The lateral reticular nucleus in the cat

Summary Intracellular recording from neurones in the lateral reticular nucleus (LRN) demonstrated that, in addition to the previously identified excitatory ipsilateral forelimb tract (iF tract) (Clendenin et al. 1974c) there is an inhibitory tract mediating information from the ipsilateral forelimb to the LRN. The excitatory and inhibitory tracts were similarly organized. The tract neurones were monosynaptically activated by affcrents in the ipsilateral forelimb and projected to the same area of the LRN. They will be considered as excitatory and inhibitory components of the iF tract and denoted the excitatory and inhibitory iF tract (EiF and IiF tracts). Stimulation of the descending ipsilateral dorsolateral funiculus (iDLF) in the C3 segment evoked disynaptic EPSPs and IPSPs in LRN neurones contacted by the EiF and IiF tracts. The responses in individual LRN neurones evoked from the iDLF were similar to the responses evoked from the forelimb nerves suggesting that the EiF and IiF tracts are monosynaptically activated by fibres in the iDLF. The dorsal portion of the magnocellular part of the LRN constituted the main termination area of both the EiF and IiF tracts. Neurones in this area have previously been shown to project ipsilaterally to lobule V in the pars intermedia of the cerebellar anterior lobe and to the paramedian lobule (Clendenin et al. 1974a). IPSPs evoked from the IiF tract in LRN neurones outside the main termination area had smaller amplitudes and longer latencies. This finding suggests that these responses were generated by thin axon collaterals given off from dorsally located stem axons.     

The lateral reticular nucleus in the cat

Intracellular recordings were obtained from 204 neurones in the lateral reticular nucleus (LRN). LRN neurones contacted by the bVFRT were identified by the responses evoked on stimulation of descending fibres in the contralateral ventral quadrant of the spinal cord (cVQ) at cervical (C5cVQ) and lumbar (L2cVQ) levels. Stimulation of the cVQ evoked excitatory or inhibitory responses in 124 of the 204 LRN neurones. EPSPs were evoked in 45, IPSPs in 52 and both EPSPs and IPSPs in 27 LRN neurones. The shortest latencies of the responses evoked from the cVQ indicated that both EPSPs and IPSPs were disynaptic. This finding was confirmed by direct stimulation of the ascending fibres in the ipsilateral ventrolateral funiculus at C3 (C3iVLF) or L1 (L1iVLF). In most LRN neurones activated or inhibited from the cVQ, stimulation of the iVLF evoked similar responses at a monosynaptic latency. These results indicate that the bVFRT consists of roughly equally large groups of excitatory and inhibitory neurones monosynaptically connected with the LRN. Excitatory and inhibitory bVFRT neurones had similar peripheral receptive fields and termination areas in the LRN. LRN neurones were divided into those contacted by cervical bVFRT neurones and lumbar bVFRT neurones. The former group consisted of LRN neurones responding to C5cVQ stimulation at latencies below 5 ms, whereas the latter group contained LRN neurones responding to stimulation of the L2cVQ. Cervical bVFRT neurones projected to most parts of the LRN whereas the projection of lumbar bVFRT neurones were confined to the ventrolateral part of the nucleus. Excitatory and inhibitory vVFRT neurones of each group had similar termination areas.(ABSTRACT TRUNCATED AT 250 WORDS)     

Convergence of central respiratory and locomotor rhythms onto single neurons of the lateral reticular nucleus

