Somatosensory research最新文献

The intensity of sensations of warmth and cold was measured psychophysically at 12 loci on the face and in the mouth in 20 human subjects. Significant differences were found among areas in the relative sensitivity to both cooling and warming, although the range of sensitivities was greater for warming than for cooling. Except for the vermilion lip and the tongue tip, oral regions were significantly less sensitive to warming than were facial regions. No such difference was found for cooling. The most posterior location tested on the hard palate, for example, exhibited a suprathreshold sensitivity to cooling that equaled or surpassed that of most locations on the face. The tongue tip and vermilion lip possessed relatively high sensitivity to both warming and cooling, with the former locus emerging as the most thermally sensitive oral area so far tested.

Injections of horseradish peroxidase (HRP) were used to study the connections of the first and second somatosensory areas (SI and SII) in tree shrews. The locations of callosally projecting neurons in SI were determined by placing large injections of HRP in the SI region of one cerebral hemisphere and determining the organization of SI of the other cerebral hemisphere with microelectrode mapping. Many callosally projecting neurons were revealed in lateral SI representing the face, especially the glabrous nose. A sparse scattering of callosally projecting neurons were located more centrally in SI in portions representing the forepaw; these neurons tended to be in cortex devoted to the dorsal hand and pads of the palm rather than the digits. Part of medial SI, representing the forelimb and trunk, had a moderately dense distribution of callosally projecting neurons. More restricted injections in SI indicated that callosally projecting neurons were largely within comparable portions of contralateral SI, although a few neurons projecting callosally to SI were located in SII and cortex caudal and rostral to SI. Large injections of HRP in SII labeled neurons throughout contralateral SII, including representations of the forepaw and hindpaw. More restricted injections in SII labeled neurons in somatotopically comparable parts of the contralateral SII. A few labeled neurons were also seen in somatotopically matched parts of contralateral SI. The results also demonstrated strong somatotopically organized connections between SI and SII of the same hemisphere, and connections of SI and SII with adjoining subdivisions of parietal and frontal cortex. The major thalamic projections to both SI and SII originated in the ventroposterior nucleus.

In whatever mammalian receptor system Merkel cells are found, they are always associated with a characteristic slowly adapting response. The role of Merkel cells in the transduction process of slowly adapting Type I cutaneous mechanoreceptors (SAI receptors or touch domes) of rats and cats was investigated by mechanical and electrical stimulation of SAI receptors and their afferent fibers in an O2-depleted environment. Circulatory hypoxia was produced either by ventilating animals with N2 or by recirculating venous blood around a limb. In both these experimental preparations, the results obtained were identical. For receptor failure to occur, it was found necessary to have an O2-depleted environment on the limb surface. This was achieved by passing N2 into a gas-tight polythene sock placed over the limb. Replacement of N2 within the polythene sock with O2 was sufficient to bring about receptor recovery, irrespective of arterial blood PO2 levels. There was an inverse linear relationship between receptor response and time when touch domes were stimulated with N2 around the limb. In contrast, the replacement of N2 around the limb with O2 produced an exponential increase in the response with time. Correlated with receptor failure was a significant reduction in the number of dense-cored vesicles normally found in the Merkel cell cytoplasm adjacent to the nerve ending innervating the cell. Receptor recovery was associated with a return in the number of dense-cored vesicles back to that found in control cells. Hypoxia had no effect on the level of electrical stimulation necessary to initiate an action potential in the afferent fiber, even though the response of SAI receptors to mechanical stimulation had ceased. The results indicate that Merkel cell dense-cored vesicles are necessary for the characteristic slowly adapting response of SAI mechanoreceptors and that this may be due to the secretion of a transmitter substance stored within the vesicles.

