Somatosensory research最新文献

The contributions of viscoelastic properties of squirrel monkey glabrous skin to slowly adapting Type I (SAI) mechanoreceptive afferent fiber discharge were examined in the present study. Individual fibers of the median and ulnar nerves were isolated by microdissection in six monkeys anesthetized with pentobarbital sodium. Utilizing mechanical stimulation and data analysis techniques identical to those of a previous study of raccoon glabrous skin and its mechanoreceptors (Pubols, 1982a; Pubols and Maliniak, 1984), we studied and compared responses to punctate mechanical stimuli controlled with respect to force or displacement. Squirrel monkey glabrous skin was found to be more compliant than raccoon glabrous skin, in that a given force applied to either a digital or a palmar skin pad produced a greater displacement of squirrel monkey skin. Skin displacement increased approximately linearly with increasing forces at the beginning of static stimulation, but over time (at least up to 20 sec), the relationship became negatively accelerated. Absolute-force thresholds of individual SAI units were significantly lower in squirrel monkey (mean = 122 mg, range = 48-340 mg) than in raccoon (mean = 484 mg, range = 70-1,290 mg). However, absolute-displacement thresholds were insignificantly lower (squirrel monkey: mean = 17.24 microns, range = 5-30 microns; raccoon: mean = 30 microns, range = 5-185 microns). Application of suprathreshold forces (range = 1-20 g) and displacements (range = 500-1,000 microns) revealed greater interunit variability in response to maintained stimulation than previously found in raccoon. In 8 out of 15 fibers, the rate of adaptation was significantly greater during constant-displacement than during constant-force stimulation; in 4 cases there was no significant difference; and in 3 cases the rate of adaptation was significantly greater during constant-force stimulation. Potential sources of interunit variability include surface topography of the hand, properties of cutaneous and subcutaneous tissues in the vicinity of the receptor, and experimental variables such as stimulus amplitude and rate of stimulus onset. It is suggested that both regional and species differences in functional properties of cutaneous mechanoreceptors are more likely attributable to differences in mechanical properties of skin and subjacent tissues than to any inherent differences in receptor properties.

The effect of human skin temperature on electrocutaneous sensitivity was examined using brief capacitive discharges. Stimuli were designed to ensure that sensory effects would be independent of skin resistance and would reflect underlying neural excitability as closely as possible. Skin temperature was manipulated by immersing the forearm in circulating hot or cold air. Detection thresholds on the arm and fingertip were raised by cooling, but were not altered by heating. Temperature-related sensitivity shifts were described by the same multiplicative factors for both threshold and suprathreshold levels. The temperature coefficient (Q10) for cutaneous sensitivity under these conditions was approximately 1.3.

Somatotopic organization was examined for 203 dorsal horn cells in spinal segments C6 and C7 of chloralose-anesthetized cats. The ventral paw and toe area were represented medial to a smaller area with input from the dorsal paw. Representation of the ventromedial forelimb was rostral to that of the paw, while the shoulder and dorsolateral limb were represented caudal to it. In 13 out of 22 electrode tracks in which three or more cells were found, the location of receptive fields progressively changed for successively recorded cells. Receptive fields on the paw were closer together and overlapped more than those on the proximal limb. Receptive fields that included glabrous skin were found for only 7 of 203 cells; all were located in the medial third of the C7 dorsal horn. It appears that glabrous skin is underrepresented in the dorsal horn; this may be compensated for a higher levels by input from the lemniscal system. The response characteristics of 172 dorsal horn neurons were examined. Of these units, 135 (78%) had cutaneous receptive fields. An additional 37 cells (22%) responded to manipulation of muscle or tendon and were classified as deep (D) cells. The cells with cutaneous receptive fields were classified as low-threshold (LT) cells (38%), high-threshold (HT) cells (20%), and wide-dynamic-range (WDR) cells (20%). Alternatively, using cluster analysis, 57 cells with cutaneous receptive fields were classified as one of five mechanical types. Type 1 cells responded primarily to low-threshold input, while the other four types fired in characteristic patterns in response to a combination of innocuous and noxious stimuli. LT cells were located more superficially in the spinal cord than the other classes; their average depth below the cord surface was 1.9 mm. WDR cells (mean = 2.1 mm) were located below the LT cells and above the HT and D cells (mean = 2.6 mm).

