Activity of rubrospinal neurons in the monkey Macaca mulatta

Action potentials were recorded from single cells of the red nucleus; they were evoked by antidromic stimulation of the rubrospinal tract and by stimulation of the contralateral nucleus interpositus of the cerebellum in monkeys. It was demonstrated that axons go to the spinal cord not only from the large cells of the caudal pole but also from the neurons of the micro cellular part of the red nucleus. The speed of conduction along axons of rubrospinal neurons of monkeys ranges from 17.4–108.8 m/ sec (predominantly 60–90 m/sec). Experiments with stimulation of the cerebellar nucleus interpositus pointed to the existence of a monosynaptic interposito-rubral connection in monkeys.Translated from Zhurnal Évolyutsionnoi Biokhimii i Fiziologii, Vol. 12, No. 1, pp. 56–62, January–February, 1976.     

Integration in descending motor pathways controlling the forelimb in the cat

Summary Recording was made in the C3-C4 segments from cell bodies of propriospinal neurones identified by their antidromic activation from more caudal segments. Monosynaptic excitatory effects from descending motor pathways and primary afferents were investigated by electrical stimulation of higher motor centres and peripheral nerves in the forelimb and neck.The cell bodies were located mainly laterally in Rexed's layer VII. Threshold mapping for single axons showed that they descend in the lateroventral part of the lateral funicle. Antidromic stimulation at different spinal cord levels showed that some neurones terminated in the forelimb segments, others in the thoracic cord or in the lumbar segments. Terminal slowing of the conduction velocity suggested axonal branching over some segments.Monosynaptic EPSPs were evoked in the neurones by stimulation of the contralateral pyramid, red nucleus and dorsal tegmentum-superior colliculus. It is concluded that corticospinal, rubrospinal and tectospinal fibres project directly to both short and long propriospinal neurones. There was marked frequency potentiation in tectospinal synapses. Convergence from two descending tracts was common and in half of the tested cells all three tracts contributed monosynaptic excitation. Experiments with collision of descending volleys and antidromic volleys from the brachial segments demonstrated that the corticospinal and rubrospinal monosynaptic projection to the propriospinal neurones is by collaterals from fibres continuing to the forelimb segments.Stimulation of cervical primary afferents in the dorsal column gave monosynaptic EPSPs in somewhat less than half of the tested propriospinal neurones. The further analysis with stimulation of forelimb nerves and C2-C3 dorsal rami showed that monosynaptic EPSPs may be evoked from low threshold cutaneous and group I muscle afferents in the forelimb and from C2-C3 neck afferents entering close to the spinal ganglia, possibly from joint receptors. Convergence from cervical afferents and at least two of the above descending tracts was common.It is postulated that the propriospinal neurones previously indirectly defined by their action on motoneurones as relaying disynaptic excitation from higher motor centres to forelimb motoneurones (Illert et al., 1977) belong to those neurones of the C3-C4 propriospinal systems which terminate in the cervical enlargement. The function of the neurones projecting beyond the upper thoracic segments is discussed.Supported by the Deutsche ForschungsgemeinschaftIBRO/UNESCO Fellow     

Descending projections of Forel's field H neurones to the brain stem and the upper cervical spinal cord in the cat

