Effectiveness of ascending synaptic influences from various spinal funiculi on reticulospinal neurons in cats

Activity of reticulospinal neurons evoked by stimulation of the ventral, ventrolateral, dorsolateral, and dorsal funiculi of the spinal cord was recorded extracellularly in cats anesthetized with chloralose. Responses of 57 reticulospinal neurons, of which 22 projected into the ventral funiculus, 20 into the ventrolateral, and 15 into the dorsolateral, were studied. The functional properties (conduction velocity and refractory period) and the location of the neurons of the above-mentioned groups in the medulla did not differ appreciably. The most effective synaptic activation of all neurons was observed during stimulation of the dorsal and dorsolateral funiculi. Responses to stimulation of the dorsal funiculus had the lowest threshold. These responses arose in reticulospinal neurons of the ventral and ventrolateral funiculi after the shortest latent period. The effectiveness of synaptic influences from the dorsal and dorsolateral funiculi was identical in the group of neurons of the dorsolateral funiculus. Correlation between activity evoked by stimulation of the dorsal funiculus in reticulospinal neurons and peripheral nerves indicated that the responses appeared in these cells to stimulation of muscular (groups I and II) and cutaneous (group II) afferent fibers. The results indicate that impulses from low-threshold muscular and cutaneous afferents, which effectively activate reticulospinal neurons, are transmitted along ascending pathways of the dorsal funiculi.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 11, No. 3, pp. 254–263, May–June, 1979.     

The inhibition of medullary reticulospinal neurons during the excitation of the dorsolateral portions of the pons blocking movement and muscle tonus in rats]

Hypothalamic stimulation increasing the muscle tone in hindlimbs, and excitation of the pontine dorsolateral areas inhibiting movements and the muscle tone in rats, were studied. Hypothalamic stimulation made 36.7% of the reticulospinal neurones to discharge in the form of short-latency spikes and to increase the muscle tone. The reticulospinal cells were completely inhibited by electrical stimulation of the pontine dorsolateral areas. 23.4% of the neurones only responded to the stimulation of the pontine dorsolateral areas. 35.9% of the cells did not respond to the stimulations at all. Excitation of the pontine inhibitory areas seems to prevent the descending activating effects from the brain rostral structures to spinal motor centres.     

Inhibitory effects of reticulospinal fibers of the lateral funiculus on neurons of thoracic spinal reflex pathways

Experiments on anesthetized cats with partial transection of the spinal cord showed that reticulo-spinal fibers in the ventral part of the lateral funiculus participate in the inhibition of polysynaptic reflexes evoked by stimulation of the ipsi- and contralateral reticular formation. The reticulo-fugal wave in the ventrolateral funiculus evoked comparatively short (up to 70 msec) IPSPs in some motoneurons of the internal intercostal nerve investigated and at the same time evoked prolonged (up to 500 msec) inhibition of IPSPs caused by activation of high-threshold segmental afferents. This wave also led to the appearance of IPSPs in 14 of 91 (15.5 %) thoracic spinal interneurons studied. The duration of these IPSPs did not exceed 100 msec; meanwhile, segment excitatory responses of 21 of 43 interneurons remained partly suppressed for 120–500 msec. It is concluded that the inhibitory action of the lateral reticulo-spinal system on segmental reflexes is due to several synaptic mechanisms, some of them unconnected with hyperpolarization of spinal neurons. The possible types of mechanisms of this inhibition are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 10, No. 2, pp. 162–172, March–April, 1978.     

