Brain stem reticular units: some properties of the course and origin of the ascending trajectory

Cortico-cortical evoked potentials and stimulation-elicited gamma activity preferentially propagate from lower- to higher-order visual areas

OBJECTIVE

The lower-order visual cortex in the medial-occipital region is suggested to send feed-forward signals to the higher-order visual cortex including ventral-occipital-temporal and dorsal-occipital regions. We determined how stimulation-elicited cortical-signals propagate between lower- and higher-order visual cortices, and whether the magnitudes of stimulation-elicited cortical-signals recorded in the higher-order visual cortex differed from those recorded in the lower-order one.

METHODS

We studied 10 patients with focal epilepsy who underwent extraoperative electrocorticography recording. Trains of 1-Hz stimuli with an intensity of 3 mA were delivered to an electrode pair within the medial-occipital region; then, cortico-cortical evoked-potential (CCEP) and stimulation-elicited gamma-activity at 80-150 Hz were measured in the ventral-occipital-temporal and dorsal-occipital regions. Likewise, CCEP and stimulation-elicited gamma-activity, driven by stimuli within the higher-order visual cortex, were measured in the lower-order visual cortex.

RESULTS

CCEPs generated, via feed-forward propagations, in the higher-order visual cortex were significantly larger than those generated, via feed-back propagations, in the lower-order visual cortex. Stimulation of the lower-order visual cortex elicited augmentation of gamma-activity in the higher-order visual cortex after the preceding CCEP subsided.

CONCLUSION

The propagation manners of stimulation-elicited cortical-signals differ between feed-forward and feed-back directions in the human occipital lobe.

SIGNIFICANCE

: Such difference may need to be taken into consideration for future clinical application of CCEPs and stimulation-elicited gamma-augmentation in presurgical evaluation for epilepsy surgery.

Subthreshold auditory inputs to extrastriate visual neurons are responsive to parametric changes in stimulus quality: sensory-specific versus non-specific coding

A new subthreshold form of multisensory processing has been recently identified that results from the convergence of suprathreshold excitatory inputs from one modality with subthreshold inputs from another. Because of the subthreshold nature of the second modality, descriptive measures of sensory features such as receptive field properties or location are not directly apparent as they are for traditional bimodal neurons. This raises the question of whether or not subthreshold signals actually convey sensory-specific receptive field information as seen in their bimodal counterparts, or if they represent non-specific effects such as arousal. The present experiment addressed this issue in visually-responsive neurons from the cat posterolateral lateral suprasylvian cortex (PLLS). Single-unit electrophysiological techniques were used to record neuronal responses to visual, auditory and combined visual-auditory stimuli while the intensity of stimulation in the subthreshold auditory modality was systematically altered. The results showed that subthreshold multisensory neurons were sensitive to changes in auditory stimulus intensity. These receptive field sensitivities are similar to those observed in bimodal neurons and thereby represent sensory-specific, not arousal-related responses. In addition, these results provide further support for the notion that multisensory processing occurs along a dynamic continuum of neuronal convergence patterns from bimodal to purely sensory-specific.

Physiological characterization, localization and synaptic inputs of bursting and nonbursting neurons in the trigeminal principal sensory nucleus of the rat

A population of neurons in the trigeminal principal sensory nucleus (NVsnpr) fire rhythmically during fictive mastication induced in the in vivo rabbit. To elucidate whether these neurons form part of the central pattern generator (CPG) for mastication, we performed intracellular recordings in brainstem slices taken from young rats. Two cell types were defined, nonbursting (63%) and bursting (37%). In response to membrane depolarization, bursting cells, which dominated in the dorsal part of the NVsnpr, fired an initial burst followed by single spikes or recurring bursts. Non-bursting neurons, scattered throughout the nucleus, fired single action potentials. Microstimulation applied to the trigeminal motor nucleus (NVmt), the reticular border zone surrounding the NVmt, the parvocellular reticular formation or the nucleus reticularis pontis caudalis (NPontc) elicited a postsynaptic potential in 81% of the neurons tested for synaptic inputs. Responses obtained were predominately excitatory and sensitive to glutamatergic antagonists DNQX and/or APV. Some inhibitory and biphasic responses were also evoked. Bicuculline methiodide or strychnine blocked the IPSPs indicating that they were mediated by GABA(A) or glycinergic receptors. About one-third of the stimulations activated both types of neurons antidromically, mostly from the masseteric motoneuron pool of NVmt and dorsal part of NPontc. In conclusion, our new findings show that some neurons in the dorsal NVsnpr display both firing properties and axonal connections which support the hypothesis that they may participate in masticatory pattern generation. Thus, the present data provide an extended basis for further studies on the organization of the masticatory CPG network.

Alz-50 immunoreactivity in the thalamic reticular nucleus in Alzheimer's disease

Examination of the thalamic reticular nucleus (Rt) with the monoclonal antibody Alz-50 in brains of Alzheimer's disease patients reveals dense extracellular and terminal-like immunoreactivity in the absence of neurofibrillary tangles or neuritic plaques. Similar terminal-like immunoreactivity is not present in other thalamic nuclei of AD brains or in the brains of controls. Based on (1) an immunocytochemical and histopathological analysis of areas known to project to the Rt, (2) that Alz-50 immunocytochemistry reveals immunoreactive neurons, neurofibrillary tangles and neuritic plaques, and (3) evidence that Alz-50 immunoreactivity can be demonstrated in the terminal fields of immunoreactive neurons, the terminal-like immunoreactivity in the Rt probably corresponds to altered preterminal axons and terminals from degenerating basal forebrain neurons. Given the presumed physiological role of the Rt, these selective lesions could alter thalamocortical processing and contribute to the cognitive impairment in Alzheimer's disease.

