Reactions of neurons of the reticular and ventral anterior thalamic nuclei to electrical stimulation of the centrum medianum of the thalamus

Responses of 145 reticular (R) and 158 ventral anterior (VA) thalamic neurons to electrical stimulation of centrum medianum (CM) were studied in cats anaesthetized with thiopental sodium (30-40 mg/kg intraperitoneally) and immobilized with d-tubocurarine (1 mg/kg). 4.1% of R and 4.4% of VA neurons under study responded to CM stimulation by antidromic spike (latency 0.3-2.0 ms). The conduction velocity of antidromic excitation in axons of those neurons was found to be 1.7-7.6 m/s. There were neurons which responded by antidromic spike to the other thalamic nuclei stimulation as well as to CM. This fact is the electrophysiological proof of the axonal branching in these neurons. 53.8% of R and 46.9% of VA neurons responded to CM stimulation with orthodromic excitation. Two groups of cells were separated among neurons excited orthodromically. The first group neurons responded to CM stimulation by discharges composed of 6-12 spikes with frequency of 130-640 per second. The neurons of the second group generated a single spike. Inhibitory reactions were noticed only in 0.7% of R and in 4.4% of VA neurons. It is shown that afferent impulses from relay nuclei, lateral posterior nucleus and motor cortex converged to some R and VA neurons responding to CM.     

Reactions of neurons in the primary and secondary somatosensory zones of the cortex to stimulation of the ventral posterior nucleus of the thalamus]

Neuronal responses in the first and second somatosensory cortex (SI and SII) to stimulation of the ventroposterior nucleus of the thalamus (VP) were studied in experiments on cats immobilized with d-tubocurarine. 12.0% responding neurons in SI and 9.5% in SII were activated antidromically by VP stimulation. In the majority of antidromic responses the latencies did not exceed 1.0 ms. The minimal latency of orthodromic spikes was 1.5 ms in SI and 1.7 ms in SII. In SI the number of neurons whose orthodromic spike latencies did not exceed 3.0 ms was larger than neurons activated with latencies of 3.1-4.5 ms. In SII an inverse quantitative relationship between those two neuronal groups was observed. In SII a significantly larger number of neurons was excited with latencies of EPSPs ranged between 1.1-9.0 ms in SI and between 1.4-6.6 ms in SII and the latencies of IPSPs between 1.5-6.8 ms in SI and 2.2-9.4 ms in SII. The importance of different pathways for excitatory and inhibitory VP influences to neurons of SI and SII is discussed.     

Neuronal activity of cat thalamus reticular and relay nuclei

The interaction between the unit activity of the thalamic reticular and relay nuclei was investigated on cats immobilized with tubarine. It was shown that unit activity of the relay nuclei can be significantly modulated by stimulation of the n. reticul. By simultaneous recording (with two microelectrodes) of the activity of thalamic reticular neurons the relay unit activity can be suppressed; during peripheral and cortical stimulations the alternation of excitation-inhibition of the thalamic reticular and relay units can be observed; excitation of thalamic reticular units can be associated with excitation of the relay nuclei units. Such types of interaction may also exist during simultaneous recording (by two microelectrodes) of the activity of the relay units and putative interneurons. Sometimes IPSPs with short latency (1 ms) were evoked in the thalamic relay nuclei during thalamic reticular stimulation. The existence of direct monosynaptic inhibition of the relay unit activity by the thalamic reticular units as well as inhibition by the activation of the putative interneurons of the specific thalamic nuclei is suggested.     

Distribution of cerebellothalamic neurons projecting to the ventral nuclei of the thalamus: An HRP study in the cat

Distribution of cerebellothalamic neurons projecting to the ventral nuclei of the thalamus was examined in the cat, using the horseradish peroxidase (HRP) method. After injections of HRP within the lateral or ventrolateral portions of the ventroanterior and ventrolateral nuclear complex of the thalamus (VA-VL), neurons labeled retrogradely with HRP were seen contralaterally in the cerebellar nuclei; many of them were situated in the nucleus interpositus anterior and nucleus interpositus posterior, and a moderate number of them were located in the nucleus lateralis. Labeled neurons in the nucleus interpositus posterior were observed mainly in the medial and ventral portions of the nucleus. On the side ipsilateral to the injections, a few labeled neurons were seen in the nucleus interpositus anterior, nucleus interpositus posterior, and nucleus lateralis. Virtually no labeled neurons were found in the nucleus medialis of the cerebellum. After HRP injections into the medial or dorsomedial portions of the VA-VL, many labeled neurons were found contralaterally in the ventral and ventrolateral portions of the nucleus interpositus posterior, as well as in the nucleus lateralis, especially in its ventral and lateral portions. On the side ipsilateral to the injections, labeled neurons in the nucleus lateralis and nucleus interpositus posterior were small in number. In the nucleus medialis only a few labeled neurons were found bilaterally in the caudal levels of the nucleus. After HRP injections centered on the ventromedial nucleus of the thalamus, many labeled neurons occurred bilaterally in the caudal portions of the nucleus medialis, with a slight contralateral preponderance, and contralaterally in the lateral and ventral portions of the nucleus lateralis. A few labeled neurons were also seen contralaterally in the ventrolateral and lateral portions of the nucleus interpositus posterior, and ipsilaterally in the nucleus lateralis.     