A single-unit population study of the feline cuneate nucleus was carried out to identify principal neuron types, their distribution within the nucleus, pattern of peripheral activation, and receptive field characteristics. Units were also tested for response to isolated dorsal column or dorsolateral funicular electrical stimulation. The nucleus was explored in a uniform pattern, and sample size was optimized by applying the search stimulus shocks to the dorsal spinal cord. Single units were defined as spinal afferents, cuneothalamic-relay (CTR) neurons, and non-cuneothalamic-relay (non-CTR) neurons. The following features were observed: The distribution within the nucleus of specific cell types agreed with cytoarchitectural studies: Spinal afferent fibers were superficial and caudal; 22% of neurons were CTR neurons; CTR neurons were most dense in the middle of the nucleus and were largely separate from non-CTR neurons. Of the 58 neurons tested for response to isolated dorsal column and dorsolateral funicular stimulation, 24% were activated from both tracts. Convergent input from the off-focus periphery (defined as other than the ipsilateral forelimb) was detected in both CTR and non-CTR neurons, most commonly from the contralateral forepaw. Several neurons were activated from three limbs. Thirty-seven percent of units were unresponsive to hair movement, touch, muscle palpation, or movement of joints. Compared to spinal fibers and non-CTR neurons, CTR neurons were most likely to have an identifiable input.

Experiments were performed on rats using neurophysiological and behavioral techniques, in an attempt to study the role played by the somatosensory cortex in the abnormal spinal neuron activity and abnormal behavior observed after brachial plexus lesions. The onsets of both phenomena occur at the same postoperative period. Cortical controls exerted on spinal dorsal horn (DH) cells were studied using a transient and reversible cortical blockade, cortical spreading depression (CSD), applied contralateral to the spinal cord recording. In 28 intact animals, 55 cells were studied during the propagation of at least two CSDs. Only 4 of these cells presented a sustained decrease in their spontaneous activity during CSD, which may correspond to transient arrest of a descending tonic cortical facilitation. In 29 animals with dorsal root sections, 161 DH cells displayed abnormal burst activity, and 52 were examined with the CSD test. Thirty-five cells presented a long-duration change in their spontaneous activity during CSD; of these, 28 showed decreased activity (suppression of descending tonic facilitation) and 7 presented increased activity (suppression of descending tonic inhibition). More DH cells were influenced by the cortex in deafferented rats (67%) than in intact rats (7%). The cortical influence was also stronger, as the hyperactive cells were frequently rendered silent during CSD. These observations suggest that the abnormal activity is partly due to a descending cortical influence. Results of a behavioral study performed on 22 rats (one control group and two experimental groups with cortical ablations) showed that the self-mutilating behavior, which develops at the same time as the abnormal DH cell activity, was reduced by unilateral cortical ablation, independent of the cortical region removed. The possible pathways involved in this cortical influence are examined in the discussion.

Two different anatomical techniques were used to obtain evidence that transection of the infraorbital (IO) nerve on the day of birth would result in reorganization of the peripheral projections of the trigeminal nerve. In 14 of 19 neonatally nerve-damaged adult rats, injection of horseradish peroxidase (HRP) directly into the IO nerve, proximal to the point of the neonatal transection, resulted in labeled cells in the ophthalmic-maxillary portion of the ganglion and labeled fibers in mandibular sensory nerves. In an additional 28 neonatally nerve-damaged adult rats, double-labeling techniques were employed to document the reorganization suggested by the HRP tracing experiments. In these experiments, one fluorescent tracer, diamidino yellow (DY), was injected directly into the regenerate IO nerve, proximal to the point of the neonatal transection; a second tracer, true blue (TB), was deposited into peripheral ophthalmic and/or mandibular fields. These combinations of injections invariably resulted in the demonstration of a small number (46-401) of double-labeled cells that were located in the ophthalmic-maxillary part of the ganglion. Identical combinations of injections in normal adult rats and the intact sides of nerve-damaged animals never produced more than 6 double-labeled cells per ganglion. In two additional series of experiments, sequential double-labeling techniques were employed to demonstrate that the multiply projecting ganglion cells probably arose in at least two ways: (1) development of non-IO projections by ganglion cells that contributed axons to the IO nerve at the time of the lesion; (2) elaboration of IO axon branches by primary afferent neurons that had non-IO projections at the time of the lesion. A final two-stage double-labeling experiment demonstrated that approximately 75% of the ganglion cells that projected to the whisker pad at birth, and survived transection of the IO nerve on the first postnatal day, regenerated axons into this trigeminal branch.