Unitary characteristics of presumptive cholinergic tegmental neurons during the sleep-waking cycle in freely moving cats

A total of 260 neurons were recorded in the rostral pontine tegmentum of freely moving cats during the sleep-waking cycle. Of these, 207 neurons (80%) were located in the dorsal pontine tegmentum containing monoaminergic and choline acetyltransferase (ChAT)-immunoreactive, or cholinergic neurons. In addition to presumably monoaminergic PS-off cells (n = 51) showing a cessation of discharge during paradoxical sleep (PS) and presumably cholinergic PGO-on cells (n = 40) exhibiting a burst of discharge just prior to and during ponto-geniculo-occipital (PGO) waves, we observed tonic (n = 108) and phasic (n = 61) neurons exhibiting, respectively, tonic and phasic patterns of discharge during wakefulness and/or paradoxical sleep. Of 87 tonic cells histologically localized in the dorsal pontine tegmentum rich in cholinergic neurons, 46 cells (53%) were identified as giving rise to ascending projections either to the intralaminar thalamic complex (n = 26) or to the ventrolateral posterior hypothalamus (n = 13) or to both (n = 9). Two types of tonic neurons were distinguished: 1) tonic type I neurons (n = 28), showing a tonic pattern and high rates of discharge during both waking and paradoxical sleep as compared with slow wave sleep; and 2) tonic type II neurons (n = 20), exhibiting a tonic pattern of discharge highly specific to the periods of paradoxical sleep. Tonic type I neurons were further divided into two subclasses on the basis of discharge rates during waking: a) rapid (Type I-R; n = 17); and b) slow (Type I-S; n = 11) units with a discharge frequency of more than 12 spikes/s or less than 5 spikes/s, respectively. Like monoaminergic PS-off and cholinergic PGO-on cells, both tonic type II and type I-S cells were characterized by a long spike duration (median: 3.3 and 3.5 ms), as well as by a slow conduction velocity (median = 1.8 and 1.7 m/s). In the light of these data, we discuss the possible cholinergic nature and functional significance of these ascending tonic neurons in the generation of neocortical electroencephalographic desynchronization occurring during waking and paradoxical sleep.

Locus coeruleus neurons projecting to the forebrain and the spinal cord in the cat

The intranuclear organization of the cat locus coeruleus neurons was investigated anatomo-physiologically. The locus coeruleus neurons project to the forebrain through the dorsal noradrenergic bundle and to the spinal cord. Horseradish peroxidase, a retrograde tracer, was pressure-injected into either the dorsal noradrenergic bundle or the ventrolateral funiculus of the high cervical cord (C1-C2). The cats (n = 12) were killed after a 2- or 3-day survival period. The frontal sections (100 micron) throughout the locus coeruleus were observed by light microscope after carrying out the diaminobenzidine reaction. The labeled locus coeruleus neurons were located predominantly in the rostral locus coeruleus proper and locus coeruleus alpha when horseradish peroxidase was injected into the dorsal noradrenergic bundle, whereas they were predominantly located in the caudal locus coeruleus alpha and subcoeruleus when horseradish peroxidase was injected into the spinal cord. In the electrophysiological experiments, cats (n = 30) were anesthetized with alpha-chloralose and two stimulating electrodes were placed stereotaxically in the dorsal noradrenergic bundle and the ipsilateral ventrolateral funiculus of the high cervical cord (C1-C2), respectively. Monophasic square-wave pulses (2.5 Hz, 100 microsecond duration, 800 microA) were delivered. A recording glass electrode, filled with 2 M NaCl saturated with Fast Green, was placed in the locus coeruleus. Neurons with different conduction velocities, which were evoked by the antidromic stimulation of the dorsal noradrenergic bundle and spinal cord, were verified in the locus coeruleus and the adjacent areas. The slow conductive neurons with a conduction velocity of less than 1 m/s had a slow firing rate (1.6 +/- 0.9/s). They were located predominantly in the rostral locus coeruleus proper and locus coeruleus alpha by the dorsal noradrenergic bundle stimulation. From the anatomical and electrophysiological experimental results, it was concluded that the conduction velocities of the horseradish peroxidase-labeled neurons observed in locus coeruleus proper and locus coeruleus alpha were mostly slow and less than 1 m/s. Most of the slow conductive neurons were considered to be noradrenergic. Neurons evoked antidromically by both the dorsal noradrenergic bundle and spinal cord stimulation were not observed.

Influence of rostral and caudal brain stem reticular formation on thalamic neurons

Single neuronal activity was recorded from the diffuse thalamic system. Influence of the rostral desynchronizing and caudal synchronizing structures of the brain stem reticular formation on these neurons was studied. Rostral stimulation produced an increase and caudal stimulation a decrease in the thalamic unit firing. A possible mechanism by which the brain stem reticular structures influence the cortical neurons is proposed on the basis of these findings.

Electrophysiological analysis of efferent neurons of cat associative parietal cortex

We have studied in acute experiments the neurons of the associative parietal cortex in the cat, using the microelectrode take-off technique. We identified the efferent neurons sending axons to the sensomotor cortex, the red nucleus, and the pontine nuclei by antidromic stimulation. We investigated the collateral branching of axons of neurons projected simultaneously into two of the formations mentioned, using the impulse collision technique. We studied the characteristics of the spatial distribution of efferent neurons in the parietal cortex.