We have analyzed the behavior of neurons of the lateral reticular nucleus (LRN) during fictive respiration and locomotion and found that some LRN neurons have both central respiratory and locomotor rhythms. Experiments were conducted on decrebrate, decerebellate, immobilized, and artificially ventilated cats, with the spinal cord transected at the lower thoracic cord. Fictive respiration and fictive forelimb locomotion were ascertained by monitoring activities from the phrenic nerve and forelimb extensor and flexor nerves, respectively. Fictive locomotion was evoked by electrical stimulation of the mesencephalic locomotor region (MRL) or sometimes occurred spontaneously. During fictive locomotion many LRN neurons fired in certain phases of the locomotion cycle; i.e., with respect to the nerve discharge of the ipsilateral forelimb they fired in either the extensor, flexor, extensor-flexor, or flexor-extensor phase. Firing of some LRN neurons was modulated synchronously with central respiratory rhythm. Neurons with inspiratory activity and those with expiratory activity were both found. More than half of these respiration-related LRN neurons had locomotor rhythm as well. The majority of the three types of LRN neurons, i.e., neurons with only locomotor rhythm, those with only respiratory rhythm, and those with both respiratory and locomotor rhythm, were antidromically activated by electrical stimulation of the ipsilateral inferior cerebellar peduncle. Electrical stimulation of the upper cervical cord showed that these LRN neurons, not only locomotion-related but also respiration-related neurons, received short latency inputs from the spinal cord. The LRN neurons studied were distributed widely in the LRN, relatively densely in the caudal two-thirds of the nucleus. No particular differences were detected between the three types of LRN neurons with respect to their location in the nucleus. These results indicate that the information about central respiratory and locomotor rhythms that is necessary for cerebellar control of the coordination between respiration and locomotion converges, at least partly, at the level of the LRN.     

The lateral reticular nucleus in the cat—II. Effects of lateral reticular lesions on posture and reflex movements

Unilateral lesion of the lateral reticular nucleus produced a postural asymmetry characterized by ipsilateral hypertonia and contralateral hypotonia of the limb extensor muscles. Soon after the operation the cat was unable to stand or walk, and it laid on one side. Within 3 days, it began to walk, but it often deviated to its contralateral side, 'frequently falling in this direction. Some compensation for both postural and motor deficits occurred in chronic preparations maintained up to 154 days after the lesion. The postural asymmetry was reversed by the following operations: (1) section of the ipsilateral VIIIth nerve, (2) electrolytic lesion of the ipsilateral Deiters' nucleus, or (3) ablation of the contralateral vermal cortex of the cerebellar anterior lobe.The lateral reticular nucleus lesion also produced, in the ipsilateral limbs, a transient loss of the proprioceptive placing reaction and a persistent deficit of the tactile placing reflex. These effects on the ipsilateral side were not reversed by the procedures described above.All of these behaviors depended on selective destruction of the lateral reticular nucleus and were not due to damage to the nearby main reticular formation or ascending and descending pathways. Moreover, the postural changes did not involve mesencephalic or higher mechanisms, since they were still observed after decerebration. The forelimbs were affected primarily by lesions involving the dorsomedial, magnocellular part of the lateral reticular nucleus, whereas the hindlimbs also were affected by lesions including the ventrolateral, parvicellular part of the lateral reticular nucleus.The postural asymmetry is attributed to interruption of the crossed spinoreticulocerebellar pathway, acting on the vermal cortex of the anterior lobe and the fastigial nucleus, while the ipsilateral loss of the placing reaction is attributed to interruption of the uncrossed spinoreticulocerebellar pathway acting on the intermediate cortex of the anterior lobe and the interpositus nucleus. The lateral reticular nucleus appears, therefore, to be composed of two, more or less independent, parts, a crossed one being related to major changes in postural tone, equilibrium and locomotion of the entire body, and an uncrossed one restricted to discrete movements of the ipsilateral limbs.     

An ipsilateral projection from the red nucleus to the lateral reticular nucleus in the cat

Summary Injections of the wheat germ agglutinin — horse-radish peroxidase complex into the lateral reticular nucleus reveal that in addition to the well known contralateral rubroreticular connection, there is also a small but clear cut ipsilateral projection. Cells of various sizes participate in this ipsilateral pathway, and the retrogradely labelled neurons lie dispersed throughout the entire red nucleus.     