Reactions of neostriatal neurons to direct electric stimulation of the optic tract and to photic stimulation in the alert cat

Responses of 98 cells in the head and body of caudate nucleus to direct electric stimulation of the optic tract and to visual stimuli were recorded extracellularly in awake chronic cats. 34.6 and 36.2% of the studied cells, respectively, have responded to these types of stimulation. Long-latency (over 40 ms for optic tract and over 80 ms for visual stimulations) excitatory responses prevailed in both cases. Small number of units responded to optic tract stimulation with short (5-14 ms) latencies. Eight of 58 tested cells were activated by both electric and visual stimuli. In this case, responses of the same cells could differ (relatively to the type of applied stimulus) either in pattern or sign. These data are discussed relative to the possible pathways for transfer of the visual information to non-specific brain structures.     

The percentage of projection neurons and local circuit neurons in different nuclei of the human thalamus

A characteristic lipofuscin pigmentation permits reliable distinction of the various neuronal types forming thalamic nuclei of the human adult. Type I projection cells with coarse and vacuolated lipofuscin granules are distinguished from type II local circuit neurons with fine and intensely stained pigment and type III neurons devoid of pigment. There is by no means a uniform ratio of projection neurons versus local circuit neurons. A high proportion of local circuit neurons is found in nuclei of the anterior complex, a medium proportion in the specific nuclei of the lateral complex, and a low proportion in the reticular nucleus. The technique and the data provided are being used as a basis for investigations of the diseased human brain.     

Prolonged unit responses in thalamic reticular, ventral, and posterior nuclei following lateral hypothalamic and midbrain reticular stimulation.

In unanesthetized postcollicular cerveau isole rats, prolonged responses over 10 sec periods following intracranial stimulation (0.2 sec, 100 Hz, 0.5 msec. 600 muA cathodal pulses) were studied with extracellular recordings of 150 nucleus reticularis and 122 ventral and posterior thalamic units. After medial forebrain bundle (MFB) or midbrain reticular (RET) stimulation, most nucleus reticularis units showed significantly decreased firing, and the MFB and RET effects converged on 64%. None was excited. In ventral and posterior thalamus, however, long-lasting unit responses were mixed. MFB and RET stimuli elicited significantly increased firing in 17% and 20% of the units, respectively, and decreased firing in 21% and 16%, respectively. Convergence of MFB- and RET-elicited responses occurred in 25% of the cells, but none showed convergence of opposite responses which might reflect believed motivational differences between the stimuli. Similar stimuli delivered to ventral medial thalamus evoked pronounced excitation of nucleus reticularis discharge, contrasting the MFB and RET effects and confirming previous reports by others. The substantial responses elicited by MFB and RET stimuli in nucleus reticularis fulfill previously proposed criteria for definition of an arousal dimension in terms of single cell activity. The similarity of MFB and RET effects in the posterior nucleus of thalamus raises questions regarding its role in nociception.     

Reactions of medial geniculate body neurons to stimulation of the auditory cortex]

Extra- and intracellular responses of pars principalis neurons in the medial geniculate body to stimulation of the first (AI), second (AII) and third (AIII) auditory cortex were studied in experiments on cats immobilized with d-tubocurarine. In geniculate neurons both antidromic (45-50%) and orthodromic (50-55%) reactions occurred in response to the auditory cortex stimulation. The latencies for antidromic and orthodromic responses were 0.3-2.5 ms and 2.0-ms, respectively. Late responses appeared with a latency of 30-200 ms. 63% of neurons responded antidromically to both AII and AI stimulation, that confirms the suggestion on the projection of a considerable number of the geniculate neurons to both auditory zones. Orthodromic responses of geniculate neurons consisted either of 1-2 spikes or a burst of 8-12 spikes with a frequency of 300-600/sec. The bursts are supposed to be the responses of inhibitory geniculate neurons. Intracellular recording showed the following responses: antidromic spikes, EPSP, EPSP-spike, EPSP-spike-IPSP, EPSP-IPSP and initial IPSP. Above 50% of initial IPSPs had the latency of 2.0-4.0 ms. They are supposed to be produced with the participation of intermediate inhibitory neurons located in the medial geniculate body.     