Morphology of neurons and axon terminals associated with descending and ascending pathways of the lateral forebrain bundle in Rana esculenta

Summary The cells of origin of afferent and efferent pathways of the lateral forebrain bundle were studied with the aid of the cobalt-filling technique. Ascending afferents originated from the lateral thalamic nucleus, central thalamic nucleus, posterior tuberculum and the cerebellar nucleus. They terminated in the anterior entopeduncular nucleus, amygdala and the striatum. Telencephalic projection neurons, which are related to the lateral forebrain bundle, were located mainly in the ventral striatum and the anterior entopeduncular nucleus, but were not so numerous in the dorsal striatum. Irrespective of their location, most of the neurons projecting axons into the lateral forebrain bundle had piriform or pyramidal perikarya. Long apical dendrites usually arborized in a narrow space, whereas widely arborizing secondary dendrites originated from short dendritic trunks. The other neurons that contributed to the lateral forebrain bundle were fusiform or multipolar cells. Striatal efferents terminated in the pretectal area and in the anterodorsal, anteroventral and posteroventral tegmental nuclei.     

Electrophysiological characteristics of reticulospinal neurones in relation to the conduction velocity of their axons

Activity was recorded intracellularly from the bodies of 87 reticulospinal neurones in the cat's gigantocellular nucleus, whose axons had a conduction velocity of 18-148 m.s-1. Slow-conducting neurones (18-45 m.s-1, 23%) were characterized by a wider action potential, higher input resistance (3.8-7.0 M omega) and a lower rheobase (1.0-1.7 nA). They were also very sensitive to changes in membrane polarity and generated regular rhythmic activity. Fast-conducting neurons (45-148 m.s-1) were characterized by a short action potential, low input resistance (0.7-2.9 M omega) and a higher rheobase (1.5-5.2 nA). When depolarizing current pulses were applied, they generated responses with action potentials with a high frequency, especially in the initial phase of depolarization, but their thresholds for the initiation of activity and steady firing were higher than in the case of slow neurones. Slow reticulospinal neurones always responded to stimulation of the spinal funiculi (mainly the dorsal funiculus) by a characteristic large postsynaptic potential on which large numbers of spike potentials were superimposed and which did not occur in fast neurones. The differences observed in membrane properties and in the character of generation of action potentials draw attention to the phasic character of fast, and the tonic character of slow, reticulospinal neurones.     

Corticofugal influences on reticulospinal neurons of the gigantocellular nucleus in cats

It was shown by intracellular recording that stimulation of the motor cortex evokes E PS Ps and I PS Ps in reticulospinal neurons of the gigantocellular nucleus of the cat medulla. The E PS Ps appeared in 94.3% and the I PS Ps in 5.7% of neurons tested. Analysis of the presynaptic pathway showed that 77.4% of E PS Ps studied arose through monosynaptic, and 22.6% through polysynaptic corticoreticular connections. By their latent period, duration, and rise time up to a maximum the monosynaptic E PS Ps were divided into two groups: "fast" and "slow." It is postulated that "fast" E PS Ps are generated in reticulospinal neurons which are activated by fast-conducting fibers and "slow" E PS Ps by slowly conducting corticobulbar fibers. I PS Ps were recorded from reticulospinal neurons that also were inhibited by stimulation of the ventral columns of the spinal cord. The hypothesis is put forward that cortical motor signals in cats can be transmitted to the spinal cord via monosynaptic and polysynaptic connections of "fast" and "slow" pyramidal neurons with reticulospinal neurons.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 8, No. 3, pp. 250–257, May–June, 1976.     

Synaptic excitation of reticulospinal neurons during the startle reaction in cats anesthetized with chloralose

Synaptic processes in reticulospinal neurons of the pons and medulla during the startle reaction evoked by somatic stimulation were investigated in cats anesthetized with chloralose. The main type of response of reticulospinalneurons was found to be PSPs involving intrareticular (proprioreticular) pathways of varied complexity: oligosynaptic (including supposedly monosynaptic) and polysynaptic. Comparison of EPSP characteristics with parameters of spino-bulbospinal (SBS) discharges recorded simultaneously in the intercostal nerves showed that polysynaptic EPSPs evoked through corresponding proprioreticular pathways were most effective in creating a descending SBS volley. About half the reticulospinal neurons of the pons and medulla were involved at any one time in the synaptic relay process during the startle reflex. The conduction velocity in axons of these neurons varied from 30 to 98 m/sec (means 64.5 Mp 16.5 m/sec). Some distinguishing features of the functional organization of the reticular "center" for the startle reaction are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 13, No. 6, pp. 594–603, November–December, 1981.     