Two series of experiments were performed to assess the effects of stimulus velocity on human subjects' perception of the distance traversed by a moving tactile stimulus. In all experiments, constant-velocity stimuli were applied to the dorsal surface of the left forearm; velocities ranging between 1.0 and 256 cm/sec were used. In some experiments the stimuli moved from distal to proximal over the skin, and in others they moved from proximal to distal. The length of skin contacted by the moving stimulus was defined by a plate having an aperture of 4.0 X 0.5 cm. In the first series of experiments, subjects were required to compare the distance traversed by a test stimulus delivered 2 sec after a standard stimulus, and also to report the on-locus and the off-locus of the brushing stimulus. In the second series of experiments, the subjects rated the perceived distance on the skin using a free-magnitude-estimation procedure. The data from both series of experiments defined the same relationship between stimulus velocity and perceived stimulus distance. More specifically, although the length of skin contacted by the stimulus was the same at all velocities, subjects' estimates of stimulus distance decreased with increasing stimulus velocity. In addition, the function relating estimates of stimulus distance to velocity was flat for velocities between 5 and 20 cm/sec, but possessed an appreciable negative slope at lower and higher velocities. It is interesting that the plateau of the relationship between perceived stimulus distance and velocity occurred within the range of velocities that human subjects employ to scan textured surfaces; it also corresponded precisely with the range of stimulus velocities at which the directional sensitivity of somatosensory cortical neurons and human subjects is optimal.

This study used antisera directed against glutamic acid decarboxylase (GAD), the biosynthetic enzyme for gamma-aminobutyric acid (GABA), to examine the light- and electron-microscopic distribution of presumed GABA-ergic synapses in the medullary homologue of the cat spinal dorsal horn, the trigeminal nucleus caudalis. At the light-microscopic level, immunoreactive terminals were concentrated in the superficial dorsal horn, laminae I and II. Colchicine was generally ineffective in revealing the distribution of cell bodies. However, in two successful cases, the majority of labeled cells were found in the magnocellular layer, ventral to the substantia gelatinosa, a region that had a lower density of immunoreactive terminals. Other labeled neurons were scattered in laminae I and II. A variety of synaptic arrangements were found at the electron-microscopic level. These derived from two types of labeled terminals. One contained both small round vesicles and large dense-cored vesicles. The second contained small round and pleomorphic vesicles. Some immunoreactive GAD terminals contained a few flat vesicles. Labeled terminals predominantly formed axodendritic synapses, via symmetrical contacts. Several axoaxonic arrangements were also observed. In most cases, the GAD terminal (which did not contain dense-cored vesicles) was presynaptic to another vesicle-containing profile, including the scalloped central terminal thought to derive from primary afferents. Another population of labeled GAD terminals was found postsynaptic to unlabeled vesicle-containing profiles, including central terminals. These data indicate that inhibitory GABA-ergic controls in the trigeminal nucleus caudalis involve both presynaptic and postsynaptic mechanisms and are probably mediated via direct contacts onto ascending projection neurons, as well as via synaptic contacts onto nociceptive primary afferent fibers. The transmission of nociceptive messages by neurons of the spinal cord dorsal horn and trigeminal nucleus caudalis is subject to a variety of segmental and supraspinal controls. Pharmacological and electrophysiological studies have implicated the biogenic amines serotonin and norepinephrine, and the endogenous opioid peptides enkephalin and dynorphin, in those controls (Basbaum and Fields, 1978, 1984; Basbaum et al., 1983; Basbaum, 1985).(ABSTRACT TRUNCATED AT 400 WORDS)

Projections to the ventroposterior (VP) complex of the thalamus were investigated after [3H]proline injections were placed in electrophysiologically identified parts of the representations of the body surface in areas 3b and 1 of somatosensory cortex of cebus monkeys. Injections placed in somatotopically matched parts of the representations in area 3b and 1 labeled similar parts of VP. Evidence was provided for the representation of the face medially in ventroposterior medial nucleus (VPM), the hand and foot in medial and lateral subnuclei in ventroposterior lateral nucleus (VPL), and the trunk and proximal limbs dorsally and caudally in VPL.