The role of spread of skin deformation in activating cutaneous mechanoreceptors at a distance from their threshold receptive fields (RFs) was examined in glabrous skin of the North American raccoon and the squirrel monkey. One feedback-controlled mechanical stimulus probe was used to indent the skin to a controlled depth at a constant velocity, at varying distances from a second probe, which was used to monitor vertical displacement depth and velocity at this distant site. In many instances, the monitor probe was positioned over the RF of a cutaneous mechanoreceptor, and single-unit action potentials were simultaneously recorded from individual fibers of the median or ulnar nerve. With distance from the site of stimulation, there was a systematic, monotonic decline in indentation depth and velocity; velocity fell off with distance more rapidly than depth. The degree of diminution with distance varied with the size, shape, and curvature of the digital or palm pad stimulated. Spread of indentation was more restricted on digital than on palm pads, and was more restricted across monkey skin than across raccoon skin. Spread was less with higher-velocity than with lower-velocity indentations, but was seemingly unaffected by indentation depth. As expected from the findings noted above, the number of spikes discharged by slowly adapting mechanoreceptive afferent fibers declined more rapidly with distance between stimulus site and RF for digital than for palmar RFs, in squirrel monkey than in raccoon skin, and with higher-velocity than with lower-velocity stimuli. Furthermore, the number of spikes occurring during either ramp or early static indentation phases of stimulation dropped to zero more rapidly with distance than did either vertical indentation depth or velocity. Decreases with distance in both indentation depth and velocity acted to restrict the size of suprathreshold RFs. For most units, horizontal components of mechanical stimulation subtracted from the effects of vertical components. It is suggested, on the basis of this and other studies, that many neural and perceptual phenomena usually attributed to central mechanisms of afferent inhibition may be attributable, at least in part, to mechanical properties of the skin. In addition, the present data suggest that regional variations in the two-point limen may be associated with variations in spread of mechanical deformation. The conclusion that glabrous skin and subjacent soft tissues act as a low-pass filter system provides a mechanical basis for the relative efficacy of high-frequency vibratory stimuli in tactile pattern perception.(ABSTRACT TRUNCATED AT 400 WORDS)

Multiunit microelectrode recordings were used to explore the responsiveness and somatotopic organization of the representation of the hand in area 3b of anesthetized macaque monkeys. Major findings were as follows: Recording sites throughout the hand representation were activated by low-threshold cutaneous stimulation. Simple, punctate mechanical stimuli were highly effective in activating neurons. Neurons had small, restricted receptive fields. Representations of nearly all skin surfaces of the hand were demonstrated in individual monkeys. The basic topographic pattern found in all monkeys included the following: a large sequential representation of the glabrous digits from thumb to little finger from lateral to medial in cortex, and from proximal to distal hand parts in cortex extending down the caudal bank of the central sulcus; moderately large representations of radial and ulnar pads of the palm in respective lateral and medial cortical locations in the hand representation; and a relatively small, fragmented representation of the dorsal hand and dorsal digits, with the fragments interspersed within the representation of the glabrous hand. The proportions of the proximal, middle, and distal glabrous digits varied, so that the representation of the distal phalanx sometimes approached the dorsal border of area 3b with area 1. A comparison of the present findings with previous results from macaque monkeys indicates that the above-described features have been revealed under a variety of recording and anesthetic conditions. Consistencies in previous and present results strongly support the conclusions that the hand representation in area 3b of macaque monkeys is activated by cutaneous receptors throughout; is composed of neurons with relatively simple, small, cutaneous receptive fields; includes all skin surfaces of the hand; and is somatotopic for the glabrous skin with small, discontinuous, intercalated representations of fragments of the dorsal skin.

