Somatosensory research最新文献

Previously reported anatomical and electrophysiological studies have shown that there are neurons in the lamprey's spinal cord that respond to stretching of the spinal cord. Neurons in similar locations are especially prominent in reptiles, where they form the marginal nuclei. These nuclei have been examined in snakes, and it has become apparent that the denticulate ligament is both structurally and functionally closely related to the marginal nuclei. The ligament loses collagen in a short segment of every intervertebral area, and the marginal nuclei are located only in this area. The marginal nuclei consist of a group of medium-sized neurons along the edge of the spinal cord, with a strip of neuropil separating them from the ligament; the neurons extend dendritic processes into this lateral neuropil area and give rise to long finger-like processes. In the present study, these processes were found to be longer than the ones that have been described for peripheral mechanoreceptors; they are thought to be important in sensory transduction. Closely associated with these processes were axon-like structures. They did not make any type of contact with the finger-like processes; however, an occasional synaptic-like contact, consisting of membrane specialization and a congregation of vesicles, was made with dendritic processes. The conclusion is that these finger-like processes are similar to those of peripheral mechanoreceptors, but that there is no equivalent process to the axon-like structure.

The purpose of the present experiments was to determine whether the organization of spinothalamic tract (STT) cells of adult rats was altered following the loss of most of their small-diameter peripheral afferent fibers, resulting from the neonatal administration of capsaicin. Rat pups were randomly assigned to serve as normal controls, to serve as vehicle controls, or to receive subcutaneous injections of capsaicin (50 mg/kg) on postnatal day (PND) 1, 2, 7, or 15; or an injection on PND 1, 3, and 5. When 60 days old, they were anesthetized and received 0.1-microliter thalamic injections of wheatgerm agglutinin conjugated to horseradish peroxidase (WGA:HRP) in the area of the central lateral nucleus (CL), the posterior group (PO), and the ventrobasal complex (VB), or the area of CL or VB. Following a survival time of 48 hr, the animals were perfused, and neuronal HRP reaction product was visualized with tetramethylbenzidine. The number and distribution of WGA:HRP-labeled STT neurons varied in treated animals with the time of capsaicin injection. Rats injected with capsaicin on or before PND 7 demonstrated a significant reduction of labeled STT neurons from the superficial laminae of the spinal cord. Additionally, lamina I neurons were unlabeled in animals treated before PND 7 even with large thalamic injections. Differences in the distribution of labeled STT neurons could not be demonstrated for animals injected with capsaicin on PND 7 or PND 15, though there was a decrement in the number of labeled neurons in PND 7 animals. In order to make certain that absence of labeled STT neurons was not due to some technical error or to insufficient spread of WGA:HRP at the site of injection, six injections of WGA:HRP were placed in the thalamus of PND 1 and normal adult animals. Where the dense core of reaction product did not extend caudal to the posterior commissure, WGA:HRP-positive neurons were located and distributed similarly to those cases described for large thalamic injections. Neurons in superficial laminae of the nucleus proprius and lamina I of the contralateral spinal cord were labeled where the dense core of the thalamic injection extended into the mesencephalon of PND 1 animals. These studies indicate that the number and distribution of the cells of origin of the STT are altered in adult rats following their neonatal treatment with the neurotoxin capsaicin, and that this effect is limited to a critical postnatal period.

Experiments were performed in order to study the receptive field (RF) organization and the callosal connectivity of the trunk representation zone in areas 3b and 1 of the postcentral cortex of macaque monkeys. Multiunit microelectrode recordings showed that neurons responding to tactile stimulation of bilateral RFs across the midline of the body were contained in three topographically distinct zones of the trunk map. In one zone, at the junction between cytoarchitectonic areas 3b and 1, RFs straddled the dorsal midline of the trunk. In the other two zones, one located caudally in area 1 in front of the postcentral dimple, and the other rostrally in area 3b in the depth of the posterior bank of the central sulcus, RFs straddled the ventral midline of the trunk. The first one and the other two zones are referred to here as the dorsal and the ventral midline representation zones, respectively. Elsewhere in the trunk map, neurons responded only to stimulation of contralateral RFs. The callosal connectivity of the trunk map was studied by means of the transport of horseradish peroxidase (HRP). Multiple injections of HRP in electrophysiologically identified sites of the trunk representation in one hemisphere labeled both callosal fiber terminals and callosally projecting neurons in the contralateral homotopic cortex. Dense patches of callosal neurons intensely labeled with HRP were present in the cortical regions representing the body midlines and were distributed for the most part in layer III. Some neurons lightly labeled with HRP were scattered in other zones of the trunk map. Callosal terminations were densest within the midline zones and very sparse or absent in the lateral trunk zones. Correlation of physiological and anatomical data obtained either separately or from the same animal demonstrated that cortical regions containing bilateral-field neurons also contained the highest density of labeled callosal terminations and neurons. This correlation suggests a role for the corpus callosum in the perception of the body midline, either by generating the bilateral RFs of these neurons or by coordinating the activity of the regions containing neurons with thalamically generated bilateral RFs.