Summary 1. Descending projections from Forel's field H (FFH) to the brain stem and upper cervical spinal cord were studied in cats. 2. Following implantation of HRP pellets into the spinal gray matter (C1-C3) or in the ponto-medullary reticular formation, the nucleus reticularis pontis caudalis (NRPC) or in the nucleus reticularis gigantocellularis (NRG), numerous neurones were retrogradely labelled in FFH on the ipsilateral side. In the former cases, the sizes of labelled neurones were medium-large (2040 m in diametre) while both small and medium-large neurones were labelled in the latter cases. 3. The lowest levels of spinal projection of single FFH neurones (n=70) were assessed by antidromic spikes elicited by stimulating electrodes placed in C1, C3 and C7. The majority (59%) projected to C1 (but not to C3), about 27% to C3 (but not to C7), and only 14% to C7. 4. Axonal trajectories of single FFH neurones in C1-C3 segments were investigated by antidromic threshold mapping methods. The stem axons of spinal-projecting FFH neurones descended in the ventral or in the ventrolateral funicli and the collaterals were projected to neck motor nuclei (lamina IX, Rexed 1954) and laminae V–VIII. The conduction velocities were estimated as 8–37 m/s from the antidromic latencies. 5. Axonal trajectories of 7 FFH neurones were investigated in the ponto-medullary reticular formation. All were antidromically activated from C1. In six neurones, the stem axons were located in the ventral part of the central tegmental tract and collaterals were projected to the NRPC and/or the NRG. Some of them projected to the inferior olive and the nucleus prepositus hypoglossi as well. The stem axon, in the remaining cell, was in the most dorso-medial part of the medial longitudinal fasciculus and collaterals were projected mainly to the dorsal part of the NRPC and the NRG, and also to the medial vestibular nucleus. 6. Anterograde transport of WGA-HRP injected into FFH revealed that in the upper cervical spinal cord, stem axons were found in the ventral funiculus and ventral part of the lateral funiculus. Collateral projections and presumed bouton-like deposits were observed in the laminae VI–IX, especially in their medial part. In the brain stem, dense bundles of the descending fibres were found in the central and the medial tegmental tracts and in the medial longitudinal fasciculus. FFH neurones projected densely to the caudal half of the NRPC and to the rostral half of the NRG. Extremely dense projections to the inferior olive were noted.Abbreviations AM anteromedian nucleus of the Edinger-Westphal - BCC m. biventer cervicis and complexus - CCN central cervical nucleus - CP cerebral peduncle - CTT central tegmental tract - DAB diaminobenzidine - DAO dorsal accessory nucleus of the inferior olive - DW nucleus Darkschewitsch - FFH Forel's field H - FMN fasciculus mammillothalamics - FR fasciculus retroflexus - G genu of the facial nerve - HB habenula - HRP horseradish peroxidase - INC interstitial nucleus of Cajal - IO inferior olive - LGN lateral geniculate nucleus - LHT lateral hypothalamic nucleus - MAO medial accessory nucleus of the inferior olive - MB mammillary body - MLF medial longitudinal fasciculus - MTT medial tegmental tract - MVN medial vestibular nucleus - NRG nucleus reticularis gigantocellularis - NRPC nucleus reticularis pontis caudalis - NRTP nucleus reticularis tegmenti pontis - OT optic tract - PAG periaqueductal gray matter - PC posterior commissure - PCN posterior commissure nucleus - PF parafascicular nucleus - PH nucleus prepositus hypoglossi - PHT posterior hypothalamic nucleus - PN pontine nucleus - PO principal nucleus of the inferior olive - Py pyramid - RN red nucleus - RSNs reticulospinal neurones - SN substantia nigra - SNr substantia nigra pars reticulata - sPF subparafascicular nucleus - STH subthalamic nucleus - TB trapezoid body - TMB tetramethylbenzidine - V3 third ventricle - ZI zona incerta - VI abducens nucleus/nerve - XII nucleus hypoglossi     

Branching neurones in the cervical spinal cord with axons that reach sacral segments and the lateral reticular nucleus. An electrophysiological study in the cat

Branching neurones in the cervical enlargement of the spinal cord were electrophysiologically studied in alpha-chloralose anaesthetized cats with the method of antidromic activation of axons. Stimulating electrodes were placed bilaterally at levels of lower thoracic and sacral segments and in the lateral reticular nucleus (LRN), ipsilaterally to the recording sites in C6/C7 segments. Thirty-nine out of a total one hundred neurones could be classified as bidirectional neurones with both descending and ascending collaterals. In the remaining cases only long descending projections to spinal segments were found. Comparison of conduction velocities measured in descending branches revealed no significant differences between individual neurones. On the other hand, descending collaterals of double direction neurones conducted impulses considerably faster than their axonal branches ascending to LRN. Our results suggest that parallel transmission of information to various, spinal or supraspinal centres of the nervous system is more common than reported before.     

