Stimulation Of Receptive Field Research Articles

Locomotion produced in mesencephalic cats by injections of putative transmitter substances and antagonists into the medial reticular formation and the pontomedullary locomotor strip

The purpose of this study was to determine the distribution of cells in the medial reticular formation (MRF) and the pontomedullary locomotor strip (PLS), which can induce locomotion when activated. Controlled microinjections of neuroactive substances (Goodchild et al., 1982) into the MRF or PLS were made in order to activate cell bodies in those areas. The ability of trigeminal receptive field stimulation to induce locomotion before and after drug infusion into the PLS was also assessed since the PLS and the spinal nucleus of the trigeminal nerve are similar in their anatomical distribution. Experiments were performed on precollicular-postmamillary decerebrate cats walking on a treadmill. Injections of glutamic acid (GA; 500 nmol) into the MRF produced locomotion that was antagonized by infusion of glutamic acid diethyl ester into the same spot. Decreases in the current threshold for locomotion produced by electrical stimulation of the MRF were observed when the MRF was infused with either GA (40-80 nmol), DL-homocysteic acid (DL-HCA; 200 nmol), or picrotoxin (PIC; 15 nmol). Injections of GA (100 nmol), DL-HCA (700 nmol), PIC (10-50 nmol), and substance P (2 nmol) into the PLS also produced locomotion. Locomotion produced by injections of PIC into the PLS was blocked by infusion of equal amounts of muscimol or GABA. Effective PLS injection sites were all confined to the trigeminal spinal nucleus or immediately ventral and medial to this in the adjacent lateral reticular formation. Trigeminal nerve peripheral field stimulation evoked locomotion after microinjection of PIC into the PLS, although this same facial stimulus was not effective prior to drug injection. We conclude that the MRF and PLS regions of the cat brain stem contain cells that produce locomotion when chemically stimulated, and we suggest that the PLS is closely related to or synonymous with the spinal nucleus of the trigeminal nerve. Furthermore, we suggest that stimulation of trigeminal afferents is analogous to stimulation of segmental afferent pathways in the production of locomotion (Sherrington, 1910; Jankowska et al., 1967; Afelt, 1970; Budakova, 1972; Grillner and Zangger, 1979).

The rostral part of the trigeminal sensory complex is involved in orofacial nociception

Single units responsive to noxious mechanical stimulation of orofacial receptive fields were recorded within the ventrobasal complex of the rat thalamus. The induced activities were compared before and after deafferentation of the subnucleus caudalis by a trigeminal tractotomy performed at the obex level. The receptive fields activated by noxious stimulation were classified as ‘oral’ when included in the oral, perioral or paranasal areas, and as ‘facial’ when included in facial regions distant from the oral cavity. After tractotomy, the unit responses to noxious stimulation of an oral field remained unchanged in 8 cases, decreased in 3 cases, and were suppressed in 4 cases. For units responding to noxious stimulation of a facial field, the responses were suppressed in 8 cases, decreased in two cases and remained unchanged in two other cases. So it appears that the rostral part of the trigeminal sensory complex (1) receives nociceptive afferents mainly from the oral and perioral areas and (2) is a relay in ascending pathways which convey painful sensations.

'Real-motion' cells in visual area V2 of behaving macaque monkeys.

Extracellular recordings were made in area V2 of behaving macaque monkeys. Neurons were classified into three groups: non-oriented cells, oriented cells with antagonistic areas and oriented cells without antagonistic areas in their receptive field. All neurons were tested with standard visual stimulations in order to assess whether they gave different responses to the movement of a stimulus and to the movement of its retinal image alone, when the stimulus was motionless and the animal voluntarily moved its eyes. To do this, neuronal responses obtained when a moving stimulus swept a stationary receptive field (during steady fixation) and when a moving receptive field swept a stationary stimulus (during tracking eye movements), were compared. The receptive field stimulation at retinal level was physically the same in both cases, but only in the first was there actual movement of the visual stimulus. Control trials, where the monkeys performed tracking eye movements without any intentional receptive field stimulation, were also carried out. Out of a total of 263 neurons isolated in the central 10 deg representation of area V2, 101 were fully studied with the visual stimulation described above. Most of these (83/101; 82%) gave about the same response to the two situations. About 14% (14/101) gave a good response to stimulus movements during steady fixation and a very weak one to retinal image displacements of stationary stimuli during visual tracking. We have called neurons of this type "real-motion cells" (cf. Galletti et al. 1984). None of the non-oriented cells was a real-motion one, while about an equal percentage of real-motion cells was found among the oriented cells with and without antagonistic areas. Finally, we found only 4 neurons which showed behaviour opposite to that of real-motion cells, i.e. they showed a better response to displacement of the retinal image of stationary stimuli than to actual movement of stimuli. We suggest that real-motion cells might contribute to correctly evaluating movement in the visual field in spite of eye movements and that they might allow recognition of the movement of an object even if it moves across a non-patterned visual background. Present data on area V2, together with similar results observed in area V1 (Galletti et al. 1984; Battaglini et al. 1986), support the view that these two cortical areas analyse the movement in a parallel fashion along with many other characteristics of the visual stimulus.