Physiological studies of brainstem reticular connectivity. II. Responses of mPRF neurons to stimulation of mesencephalic and contralateral pontine reticular formation

The connectivity between medial pontine reticular formation (mPRF) and the contralateral mPRF and between mPRF and the mesencephalic reticular formation (MRF) was studied by intracellular recordings of mPRF neuronal responses to microstimulation of the contralateral gigantocellular field (cFTG) portion of mPRF and ipsilateral MRF in unanesthetized, undrugged cats. There was a very high percentage (75-86%) of monosynaptic latency postsynaptic potentials (PSPs) in mPRF neurons in response to microstimulation of cFTG and MRF, and most PSPs (72-82%) were excitatory ones (EPSPs). The initial EPSPs from cFTG stimulation were characterized by a rapid rise time and a relatively constant latency, while those from MRF had a less rapid rise time and a longer plateau; EPSPs from both sites frequently led to spike potential generation. In contrast, the percentage of initial monosynaptic inhibitory PSPs (EPSPs) was less than 4% from each of these regions, statistically significantly less than that from bulbar FTM and bulbar FTG stimulation (about 12%) reported in the companion paper. Injection of depolarizing current in mPRF neurons unmasked hyperpolarizing PSP responses to stimulation that followed initial depolarizing PSPs. Intracellular HRP labeling indicated that these data were from recordings from neurons with 20-100 microns diameters, with 80% greater than 40 microns. Neurons with a different discharge pattern for this area of the pons, a stereotyped burst pattern, were recorded just ventral to mPRF; this discharge pattern resembled that found in inhibitory interneurons in other central nervous system regions. There were no differences in the density and pattern of orthodromic PSPs between those mPRF neurons that were antidromically activated from cFTG and the general population that was not antidromically activated from cFTG or other stimulated sites; this suggests, when combined with data of the companion paper, an identity of input and output elements in mPRF with respect to synaptic response properties. The high degree of connectivity between reticular regions may furnish a substrate for functional interaction.

Physiological studies of brainstem reticular connectivity. I. Responses of mPRF neurons to stimulation of bulbar reticular formation

The connectivity between medial pontine reticular formation (mPRF) and bulbar reticular formation (BRF) was studied by intracellular recordings of mPRF neuronal responses to microstimulation of BRF in unanesthetized, undrugged cats. There was a very high percentage (75-90%) of monosynaptic latency postsynaptic potentials (PSPs) in mPRF neurons in response to microstimulation of 3 BRF areas: the magnocellular tegmental field (FTM), the bulbar gigantocellular tegmental field (BFTG), and bulbar lateral tegmental field (BFTL). The type of initial orthodromic response produced in mPRF neurons by BRF stimulation was predominantly (75-95%) a monosynaptic excitatory PSP (EPSP) which was characterized by a rapid rise time, a nearly constant latency, and often led to spike potential generation. In contrast, the percentage of initial monosynaptic inhibitory PSPs (IPSPs) was much lower for FTM (12.3%), for BFTG (12.5%) and was zero for BFTL. While microstimulation techniques alone cannot differentiate between excitation of fibers of passage and neuronal somata, the very high percentage of initial EPSPs in our data and the anatomical evidence for dense BRF to mPRF neuronal projections as compared with less dense projections from fibers passing through BRF to mPRF suggest that excitatory BRF-mPRF connections are predominant. The high degree of connectivity between BRF and mPRF may furnish an important substrate for functional interaction. Comparison of the mPRF neuronal population that was not antidromically activated by FTM microstimulation vs the mPRF neuronal population that was antidromically activated from FTM and also studied for orthodromic responsiveness showed no statistically significant differences between these populations on the parameters of percentage of monosynaptic input, monosynaptic initial EPSPs, monosynaptic initial IPSPs and presence of a PSP with a latency of less than 5 ms. For BRF connectivity this suggests an identity of mPRF input and output neurons with respect to synaptic response properties.

Inhibition of dorsal-horn cell responses by stimulation of the Kölliker-Fuse nucleus

The Kölliker-Fuse nucleus (KF) in the dorsolateral pons has been shown to be the major source of catecholamine innervation of the spinal cord. This has important implications in terms of pain control mechanisms, since catecholamine-mediated mechanisms are essential for the expression of opiate and other varieties of antinociception. This study examines the effects of KF stimulation on responses of dorsal-horn cells to innocuous and noxious cutaneous stimuli in anesthetized cats. Stimulation of the KF potently inhibits the responses of dorsal-horn cells to both noxious and innocuous stimuli. The threshold for the inhibitory effect is significantly lower for responses to noxious stimuli as opposed to innocuous stimuli. The inhibitory effect is specific to the stimulus site, as evidenced by a marked decrease in the effect following small changes in the position of the stimulating electrode in the brain stem. The latency of the effects indicates a bulbospinal conduction velocity of 4 to 5 m/sec, which is much slower than usual reticulospinal effects and is consistent with a catecholamine-mediated system. The dependence of KF-spinal inhibition on intact biogenic amines was tested by depleting the animals of these amines with reserpine pretreatment. Depletion of biogenic amines resulted in a significant decrease in the KF spinal inhibitory effects, suggesting their dependence on intact noradrenergic stores. The results of these studies are consistent with the idea that the KF-spinal system plays an important noradrenergic-dependent role in the brain-stem modulation of spinal processing of noxious, potentially painful stimuli.

