Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale
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Weak transcranial direct current stimulation (tDCS) can induce long lasting changes in cortical excitability. In the present study we asked whether tDCS applied to the left primary motor cortex (M1) also produces aftereffects distant from the site of the stimulating electrodes. We therefore tested corticospinal excitability in the left and the right M1 and transcallosal excitability between the two cortices using transcranial magnetic stimulation (TMS) before and after applying tDCS. ⋯ MEPs evoked from the right M1 were unchanged whilst the duration of transcallosal inhibition evoked from the right M1 was shortened after cathodal tDCS and prolonged after anodal tDCS. The duration of transcallosal inhibition returned to control values before the effect on the MEPs from the left M1 had recovered. These findings are compatible with the idea that tDCS-induced aftereffects in the cortical motor system are limited to the stimulated hemisphere, and that tDCS not only affects corticospinal circuits involved in producing MEPs but also inhibitory interneurons mediating transcallosal inhibition from the contralateral hemisphere.
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Early detection of delayed cerebral energy failure may be important in the prevention of reperfusion injury of the brain after severe perinatal hypoxia-ischaemia (HI). This study investigated whether monitoring of the redox state of cytochrome aa(3) (Cytaa(3)) with near infrared spectroscopy (NIRS) after severe perinatal asphyxia may allow us to detect early a compromised energy metabolism of the developing brain. We therefore correlated serial Cytaa(3) measurements (to estimate mitochondrial oxygenation) simultaneously with the (31)phosphorous-magnetic resonance spectroscopy ((31)P-MRS)-measured phosphocreatin/inorganic phosphate (PCr/Pi) ratio (to estimate cerebral energy reserve) in newborn piglets before and after severe hypoxia-ischaemia. ⋯ With severe reduction in PCr/Pi-ratio, major changes in the redox-state of Cytaa(3) also occurred: Cytaa(3) was mostly either in a reduced state (down to -6.45 micromol/L) or in an oxidised state (up to 6.84 micromol/L) at these low PCr/Pi ratios. The positive predictive value (PPV) of Cytaa(3) to predict severe reduction of the PCr/Pi ratio was 42%; the negative PPV was 87%. A similar relation was found for Cytaa(3) with histologically determined loss of neurons.
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Comparative Study
Mean sustained pain levels are linked to hemispherical side-to-side differences of primary somatosensory cortex in the complex regional pain syndrome I.
Chronic back pain as well as phantom-limb pain is characterized by a close relationship between the amount of cortical reorganization and the magnitude of pain. In patients with positively assessed complex regional pain syndrome type I (CRPS I), we found a positive correlation between representational changes of primary somatosensory cortex (SI) and mean sustained pain levels. We investigated seven right-handed patients with CRPS I of one upper limb by means of somatosensory evoked potential (SSEP) mapping. ⋯ Individual expansion of hand representation contralateral to the CRPS-affected limb was significantly correlated with mean pain intensity. Accordingly, low pain levels were linked to small representational side-to-side differences, while subjects with a distinctive hemispherical asymmetry reported the highest pain levels. Follow-up studies using functional imaging methods might be instrumental in providing a better understanding of this issue.
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To investigate the excitability of Adelta nociceptive pathways and the nature of the vertex laser evoked potentials (LEPs), we studied the recovery cycle of the P2-LEP component and compared it with that of the P200 of the somatosensory evoked potential (SEP). Using two identical CO(2)-laser stimulators, we delivered paired stimuli to two adjacent skin spots on the hand at interstimulus intervals ranging from 250 ms to 2 s. ⋯ The P200-SEP, after paired stimuli to the median nerve, showed a time course even slower than the P2-LEP ( P<0.01). Besides providing the LEP recovery curve in normal subjects, our findings indicate that the P2-LEP relays through a number of synapses similar to (or even lower than) that for the P200-SEP, thus lending further support to the view that the nociceptive P2-LEP is not an endogenous potential equivalent to the P300.
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We provide evidence that the human spinal cord is able to respond to external afferent input and to generate a sustained extension of the lower extremities when isolated from brain control. The present study demonstrates that sustained, nonpatterned electrical stimulation of the lumbosacral cord--applied at a frequency in the range of 5-15 Hz and a strength above the thresholds for twitches in the thigh and leg muscles--can initiate and retain lower-limb extension in paraplegic subjects with a long history of complete spinal cord injury. We hypothesize that the induced extension is due to tonic input applied by the epidural stimulation to primary sensory afferents. ⋯ We speculate that the volleys induced externally to the lumbosacral network at a frequency of 5-15 Hz initiate and retain an "extension pattern generator" organization. Once established, this organization would recruit a larger population of motor units in the hip and ankle extensor muscles as compared to the flexors, resulting in an extension movement of the lower limbs. In the electromyograms of the lower-limb muscle groups, such activity is reflected as a characteristic spatiotemporal pattern of compound motor-unit potentials.