Activity evoked from the mesencephalic tegmentum in descending pathways other than the rubrospinal tract

Summary Discharges evoked by mesencephalic stimulation have been recorded from the contralateral dissected dorsal and ventral quadrants in the lower thoracic region of the spinal cord in cats. The effect of this activity on different spinal cord mechanisms was analysed.After interruption of the rubrospinal tract in the lateral part of the medulla, stimulation just dorsal to the red nucleus still evokes a discharge in the contralateral dorsal quadrant. The discharge which requires repetitive stimulation is probably mediated by a two-neurone pathway. Since the stimuli giving this dorsal discharge produces inhibition of interneuronal transmission from the flexor reflex afferents to motoneurones, which is the characteristic effect of activity in the dorsal reticulospinal system (Engberg et al., 1968) it is suggested that the second order neurones belong to this pathway.Stimulation just dorsal to the red nucleus also evokes a synchronized discharge in the contralateral ventral quadrant, mediated by a two-neurone pathway. The first order neurones are at least partly of tectal origin; the second order neurones originate from the brain stem and their axons descend in the medial longitudinal fasciculus. Activity in these fibres has no detectable effect on spinal cord mechanism controlling hindlimb muscles but produces monosynaptic EPSPs in motoneurones of the upper lumbar segments.Some observations are reported regarding long propriospinal excitatory and inhibitory connexions to upper lumbar motoneurones.This work was supported by the Swedish Medical Research Council (Project No. 14X-94-07C).     

Electron microscopic demonstration of terminations of posterior thalamic axons on identified rubrospinal neurons in the rat

In the rat, the major output of the posterior thalamic nucleus (PT) ends in the ventrolateral sector of the rostral two-thirds of the red nucleus. The aim of this study is to identify the rubral cells contacted by these thalamic efferents. In a first set of experiments, an anterograde neurotracer (PHA-L) was injected into the rostral part of the red nucleus. The only structure consistently and densely labeled was the contralateral spinal cord. Therefore, in a second set of experiments, massive HRP injections were made at different cervical levels in the spinal cord in combination with either electrolytic lesion or PHA-L injection in the contralateral PT. Both anterograde and retrograde labelings obtained in the RN were examined by correlated light and electron microscopy. Our findings indicate that anterogradely degenerated or labeled axons arising from the PT form synaptic contacts on HRP-filled dendritic processes of rubrospinal neurons. The thalamo-rubral articulation is direct and seems to be mainly axo-dendritic. These results support the possible participation of the PT in the modulatory control of spinal interneurons through the rubrospinal tract.     

Spinal branching of rubrospinal axons in the cat

Summary The branching patterns of rubrospinal (RS) axons projecting to the cervical spinal cord between C3 and C8 were studied in the cat. RS neurons were identified by their antidromic responses to microstimulation of local axon branches within the cervical gray matter. Twenty-six of 58 RS neurons projecting to the cervical gray matter also sent axon branches to the thoracic spinal cord. Two out of 40 of these RS neurons also sent axon branches to the lumbar spinal cord. Using a collision technique, it was demonstrated that stem axons of rubrospinal neurons commonly sent multiple collaterals to different cervical segments.Neurons projecting to the cervical spinal cord alone were located in the dorsal quadrants of the red nucleus. Those projecting to cervical, as well as to more caudal segments, were intermingled with the former, and in slightly more ventral portions of the red nucleus. The presence of RS neurons projecting to widely separate levels of the spinal cord suggests that individual RS neurons may be capable of ultimately influencing two or more different motoneuron pools.     

