Raimond Emmers

Separate Relays of Tactile, Pressure, Thermal, and Gustatory Modalities in the Gat Thalamus.

Summary An analysis was made of response patterns of 60 neurons in the cat thalamus which were modified by stimulation of limited loci of the tongue with tactile, pressure, thermal, and taste stimuli. This analysis revealed the following. 1. Except for the activity elicited by tongue stimulation with ice water, responses of the neurons were modality specific. 2. Tactile stimuli activated certain thalamic neurons which were grouped in a separate cluster from those responding to pressure. Such an organization indicated the existence of separate sensory systems for touch and pressure. 3. Neurons were found which were activated by lowering of the tongue temperature within a particular range. They did not respond to ice water if the tongue was preceded to 22°C. 4. Taste is relayed in a particular nucleus of the cat thalamus. Anatomically this nucleus has been described in the human thalamus as the nucleus semilunaris accessorius. It is suggested that this term be adopted for designating a homologous nucleus in the cat thalamus. 5. Since responses to taste stimuli were conveyed to the thalamus ipsilaterally, an ipsilateral pathway should exist. The anatomical description of such a pathway awaits future work.

Thalamic mechanisms that process a temporal pulse code for pain

The sequential ordering of spikes emitted by single thalamic neurons which respond to noxious stimulation was studied using rats anesthetized with a mixture of alpha-chloralose and urethane. An electrical stimulus applied to the sciatic nerve contralateral to the thalamic recording site fired single thalamic SII neurons with a short latency spike burst and with long latency spikes which occurred at relatively fixed intervals. On repetition of stimulation, the short latency spike burst formed a high amplitude peak on sequential spike density histograms (I of Fig. 2B); the long latency spikes formed peaks of relatively low amplitude (M1, M2, M3 of Fig. 2B). Histograms of touch and light pressure relay neurons found within the thalamic SII differed conspicuously from that of Fig. 2B. Further experiments revealed that the I peak contained frequency coded information about the stimulus intensity, whereas the M peaks with their temporal relationship to the I peak coded information pertaining to a particular sensory modality. The M peaks are formed by timed firing in a positive feedback loop found between the thalamic SII and the nucleus centrum medianum-nucleus parafascicularis (CM-Pf) neurons. Consequently, the M peaks can be abolished without losing the I peak by a lesion placed in a portion of the CM-Pf complex or by the administration or morphine which is able to disorganize the timing mechanism of the feedback loop. Therefore, it is reasonably certain that the modality coded by the M peaks is pain.

Interaction of neural systems which control body water

Abstract The method of electrophysiological unit recording was used to study the interaction of neurons among the following nuclear masses: (1) the nucleus semilunaris accessorius thalami (nSA), which relays gustatory and splanchnic afferents in the thalamus, (2) the nucleus supraopticus hypothalami (nSoH) and the nucleus paraventricularis hypothalami (nPv) which are known to control water retention, and (3) the area lateralis hypothalami (ALH) and the nucleus entopeduncularis (nEp) which are implicated in the control of water intake. Neuronal activity of these nuclei was altered by intracarotid infusions of a 3% NaCl solution or distilled water, or by electrical pulses applied via a concentric stimulating electrode to the nSA or to neurons of the ALH-Ep or the nSoH. It was found (Fig.9) that neurons of the nSoH and nPv function as osmoreceptors (Fig. 9.1, 9.2) whereas those of the ALH-Ep do not. The activity of the latter depends on the interaction of tonic excitatory and inhibitory influences (Fig. 9.3, 9.4) which are relayed to the ALH-Ep neurons via the nSA. Therefore, oropharyngeal stimulation during water intake will gradually alter the activity of the ALH-Ep neurons. This function can serve as a metering device of water consumption. A reverberatory negative feedback circuit (Fig. 9.5, 9.6) provides for interaction between the two hypothalamic control systems suggesting the explanation for many interesting phenomena observed with the regulation of water balance.