Spinomesencephalic tract neurons in the cat spinal cord were retrogradely labeled following injections of wheatgerm agglutinin conjugated to horseradish peroxidase into the region of the midbrain parabrachial area. Labeled cell bodies were concentrated in lamina I, bilaterally. A more scattered distribution was observed in lamina V and deeper laminae. Bilateral lesions of the dorsolateral funiculus (DLF) at thoracic levels eliminated labeling of lamina I neurons below the lesions, but had no effect on the labeling of the neurons in deeper laminae. Injections of colchicine into the spinal white matter caused the label to accumulate intra-axonally and revealed labeled axons bilaterally in the DLF and ipsilaterally in the ventrolateral and ventral funiculi.

The response properties of spinothalamic tract (STT) cells in the dorsal horn of the cervical spinal cord were examined in chloralose-anesthetized cats. The activity of 56 STT cells located in laminae IV-VI was studied, with most activity isolated in the lateral part of the dorsal horn. The level of background activity in STT cells was low (mean = 1.2 impulses/sec; n = 26). Conduction velocity estimates for STT axons ranged from 9 to 76 m/sec (mean = 38 m/sec; n = 56) and were not correlated with the recording site in the spinal cord. Most cells were antidromically activated from an electrode in the medial part of the posterior group of nuclei in the thalamus. Excitatory receptive fields were ipsilateral to the recording site, and for 38 of 40 neurons were confined to the forelimb. Although receptive fields were often restricted to part of the paw, they did not include glabrous skin. Among 31 cells classified, four groups were identified: low-threshold (LT) cells (13%) responded to pressure and brushing of the skin; high-threshold (HT) cells (13%) responded only to noxious pinching or squeezing of the skin; wide-dynamic-range (WDR) cells (58%) responded to innocuous mechanical stimuli but had a greater response to noxious stimuli; deep (D) cells (16%) responded to manipulation of subcutaneous tissues such as muscle. Heat stimuli 30 sec in duration, in the range of 43-55 degrees C, were applied to the receptive fields of 14 neurons that included representatives from all three groups with cutaneous input. Nine neurons responded to heat with thresholds that ranged from 47 degrees to 55 degrees C (mean = 51 degrees C). The responses of these nine STT cells increased with increasing stimulus intensity in the noxious range. In the cat cervical dorsal horn, STT cells can signal the occurrence of noxious stimuli on the body surface, and, judging by the sizes of their peripheral receptive fields, are capable of signaling precise information about the location of the damage. Furthermore, some cells are able to signal the intensity of a noxious heating pulse.

Electrical activity of trigeminal central projection areas was recorded in anesthetized and chronic awake geese. Evoked potentials of telencephalic structures were studied after stimulation of the bill, quintofrontal tract (QF), and several telencephalic structures (nucleus basalis [Bas], neostriatum frontale [NF], paleostriatum augmentatum [PA], and neostriatum caudale [NC]). Short-latency evoked potentials were recorded in Bas after stimulation of the bill or QF; this finding is consistent with a direct connection between the main sensory trigeminal nucleus and Bas. Short- and long-latency evoked potentials were recorded in PA and NC after stimulation of the posterior QF. These potentials are concluded to be due to two different pathways: The shorter-latency response is produced by fibers leaving QF posteriorly, while the longer-latency response is derived from fibers traveling along QF, relaying first in Bas and then in NF. From Bas and NF, two pathways convey impulses to NC; only one is relayed in PA.