Selective effects of (−)-baclofen on spinal synaptic transmission in the cat

Summary When ejected microelectrophoretically near spinal interneurones of cats anaesthetised with pentobarbitone and under conditions where postsynaptic excitability was maintained artificially at a constant level, (–), but not (+), -baclofen selectively reduced monosynaptic excitation by impulses in low threshold muscle (Ia and Ib) and cutaneous (A) afferents. Polysynaptic excitation of interneurones and Renshaw cells by impulses in higher threshold afferents was less affected, and baclofen had little or no effect on the cholinergic monosynaptic excitation of Renshaw cells. Glycinergic and gabergic inhibitions of spinal neurones were relatively insensitive to baclofen. These stereospecific actions of baclofen, produced by either a reduction in the release of excitatory transmitter or postsynaptic antagonism, suggest that Ia, Ib, and A afferents may release the same excitatory transmitter which differs from that of spinal excitatory interneurones.Microelectrophoretic (–), but not (+), -baclofen also reduced primary afferent depolarization of ventral horn Ia extensor afferent terminations produced by impulses in low threshold flexor afferents, without altering either the electrical excitability of the terminations or their depolarization by electrophoretic GABA or L-glutamate. This stereospecific action of baclofen is interpreted as a reduction in the release of GABA at depolarizing axo-axonic synapses on Ia terminals.     

Cholinergic mechanisms in the reticular control of transmission in the cat lateral geniculate nucleus

1. We examined the hypothesis that the ascending reticular arousal system influences thalamic transmission through a cholinergic mechanism. Extra- and intracellular recordings were obtained from neurons of the dorsal lateral geniculate nucleus (LGNd) and the perigeniculate nucleus (PGN) of cats anesthetized either with N2O and pentobarbital or with N2O and halothane. We compared the effects that electrical stimulation of the mesencephalic reticular formation (MRF) and ionophoretically applied acetylcholine (ACh) have on spontaneous and evoked activity of individual neurons and tested whether these effects could be antagonized by ionophoretic administration of the muscarinic receptor blocker scopolamine. The effects of ionophoretically applied glutamate (GLU), N-methyl-D-aspartate, and bicuculline were examined in addition. 2. The prominent effects in LGNd relay cells of both ACh application and of MRF stimulation were an enhancement of the resting discharge, a facilitation of the excitatory responses to light, a reduction of the amplitude and duration of evoked inhibitory episodes, and a blockade of postinhibitory rebound burst. These latter effects resembled those induced with bicuculline. Under barbiturate anesthesia neither ACh application nor MRF stimulation elicited discharges when the excitatory input from the retina was blocked. Ionophoretic application of hte muscarinic antagonist scopolamine abolished the effects of ACh ionophoresis in all relay cells tested (n = 20), and in 10 cells it also antagonized completely the effects of MRF stimulation. In the remaining cells scopolamine reduced the effects of MRF stimulation. 3. Increasing the depth of anesthesia reduced or abolished the effects of ACh application and MRF stimulation on the cells' resting activity but did not interfere with the facilitation of evoked responses. 4. The effects of the excitatory amino acids GLU and NMDA differed from those of MRF stimulation and ACh application, since the former always enhanced both spontaneous and evoked discharges but neither shortened phases of evoked inhibition nor abolished postinhibitory rebound bursts. 5. There was a high correlation between the effectiveness of MRF stimulation and ACh application in individual neurons. On the average, the facilitation of evoked responses was more pronounced in X- than in Y-cells, and the fraction of cells responding with an increase of resting activity to both procedures was considerably higher among X- than among Y-cells.(ABSTRACT TRUNCATED AT 400 WORDS)     

A projection from the periaqueductal grey to the lateral reticular nucleus in the cat

Summary A projection from the periaqueductal grey (PAG) to the lateral reticular nucleus (NRL) in the cat was demonstrated by means of retrograde transport of the wheat germ agglutinin-horseradish peroxidase complex. The connection has its main origin ipsilaterally in the ventral part of the caudal PAG, but scanty projections from other parts of the PAG were also found. The neurons projecting to the NRL are of varying shapes and sizes, but most cells have a maximum diameter of less than 20 m. The findings are discussed in relation to the other afferent and efferent connections of the NRL.     