Activation of neurons in the gracile nucleus by two afferent pathways in the rat

Abstract Multiunit recordings of responses to tactile stimulation of the hind leg or to electrical stimulation of the dorsal funiculi were made in the gracile nucleus of rats anesthetized with Nembutal. The responses were then abolished by discrete lesions made in the spinal cord. All lesions and recording sites were reconstructed histologically. It was found that cells of the gracile nucleus responded to stimulation of the dorsal funiculi with both a short-latency and a long-latency response. Following a lesion of the dorsal funiculi, stimulation at a site above the lesion evoked only the short-latency response, and stimulation below the lesion evoked only the long-latency response. In addition, responses to tactile stimulation of the hind leg could still be recorded in the gracile nucleus. Following a lesion of the dorsal part of the lateral funiculus in addition to the dorsal funiculus lesion, responses both to tactile stimulation of the hind leg and to electrical stimulation of the dorsal funiculi below the lesion were abolished. Further experimentation indicated that the pathway ascending in the dorsal funiculi projects mainly to the caudal portion of the gracile nucleus, whereas the pathway ascending in the dorsal part of the lateral funiculus projects mainly to the rostral portion of this nucleus. It is likely that the pathway ascending in the dorsal part of the lateral funiculus belongs to the spinocervical tract.

Dual alterations of thalamic nociceptive activity by stimulation of the periaqueductal gray matter

Abstract Effects produced by electrical stimulation of the mesencephalic periaqueductal gray matter (PAG) on single nociceptive neurons of the somesthetic thalamus were studied in rats anesthetized with α-chloralose and urethane. Stimulation of the PAG excited the nociceptive neurons, and, although the probability of firing them with a single pulse was low, trains of pulses increased their preexisting activity considerably and lastingly. Poststimulus spike density histograms of the activity evoked by stimulation of the sciatic nerve displayed several peaks which have been analyzed previously. In brief, a short-latency, high-count peak was related to the intensity of stimulation (I peak); three to four long-latency, low-count peaks were characteristic of nociception (modality, or M peaks). PAG stimulation modified mainly the M peaks. With stimulation of the caudal portion of the PAG, fusion of the M peaks was observed and persisted. With stimulation of its rostral portion each M peak was augmented. At several PAG sites, however, increasing the stimulus frequency and pulse-train duration converted the augmentation effect into fusion of the M peaks. Thalamic neurons that relay tactile afferents were not fired by stimulation of the PAG, although after such a stimulation they began to fire spontaneously. These results parallel the dual behavioral effects previously observed with PAG stimulation. When repeated trains of pulses were used, analgesia was induced that outlasted the period of stimulation. However, with stimulation of the rostral PAG, behavioral agitation and lowering of the nociceptive thresholds were observed. It is likely that augmentation of the M peaks represents central sensitization to noxious stimulation, and fusion of the M peaks represents a conversion of pain into less pain-like sensations. These effects may be mediated via a pathway that connects the PAG with the ventrobasal and the parafascicular nuclei of the thalamus.

TONIC CONTROL OF WATER INTAKE VIA THE THALAMIC TASTE NUCLEUS

This report addresses itself to ( 1 ) the origin of tonic activity in the thalamic taste nucleus, and (2) the influence of this activity on certain hypothalamic neurons that control water intake. The first topic will be elucidated by appraising the activity of single neurons in the cat thalamic taste nucleus; the second will be analyzed by describing a set of experiments in which several groups of rats demonstrated their inability to regulate water intake after their thalamic taste nuclei were destroyed.

Spinal afferents to SI and SII of the rat thalamus.

Abstract Spinal pathways projecting to thalamic SI and SII regions were delineated by by the following procedure: A site was localized stereotactically in SII which responded to tactile stimulation of the hind leg; input was then interrupted by partial ablation of the spinal cord or lower medulla; then a site in SI was localized which responded to tactile stimulation of the same peripheral field, and further ablation of the spinal cord was performed to abolish this response. In one series of experiments, localization of the hind leg region was done in SI first, and in addition, electrical stimulation of the dorsal funiculi was performed to investigate the possibility of interactions between collaterals of dorsal funicular fibers and the cells of origin of other ascending pathways. Results indicated that input to SI ascends in the dorsal funiculus, whereas input to SII ascends in the ventral quadrant (both corssed and uncrossed) and in the dorsal part of the lateral funiculus. The pathway to SI represents the classical dorsal funiculusmedial lemniscus system. Pathways ascending in the ventral quadrant appear to be equivalent to the spinothalamic tract, and the pathway ascending in the dorsal lateral funiculus appears to correspond to the spinocervical tract of the cat. Further, it was ascertained that collaterals of dorsal funicular fibers synapse with neurons which form the spinocervical tract and with those which form the spinothalamic tract.