Interaction of heterosensory afferent impulses in the neurons of the thalamic associative nuclei]

The interaction of visual, auditory and cutaneous afferent signals on neurons of pulvinar (Pulv), posterior lateral (LP) and mediodorsal (MD) thalamic nuclei was studied in acute experiments on cats anaesthetized with a mixture of nembutal and chloralorse. The first (more frequent) type of the interaction was characterized by inhibition or inhibition followed by facilitation of the response; the second--by facilitation or facilitation followed by inhibition of neuronal activity. The capacity for interaction in MD neurons was less expressed than in cells of the Pulv-LP complex (P less than 0.05). The interaction could be better seen in three-sensory elements. Certain peculiarities were found in the interaction of afferent signals coming from different peripheral receptive areas in neurons of both structures.     

Localization of neurons afferent to the optic tectum in longnose gars

Afferent pathways to the optic tectum in the longnose gar were determined by unilateral tectal injections of HRP. Retrogradely labeled cells were observed in the ipsilateral caudal portion of the rostral entopeduncular nucleus and bilaterally in the rostral half of the lateral zone of area dorsalis of the telencephalon. The following diencephalic cell groups were also labeled following tectal injections: the ipsilateral anterior, ventrolateral, and ventromedial thalamic nuclei, the periventricular pretectal nucleus, and the central pretectal nucleus (bilaterally); the ventromedial thalamic and central pretectal nuclei revealed the largest number of labeled cells. At midbrain levels, retrogradely labeled cells were seen in the ipsilateral torus longitudinalis, nucleus isthmi, and accessory optic nucleus; cells were labeled bilaterally in the torus semicircularis and a rostral tegmental nucleus. Only a few cells were labeled in the contralateral optic tectum, suggesting that few of the fibers of the intertectal commissure are actually commissural to the tectum. At hindbrain levels, retrogradely labeled cells were seen bilaterally in the locus coeruleus, the superior, medial, and inferior reticular formations, the eurydendroid cells of the cerebellum, and the nucleus of the descending trigeminal tract; the contralateral dorsal funicular nucleus also exhibited labeling. Clearly, the tectum in gars receives a substantial number of nonvisual afferents from all major brain areas, most of which have been reported in other vertebrates. The functional significance of these afferent sources and their probable homologues in other vertebrate groups are discussed.     

Responses of feline medial medullary reticular formation neurons with projections to the C5-C6 ventral horn to vestibular stimulation

Prior studies have shown that the vestibular system contributes to adjusting respiratory muscle activity during changes in posture, and have suggested that portions of the medial medullary reticular formation (MRF) participate in generating vestibulo-respiratory responses. However, there was previously no direct evidence to demonstrate that cells in the MRF relay vestibular signals monosynaptically to respiratory motoneurons. The present study tested the hypothesis that the firing of MRF neurons whose axons could be antidromically activated from the vicinity of diaphragm motoneurons was modulated by whole-body rotations in vertical planes that stimulated vestibular receptors, as well as by electrical current pulses delivered to the vestibular nerve. In total, 171 MRF neurons that projected to the C5-C6 ventral horn were studied; they had a conduction velocity of 34+/-15 (standard deviation) m/sec. Most (135/171 or 79%) of these MRF neurons lacked spontaneous firing. Of the subpopulation of units with spontaneous discharges, only 3 of 20 cells responded to vertical rotations up to 10 degrees in amplitude, whereas the activity of 8 of 14 neurons was affected by electrical stimulation of the vestibular nerve. These data support the hypothesis that the MRF participates in generating vestibulo-respiratory responses, but also suggest that some neurons in this region have other functions.     

Reactions of cat caudate nucleus neurons to cortical stimulation before and after destruction of nonspecific thalamic nuclei

In experiments on immobilized cats extracellular responses of caudate nucleus neurons were recorded during electrical stimulation of anterior sigmoideus and suprasylvius gyri. In the first set of experiments performed on cats with an intact thalamus the responses to stimulation of both cortical areas represented initial excitation (sometimes absent) followed by inhibition and late activation. Some qualitative and quantitative differences in the neuronal responses to stimulation of these cortical areas were found. The second set of experiments was performed on cats with preliminary destroyed unspecific thalamic nuclei. Changes were observed only in time course of late caudate neuronal responses.