Mechanical response properties of ventroposterior medial thalamic neurons in the alert monkey.

The properties of trigeminal cutaneous thalamic neurons were explored in alert cynomolgous monkeys to determine receptive field and response characteristics. Two monkeys received juice reward for sitting quietly while an investigator probed the monkey's face with mechanical stimuli. Extracellular single unit recordings were made from the ventroposterior medial thalamic nucleus (VPM), and mechanical response properties were evaluated for each cell having an extraoral cutaneous receptive field. Of 89 cells examined, 90% responded best to innocuous tactile stimulation, and were classified as low threshold. The other 10%, classified as wide dynamic range, showed a graded response to increasingly intense stimuli, with a maximum discharge to noxious pinch. Of the low threshold neurons, most exhibited excitatory responses, with about half being rapidly adapting and the others slowly adapting. The spontaneous activity of 11% of the low threshold neurons was inhibited by stimulation of the neuron's receptive field. There was no systematic difference in receptive field size for the various types of neurons, but the receptive fields of wide dynamic range cells were smaller than those previously observed in trigeminopthalamic neurons of the medullary dorsal horn. The wide dynamic range and inhibitory low threshold neurons were located primarily in the caudal third of VPM, while the excitatory low threshold neurons were located throughout. In summary, response characteristics of VPM neurons show more diversity in the alert monkey than has been reported in paralyzed and/or anesthetized animals.

Acetylcholine permits long-term enhancement of neuronal responsiveness in cat primary somatosensory cortex

Acetylcholine (ACh) was administered iontophoretically to single neurons in cat somatosensory cortex. Using extracellular recording techniques, neuronal responsiveness was determined at regular intervals from the number of action potentials produced either by iontophoretically applied glutamate or by tactile stimulation of the cutaneous receptive field. The responses were altered in only 21% ( 1361) of the neurons following the application of ACh alone. In contrast, 75% ( 6688) of the neurons displayed altered responses during administration of ACh simultaneously with either iontophoretically administered glutamate or with tactile stimulation of the receptive field. Forty-seven percent ( 2962) of the responses potentiated in the presence of ACh remained enhanced for periods lasting from 8 min to over 1 h. The responsiveness of cortical neurons to afferent inputs changes during the reorganization of somatotopic maps that occurs after deafferentation, and perhaps during some forms of learning. As ACh has been implicated in some of these processes, it may be that the changes in responsiveness observed here following iontophoretically applied ACh are similar to those which facilitate the acquisition of neuronal responses to altered or novel afferent inputs.

Bilateral receptive fields and callosal connectivity of the body midline representation in the first somatosensory area of primates.

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.

Manipulation reach and visual reach neurons in the inferior parietal lobule of the rhesus monkey

The characteristics of inferior parietal lobule (IPL) ‘reach cell’ responses were studied in the fixating monkey in order to obtain a clue to the function of this area in manipulative behavior. Two subclasses of reach neurons, one showing visual receptive field sensitivity, the other lacking such responsiveness are proposed as correlates of two types of reach behavior, one that takes place with, the other without visual guidance, respectively. Neuronal firing increment and/or firing decrement responses, stimulus-bound to reach behavior were obtained from either type of cell. These were presented as equivalent modes of signal encoding, on the basis of the transformability of neuronal firing increment to firing decrement response as a result of preconditioning stimulation of the oral pulvinar nucleus. Many of the responses obtained were bimodal and some were biphasic, i.e. consisting of incremental and decremental firing phases. As each such component was independently modifiable, they were regarded different signal entities. The multicomponental nature of the visual reach neuronal response was manifested when the reaching movement was into the receptive field of the neuron. Responses consisted then of an initial component, presumably visual, prefixed to a response component stimulus-bound to the reach-invoking stimuli. The early component occupied the same position as the visual response of the cell obtained with visual receptive field stimulation alone. Moreover, the early component could be negated by preconditioning stimulation of the LP and/or pulvinar targets, and then the residual response simulated the response of a non-visual reach neuron. In addition, mean latencies of the visual reach neurons were shorter, i.e. 147 ± 32 ms as compared to 203 ± 62 ms of the non-visual reach cells. Observations made here clearly indicate the participation in a rather unique way of the LP and pulvinar posterior thalamic nuclei in IPL reach function.