Prolactin-secreting neurons in the dorsolateral hypothalamus in Sprague-Dawley rats

By means of immunocytochemical techniques ovine prolactin like immunoreactivity (oPRL-LIR) has been demonstrated in the perikarya located around fornix in the dorso-lateral part of the rat hypothalamus. No PRL-LIR was observed in the arcuate n. perikarya. Immunoreactive fibers were present in the hypothalamus, medial thalamus, accumbens and amygdaloid nuclei.     

Distribution of reticulospinal neurons in the chicken by retrograde transport of WGA-HRP

To determine the distribution of reticulospinal (RS) neurons in the chicken, WGA-HRP was injected into the cervical or lumbosacral enlargement either unilaterally or bilaterally. The brainstem reticular nuclei sent largely descending fibers to both the spinal enlargements. The mesencephalon (medial and lateral mesencephalic reticular formation) and the rostral pons (nucleus reticularis [n.r.] pontis oralis) project mainly to the cervical enlargement. RS neurons were mainly distributed from the pontomedullary junction to the rostral medulla including n. r. pontis caudalis and pars gigantocellularis, n. r. gigantocellularis, n. r. parvocellularis, n. r. paragigantocellularis, and n. r. subtrigeminalis. It is suggested that the majority of these neurons send axons at least as far as the lumbosacral enlargement. In the lower medulla, RS neurons were distributed in the dorsal and ventral parts of the central nucleus of the medulla.     

Perturbations of the vesicle cycle in reticulospinal synapses of axons in lamprey after presynaptic microinjections of GTPgammaS

The synaptic vesicle cycle sustains neurotransmission and keeps pace between exo- and endocytosis in synapses. GTP-binding proteins function as key regulators of this cycle. The large GTPase dynamin is implicated in fission of clathrin-coated vesicles from the presynaptic membrane during endocytosis. The present study addresses the effect of the non-hydrolysable GTP analog, GTPgammaS, on the assembly of the dynamin fission complex in situ. Intraaxonal microinjections of GTPgammaS induced distinct ultrastructural changes in synapses: the number of synaptic vesicles at active zones was reduces, and the number of docked vesicles was increased; at the same time the number of clathrin-coated intermediates at the synaptic endocytic zone was increased, indicating that synaptic vesicle recycling was inhibited. Clathrin-coated intermediates with unusual shape were found. At low concentrations of GTPgammaS they were represented by long tubules decorated by spirals containing dynamin and clathrin-coated vesicles on the top. At high concentrations of GTPgammaS the tubulular structures were shorted and branched. The pitch of the spiral and tubule's diameter were significantly reduced (23.1 +/- 0.4 and 19.0 +/- 0.5 nm, respectively, as compared to those at low concentration of GTPgammaS, 26.6 +/- 0.4 and 23.3 +/- 0.4 nm; P < 0.001). We suggest that these structural changes correspond to distinct steps in the fission reaction. A model is proposed. It implies that the fast GTP hydrolysis leads to an increase in length of the spiral due to the straightening of the dynamin dimmers, composing the spiral. This leads to a fast increase both in the pitch and the diameter of the helix. The shift in diameter breaks the local hydrophobic interactions between the inner and the outer leaflets of the lipid membrane at the sites of dynamin binding. Stretching of the spiral leads to an expansion of the neck in the longitudinal direction and promotes severing of the membrane that subsequently results in the release of the clathrin-coated vesicle.     

