Journal of neurophysiology
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We recorded, with the microneurography technique, single-unit impulses from nine cutaneous mechanoreceptive afferents with conduction velocities in the C range and receptive fields in the hairy skin of the forearm. The units responded with high impulse rates to light touch and had low monofilament thresholds. The geography of receptive fields was explored with a scanning method: a lightweight probe with a small and rounded tip was made to scan the field area in a series of closely adjacent tracks while single-unit activity was recorded. ⋯ The fields were roughly round or oval in shape with no preferred orientation. The size of the response differed between individual sensitive spots in a field, suggesting a highly nonuniform terminal organization. The properties of the fields seem consistent with a role of tactile C afferents to provide information about pleasant touch and skin-to-skin contacts to central structures controlling emotions and affiliative behavior.
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We investigated electrophysiological changes in chronically axotomized and neighboring intact dorsal root ganglion (DRG) neurons in rats after either a peripheral axotomy consisting of an L5 spinal nerve ligation (SNL) or a central axotomy produced by an L5 partial rhizotomy (PR). SNL produced lasting hyperalgesia to punctate indentation and tactile allodynia to innocuous stroking of the foot ipsilateral to the injury. PR produced ipsilateral hyperalgesia without allodynia with recovery by day 10. ⋯ These changes included (in relation to sham values) higher input resistance, lower current and voltage thresholds, and action potentials with longer durations and slower rising and falling rates. The incidence of spontaneous activity, recorded extracellularly from dorsal root fibers in vitro, was significantly higher (in relation to sham) after SNL but not after PR, and occurred in myelinated but not unmyelinated fibers from both L4 (9.1%) and L5 (16.7%) DRGs. We hypothesize that the changes in the electrophysiological properties of axotomized and intact DRG neurons after SNL are produced by a mechanism associated with Wallerian degeneration and that the hyperexcitability of intact neurons may contribute to SNL-induced hyperalgesia and allodynia.
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Motor cortex stimulation has both excitatory and inhibitory effects on ipsilateral muscles. Excitatory effects can be assessed by ipsilateral motor-evoked potentials (iMEPs). Inhibitory effects include an interruption of ipsilateral voluntary muscle activity known as the silent period (iSP) and a reduction in corticospinal excitability evoked by conditioning stimulation of the contralateral motor cortex (interhemispheric inhibition, IHI). ⋯ Target muscle activation decreased IHI at 8-ms ISI but had little effect on IHI at 40-ms ISI. iSP and IHI at 8-ms ISI did not correlate at any stimulus intensities and current directions tested, and factor analysis showed that they are explained by different factors. However, active IHI at 40-ms ISI was explained by the same factor as iSP. The different directional preference for cMEP compared with iMEP and IHI suggests that these ipsilateral effects are mediated by populations of cortical neurons that are different from those activating the corticospinal neurons. iSP and IHI do not represent the same phenomenon and should be considered complementary measures of ipsilateral inhibition.
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Substance P (SP) is an undecapeptide that is co-localized with conventional transmitters in the nucleus accumbens (NAc). Its neurochemical and behavioral effects resemble those of cocaine and amphetamine. How SP accomplishes these effects is not known, partly because its cellular and synaptic effects are not well characterized. ⋯ In addition, the SP-induced synaptic depression was blocked by an adenosine A1 receptor blocker 8-cyclopentyltheophylline (8-CPT) but not the N-methyl-D-aspartate (NMDA) receptor antagonist D-APV. These data show that SP, by activating presynaptic NK1 receptors, depresses excitatory synaptic transmission indirectly by enhancing extracellular dopamine and adenosine levels. Since the cellular and synaptic effects of SP resemble those of cocaine and amphetamine, it may serve as an endogenous psychogenic peptide.
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A total of 20 right-handed subjects were asked to perform a grasp-lift-and-hold task using a precision grip. The grasped object was a one-degree-of-freedom manipuladum consisting of a vertically mounted linear motor capable of generating resistive forces to simulate a range of object weights. In the initial study, seven subjects (6 women, 1 man; ages 24-56 yr) were first asked to lift and hold the object stationary for 4 s. ⋯ Vision of the grasping hand did not significantly correct the finger misalignment after digital anesthesia. Taken together, these results suggest that mechanoreceptors in the fingertips signal the source and direction of pressure applied to the skin. The nervous system uses this information to adjust the fingers and direct the pinch forces optimally for grasping and object manipulation.