Tectospinal neurons in the cat have discharges coding gaze position error

Tectospinal neurons (TSNs) in the caudal superior colliculus of the alert behaving cat have a sustained discharge that depends on the magnitude and direction of the vector between a food target and the visual axis. Each TSN has its ‘optimal’ vector for which it will be activated at a maximum discharge rate independent of whether the animal's head is free or fixed. The discharge is not due to prolonged stimulation of a TSN's receptive field since the discharge persists even when the target is not visible.

Visceral and somatic afferent convergence onto neurons near the central canal in the sacral spinal cord of the cat.

One hundred and sixty extracellularly and intracellularly recorded unitary discharges from the sacral or caudal spinal segments of 30 anemically decerebrated cats were studied to examine the effects of somatic and visceral afferent stimulation on neurons near the central canal (CC). The recorded unitary activity was histologically verified (by dye marks or horseradish peroxidase, HRP) as having come from the gray matter surrounding the CC that approximates Rexed's lamina X. In the absence of intentional stimulation or apparent injury by the recording electrode, 62% of the units exhibited ongoing discharges. Each unit was tested for responses to the stimulation of somatic (cutaneous and subcutaneous) and visceral (bladder and colon) structures. Seventy-six (48%) of the units responded exclusively to the stimulation of somatic receptive fields, and 10 (6%) of the units were selectively responsive to stimulation of the pelvic viscera. The activity of the remaining 74 (46%) was influenced by activity in both somatic and visceral afferent fibers. Eighteen of the 160 neurons were intracellularly marked with HRP. Based on perikaryal size and dendritic extent, it was possible to divide these cells into two partially overlapping groups. One group consisted of seven neurons with small to medium-sized perikarya, dendritic arbors largely restricted to the gray matter surrounding the CC, and small, singular somatic receptive fields. The second group comprised 11 cells with medium to large-sized soma and dendrites extending out of lamina X. These larger neurons usually possessed multiple, widely distributed somatic receptive fields. The principal finding of the present study is that in the sacral spinal cord many cells near the CC receive primary afferent inputs converging from a wide range of receptor types in somatic and visceral structures. Such neurons are capable of integrating afferent information from somatic structures on both sides of the body with information originating in pelvic viscera and midline regions such as the genitals.

Failure of ES 52, a highly potent enkephalinase inhibitor, to affect nociceptive transmission by rat dorsal horn convergent neurones

The effects of ES 52, a highly potent derivative of the enkephalinase (enkephalin-dipeptidylcar☐ypeptidase) inhibitor thiorphan, were studied on nociceptive activities of dorsal horn convergent neurones in the anaesthetized rat. Neither the C-fibre component of the responses elicited by supramaximal electrical stimulation of the hindpaw excitatory receptive fields nor diffuse noxious inhibitory controls triggered by immersion of the tail in 46–48 °C waterbaths, were affected by ES 52. Thus we conclude that, in our experimental conditions, modulations of the transmission of nociceptive messages at the spinal level are not greatly modified by specifically blocking the degradation of enkephalins. If a major role for enkephalinase (vs aminopeptidase) in the catabolism of enkephalins at the spinal level can be confirmed, then comparison of the present data with our previous results obtained using the opioid antagonist naloxone, might suggest a predominant role for proenkephalin B products (i.e. dynorphins and/or α-neo-endorphin) in modulating nociceptive transmission in the spinal cord.