The lateral reticular nucleus in the cat IV. Activation from dorsal funiculus and trigeminal afferents

1. The activation of neurones in the mLRN (major portion of lateral reticular nucleus comprising its parvi- and magnocellular parts) be a spinal path ascending in the dorsal funiculus (DF) and by trigeminal afferents has been studied. 2. Stimulation of the DF at C3 activated about one half of the mLRN neurones. The latencies were 2-28 ms. In experiments with the spinal cord interrupted at C3 except for the DF it was shown that cutaneous and high threshold muscle afferents in mainly forelimb nerves were effective. The latencies of the responses to nerve stimulation were 8-27 ms. 3. Stimulation of trigeminal afferents evoked a response in about one third of the mLRN neurones. The latencies were 2-27 ms. 4. Activation from the DF- and trigeminal paths occurred often in the same mLRN neurones and the neurones activated from the two paths had a similar location in the nucleus and a similar termination in the cerebellar cortex. 5. The DF- and trigeminal paths had similar properties. Activation was evoked from both ipsilateral and contralateral nerves. Fast adapting hair receptors were commonly effective. 6. Evidence is presented indicating that the DF- and trigeminal paths share a common final path to the mLRN neurones which is formed by brain stem interneurones intercalated between the DF- and trigeminal nuclei and the mLRN. It is suggested that these interneurones represent a supraspinal motor centre. 7. Activation from the DF- and trigeminal paths occurred with unequal frequency among groups of mLRN neurones activated from different spinal paths ascending in the ipsilateral lateral funiculus (cf. Clendenin et al., 1974a).     

Projections from the lateral reticular nucleus to the cerebellar cortex and nuclei in the cat

Summary The fiber projection from the lateral reticular nucleus (LRN) to the cerebellum was examined in the cat. Electrolytic lesions were placed in a confined area of the LRN using a parapharyngeal approach, and the ensuing degeneration was studied in sections stained by the Nauta and the Fink-Heimer methods.Fibers from the LRN ascend the ipsilateral restiform body, terminating bilaterally but chiefly in the ipsilateral cerebellum. In the ipsilateral cortex projections were found to lobules I to V, with denser terminations in sublobules IVb to Ve. The projections are stronger in the intermediate-lateral zones than in the vermis proper. There is also a dense projection to sublobules VId and VIf and to the medialmost part of the simple lobule (HVI). Scanty termination was seen in the medialmost part of crus I. There is a moderate projection to the caudalmost folium of sublobule VIIb and to the rostral folia of sublobule VIIIa and the paramedian lobule. The contralateral projection by fibers crossed within the cerebellum is far less dense but clearcut in the anterior lobe, the rostral folia of lobule VI and the medial part of the simple lobule.In the ipsilateral nuclei strong projections were found to rostral portions of the medial nucleus (M) and the caudal two thirds of the anterior interpositus nucleus (IA) with predominance in the lateral part of the latter. A focal projection was found to rostrodorsal portions of the posterior interpositus nucleus (IP). No projection was found to the lateral nucleus (L). Contralaterally there is a weak projection to the rostral part of M and the medial parts of IA and IP.Abbreviations AL Anterior lobe - BP Brachium pontis - CD Subnucl. dorsalis of the nucl. medullae oblongatae centr. - CV Subnucl. ventralis of the nucl. medullae oblongatae centr. - Cr. I Crus I - Cr. II Crus II - F Flocculus - GL Granular layer - IA Anterior interpositus nucleus - IO Inferior olive - IP Posterior interpositus nucleus - LRN Lateral reticular nucleus - L Lateral nucleus - M Medial nucleus - ML Molecular layer - Pfd Paraflocculus dorsalis - Pfv Paraflocculus ventralis - Pm Paramedian lobule - RB Restiform body - SL Simple lobule - SMP Subnucleus medialis parvocellularis - ST Nucleus of the spinal trigeminal tract - XII Hypoglossal nerve nucleus     

Glutamate activation of cat thalamic reticular nucleus: effects on response properties of ventroposterior neurons