Reticulo-olivary circuits: an intracellular HRP study in the rat

Intracellular recordings were obtained from medullary reticular neurons subsequent to electrical stimulation of the ipsilateral or contralateral inferior cerebellar peduncle (ICP) and/or the midbrain. After recording physiological data, the neurons were intracellularly injected with horseradish peroxidase (HRP). Thirty-four HRP filled neurons were subjected to light microscopic analysis. They could be divided into two general groups: those which extend dendritic processes into the neuropil of the inferior olivary complex (n = 19); and those that have no anatomical relationship to the inferior olive (n = 15). These two populations of reticular neurons differ in their distribution, morphological characteristics and physiological responses. Neurons which extend dendritic processes into the inferior olive are located within 200 microns of the dorsal border of this nuclear complex, between the exiting fibers of the XIIth nerve and the raphe. The cell bodies are located in the nucleus reticularis gigantocellularis and are fusiform or multipolar in shape. Their dendrites extend for long distances in the mediolateral direction; are thin and relatively spine-free except at their distal tips where spines and varicose appendages are evident. Physiologically, midbrain stimulation elicits a fast rising hyperpolarization which is identified as an inhibitory postsynaptic potential. However, only rarely is a response observed subsequent to stimulation of either the ipsilateral or contralateral ICP. Dendrites from 4 neurons from the first group of reticular cells were analyzed at the ultrastructural level. Based on random and serial thin sections, the following features were noted: they contain numerous mitochondria when compared to olivary dendrites; they contribute to the postsynaptic elements within olivary synaptic clusters (glomeruli); and they exhibit focal clusters of synaptic vesicles although conventional synaptic complexes have not been observed. Reticular neurons of the second group, those that do not extend dendritic processes into the inferior olive, are located either lateral to the XIIth nerve or at distances greater than 200 microns from the dorsal border of the inferior olivary complex. Their cell bodies and dendrites are comparable morphologically to the reticular neurons whose dendrites do arborize in the inferior olive. However, rarely are the distal tips of their dendrites characterized by spines or varicose appendages. Physiologically, this population of reticular neurons respond to midbrain stimulation with a low amplitude, short latency depolarizing potential which is interrupted by a hyperpolarizing potential.(ABSTRACT TRUNCATED AT 400 WORDS)

Ascending reticular activating system in the rat: a 2-deoxyglucose study

The autoradiographic [14C]2-deoxyglucose (2-DG) method was used to map ascending pathways which are influenced by arousing electrical stimulation of the midbrain reticular formation (RET) in alert rats. The major finding was that RET stimulation produces selective patterns of metabolic activation and suppression in discrete brain regions. The regions activated were limited to specific intralaminar, medial and anterior thalamic nuclei, and to the entire auditory system. Conversely, a large suppression of 2-DG uptake was observed in most of the cerebral cortex, limbic and extrapyramidal structures, whereas at the same time some brain regions were left unaffected. Striking similarities were found between the functional pathways affected differentially by RET stimulation and well-defined cholinergic pathways which originate in the midbrain tegmentum. Structures which showed metabolic activation are part of the dorsal cholinergic pathway, whereas the regions suppressed are part of the ventral cholinergic pathway and its higher-order projections. The results support the conclusion that cholinergic pathways represent the thalamic and extrathalamic divisions of the reticular activating system. Our observations provide the first anatomical demonstration that RET stimulation has widespread and differential effects on cerebral metabolism. They also support the concept that arousing electrocortical desynchronization involves reticular activation of thalamocortical neurons, which in turn have widespread suppressive influences on cortical metabolism.

Thalamic projections of nucleus reticularis thalami of cat: a study using retrograde transport of horseradish peroxidase and fluorescent tracers

The retrograde transport of horseradish peroxidase (HRP) and fluorescent tracers after injections in various thalamic nuclei was used to investigate the relative density of retrogradely labeled cells in different districts of reticularis thalami (RE) nuclear complex of cat. The RE nucleus was left virtually free of labeling only after injections localized into the anterior nuclear group; in those experiments, heavy retrograde labeling was obtained in mammillary nuclei. The major targets of RE cells proved to be centralis lateralis-paracentralis (CL-PC) and centrum medianum-parafascicularis (CM-PF) intralaminar nuclei. The projections to various intralaminar nuclei mainly arise in the rostral pole and rostrolateral part of RE nucleus and are reciprocal to intralaminar-RE pathways disclosed by Jones ('75). The RE territories labeled following injections in relay and associational nuclei are more restricted and are located contiguously and slightly anteriorly to a given nucleus. There was a very small proportion of doubly labeled RE cells after injections with fluorescent tracers in different nuclei. This was not due to a technical failure since many double-labeled neurons were found in the same material in medial globus pallidum after thalamic and midbrain injections (see companion paper by Parent and Steriade, '84). We conclude that most individual RE axons arborize in only one thalamic nucleus or nuclear group. An additional finding was the existence of intralaminar-to-relay (CL-PC to VA-VL) projections.

Depth evoked potential and single unit correlates of vertex midlatency auditory evoked responses

The rostral brainstem and thalamus of awake cats were studied for depth correlates of a surface-recorded midlatency auditory evoked potential, wave "A', with a latency range of 17-25 ms. In response to clicks or pure tones, midlatency potentials were recorded from the level of the cuneiform nucleus (14-15 ms latencies) forward through the medial tegmentum to the level of the intralaminar thalamic nuclei centralis lateralis (CL) and center median (CM) (17-19 ms latencies). While this medial projection system to the thalamus involved primarily CL and CM, slightly longer latency responses were also found in nucleus ventralis anterior (VA) and ventralis lateralis (VL). A ventral diencephalic response was characterized by latencies averaging 0.5-1.2 ms less than those from the dorsal thalamic regions. Both surface and depth midlatency potentials showed comparable sensitivity to rate of stimulation. At click rates above 1/s, peak amplitudes diminished, and for rates greater than 10/s, both surface and depth midlatency responses were abolished. This rate sensitivity differs from that of the auditory brainstem responses (ABRs), which are essentially unchanged at rates of 20/s. Whereas ABRs are unaffected by surgical levels of sodium pentobarbital, the surface and depth midlatency potentials are replaced by a deep negativity within minutes following administration of anesthesia. Extracellular recordings of acoustically responsive single units in CL and CM demonstrated latencies comparable to the CL and CM field potential latencies. Both the units and field potentials were similarly rate sensitive. Each auditory unit showed a best frequency response, but none demonstrated somatosensory convergence. Bilateral aspiration of the inferior colliculus did not abolish the midlatency depth or surface responses. Rather, recordings in CL and CM suggested response enhancement over a two week postoperative period. Taken together these data suggest that the midlatency vertex potential, wave "A', reflects a generator system which projects from cuneiform nucleus, through the medial tegmentum to the medial thalamus, particularly to CL and CM. The functional significance of this medial auditory projection system remains to be determined. It could mediate physiological correlates of "state', modulate sensory input or motor output, or it could provide an integrative mechanism for the focusing of auditory attention.