Time-Dependent Changes in the Functional Organization of Somatosensory Cerebral Cortex following Digit Amputation in Adult Raccoons

Surgical removal of the third forepaw digit in raccoons causes both long-term and short-term changes in functional organization within the digit 3 primary somatosensory (SmI) cortex. Previous studies have shown that 36-52 weeks following amputation in infant raccoons, neurons within the digit 3 cortical territory had become responsive to cutaneous stimulation of "new" forepaw regions adjoining the digit stump (Carson et al., 1981; Kelahan et al., 1980, 1981); the "novel" receptive fields (RFs) were often larger than normal and revealed no orderly somatotopic organization. In the present study, the cortical effects of digit 3 removal were examined in adult raccoons. Within 36 weeks after amputation, the digit 3 zone was also found to be reactivated by "novel" inputs from the forepaw, with no strictly topographic representation of the "new" skin fields. The basic features of cortical reactivation were very similar in animals amputated as adults and as infants, except that the former typically had larger neuronal RFs than the latter. Short-term cortical changes were studied in adult raccoons within 1 day and between 1 and 4 weeks after amputation: Significant time-dependent differences were found in the reactivated digit 3 territory. Within 1 hr following amputation, some cells in the digit 3 zone began to respond to low-intensity cutaneous stimulation of "new" forepaw regions, limited almost exclusively to digits 2 and 4. Neuronal RFs tended to be larger than normal and showed no strictly topographic organization. One to 4 weeks following amputation, the condition of the digit 3 zone differed dramatically from that found immediately and long after amputation--the majority of responsive neurons could be excited only by high-intensity stimulation of small RFs on the digit 3 stump; relatively few cells were sensitive to low-intensity stimulation of adjacent, intact skin regions. Again, no true somatotopic organization was evident. The combined results of these experiments indicate that within 36 weeks following removal of a digit in raccoons, the deprived SmI cortical sector undergoes a dynamic sequence of changes in functional organization: Neurons that are normally excited by stimulation of digit 3 first become responsive primarily to stimulation of digits 2 and 4 (within 1 day after amputation), then to the digit 3 stump (from at least 1-4 weeks after amputation), and finally again to digits 2 and 4 (within at least 36 weeks after amputation).(ABSTRACT TRUNCATED AT 400 WORDS)

Extra- and intracellular recordings from dorsal column postsynaptic spinomedullary neurons in the cat

Dorsal column postsynaptic (DCPS) spinomedullary neurons in the dorsal horn of spinal segments L6-S1 of adult cats anesthetized with sodium pentobarbital were identified by antidromic stimulation of cervical dorsal columns that were dissected free of, and electrically isolated from, the rest of the spinal cord. The neurons were categorized with respect to natural stimulation of their cutaneous receptive fields. An equal number of low-threshold mechanoreceptive and wide-dynamic-range neurons were found. No DCPS neurons could be classified as nociceptive-specific. All neurons received input from low-threshold mechanoreceptors with myelinated axons. There was no evidence that any neurons received monosynaptic input from unmyelinated, primary afferent fibers. The average conduction velocity of the antidromic responses was 45.7 m/s. Nearly half of the DCPS cells showed an antidromic spike followed by synaptically driven responses that were probably evoked by antidromic invasion into the intraspinal collaterals of A-beta primary afferent fibers that ascended the dorsal columns. Intracellularly recorded synaptic responses of DCPS neurons to dorsal column and receptive field stimulation usually consisted of an EPSP with overriding spike potentials followed by a prolonged IPSP whose amplitude decreased markedly as the stimulus frequency was increased in the range of 5 to 30 Hz. The results indicate that DCPS neurons constitute a projection system capable of signaling innocuous and tissue-damaging mechanical stimuli. The DCPS projection may play a role in the modulation of touch and pain perception.

Receptive field characteristics of neurones in striate cortex of newborn lambs and adult sheep

The properties of cells of the striate visual cortex (V1) have been studied in the normal adult sheep and in new-born lambs without visual experience, the majority of cells in the lamb are orientation specific, but 20% are non-oriented compared to only 3% cells in the adult. In the lamb there was little or no facilitation of binocularly-driven cells by simultaneous stimulation of both receptive fields. Cells which responded only to binocular stimulation of particular disparities (‘obligate binocular’ cells) were rarely encountered. In the adult, 15% of the sample were obligate binocular cells and a further 28% showed binocular facilitation. Simple and complex receptive fields were found in similar proportions in both new-born lambs and adult sheep. End-stopped cells comprised 17% of the sample in adults but only 2% in the lambs. Direction sensitive cells were found more frequently in the sheep (21% of cells) than in the lamb (4% of cells). It is concluded that facilitatory processes in binocular cells and inhibitory mechanisms generally, seem much less developed in the lamb.

Electrophysiological response properties of spinoreticular neurons in the monkey.