Calcitonin gene-related peptide (CGRP) is a recently characterized neuroactive substance that is expressed in a large proportion of small- to medium-diameter sensory ganglion neurons whose central terminals lie in the superficial spinal and medullary dorsal horn. This restricted distribution within the peripheral nervous system suggests a prominent role for the peptide in nociceptive processing. The mammalian tooth pulp, which receives a relatively homogeneous afferent input from thin (putative nociceptive) fibers originating from this subpopulation of trigeminal ganglion cells, thus affords an ideal target zone in which to examine peripheral nociceptive mechanisms. The large percentage of these neurons displaying CGRP-like immunoreactivity (CGRP-LI) furthermore provides a valuable tool to study its thin-fiber afferent innervation. CGRP-LI has been localized within intact, decalcified specimens of rat, cat, monkey, and human teeth and associated dental structures. A remarkably robust CGRP-LI innervation of molar pulp and dentin was revealed in all species, with fibers coursing both in fascicles and individually, in variable relation to blood vessels and pulpal stroma. Our methods enabled tracing of a large number of axons through Raschkow's plexus and odontoblast layer into dentinal tubules. Paralleling anterograde axonal transport studies, a greater share of fibers was found in coronal vis-à-vis radicular dentin. In the rat, this fiber pattern stood in contrast both to incisor dentin, which appeared devoid of CGRP-LI, and to the abundant labeled axons in gingiva and periodontal tissues. Surgical deafferentation of rat mandible resulted in widespread depletion of CGRP-LI, while superior cervical ganglionectomy was without effect, confirming the sensory nature of the CGRP-LI fibers. Neonatal capsaicin treatment greatly attenuated the immunostaining, providing evidence for CGRP-LI localization in chemosensitive unmyelinated afferents. The great density of CGRP-LI axons demonstrated is considered in contrast to the restricted range and extent of sensory stimuli to which teeth are presumably subjected, and in relation to the diverse ongoing trophic, regulatory, and reparative processes in tooth structures. It is therefore suggested that these fibers may be subserving prominent efferent roles in dental pulp not directly related to nociception.

The magnitude and duration of itch sensation produced by intracutaneous injection of histamine were determined for humans with the procedure of magnitude estimation scaling. Thirteen subjects received a 10-microliter intracutaneous injection of histamine at doses of 0.0001, 0.001, 0.01, 0.1, 1, and 10 micrograms into the volar forearm; eight of these subjects also received a 100-microgram dose. One subject received multiple injections over several weeks to determine the reliability of the magnitude estimates of itch. Following each injection, the area of flare and duration of itch were also determined. Intracutaneous injection of histamine produced a pure sensation of itch, without pain. The magnitude of itch increased in a dose-dependent fashion. The lowest histamine dose that produced itch greater than the itch produced by vehicle was 0.01 micrograms. The greatest itch was produced by the 100-microgram dose. A power function fitted to the mean magnitude estimates had an exponent of 0.17, indicating a negatively accelerating relation between the magnitude of itch and histamine dose. The one subject who received histamine over several weeks gave fairly reproducible estimates of itch magnitude. The duration of itch and the area of flare also increased in a dose-dependent fashion. The lowest dose of histamine that produced a duration of itch longer than the itch produced by the vehicle was 0.1 microgram, while the 100-microgram dose produced the longest duration of itch. Although the area of flare increased with each increase in dose from 0.1 to 10 micrograms, the areas of flare produced by 10 and 100 micrograms of histamine did not differ. These results indicate that humans can scale the magnitude of itch produced by histamine in a dose-dependent manner. In addition, the duration of itch and the area of flare produced by histamine are dose-dependent, confirming results of previous investigators. Intracutaneous histamine is easily quantifiable and may thus be a useful stimulus in neurophysiological studies of the peripheral neural mechanisms of itch.