Diencephalic projections from the pontine reticular formation: autoradiographic studies in the cat

Ascending projections to the diencephalon from the pontine reticular formation were studied in the cat by autoradiographic techniques. Projections from both rostral and caudal pontine regions ascend to the caudal diencephalon and divide into two components; a dorsal leaf terminates primarily in the thalamic intralaminar complex and a ventral leaf terminates in the subthalamic region. The relative densities of the two terminal regions vary with the injection site. Fibers originating in the caudal pons (nucleus reticularis pontis caudalis) terminate relatively heavily in the intralaminar nuclei of the dorsal thalamus, particularly the centre median, central lateral, central dorsal and paracentral nuclei, and also the dorsal medial nucleus. Relatively sparse termination occurs in the subthalamic region. In contrast, fibers from the rostral pons (nucleus reticularis pontis oralis) terminate relatively heavily in the subthalamic region, including the zona incerta, the fields of Forel, the ventral part of the thalamic reticular complex, and the lateral hypothalamus. Relatively sparse termination occurs in the dorsal thalamus, but includes the centre median, parafascicular, central lateral, paracentral and dorsal medial nuclei. These data are discussed with regard to reticular control of forebrain activity and the role of the classic dorsal and ventral components of ascending reticular projections.

Brain stem generation of the hippocampal EEG

In summary, based on unit recording studies in behaving animals it appears that the brain stem cells best suited for a role in theta generation are the pontine RF neurons in the rat (Vertes, 1980, 1981) rabbit (Klemm, 1970) and dog (Arnolds, 1977) that fire selectively during the identical states in which theta is present in the hippocampus and whose discharge characteristics closely parallel properties attributed to hippocampal theta (Vertes, 1981). The omission of any description of RF cells in the behaving cat with theta-like properties (McCarley and Hobson, 1971; Hobson et al., 1974; Siegel et al., 1977, 1979) could stem from any of the following factors: (1) species differences; (2) differences in RF recording sites; (3) failure of the cat studies to specifically evaluate RF cell discharge in relation to hippocampal theta. The only other brain stem nucleus directly implicated in theta generation in unit recording studies was the raphe magnus (Sheu et al., 1974). The firing of cells within other monoaminergic nuclei including the locus coeruleus was unrelated or only loosely related to states of hippocampal synchronization or desynchronization (Sheu et al., 1974; Chu and Bloom, 1973, 1974; Hobson et al., 1975; Foote and Bloom, 1979; Aston-Jones and Bloom, 1981; Heym et al. 1981).

Unit responses of intralaminar thalamus to midbrain and medullary stimulation and effects of conditioning caudate and hippocampal stimuli

Single units responding to heterotopic somatic stimuli, on extracellular recording in thalamic intralaminar and neighbouring nuclei, also responding to stimulation of the midbrain tegmentum or the medullary magnocellular reticular formation. Consideration of response latencies suggested that some monosynaptic projections from both midbrain and medulla may be received in nuclei centralis lateralis, centrum medianum-parafascicularis complex, and medial ventralis lateralis. Responses to brainstem of nuclei medialis dorsalis, lateralis posterior were of considerably longer latency. There was no correlation between shortness of latency and following-rate of unit responses; the ability of intralaminar neurons to follow rapidly-repeated brainstem stimuli is inferred to be limited by inhibitory processes rather than by synaptic interruptions in the afferent pathway. Conditioning stimuli to caudate nucleus or hippocampus suppressed most intralaminar responses to midbrain stimuli, the shortest-latency responses included, suggesting that inhibitory effects could be exerted at the thalamic level, perhaps directly on the responsive neurone.

Ascending projections of the brain stem reticular formation in a nonmammalian vertebrate (the lizard Varanus exanthematicus), with notes on the afferent connections of the forebrain