Extracellular recordings were made from 29 spinoreticular cells in the spinal cords of anesthetized monkeys. The cells were in either the cervical or the lumbar enlargement, and they were identified by antidromic activation from the medial part of the pontomedullary reticular formation. More spinoreticular neurons were sampled in the cervical than in the lumbar cord. Most of the cells were contralateral to the side from which antidromic activation was observed, but a higher proportion of the spinoreticular neurons in the cervical enlargement than in the lumbar enlargement was ipsilateral to the antidromic stimulus. Three cells in the lumbar cord were antidromically activated not only from the reticular formation but also from the contralateral thalamus, confirming that some spinoreticular projections are formed by collaterals from spinothalamic cells. Most of the spinoreticular neurons were in the ventral horn in laminae VII and VIII, although a few were in laminae IV-VI. Nearly half of the spinoreticular cells in the sample could not be activated by any form of peripheral stimulation tested. The other cells could be activated by stimulation of receptive fields that varied from small to large, that were sometimes bilateral regions of the skin or deep tissues. Although some spinoreticular cells could be classified as low threshold or wide dynamic range, the largest proportion were high threshold, requiring noxious stimulation for their activation. Descending volleys resulting from stimulation in the reticular formation could often be shown to inhibit or to excite spinoreticular neurons. It can be concluded that at least some spinoreticular neurons may play a role in nociception.

Single unit receptive fields in rabbit primary binocular cortex.

The receptive fields of 125 single units recorded from the binocular region of rabbit primary visual cortex have been analysed. The population of 43% radially symmetric, 23% directional, and 23% orientation selective units is similar to that of rabbit monocular visual cortex. The relative scarcity of orientation selective units and the absence of orientation columns differentiates rabbit from cat primary visual cortex. However, the majority of binocular units had similar receptive fields in each eye and very unconventional receptive fields were not encountered. Tested binocular units demonstrated summation upon simultaneous stimulation of both receptive fields. In conjunction with findings reported elsewhere, these results suggest that rabbit and cat possess a similar provision for binocular vision in spite of some differences in their cortical organisation.

Response of forelimb guard hair afferent units to air-jet stimulation of entire receptive field.

1. In anesthetized, immobilized cats, individual axonal responses to movement of forelimb G hairs were recorded in the ipsilateral cuneate fasciculus at C1-C2 with a glass-insulated tungsten microelectrode. Several test criteria were used to identify primary afferent axons among the recorded neurons. 2. The output discharge in the parent sensory axon during continuous air-jet stimulation of the entire receptive field was irregular and was usually maintained. Criteria for statistical stability were introduced, permitting an analysis of the response as a stable point process. 3. The main interspike interval during the response to maximal air-jet stimulation ranged in different units from 4 to 100 ms, with a transition zone near 20 ms (50/s) between short and long mean interval units. Long and short mean interval units tended to be related to G2- and G1-type hair receptors, respectively. 4. In both short and long mean interval units, the interspike-interval distribution (IID) had a dead time (usually 2-4 ms), a sigmoidal rising limb, a single mode, and a monotonic falling limb, a single mode, and a monotonic falling limb. In short mean interval units, the decrement of the falling limb was described by a single exponential. 5. The value of the standard deviation of the IID was a little less than that of the mean interval minus dead time, the inequality probably resulting from the finite duration of the rising limb and, in long mean interval units, the nonexponential falling limb. 6. Joint interval analysis revealed that successive intervals were independent except in short mean interval units, where the shortest intervals tended to be followed by intervals shorter than the mean. 7. The expectation density function did not reveal any periods in the output discharge. In short mean interval units, a constant mean (planteau) level was attained after a finite rise time and usually one or more overshooting-undershooting sequences. In long mean interval units, the expectation density function monotonically rose to the plateau level. 8. Reducing the air-jet intensity caused an increase in mean interval of discharge and in standard deviation of the IID and slowed the rise time of the expectation density function to the plateau level. 9. Similarities and differences between the whole-field response to air-jet stimulation and the Poisson process are discussed.

