the younger group, in the older one P27 latency and N20-P27 interpeak latency were about 2 ms longer, and P27-N35 and P27-P45 interpeak latencies were significantly decreased. These findings suggest that N20 and P27 are generated by different structures and that the subsequent components do not depend on P27.     

Scalp topography and distribution of cortical somatosensory evoked potentials to median nerve stimulation

Topographies and distributions of cortical SEPs to median nerve stimulation were studied in 8 normal adults and 5 neurological patients. SEPs recorded from C4, P4, Pz, T6-A1A2 derivations to left median nerve stimulation were composed of 2 early negative (N16, N20) and 2 positive components (P12, P23), whereas those recorded from frontal electrodes (Fz, Fp1, Fp2) disclosed 2 early negativities (N16, N24) and 2 early positivities (P12, P20). N20 and P20, and P23 and N24, reversed across the rolandic fissure with no significant difference in their peak latencies. P23 was of slightly shorter latency at C4 than at more posterior electrodes (P4, T6, Pz).In 3 patients with complete hemiplegia but normal sensation, all the early SEP components were normal in scalp distribution and peak latencies except for a decrease of N24 amplitude. In 2 patients with complete hemiplegia and sensory loss no early cortical SEPs were seen. These findings suggest that N20 and P20 are generated as a single horizontal dipole in the central fissure, whereas P23 and N24 are a reflection of multiple generators in pre- and postrolandic regions.     

Dynamics of changes in somatosensory input to the motor cortex following lesion of somatosensory cortical areas in dogs

The pattern of change produced in somatosensory evoked potential (EP) in the forelimb projection area within the motor cortex (MI) following lesion of the projection area of the same limb in the somatosensory cortex (SI) or in parietal cortex area 5 was investigated during chronic experiments on waking dogs. Amplitude of the initial positive — negative wave of EP declined to 28–63% of preoperational level in all cases. No significant recovery of EP was noted for three weeks. Thus, a correlation between change in EP and spontaneous recuperation of the precision motor response occurring within two weeks after lesion of the SI did not exist. Nor was EP reinstated in the MI after ablation of area 5, despite complete but gradual reinstatement of EP (after an initial decline to 53%) in the nearby SI region. This protracted depression of EP seems to have been associated with breakdown of somatotopic sensory input from the SI or from area 5 to the MI, since EP in the motor cortex of the intact hemisphere and the hindlimb projection area within the MI on the lesioned side either remained unchanged or recovered within a week or two.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 22, No. 1, pp. 61–68, January–February, 1990.     

Monitoring of subcortical and cortical somatosensory evoked potentials during carotid endarterectomy: comparison with stump pressure levels

Monitoring of multichannel somatosensory evoked potentials (SEPs) has been performed in 40 cases of carotid endarterectomy (CEA). SEPs were obtained after median nerve stimulation at wrist, recording from 2nd cervical and from the scalp parietal (ipsi- and contralateral) and central (contralateral) positions. The reduction of CBF due to clamping of the carotid artery provoked SEP abnormalities in 10 of the 40 cases. None of the 30 patients with unmodified SEPs developed post-surgical neurological sequelae.SEP alterations were characterized exclusively by amplitude decrements and latency increases of the cortical components, the subcortical ones being unaffected. In 5 of these patients, SEPs returned to normal values before the end of the intervention and no neurological deficit was observed on awakening. In the remaining 5 cases SEPs retained their abnormalities and patients developed post-surgery neurological sequelae (4 immediately, 1 the day after).SEP alterations affected parietal and central components to a similar extent; however, in a few cases cerebral blood flow deficits provoked by carotid clamping modified differently the central P22 and the parietal N20–P25 waves.Comparisons with stump (back) pressure in the carotid artery revealed a higher sensitivity of the SEP technique in detecting vascularization problems due to carotid clamping.The time course of the appearance of SEP abnormalities seems to discriminate alterations secondary to collateral revascularization from those determined by embolization.