In the present study an attempt has been made to analyze the ascending reticular projections in the lizard Varanus exanthematicus by means of the horseradish peroxidase (HRP) technique. Reticular projections ascending to the telencephalon were found to arise in the mesencephalon, but not caudal to the mesorhombencephalic border. HRP injections into the dorsal thalamus have demonstrated retrogradely labeled cells in the mesencephalic reticular formation, particularly at the level of the oculomotor nerve and in the medial magnocellular zone of the rhombencephalic reticular formation, predominantly rostrally. HRP infiltrations at the mesodiencephalic border damaged most of the fibers passing beyond this junction, resulting in the uptake of HRP by the damaged axons and subsequent labeling of the cell bodies or origin of ascending reticular projections to the diencephalon and telencephalon. From a comparison of cell-labeling patterns in cases of HRP injections of, respectively, the dorsal thalamus and the mesodiencephalic border, it seems likely that the nucleus reticularis medius and more sparsely the nucleus reticularis inferior project to ventral diencephalic structures (ventral thalamus and hypothalamus), whereas the midbrain reticular formation and the rostral parts of the rhombencephalic reticular formation (nuclei reticulares isthmi and superior) project to both the dorsal thalamus and more ventral diencephalic structures. Projections arising throughout the rhombencephalic reticular formation, but predominantly in the nucleus reticularis inferior, were found to ascend to the midbrain reticular formation. The present experimental data in the lizard Varanus exanthematicus are comparable to the findings in mammals, with the exception of the reticulo-oculomotor pathways which have not been analyzed so far in reptiles. In addition to the aforementioned ascending reticular projections, the present study has demonstrated projections ascending from monoamine cell groups, various diencephalic structures, as well as from neuronal groups involved in somatosensory, auditory, and gustatory systems. Projections were found from the locus coeruleus and the nucleus raphes superior to the telencephalon, as well as from the substantia nigra and the presumable reptilian homologue of the mammalian ventral tegmental area to the basal forebrain and the dorsal thalamus. Bilateral projections were demonstrated from the principal trigeminal nucleus to the telencephalon, reminiscent of the quintofrontal tract of birds. Ascending projections to the diencephalon were found to originate bilaterally in the descending trigeminal nucleus and the dorsal funicular nucleus. Auditory projections to the midbrain arise bilaterally in the superior olivary complex and in the cochlear nuclear complex. Finally, the ascending gustatory pathway arising in the nucleus of the solitary tract was found to project to the "parabrachial region," which in its turn has extensive projections to the forebrain.

Conative regulation of cortical activity by the reticular formation, hypothalamus, and thalamus

Evolution of neural regions suggests the requirement for a common format for information units exchanged among regions. Short-term memory experiments suggest a format of six or seven items. A similar number of configurations of primary cerebral interactions is proposed, associated with arousal, sleep, approach, withdrawal, perseveration, alert scanning, and commanded by the reticular formation. A comparable number of basic states (feeding, mating, grooming, shelter-seeking, fighting, etc.) is proposed as operating within these configurations, under regulation by the hypothalamus. Conative variables from this region are transformed into patterns of regulation for local cortical populations by the thalamus. Elaboration of these configurations and states by higher structures leads to new forms of cortical activity only loosely coupled to brain stem systems.

Efferent projections of the deep mesencephalic nucleus (pars lateralis) in the rat

The projections of the lateral part of the deep mesencephalic nucleus (DMN) were traced by autoradiography and retrograde horseradish peroxidase (HRP) techniques. At the level of the DMN, projections from its lateral part crossed the midline and terminated in the medial and lateral part of the contralateral DMN. Furthermore, two labeled tracts passed rostrally from the lateral part of the DMN. One tract coursed dorsolaterally from the lateral DMN to terminate in the ipsilateral lateral thalamic nucleus. The second tract coursed ventrally and rostrally over the substantia nigra toward the ipsilateral zona incerta. At the caudal part of the zona incerta these fibers divided into two bundles. One bundle coursed superiorly to terminate bilaterally in the mediodorsal nucleus of the thalamus. The second bundle of fibers passed anteriorly to enter the ipsilateral zona incerta. Some of these fibers terminated upon neurons of the zona incerta and the ventromedial part of the subthalamic nucleus. The remaining fibers within the zona incerta coursed anteriorly to enter the internal capsule. These fibers terminated in the entopeduncular nucleus and medial part of the globus pallidus. These findings indicate that the lateral part of the DMN is likely to be involved in the ascending activating system of the reticular formation by connections with thalamic nuclei. Furthermore, the lateral part of the DMN may play a part in suprasegmental motor control via connections with rostral brain stem motor centers.

A new aspect of the collision test principle: cell body distance from recording site

A theoretical method is described for estimating the distance between a spike recording-site, possibly axonal, and the corresponding cell body of unknown location. The method requires that an orthodromic spike be recorded following an antidromic spike, with estimation of a collision interval analogous to that used for establishing antidromicity. To calculate the distance between recording-site and cell body, values are needed for the collision interval between antidromic and succeeding orthodromic spikes, the refractory period of the spike, and the antidromic conduction speed. Problems may arise in determining the last value. The method is illustrated with antidromic spikes recorded in the medial thalamus of the cat upon stimulating the caudate nucleus.

Basal forebrain facilitation of reflex swallowing in the cat

In adult cats anaesthetized with urethane, electrical and chemical stimulation of the basal forebrain facilitated reflex swallowing elicited by electrical stimulation of the superior laryngeal nerve. A systematic stereotaxic mapping study using electrical stimulation revealed that the facilitatory sites were distributed along the course of the ansa peduncularis, specifically its rostral forebrain and hypothalamic components associated with the anterior amygdalar area, substantia innominata, lateral preoptic area, anterior hypothalamus and nucleus accumbens. By means of acute discrete radiofrequency lesions, the descending pathways mediating facilitatory influences from the nucleus accumbens and the amygdala to the brain stem were found to traverse the lateral hypothalamus. Ventral tegmental facilitatory sites in the midbrain are likely to be associated with these descending pathways; however, there is evidence for independent participation of this region of the brain in the control of swallowing. Chemical stimulation by means of microinjections of dopamine and apomorphine into the amygdala and nucleus accumbens also enhanced reflex swallowing. It is concluded that the results of this investigation implicate the basal forebrain as a site of integration of viscero-olfacto-gustatory information needed for the enactment of ingestive behaviour.