Analysis of potentials induced in red nucleus neurones from the somaesthetic pathway stimulated at the bulbar level

A somaesthetic pathway to the magnocellular red nucleus (RNm) via relays other than corticoor cerebello-rubral relays was previously found to exist in the cat. At the brainstem level, the ascending spinorubral fibres follow the medial lemniscus (LM). The present paper aims at describing in detail and evaluating the quantitative importance of the short-latency responses in RNm cells after microstimulation performed in the LM through a monopolar electrode. The RNm cells, tested intracellularly in cats anaesthetized with -choralose and placed in a stereotaxic device, were identified by their antidromic activation to stimulation of the rubrospinal tract in the cervical cord. It was established that single-shock stimulation below 100 A current delivered to the LM induced short-latency postsynaptic potentials (PSPs) in 87% of all the rubrospinal cells tested. The responses were indeed due to activation of LM fibres, as demonstrated by different tests: the location of the electrode tip in the LM was verified by recording, with the same electrode, the potentials evoked by stimulating the dorsal columns of the cord. The site was later confirmed histologically. The absence of stimulus spread from the LM to the underlying pyramidal tract was systematically checked by simultaneously recording the responses evoked in RNm cells and in the motor cortex. Monosynaptic excitatory responses (EPSPs) were evoked in RNm cells with a minimum stimulating current of less than 20 A in the LM and a mean threshold of 42 A. Disynaptic inhibitory potentials (IPSPs) were evoked in 23% of these cells with single-pulse stimulation. These latter responses showed a temporal facilitation with short trains of three pulses, which indicated that they were transmitted through inhibitory interneurones. Recordings were also performed from presumed LM fibre terminals running inside the RNm. The results therefore confirm the existence of strong lemniscal projections to RNm and demonstrate that they transmit both excitatory and inhibitory messages to rubrospinal cells. As the somaesthetic pathway to the RNm was previously found to come from the spinal cord, where it is located in the ventral portion, the present results also confirm that the LM is composed of fibres originating not only from neurones in the dorsal column nuclei, but also from cells placed at the segmental levels of the cord. The presumed sensorimotor function of this ascending pathway is discussed.     

Axonal trajectories of single Forel's field H neurones in the mesencephalon,pons and medulla oblongata in the cat

Summary We studied axonal trajectories of single Forel's field H (FFH) neurones (n= 19) in the mesencephalon, pons and medulla by systematic antidromic threshold mapping in cats and differentiated them into two major types. Type I neurones were characterized by projections to the oculomotor nucleus (IIIn) and type II neurones by lack of projections to the IIIn. 2. Type I neurones (11/19) were further classified into three subtypes by the lowest level of projections; type Ic (n = 3) which projected to the cervical cord and type Ib (n = 7) which terminated at the ponto-medullary level and type Ia (n = 1) at more rostral level. In the mesencephalon, stem axons passed just lateral to the IIIn and projected collaterals to the IIIn and the ventral part of the periaqueductal gray matter. In the lower brain stem, stem axons of type Ib and Ic neurones passed in the dorsal part of the reticular formation or in the medial longitudinal fasciculus and projected collaterals to the dorsal part of the nucleus reticularis pontis caudalis (NRPC) and the nucleus reticularis gigantocellularis (NRG) and the reticular formation underlying the nucleus prepositus hypoglossi (PH) and the raphe region. Projections to the superior colliculus were observed in two cases. 3. Type II neurones (8/19) were classified into 2 type IIb projecting to the ponto-medullary reticular formation and 6 type IIc projecting to the cervical spinal cord. In the mesencephalon, stem axons passed through a more lateral region than those of type I and projected collaterals to the mesencephalic reticular formation and the red nucleus. In the lower brain stem, the stem axons passed in the ventral part of the reticular formation corresponding to the central tegmental tract and projected collaterals to the ventral part of the NRPC and NRG. Projections to the interstitial nucleus of Cajal, the inferior olive and the reticular formation underlying the PH were also observed. 4. The dorsal and ventral location of, respectively, stem axons of type I and type II neurones in the lower brain stem was confirmed in a larger number of neurones in experiments with restricted mapping. 5. There was not much difference in location of cell bodies of type I (totally n = 50) and type II (n = 46) neurones. The proportion of spinal-projecting neurones were larger in type II (21/46, 46%) than in type I (7/50, 14%) neurones.     