Microelectrodes were used to record the extracellular activity of 80 single neurons of the main cuneate nucleus (MCN) of raccoons anesthetized with either methoxyflurane or pentobarbital sodium. All 80 MCN neurons had peripheral receptive fields (RFs) that lay entirely on the glabrous surfaces of the forepaw and were responsive to light mechanical stimulation. Neurons were characterized according to the nature of their response to mechanical stimulation of their RFs, as well as to their response to electrical stimulation of the contralateral thalamic ventrobasal complex (VB). All antidromically activated neurons (64% of sample) were histologically verified as falling within the clusters region of the MCN, while synaptically activated neurons (19% of sample), as well as neurons not responsive to VB stimulation (17% of sample), were located in both the clusters and the polymorphic regions. Antidromically activated neurons typically responded with a single fixed-latency spike, although a few responded with a burst of 3 or more spikes. Others responded with a single antidromic spike followed by a train of synaptically activated spikes. In these latter neurons, it was often possible to block the synaptic spikes selectively. MCN neurons were classed according to their response to controlled mechanical stimuli as rapidly adapting (RA), slowly adapting (SA), or Pacinian (Pc). The proportions of neurons falling into these categories did not vary significantly with the type of response to thalamic stimulation, and the overall percentages were 56% RA, 24% SA, and 20% Pc. These figures are very similar to those previously obtained in a sample of primary afferent fibers of the raccoon cervical cuneate fasciculus (L. M. Pubols and Pubols, 1973). Absolute displacement, displacement velocity, and force thresholds, which ranged between 4 and 326 micron, 0.01 and 16.3 micron/msec, and 120 and 3600 mg, respectively, are comparable to those previously found for primary afferents supplying mechanoreceptors of the glabrous surfaces of the raccoon's forepaw. Neither displacement nor force thresholds differed for RA versus SA neurons; however, displacement velocity thresholds were significantly lower for SA than for RA neurons.

The retrograde transport of horseradish peroxidase (HRP) was used to study the distribution of perikarya in the dorsal root ganglia (DRGs). Injections of HRP subcutaneously into a small area of the foreleg, flank, perineum, the central pad of the forepaw, muscles of the foreleg, the wall of the urinary bladder, and mucosa of the rectum resulted in many retrogradely labeled perikarya in one DRG. Labeled perikarya were distributed in the ganglia proximally to distal elongated slabs or columns, especially in cases of subcutaneous injections. A similar slab, or columnar distribution, of HRP-labeled perikarya was noticed when the tracer was injected into the spinal cord preceded by the transection of all dorsal root filaments but one. Perikarya located along the lateral border of the ganglion were labeled through rostral filaments, and perikarya distributed along the medial border were labeled through caudal filaments. A segmental somatotopic map has been conceived for the DRG as an intermediate territory between the periphery and the spinal cord.

This study was aimed at measuring the kinetics of retrograde death among primary sensory neurons axotomized by transection of the ipsilateral sciatic nerve in adult rats. Using electrophysiological and retrograde transport methods, we first determined that most sciatic afferents enter the spinal cord along the L4 and L5 dorsal roots (DRs), and that about 54% of the cells in the L4 and L5 dorsal root ganglia (DRGs) project an axon into the sciatic nerve. Knowing this value, we could then calculate the rate of loss of axotomized neurons from the overall rate of neuron loss in the DRGs at different times after the lesion. Following unilateral sciatic neurectomy, we found a steady falloff in the ratio of DRG neurons on the operated versus the intact control sides in cresyl-violet-stained serial paraffin sections. We were surprised to note, however, that on the control side there was a steady increase in the cell count with age. Counts done on a series of unoperated rats of various ages confirmed this natural increase. Overall, new neurons accrete at an average rate of 18.1 cells per day to the combined L4 and L5 DRGs, nearly doubling their numbers during the adult life of the animal. The new cells add mostly to the small-diameter neuronal compartment. Evidence from neonatally operated rats indicates that the decline in the ratio of neurons in operated versus control DRGs following sciatic nerve section in the adult results more from a halt in the accretion of new neurons to the sciatic compartment than from frank cell death. From our data, we calculate that the loss of axotomized neurons occurs at a rate of only about 8% per 100 postoperative days.