Effects of chronic descending tractotomy on the response patterns of neurons in the trigeminal nuclei principalis and oralis

Unilateral, descending, trigeminal tractotomy was carried out on adult cats using aseptic technique. Unit activity, evoked by bipolar noxious stimulation of the tooth pulp and by innocuous mechanical stimulation of other receptive fields, was recorded from neurons in the trigeminal principalis and oralis nuclei, 7 to 13 weeks after tractotomy. Seventy neurons from the lesion side of the brain stem and 59 from the control side were studied. All units responded to pulpal stimuli and the great majority to mechanical stimuli. Three general response configurations were produced by each of the two forms of stimulation: brief bursts of 1 to 5 spikes, trains of 6 to 20 spikes, and multiple component discharges consisting of sequences of brief bursts. No statistically significant differences between the two populations were found for peripheral field size or location, for discharge characteristics such as latency, configuration, or mean density, or for responsiveness to the two types of stimuli. However, a statistically greater proportion of control units fired dense impulse trains (more than eight spikes) to intense pulpal stimuli. The results provide evidence that the response patterns of most somatosensory neurons in nuclei principalis and oralis are unaffected by descending trigeminal tractotomy and that stimulus modality is not coded by the response characteristics of polymodal units in these nuclei. Although some modulatory influence is exerted on polymodal neurons in the rostral nuclei by the trigeminal nucleus caudalis, it appears doubtful that this is a significant factor in determining whether a peripheral stimulus is perceived as noxious or innocuous.

Inhibition by etorphine of the discharge of dorsal horn neurons: Effects on the neuronal response to both high- and low-threshold sensory input in the decerebrate spinal cat

The effect of etorphine on the receptive field characteristics of neurons sensitive to brush-touch-pinch and brush-touch was examined. It was found that, aside from the depression of the neuronal response to pinch and the depression of the Ad- and C-fiber responses evoked by receptive field stimulation, etorphine at the same doses attenuated the response to brush-touch as well in about 20% of the cells. Close examination of this phenomenon revealed, first, that in these cells there was a small but detectable reduction in the extent of the brush-touch field. Second, it was found that the response to A-fiber volleys produced by stimulating beyond the edge of the natural brush-touch receptive field produced an evoked discharge which was strongly inhibited by etorphine, unlike the A-fiber input, which derived from electrical stimulation in the center of the brush-touch field. The lack of effect on A-fiber input derived from the center of the brush-touch field was not due to stimulus strength, as no effect of etorphine was observed at threshold currents. In any case, the inhibition of either high- or low-thresh-old input was pharmacologically specific, i.e., it was dose dependent, antagonized by naloxone, produced by two opiate agonists (morphine and etorphine), and not produced by dextrophan. These data suggest that, although the most prominent effect on cellular discharge is that related to high-threshold stimulation and small-fiber input, a measurable degree of depression could be detected at low doses of etorphine in cells which were responsive only to innocuous stimuli.

Unit responses in the ventrobasal thalamus (VPL) of the cat to bradykinin injected into somatic and visceral arteries

Under light to moderate sodium pentobarbital anesthesia one half of the thalamic neurons displaying place and modality specific responses to low intensity mechanical stimulation of contralateral receptive fields also responded, by excitation or by inhibition, to bradykinin injections into somatic and visceral arteries. Partial or complete somatovisceral convergence was commonly seen in neurons with otherwise typical lemniscal properties. The response intensities varied considerably for different neurons; the responses to somatic bradykinin were always stronger than to visceral injections. The time course of the response was comparable to that seen in peripheral nerves, other central sites, and the behavioral reaction to bradykinin. Control experiments indicated the independence of the bradykinin responses from variations in blood pressure or general arousal effects. It is suggested that modulation of ventrobasal thalamic activity may be an important factor in central nociceptive processing.

Taste responses in the facial lobe of the carp, Cyprinus carpio L.

Single unitary discharges in the facial lobe of the carp, Cyprinus carpio L., were studied electrophysiologically in response to chemical stimulation of the chemoreceptors of the facial skin. Of 258 neurons recorded, 95% were facilitated by taste stimuli and 5% were inhibited. According to responsiveness to the 4 basic tasting substances, the taste neurons were classified into 5 types: type I which was activated by one stimulus (43% of total neurons), type II by two (35%), type III by three (12%), type IV by 4 (5%), and type V which was inhibited by certain qualities (5%). Statistical analysis revealed that responsiveness of taste neurons differed somewhat in receptive fields of the face. The taste neurons in the barbel and lip areas of the facial lobe showed variable responsiveness to chemical solutions, but those in other areas belonged generally to types I or II and did not respond to sucrose. Sodium chloride solution was the most effective among the 4 basic stimuli in this experiment. About 90% of the taste neurons also responded to mechanical stimulation of taste receptive fields. Tactile neurons in the facial lobe were localized in a topographical manner and the taste neurons were arranged in the tactile areas, but chemotopical organization was not recognized in the present study.