Response properties of electrosensory neurons in the lateral mesencephalic nucleus of the paddlefish

Many fishes and amphibians are able to sense weak electric fields from prey animals or other sources. The response properties of primary afferent fibers innervating the electroreceptors and information processing at the level of the hindbrain is well investigated in a number of taxa. However, there are only a few studies in higher brain areas. We recorded from electrosensory neurons in the lateral mesencephalic nucleus (LMN) and from neurons in the dorsal octavolateral nucleus (DON) of the paddlefish. We stimulated with sine wave stimuli of different amplitudes and frequencies and with moving DC stimuli. During sinusoidal stimulation, DON units increased their firing rate during the negative cycle of the sine wave and decreased their firing rate to the positive cycle. Lateral mesencephalic nucleus units increased their rate for both half cycles of the sine wave. Lateral mesencephalic nucleus units are more sensitive than DON units, especially to small moving dipoles. Dorsal octavolateral nucleus units respond to a moving DC dipole with an increase followed by a decrease in spike rate or vice versa, depending on movement direction and dipole orientation. Lateral mesencephalic nucleus units, in contrast, increased their discharge rate for all stimuli. Any change in discharge rate of DON units is converted in the LMN to a discharge rate increase. Lateral mesencephalic nucleus units therefore appear to code the presence of a stimulus regardless of orientation and motion direction.     

Synaptic activation of thoracic interneurons by reticulospinal fibers of the lateral funiculus

Synaptic responses of different functional groups of interneurons in segments T10 and T11 to stimulation of the ipsilateral and contralateral medullary reticular formation were investigated in anesthetized cats with only the ipsilateral lateral funiculus remaining intact. Activation of reticulospinal fibers of the lateral funiculus with conduction velocities of 30–100 m/sec was shown to induce short-latency and, in particular, monosynptic EPSPs in all types of cells tested: in interneurons excited by group Ia muscle afferents, in cells activated only by high-threshold cutaneous and muscle afferents (afferents of the flexor reflex), in cells activated mainly by descending systems, and, to a lesser degree, in neurons connected with low-threshold cutaneous afferents. These cell populations are located mainly in the central and lateral parts of Rexed's lamina VII. Most neurons in laminae I–V of the dorsal horn, except six cells located in the superficial layers of the dorsal horn, received no reticulofugal influences. The functional organization of connections of the lateral reticulospinal tract with spinal neurons is discussed and compared with the analogous organization of the medial reticulospinal tract, and also of the "lateral" (cortico- and rubrospinal) descending systems.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 10, No. 2, pp. 150–161, March–April, 1978.     

Soma position is correlated with time of development in three types of identified reticulospinal neurons

The relationship among neuronal type, position, and time course of development of identified neurons was examined in the zebrafish (Brachydanio rerio). The cells studied, the reticulospinal neurons Mauthner, MiM1, and MiV1, are located within the same small region in the hindbrain, differ stereotypically in their positions within this region and also in their axonal projections. All of the cell types were generated and had initiated axonal outgrowth by the second day after fertilization. The time that these events occurred was specific for each cell type, with axonal outgrowth occurring about 10 hr after the neuronal birthday. Furthermore, the time of the events varied systematically according to the dorsoventral location of the neuron within the set.     

Ascending and descending projections of the lateral vestibular nucleus in the rat

The tracer neurobiotin was injected into the lateral vestibular nucleus in rat and the efferent fiber connections of the nucleus were studied. The labeled fibers reached the diencephalon rostrally and the sacral segments of the spinal cord caudally. In the diencephalon, the ventral posteromedial and the gustatory nuclei received the most numerous labeled fibers. In the mesencephalon, the inferior colliculus, the interstitial nucleus of Cajal, the nucleus of Darkschewitch, the periaqueductal gray matter and the red nucleus received large numbers of labeled fibers. In the rhombencephalon, commissural and internuclear connections originated from the lateral vestibular nucleus to all other vestibular nuclei. The medioventral (motor) part of the reticular formation was richly supplied, whereas fewer fibers were seen in the lateral (vegetative) part. In the spinal cord, the descending fibers were densely packed in the anterior funiculus and in the ventral part of the lateral funiculus. Collaterals invaded the entire gray matter from lamina IX up to lamina III; the fibers and terminals were most numerous in laminae VII and VIII. Collateral projections were rich in the cervical and lumbosacral segments, whereas they were relatively poor in the thoracic segments of the spinal cord. It was concluded that the fiber projection in the rostral direction was primarily aimed at sensory-motor centers; in the rhombencephalon and spinal cord, fibers projected onto structures subserving various motor functions.     