Cortical foci in which stimulation produced movement in either the forelimb or hindlimb were isolated in rats. In each experiment, two foci were selected: one for movement in the forelimb, and the other in the hindlimb. Stimulation was subsequently reduced in order to avoid eliciting a movement, and the effects of this stimulation on activity of gracile and cuneate neurons were examined. Both excitation and inhibition were observed and were found to be arranged in a somatotopic manner. Excitation was almost exclusively obtained when the receptive field (RF) of a given neuron corresponded to the body surfaces overlying the joints involved in the cortically evoked movement. A high percentage of neurons with RFs on body surfaces corresponding to, or adjacent to, the region of cortically induced movement were inhibited, while the activity of neurons with RFs distant to the site of movement was seldom modified. These results suggest that cortical influences exerted on the dorsal column nuclei (DCN) in rats are organized in a somatotopic manner.

Experiments were performed in order to investigate, at the light- and electron-microscopic levels, the transneuronal transport of wheatgerm agglutinin conjugated with horseradish peroxidase (WGA:HRP) in the somatosensory system of rats. In five adult albino rats, various amounts of WGA:HRP at different concentrations were bilaterally injected in the dorsal column nuclei (DCN). In one additional animal, WGA:HRP was injected in one side, whereas free HRP was injected in the contralateral DCN. In another five rats, WGA:HRP was injected in the first somatosensory cortex (SI). The postinjection survival time of the animals ranged from 30 to 48 hr. The histochemical visualization of the enzyme was performed using either paraphenylenediamine-pyrocatechol (PPD-PC) or tetraethylbenzidine (TMB) as chromogens on adjacent horizontal serial sections. All the reacted samples were studied at the light-microscopic level, and sections from four animals were processed for the ultrastructural investigation. After DCN injections, a massive anterograde labeling was always observed in nucleus ventralis posterolateralis (VPL) of the thalamus, where also labeled neurons and glial cells were detected at both the light- and the electron-microscopic levels. Labeled neurons and terminals in the lateral border of nucleus reticularis (Re) of the thalamus were also observed after either DCN or SI injection of WGA:HRP. Our results show that WGA:HRP is effectively transported not only anterogradely and retrogradely through the somatosensory system of the rat, but also transneuronally. The transneuronal transfer of the tracer seems to be mainly related to the postlabeling survival time of the animal, and it does not occur when free HRP is injected. In controlled experimental conditions, WGA:HRP can therefore be useful for tracing secondary projections. Moreover, in the present report, the existence of a mediolateral arrangement of the projections of the somatosensory-related area of Re toward VPL is directly demonstrated. As for the histochemical procedure employed, differences in sensitivity are shown between PPD-PC and TMB, although the same general pattern of labeling is present with both chromogens.

Primary afferent units in the saphenous nerve of cats, functionally identified as A-delta myelinated nociceptors, were tested for their responses to stimulation of the sympathetic trunk. The units were subdivided functionally into A-mechano-heat receptors (AMHs), which respond to both noxious heat and pressure, and high-threshold mechanoreceptors (HTMs), which respond only to pressure. No units of either subdivision were activated by sympathetic stimulation (SS) prior to noxious heating of their receptive fields. However, six of the seven AMH units with the highest mechanical thresholds (greater than 5 g von Frey) were activated by SS alone (10 Hz) after they had been sensitized by noxious heating of their receptive fields. Sensitized AMH units with lower mechanical thresholds (less than 5 g) were generally not activated by SS alone (1 of 22 units), and their responses to warming of their receptive fields were not altered by SS. The excitatory sympathetic action on AMH units was abolished by alpha- but not beta-andrenergic blockade in the two units tested. HTMs were unresponsive to SS even after repeated noxious heating of their receptive fields (15 units tested). The results of this study indicate that relatively high rates of sympathetic efferent activity (10 Hz) can induce firing in a small population of AMH receptors in damaged skin, specifically those units with high mechanical thresholds. This sympathetically evoked activity might trigger or exacerbate pain associated with skin damage; however, functional conclusions are difficult to draw, because of the scarcity of such units and the fact that the responses in some were brief and of low firing rates.

We used wheatgerm agglutinin conjugated to horseradish peroxidase (WGA:HRP) as an anterograde tracer to label the terminals of the lemniscal, spinothalamic, and trigeminothalamic pathways in the ventrobasal complex of the rat thalamus (VB). The use of benzidine dihydrochloride (BDHC) as the chromogen allowed us to view the labeled profiles with the electron microscope and permitted us to compare the morphology of the terminals from the various pathways. We found that all the labeled somatosensory pathways terminate in the VB in the form of large terminals that contain round synaptic vesicles and make numerous asymmetrical synaptic contacts, usually with dendritic protrusions and proximal dendrites. The present results demonstrate that pathways conveying noxious and non-noxious somatosensory information terminate upon thalamic neurons with synaptic terminals having similar morphological features.