Behavioral functions of the reticular formation

Studies of the behavioral correlates of activity in reticular formation cells, usually performed in restrained animals, have found units whose discharge relates to sensory stimuli, pain and escape behavior, conditioning and habituation, arousal, complex motivational states, REM sleep, eye movements, respiration and locomotion. Units with these different behavioral correlates were found in the same anatomical areas. Most studies report that a large proportion of encountered cells related to the behavior being studied. If one adds up the reported percentages, the total far exceeds 100%. Therefore it appears that many investigators are looking at the same cells and reaching very different conclusions about their behavioral roles. On the basis of observations in unrestrained cats, it is hypothesized that discharge in most RF cells is primarily related to the excitation of small groups of muscles. This hypothesis can parsimoniously explain many previous observations on the behavioral correlates of these cells, and is consistent with anatomical, physiological and phylogenetic studies of the reticular formation. The hypothesized simplicity of reticular formation unit function is contrasted with the complexity of the behavioral functions mediated by the RF, and the implications of this contrast discussed.

Pain pathways and mechanisms

The current concepts of the anatomy of the pain pathways have been described and the mechanism by which they function has been discussed.

The course of direct projections from the abducens nucleus to the contralateral medial rectus subdivision of the oculomotor nucleus in the cat

We have used autoradiography (tritiated leucine) to investigate the projections of a number of nuclear groups of the cat pons. Some cells of the abducens nucleus have axons that cross the midline, ascend in the opposite median longitudinal fasciculus (MLF) and synapse on the cells of the oculomotor complex which have been identified by others as those innervating the medial rectus muscle.

Depression of evoked potentials in rat thalamic ventro-basal complex and somatosensory cortex after reticular stimulation

Experiments on unanesthetized rats immobilized with D-tubocurarine showed that electrical stimulation (100/sec) of the central gray matter and the mesencephalic and medullary reticular formation considerably depressed potentials in the somatic thalamic relay nucleus and somatosensory cortex evoked by stimulation of the forelimb or medial lemniscus. The mean threshold values of the current used for electrical stimulation of these structures did not differ significantly and were 70 (20--100), 100(20--120), and 120 (50--200) muA, respectively. On comparison of the amplitude-temporal characteristics of inhibition of evoked potentials during electrical stimulation of the above-mentioned structures by a current of twice the threshold strength, no significant differences were found. Immediately after the end of electrical stimulation the amplitude of the cortical evolved potential and the post-synpatic components of the thalamic evoked potential was 50--60% (P less than 0.01) below the control values. The duration of this depression varied from 0.5 to 1 sec. An increase in the intensity of electrical stimulation of brain-stem structures to between three and five times the threshold led to depression of the presynaptic component of the thalamic evoked potential also. Depression of the evoked potential as described above was found with various ratios between the intensities of conditioning and testing stimuli.

Localization of calcium channels in Paramecium caudatum

1. Electrical recordings from Paramecium caudatum were made after removal of the cilia with chloral hydrate and during ciliary regrowth to study the electrical properties of that portion of the surface membrane enclosing the ciliary axoneme. 2. Removal of the somatic cilia (a 50% reduction in membrane surface area) results in an almost complete elimination of the regenerative Ca response, all-or-none Ba2+ spike, and delayed rectification. 3. A twofold increase in input resistance resulted from the 50% reduction in membrane surface area. 4. The electrical properties remained unchanged, despite prolonged exposure to the chloral hydrate, until the cilia were mechanically removed. 5. Restoration of the Ca response accompanied ciliary regrowth, so that complete excitability returns when the cilia regain their original lengths. 6. It is concluded that the voltage-sensitive Ca channels are localized to that portion of surface membrane surrounding the cilia. 7. Measurements of membrane constants before and after deciliation and estimations of the cable constants of a single cilium suggest that the cilia of Paramecium may be fully isopotential along their length and with the major cell compartment.

Brain stem stimulation and the acetylcholine-evoked inhibition of neurones in the feline nucleus reticularis thalami

1. In cats anaesthetized with halothane and nitrous oxide, the responses to iontophoretically applied acetylcholine (ACh) and to high-frequency stimulation of the mid-brain reticular formation (MRF) were tested on spontaneously active neurones in the nucleus reticularis thalami and underlying ventrobasal complex.2. The initial response to MRF stimulation of 90% of the ACh-inhibited neurones found in the region of the dorsolateral nucleus reticularis was an inhibition. Conversely, the initial response of 82% of the ACh-excited neurones in the ventrobasal complex was an excitation. Neurones in the rostral pole of the nucleus reticularis were inhibited by both ACh and RMF stimulation.3. The mean latency (and s.e. of mean) for the MRF-evoked inhibition was 13.7 +/- 3.2 ms (n = 42) and that for the MRF-evoked excitation, 44.1 +/- 4.2 ms (n = 35).4. The ACh-evoked inhibitions were blocked by iontophoretic atropine, in doses that did not block amino acid-evoked inhibition. In twenty-four ACh-inhibited neurones the effect of iontophoretic atropine was tested on MRF-evoked inhibition. In all twenty-four neurones atropine had no effect on the early phase of MRF-evoked inhibition but weakly antagonized the late phase of inhibition in nine of fourteen neurones.5. Interspike-interval histograms showed that the firing pattern of neurones in the nucleus reticularis was characterized by periods of prolonged, high-frequency bursting. Both the ACh-evoked inhibitions and the late phase of MRF-evoked inhibitions were accompanied by an increased burst activity. In contrast, iontophoretic atropine tended to suppress burst activity.6. The possibility is discussed that electrical stimulation of the MRF activates an inhibitory cholinergic projection to the nucleus reticularis. Since neurones of the nucleus reticularis have been shown to inhibit thalamic relay cells, activation of this inhibitory pathway may play a role in MRF-evoked facilitation of thalamo-cortical relay transmission and the associated electrocortical desynchronization.