Modifications of sensory modality codes by stimuli of graded intensity in the cat thalamus

Abstract Recently it has been demonstrated that each sensory modality of lingual afferents elaborates its own pattern of spike activity. These patterns can be regarded as modality codes. The processing of such codes takes place only with high intensity stimulation. Therefore, the question arose as to how changes in the stimulus intensity influence the coding of afferent impulses. The method of study consisted of driving single thalamic neurons by applying square-wave pulses at 1/sec to the appropriate peripheral receptive field on the tongue and recording extracellular spike activity from neurons which relayed separately touch, pressure, thermal or gustatory modalities in the thalamus. Spikes which occured during 1000 consecutive post-stimulus periods were accumulated by superimposing the moment of stimulation in a digital computer. The accumulated counts per address were typed put by a read-out printer and were plotted on a dot-plotting autograph. Results indicated that stimulus intensity was frequency-coded within the initial burst of spike activity which followed any single stimulus. As the stimulation was intensified, the latency of the response decreased and the number of spikes which formed the first burst or cluster of activity increased. Spikes which formed the second and the third clusters did not elaborate a frequency code, although shifts in their latencies and changes in their range of scatter occured. Apparently, the first spike cluster in any of the four sensory codes process information about stimulus intensity, the late clusters pertain to modality coding. Some interaction between the frequency and modality codes does exist, however, suggesting the neural mechanism for observations regarding altered perception of stimulus quality with a change in stimulus intensity.

Role of thalamic gustatory nucleus in diet selection by normal and parathyroidectomized rats.

Summary Young male Sherman rats were allowed to select individual components of their diet from 9 sources. After the selection of dietary items became relatively stable, a group of the animals was parathyroidectomized to intensify their need for one dietary component, calcium, while the rest of the animals served as unoperated controls. The role of thalamic gustatory neurons in mediating the response to dietary needs was then determined by destroying the thalamic gustatory nucleus in a certain number of the Ptx and the unoperated animals. Thalamie lesions which spared the gustatory neurons were also made to provide for adequate controls. If the thalamic lesions destroyed gustatory neurons in Ptx rats, the animals significantly reduced calcium intake. This reduction was sufficiently large to cause tetany and death in some of the animals. Changes in the consumption of sodium dibasic phosphate, sodium chloride, and distilled water also occurred. Destruction of the thalamic gustatory nucleus in previously normal rats did not diminish calcium intake significantly but the consumption of this item as well as other components of the diet became highly variable. These findings indicate that the gustatory system usually exerts a profound influence on the selection of diet not only under special circumstances but also in normal rats.

Changes in thalamic nociception resulting from morphine- and meperidine-dependence in rats.

Rats were injected with progressively increasing doses of morphine or meperidine during a period of 3 to 40 days. From this colony of animals individual rats were used at 3- to 4-day intervals for electrophysiologic experiments to analyze the activity of nociceptive neurons in the somesthetic thalamus. After an i.p. injection of chloralose-urethane and the appropriate preparation for a stereotaxic microelectrode penetration of the thalamus, a nociceptive neuron was identified in the nucleus ventralis posterolateralis by its unique spacing of spike potentials emitted in response to pricking the foot with a pin. In addition to the short-latency response that formed a high activity peak on poststimulus time histograms, spikes following the stimulus up to 500 ms also formed activity peaks. Single-pulse stimulation of the sciatic nerve evoked the same response as pinpricks, but innocuous stimuli (pin shielded with a piece of cork) evoked a response without the late activity peaks. Only neurons that exhibited this differential response were regarded as nociceptive. Their response and spontaneous activity were accumulated separately on a digital computer. Following this, naloxone was infused i.v. and the computer accumulations were repeated. It was found that during naloxone-precipitated narcotic withdrawal, innocuous stimuli evoked responses indicative of pain; the nociceptive system was sensitized. Furthermore, a small dose or morphine or meperidine heightened the sensitization. This action of the narcotic agents was reversed by 5-hydroxytryptophan, which assisted the narcotics in suppressing pain in morphine- or meperidine-dependent rats but had no demonstrable effect in control animals. The spontaneous tonic activity of the nociceptive neurons of the somesthetic thalamus was high in rats exhibiting narcotic dependence. Naloxone decreased the count, but not to the value of the control animals. The sensitization of nociception can be explained by a decreased action of a neural pathway that descends from the periaqueductal gray matter via the nucleus raphe magnus to the spinal cord and there blocks the excitation of the spinothalamic tract cells by A-delta and C fibers. The mechanisms that increase the spontaneous activity of the thalamic nociceptive neurons remain unclear.