Electrotonic coupling between neurons in the rat lateral vestibular nucleus

Summary Correlation of morphological and electrophysiological data strongly suggest that in rat, the giant cells of the lateral vestibular nucleus (L.V.N.) are electrotonically coupled. 1. in addition to active zones large terminals synapsing on the perikaryon and/or the main dendritic trunk of the cells bear gap junctions which are interpreted as low electrical resistance pathways between neurons. 2. electrical activity of the giant cells was recorded intracellularly as the vestibulo-spinal tract was stimulated. Graded antidromic stimulation produced graded antidromic depolarizations (G.A.Ds) in 69% of cells with high threshold axons. 3. the latency of the G.A.Ds was too short to allow for chemical transmission through afferents or recurrent collaterals. 4. collision experiments demonstrated that directly evoked spikes blocked the antidromic spikes but did not block the G.A.Ds which thus were accounted for by activation of cells others than the impaled ones. 5. lesion experiments indicated that afferent fibers from the spinal cord terminate exclusively in the dorsal part of the L.V.N. Since G.A.Ds were recorded all throughout the nucleus, they were not excitatory post synaptic potentials (EPSPs) from spinal afferents. 6. when the strength of the spinal cord stimulation was increased EPSPs were also generated but they were distinct from the G.A.Ds by their latencies, time course and maximum amplitude. 7. since no direct contact is observed between neurons it is inferred that, as in other documented cases, coupling between giant cells is mediated by way of presynaptic fibers.Maitre de Recherches à l'INSERM.     

The significance of the recurrent collateral in the structural organization of nervous centers

Summary Experiments were performed on cats and albino rats to investigate the synaptic connections of recurrent coilaterals of the axons of neurones representing exit elements of the nerve centers. The method was to cause a retrograde degeneration of the neurones, and therefore also of their axon collaterals by section of the corresponding nervous pathways.From the results obtained it could be deduced that collaterals of the motoneurone axons of the spinal cord form synaptic connections with Renshaw cells. The axons of the ganglion cells of the retina also have collaterals which form a connection with the amacrine cells. Recurrent collaterals of the mitral cell axons of the olfactory bulbs are connected with the small cells lying at the basis of the mitral cell layer. Axons of the cells of the vagal motor nucleus also possess collaterals which form a connection with small cells of the nucleus. However, they may terminate on the cell bodies of the effector neurones. The combined evidence from these experiments and those reported in the literature lead to the conclusion that the exit element of a nervous center is a peculiar neurone circuit involving the recurrent collateral. Its significance is still unknown, and remains to be studied.(Presented by Active Member AMN SSSR V. V. Parin) Translated from Byulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 53, No. 2, pp. 106–110, February, 1962     

Electrophysiological demonstration of a dentato-rubrospinal projection in cats

Electrophysiological experiments were performed in cats anesthetized with alpha-chloralose to demonstrate the existence of a dentato-rubrospinal projection. In one series of experiments lesions of the red nucleus were found to eliminate a response recorded at C-5 in the spinal cord following stimulation in the dentate nucleus. This response was unaffected by lesions placed in the interposed nuclei adjacent to the region of the dentate nucleus in which the stimulating electrode was located. Other experiments demonstrated that this response was evoked only when the stimulating electrode was located at the edge of or within the dentate nucleus. Together these studies show that stimuli applied in the dentate nucleus evoke a short latency response mediated by the red nucleus which does not result from current spread to the interposed nuclei.The effect of dentate stimulation on identified rubrospinal neurons was evaluated using extracellular and intracellular recordings. Rubrospinal neurons were identified by their antidromic activation from the spinal cord. In several of these neurons, stimulating in the dentate nucleus evoked short-latency synaptically mediated responses. Intracellular recording revealed that dentate stimulation evoked graded depolarizations in rubrospinal neurons with a mean latency of 1.4 ms. These findings indicate that the output of the dentate nucleus directly activates a component of the rubrospinal projection.     