Determination of antidromic excitation by the collision test: problems of interpretation

Extracellular unit recordings were made from pontine reticular neurons in the cat and cells of the motor cortex in monkeys. In all cases, the characteristics of responses to electrical stimulation were studied using the tests of invariance of latency, high frequency following, and collision for determining the orthodromic or antidromic nature of the responses. The results of these tests show that their conclusions are not always consistent. A systematic error was found between the experimental and predicted values of the collision interval. It is argued that this error is due to differences in the application of measured parameters in calculating the collision interval. The collision test can be considerably improved by repeating the test with stimuli of progressively greater strengths.

Brain stem reticular units: synaptic responses to stimulation within the ascending reticular pathways

When the ascending reticular axonal system is stimulated, the responses of distal structures (e.g., the cerebral cortex) appear to outlast the stimulus; these longlasting effects could reflect the intrinsic nature of the distal structure, or the response could reflect an interaction among the reticular cells which tends to prolong the effects of stimulation. To examine the latter hypothesis, single units with ascending axons (projecting units) were recorded in the cat rostral rhombencephalon in acute experiments conducted under halothane-nitrous oxide anesthesia. Stimulation of areas to or through which axons of reticular neurons projected (midbrain tegmentum and lower tectum, medial thalamus, and basal forebrain) produced a consistent and specific response which was elicited only from these areas: suppression of spontaneous activity which was typically elicited from several areas having ascending axons. One-half of these responses were accompanied by a short latency-single spike synaptic excitation. Stimulating areas more than 1.0 mm from the ascending trajectory never produced this response, whereas the number of responses was directly related to the number of projecting axons identified in any one experiment from a given site. Thus, the predominant effect of stimulating within the ascending axonal trajectory was suppression of spontaneous activity in the projecting units, not an 'en cascade' activation of these units; on the contrary, the only type of excitation encountered was a single, short latency spike. Therefore, any effects of stimulation within the ascending reticular pathway which appear to outlast the stimulus (as previously described in the literature) cannot be ascribed to a reverberating (excitatory) circuit among projecting units. A possible source of the synaptic responses of projecting units is a retrograde activation of collaterals interconnecting the reticular cells. If such interaction exists, it is specifically distributed among cells with ascending axons, as the responses were only observed in a very few units not identified by antidromic excitation; however, other evidence is adduced to support the belief that these few units were projecting units whose axons were beyond the reach of the stimulating electrodes. Futhermore, the axons may be bundled such that units with axons nearest that of a given projecting unit give rise to the most extensive synaptic interactions; the activation of these nearby axons suppresses spontaneous activity, while axons farther away have a greater possibility of being excitatory in nature. Should such a medium for interaction exist, reticular collateral interactions might be seen to exist specifically for the purpose of decreasing the activity of cells destined for similar rostral target structures.

Autoradiographic studies of the projections of the midbrain reticular formation: ascending projections of nucleus cuneiformis

The ascending projections of the cuneiform nucleus in the cat were traced by autoradiography in the transverse and sagittal planes following stereotaxically placed injections of (3)H-leucine. The ascending fibers are almost exclusively ipsilateral and enter the diencephalon as a wide radiation. At the mesodiencephalic junction fibers enter the nucleus of the posterior commissure and pretectal nuclei, and others cross in the posterior commissure to distribute to these structures on the contralateral side. More ventrally directed fibers distribute to the fields of Forel and then spread into the posterior hypothalamus and zona incerta. At the caudal level of the ventral thalamic group, the ascending fibers diverge and follow two separate courses. One division of fibers continues forward beneath the ventral thalamic group and distributes to the zpna incerta and dorsal hypothalamic area. It rapidly diminishes in size as it attains more rostral levels where it is found in the bed nuclei of the stria terminalis and the anterior commissure. Other fibers of this division spread laterally to innervate the ventral lateral geniculate nucleus, the lateral hypothalamus, and preoptic area, and still others follow the entire confirmation of the thalamic reticular nucleus. The second division of fiber ascends through midline and intralaminar nuclei, completely encircling the mediodorsal nucleus, which is uninnervated except for a small ventral region. The distribution of this division is heaviest to the paraventricular, parafascicular, and central dorsal nuclei. Neither division is conspicuous rostral to the anterior commissure. No projections to neostriatum or specific thalamic nuclei were evident.

Which elements are excited in electrical stimulation of mammalian central nervous system: a review

(1) There are data on the amount of current necessary to stimulate a myelinated fiber or cell body and/or its axon a given distance away from a monopolar electrode over the entire range of practical interest for intracranial stimulation. Data do not exist for other electrode configurations. (2) Currents from a monopolar cathode of more than 8 times threshold may block action potentials in axons. Therefore, only axons lying in a shell around the electrode are stimulated. Elements very close to the electrode may not be stimulated. Close to an electrode small diameter axons may be stimulated and larger ones may not be. (3) Most, and perhaps all, CNS myelinated fibers have chronaxies of 50-100 musec. When gray matter is stimulated, the chronaxie is often 200-700 musec. It is not clear what is being stimulated in this case. Current-duration relations should be determined for many more responses. (4) There are no current-distance or current-duration data for central finely myelinated or unmyelinated fibers. (5) It takes less cathodal current than anodal to stimulate a myelinated fiber passing by a monopolar electrode. When a monopolar electrode is near a cell body, on the opposite side from the axon, often the lowest threshold is anodal, but sometimes cathodal. Stimulation of a neuron near its cell body is not well understood, but in many cases the axon is probably stimulated. (6) Orientation of cell body and axons with respect to current flow is important. For an axon it is the component of the voltage gradient parallel to the fiber that is important. (7) The pia has a significant resistance and capacitance. Gray matter, white matter, and cerebrospinal fluid have different resistivities, which affect patterns of current flow. (8) More is known about stimulation of mammalian CNS than most workers are aware of. Much of what is unknown seems solvable with current methods.