Arun K. Senapati

Spinal dorsal horn neuron response to mechanical stimuli is decreased by electrical stimulation of the primary motor cortex

Motor cortex stimulation (MCS) has been used clinically as a tool for the control for central post-stroke pain and neuropathic facial pain. The underlying mechanisms involved in the antinociceptive effect of MCS are not clearly understood. We hypothesize that the antinociceptive effect is through the modulation of the spinal dorsal horn neuron activity. Thirty-two wide dynamic range spinal dorsal horn neurons were recorded, in response to graded mechanical stimulation (brush, pressure, and pinch) at their respective receptive fields, while a stepwise electrical stimulation was applied simultaneously in the motor cortex. The responses to brush at control, 10 V, 20 V, and 30 V, and recovery were 11.5+/-1.6, 12.1+/-2.6, 11.1+/-2.2, 10.5+/-2.1, and 13.2+/-2.5 spikes/s, respectively. The responses to pressure at control, 10 V, 20 V, and 30 V, and recovery were 33.2+/-6.1, 22.9+/-5.3, 20.5+/-5.0, 17.3+/-3.8, and 27.0+/-4.0 spikes/s, respectively. The responses to pinch at control, 10 V, 20 V, and 30 V, and recovery were 37.2+/-6.4, 26.3+/-4.7, 25.9+/-4.7, 22.5+/-4.3, and 35.0+/-6.2 spikes/s, respectively. It is concluded that, in the rat, electrical stimulation of the motor cortex produces significant transient inhibition of the responses of spinal cord dorsal horn neurons to higher intensity mechanical stimuli without affecting their response to an innocuous stimulus.

Electrical stimulation of the primary somatosensory cortex inhibits spinal dorsal horn neuron activity

Cortical stimulation has been demonstrated to alleviate certain pain conditions. The aim of this study was to determine the responses of the spinal cord dorsal horn neurons to stimulation of the primary somatosensory cortex (SSC). We hypothesized that direct stimulation of the SSC will inhibit the activity of spinal dorsal horn neurons by activating the descending inhibitory system. Thirty-four wide dynamic range spinal dorsal horn neurons were recorded in response to graded mechanical stimulation (brush, pressure, and pinch) at their respective receptive fields while a stepwise electrical stimulation (300 Hz, 0.1 ms, at 10, 20, and 30 V) was applied in the SSC through a bipolar tungsten electrode. The responses to brush at control, 10 V, 20 V, 30 V, and recovery were 16.0 +/- 2.3, 15.8 +/- 2.2, 14.6 +/- 1.8, 14.8 +/- 2.0, and 17.0 +/- 2.2 spikes/s, respectively. The responses to pressure at control, 10 V, 20 V, 30 V, and recovery were 44.7 +/- 5.5, 37.0 +/- 5.6, 29.5 +/- 4.8, 31.6 +/- 5.2, and 43.2 +/- 5.7 spikes/s, respectively. The responses to pinch at control, 10 V, 20 V, 30 V, and recovery were 58.1 +/- 7.0, 42.9 +/- 5.5, 34.8 +/- 3.9, 34.6 +/- 4.4, and 52.6 +/- 6.0 spikes/s, respectively. Significant decreases of the dorsal horn neuronal responses to pressure and pinch were observed during SSC stimulation. It is concluded that electrical stimulation of the SSC produces transient inhibition of the responses of spinal cord dorsal horn neurons to higher intensity mechanical stimuli without affecting innocuous stimuli.

Inhibition of spinal cord dorsal horn neuronal activity by electrical stimulation of the cerebellar cortex

The cerebellum plays a major role in not only modulating motor activity, but also contributing to other functions, including nociception. The intermediate hemisphere of the cerebellum receives sensory input from the limbs. With the extensive connection between the cerebellum to brain-stem structures and cerebral cortex, it is possible that the cerebellum may facilitate the descending system to modulate spinal dorsal horn activity. This study provided the first evidence to support this hypothesis. Thirty-one wide-dynamic-range neurons from the left lumbar and 27 from the right lumbar spinal dorsal horn were recorded in response to graded mechanical stimulation (brush, pressure, and pinch) at the hind paws. Electrical stimulation of the cerebellar cortex of the left intermediate hemisphere significantly reduced spinal cord dorsal horn neuron-evoked responses bilaterally in response to peripheral high-intensity mechanical stimuli. It is concluded that the cerebellum may play a potential antinociceptive role, probably through activating descending inhibitory pathways indirectly.

Near infrared and visible spectroscopic measurements to detect changes in light scattering and hemoglobin oxygen saturation from rat spinal cord during peripheral stimulation.

In this study, we investigated dynamic changes in light scattering and hemoglobin oxygen saturation (S(sc)O(2)) on the rat spinal cord due to peripheral electrical stimulation by measuring near infrared (NIR) and visible spectroscopy, respectively. The spectral slope in the wavelength region between 700 and 900 nm is used as an index (S(NIR)) to quantify light scattering. With a 100-mum (source-detector separation) fiber-optic needle probe, optical reflectance was measured from the left lumbar region, specifically LL5, of the spinal cord surface at a height of 575 mum from the spinal cord surface. Graded electrical stimulations from 20 to 50 V, in increments of 10 V, were given to the plantar surface of the rat left hind paw for a period of 20 s. Changes in both light scattering (S(NIR)) and S(sc)O(2) were determined as a difference between the baseline and the maximum of slope value and hemoglobin oxygen saturation, respectively, during the stimulation period. There were significant differences in both S(NIR) and S(sc)O(2) during stimulation, with the average percentage changes of 10.9% and 15.5%, respectively. We observed that both S(NIR) and S(sc)O(2) measured at the spinal cord are insensitive to the intensity of the electrical stimulus, which is possibly caused by the nonlinear process of neurovascular coupling. Our finding essentially indicates that peripheral electrical stimulation results in significant changes in both light scattering and hemoglobin oxygen saturation on the rat spinal cord, and ignoring light scattering changes could lead to possible negative offsets of hemodynamic parameters (oxy-, deoxy-, and total hemoglobin concentrations) obtained in the functional optical imaging in the brain.

Determination of hemoglobin oxygen saturation in rat sciatic nerve by in vivo near infrared spectroscopy.

The purpose of this study is to investigate the values of hemoglobin oxygen saturation in the sciatic nerve of the rat following spinal nerve ligation. An optical spectroscopic technique along with a fiber optic probe was used to test the hypothesis that demyelination and degeneration after nerve injury lead to a significant decrease in the percentage of hemoglobin oxygen saturation. A modified spinal nerve ligation method was used to induce the degeneration, and three types of ligation on left spinal nerve (L4, L4 and L5, L5) were performed in rats. The optical reflectance measurements were taken from the left and right sciatic nerves on postoperative days 1, 4, 7, and 14. No significant difference was found among the three types of ligation, nor was between left and right sciatic nerve at postoperative day 1. Significant decreases in oxygen saturation percentages were found between left and right sciatic nerves at postoperative days 4, 7, and 14. This study continues to show the effectiveness of optical methods in determining/differentiating tissue properties, providing an excellent and robust in vivo technique that can have a potential clinical application in detecting demyelination and degeneration of the nervous system.

Detection of changes in rat spinal cord due to peripheral stimulation using NIR reflectance spectroscopy

Near infrared reflectance spectroscopy showed significant changes in light scattering and hemoglobin oxygen saturation on the ipsilateral side of the rat spinal cord during electrical stimulation of the plantar surface on the hind paw.