The thalamic reticular nucleus (RTN) exerts an inhibitory influence upon the dorsal thalamus. During wakefulness and arousal, RTN neurons fire tonically, whereas during slow-wave sleep they fire rhythmic high frequency bursts. The effects produced by RTN inhibition upon the activity of dorsal thalamic neurons will therefore vary in relation to the firing mode of the RTN neurons. In the present study, we compared the effects of oscillating RTN neurons and of RTN neurons tonically activated with glutamate on the response profiles of single units reacting to controlled cutaneous stimulation in cat ventroposterior lateral thalamic nucleus (VPL). Experiments were performed under light barbiturate anesthesia and prior to the glutamate activation of the RTN, both RTN and VPL neurons showed spontaneous bursting patterns of activity consistent with the oscillatory mode. Typically, a cutaneous stimulus evoked a short latency excitatory response in VPL followed by a period of complete inhibition termed post-stimulus inhibition (PSI). In many neurons, the PSI was followed by a period of increased activity termed post-inhibitory excitation (PIE). Ejection of glutamate in the identified somatosensory division of the RTN shifted the oscillatory firing of its neurons to a high tonic mode and usually resulted in a decrease in VPL neuronal activity. Significant variations were observed in the occurrence and the magnitude of the effects among the different components of neuronal activity examined. Tonic activation of the RTN resulted in a significant reduction of ON- and OFF-PIEs in 81% of cases (30/37) and of spontaneous activity in 67% (22/ 33). In contrast, the response to a cutaneous stimulus was decreased in only 29% of cases (17/59) and was significantly increased in 24% (14/59). Tonic activation of the RTN by glutamate resulted in little change in the firing pattern of VPL neurons, and both short and long spike intervals were affected in a similar proportion. We conclude that the components of VPL neuronal activity most affected by switching RTN neurons from the oscillatory to the tonic mode are those normally dependent upon RTN neuronal oscillation. The present results also suggest that lowering background activity, such as occurs during the transition from sleep to wakefulness, is a factor leading to increase in the responsiveness of dorsal thalamic neurons.     

The GABA-immunoreactive neurons in the interlaminar regions of the cat lateral geniculate nucleus: light and electron microscopic observations

Summary GABA-immunoreactive cells located in the interlaminar zone between the A and A1 laminae of cat LGN were studied at the LM and EM levels. The mean perikaryal size of these neurons was larger than that of GABA-immunoreactive cells in the A-laminae of LGN. Interlaminar GABA+ cells examined in plastic semithin sections of LGN after massive injections of HRP in the striate and extrastriate visual cortex were not retrogradely labeled with reaction products (as previously reported for the GABA+ cells in the laminar regions of LGN) suggesting that these cells do not project to the visual cortex. Serial EM analysis of two partially reconstructed interlaminar GABA+ cells showed that they receive synaptic inputs from RLD terminals of axon collaterals of geniculo-cortical relay cells, from cortical (RSD) terminals, from inhibitory (F) axon terminals, and from other undetermined terminals, but not from retinal (RLP) axon terminals. These data suggest that the GABAergic cells in the interlaminar zones of LGN participate as interneurons in recurrent inhibitory circuits in LGN. The synaptic inputs to these cells and ultrastructural features, notably somatic spines and dendrites oriented predominantly orthogonal to the projection lines in LGN, are similar to those of neurons of the perigeniculate nucleus.     

Trigeminal excitation of dorsal neck motoneurones in the cat

Summary Excitation of dorsal neck motoneurones evoked by electrical stimulation of primary trigeminal afferents in the Gasserian ganglion has been investigated with intracellular recording from -motoneurones in the cat. Single stimulation in the Gasserian ganglion ipsi-and contralateral to the recording side evoked excitatory postsynaptic potentials (EPSPs) in motoneurones innervating the lateral head flexor muscle splenius (SPL) and the head elevator muscles biventer cervicis and complexus (BCC). The gasserian EPSPs were composed of early and late components which gave the EPSPs a hump-like shape. A short train of stimuli, consisting of two to three volleys, evoked temporal facilitation of both the early and late EPSP components. The latencies of the gasserian EPSPs ranged from 1.6 to 3.6 ms in SPL motoneurones and from 1.6 to 5.8 ms among BCC motoneurones. A rather similar latency distribution between 1.6 and 2.4 ms was found for ipsi- and contralateral EPSPs in SPL and BCC motoneurones, which is compatible with a minimal disynaptic linkage between primary trigeminal afferents and neck motoneurones. Systematic transections of the ipsi- and contralateral trigeminal tracts were performed in the brain stem between 3 and 12 mm rostral to the level of obex. The results demonstrate that both the ipsi- and contralateral disynaptic and late gasserian EPSPs can be mediated via trigeminospinal neurones which take their origin in the nucleus trigeminalis spinalis oralis. Transection of the midline showed that the contralateral trigeminospinal neurones cross in the brain stem. Systematic tracking in and around the ipsilateral trigeminal nuclei demonstrated that the axons of ipsilateral trigeminospinal neurones descend just medial to and/or in the medial part of the nucleus. Spinal cord lesions revealed a location of the axons of the ipsilateral trigeminospinal neurones in the lateral and ventral funiculi. Interaction between the ipsi- and contralateral gasserian EPSPs showed complete summation of the disynaptic EPSP component, while the late components were occluded by about 45%. These results show that the disynaptic EPSPs are mediated by separate trigeminospinal neurones from the ipsi- and contralateral side, while about half of the late EPSPs are mediated by common neurones which receive strong bilateral excitation from commissural neurones in the trigeminal nuclei. Spatial facilitation was found in the late gasserian EPSP but not in the disynaptic gasserian EPSP by conditioning stimulation of cortico- and tectofugal fibres. Disynaptic pyramidal and tectal EPSPs, which are mediated by reticulospinal neurones, were facilitated by a single stimulation in the gasserian ganglion at an optimal interval of 2 ms. It is suggested that primary trigeminal afferents can excite the reticulospinal neurones via a disynaptic trigeminoreticular pathway.     