Evidence that descending cortical axons are essential for thalamocortical axons to cross the pallial-subpallial boundary in the embryonic forebrain

Developing thalamocortical axons traverse the subpallium to reach the cortex located in the pallium. We tested the hypothesis that descending corticofugal axons are important for guiding thalamocortical axons across the pallial-subpallial boundary, using conditional mutagenesis to assess the effects of blocking corticofugal axonal development without disrupting thalamus, subpallium or the pallial-subpallial boundary. We found that thalamic axons still traversed the subpallium in topographic order but did not cross the pallial-subpallial boundary. Co-culture experiments indicated that the inability of thalamic axons to cross the boundary was not explained by mutant cortex developing a long-range chemorepulsive action on thalamic axons. On the contrary, cortex from conditional mutants retained its thalamic axonal growth-promoting activity and continued to express Nrg-1, which is responsible for this stimulatory effect. When mutant cortex was replaced with control cortex, corticofugal efferents were restored and thalamic axons from conditional mutants associated with them and crossed the pallial-subpallial boundary. Our study provides the most compelling evidence to date that cortical efferents are required to guide thalamocortical axons across the pallial-subpallial boundary, which is otherwise hostile to thalamic axons. These results support the hypothesis that thalamic axons grow from subpallium to cortex guided by cortical efferents, with stimulation from diffusible cortical growth-promoting factors.     

Mitochondrial biogenesis in the axons of vertebrate peripheral neurons

Mitochondria are widely distributed via regulated transport in neurons, but their sites of biogenesis remain uncertain. Most mitochondrial proteins are encoded in the nuclear genome, and evidence has suggested that mitochondrial DNA (mtDNA) replication occurs mainly or entirely in the cell body. However, it has also become clear that nuclear-encoded mitochondrial proteins can be translated in the axon and that components of the mitochondrial replication machinery reside there as well. We assessed directly whether mtDNA replication can occur in the axons of chick peripheral neurons labeled with 5-bromo-2'-deoxyuridine (BrdU). In axons that were physically separated from the cell body or had disrupted organelle transport between the cell bodies and axons, a significant fraction of mtDNA synthesis continued. We also detected the mitochondrial fission protein Drp1 in neurons by immunofluorescence or expression of GFP-Drp1. Its presence and distribution on the majority of axonal mitochondria indicated that a substantial number had undergone recent division in the axon. Because the morphology of mitochondria is maintained by the balance of fission and fusion events, we either inhibited Drp1 expression by RNAi or overexpressed the fusion protein Mfn1. Both methods resulted in significantly longer mitochondria in axons, including many at a great distance from the cell body. These data indicate that mitochondria can replicate their DNA, divide, and fuse locally within the axon; thus, the biogenesis of mitochondria is not limited to the cell body.     

Local and global motion preferences in descending neurons of the fly

For a moving animal, optic flow is an important source of information about its ego-motion. In flies, the processing of optic flow is performed by motion sensitive tangential cells in the lobula plate. Amongst them, cells of the vertical system (VS cells) have receptive fields with similarities to optic flows generated during rotations around different body axes. Their output signals are further processed by pre-motor descending neurons. Here, we investigate the local motion preferences of two descending neurons called descending neurons of the ocellar and vertical system (DNOVS1 and DNOVS2). Using an LED arena subtending 240° × 95° of visual space, we mapped the receptive fields of DNOVS1 and DNOVS2 as well as those of their presynaptic elements, i.e. VS cells 1–10 and V2. The receptive field of DNOVS1 can be predicted in detail from the receptive fields of those VS cells that are most strongly coupled to the cell. The receptive field of DNOVS2 is a combination of V2 and VS cells receptive fields. Predicting the global motion preferences from the receptive field revealed a linear spatial integration in DNOVS1 and a superlinear spatial integration in DNOVS2. In addition, the superlinear integration of V2 output is necessary for DNOVS2 to differentiate between a roll rotation and a lift translation of the fly.