Neuronal circuitry in the nucleus lateralis of the cerebellum

Summary This study summarizes the nervous circuitry of the lateral nucleus from evidence presented in the previous papers in this investigation. The large and small neurons receive three types of extracerebellar afferent fibers—collaterals of climbing fibers and mossy fibers on their way to the cerebellar cortex, and fluorescent CAT fibers of two varieties. The large projection neurons receive a large inhibitory corticonuclear input via synapses of Purkinje cell axons upon their somatic and dendritic surfaces, as well as upon initial axonal segments. On their way into the superior cerebellar peduncle, the axons of these neurons send recurrent collateral branches, which provide a positive feedback circuit within the nucleus. The small inhibitory interneurons depend to a large extent on the activating input upon their dendrites; their corticonuclear input from Purkinje cells is less numerous than that of large neurons. Their myelinated axons provide a rapid system of internal inhibitory control. In cases in which the axons are long and leave the nucleus, like those emitted by the small neurons in the medial hilus zone, they may provide a route of internuclear communication.The arrangements of neurons in the columnar zone are reviewed. The large neurons are oriented at an angle to incoming extracerebellar fibers and present a profile to the Purkinje cell axon. The interneuronsare tilted at ninety degrees to the dendritic trees of the large neurons, and they receive a larger extracerebellar afferent input. Whereas the corticonuclear input to the large columnar neurons probably expresses a ratio of one Purkinje cell to one large neuron, the tilt assumed by the small neurons may increase this ratio. Electron microscopy has shown that small neurons receive a smaller corticonuclear input than large neurons. Elsewhere in the swirled zones of the nucleus, these influences are less precisely oriented, and therefore less restrictive interplay exists between corticonuclear influences and the large multipolar neurons.Supported in part by U.S. Public Health Service grants NS10536, NS03659, Training grant NS05591 from the National Institute of Neurological Diseases and Stroke, and a William F. Milton Fund Award from Harvard University.     

The central cervical nucleus in the cat. I

Summary The central cervical nucleus (CCN) in the upper cervical cord of the cat was studied with the Golgi rapid, Golgi Cox, and Golgi Kopsch methods. The majority of the neurones were large, multipolar, with dendrites radiating out from the nucleus dorsolaterally, laterally, and ventrally. Occasionally, small triangular or spindle shaped neurones were seen with dendrites following the rounded contour of the CCN. The axons from the large neurones were mostly directed towards the anterior commissure. Occasionally, the initial part of the axons from the smaller neurones were observed. They gave off collaterals within the CCN. The bulk of afferents came from the dorsal funiculus and spread out to form an afferent plexus around the CCN neurones. A small number of afferents was observed from the lateral funiculus. They were thinner than those from the dorsal funiculus and entered the CCN from its lateral aspect. The afferents from the dorsal funiculus had the same orientation as the dorsolaterally directed dendrites and the lateral funicular afferents the same as the laterally oriented dendrites.