Properties of vestibular neurones projecting to neck segments of the cat spinal cord

1. Vestibular neurones projecting to the upper cervical grey matter (vestibulocollic neurones) were identified by localized microstimulation in the C3 segment of the cat spinal cord.

2. The neurones were found in the lateral (Deiters'), medial and descending nuclei bilaterally and projected to the spinal cord in the lateral and medial vestibulospinal tracts (LVST and MVST). Ipsilateral axons of Deiters' neurones were mostly in the LVST, axons of medial and descending neurones in the MVST; a few Deiters' neurones had axons in the MVST; some descending neurones had axons in the LVST. Most axons of contralateral neurones were in the MVST.

3. The axons of 62% of ipsilateral vestibulocollic Deiters' neurones not only gave off a collateral to C3, but also extended as far as the cervical enlargement (`branching'); some of these neurones projected as far as the upper thoracic cord, almost none to the lumbar cord. Ipsilateral descending nucleus neurones branch in the same fashion, but there is no branching in the relatively small medial nucleus population.

4. A large majority of vestibulocollic neurones receive monosynaptic excitation from the ipsilateral labyrinth and a number are inhibited by stimulation of the contralateral labyrinth (commissural inhibition). It is possible that commissural inhibition acts on a broad population of vestibular neurones involved in the control of eye, head and trunk movement.

5. Vestibulocollic neurones do not make up a homogeneous population acting only on the neck. Instead it is likely that subpopulations, for example branching and non-branching neurones, have different functions.

     

Distribution and peculiarities of axon collateral branching of rubro-spinal neurones in brainstem structures

Peculiarities of axon branching and distribution of rubro-spinal neurones in various brainstem structures were studied in acute cats using the technique of intracellular recording of antidromic action potentials as well as collision testing. Axon collaterals of rubro-spinal neurones into the main sensory trigeminal nucleus, facial nerve nucleus, descending (inferior) vestibular nucleus, lateral reticular nucleus, external cuneate nucleus, gracile and main cuneate nuclei were identified. Correlation between the antidromic impulse conduction time along the stem axon before and after collateral branching and the time of impulse conduction in the collaterals themselves was analysed. The number of axon collaterals of individual rubro-spinal neurones to particular brainstem structures was studied. A tendency was observed for the synchronous arrival of rubro-spinal impulses to various brainstem centres, due to an increase in conductance velocity the further away these centres were from the red nucleus.     

Recurrent Control from Motor Axon Collaterals of la Inhibitory Pathways to Ventral Spinocerebellar Tract Neurones

The effects of impulses in recurrent motor axon collaterals on transmission in different inhibitory pathways to ventral spinocerebellar tract (VSCT) neurones were investigated in the cat by conditioning of IPSPs evoked in intracellularly recorded VSCT cells. Disynaptic IPSPs from large muscle spindle (la) afferenls were depressed in many but not all VSCT cells following an antidromic stimulation of ventral roots. The effect was found in VSCT neurones which themselves did not receive recurrent inhibition from motor axon collaterals. In cells with affected la IPSPs also some polysynaptic IPSPs evoked from ipsi- and contralateral group II muscle afferents and low threshold cutaneous afferents were depressed by a ventral root volley as well as disynaptic IPSPs from fibres descending on the ipsilateral side of the spinal cord. In unanesthetized preparations recurrent facilitatory potentials similar to those in motoneurones were evoked in VSCT neurones with la IPSPs. The findings indicate that some VSCT neurones receive collateral connexions from the interneurones which mediate la recipiocal inhibition to motoneurones and support the hypothesis that the VSCT conveys information about transmission in inhibitory reflex pathways to motoneurones (Lundberg 1971).     

Morphometric and experimental studies of the red nucleus in the albino rat

Cytoarchitectural observations showed that the red nucleus of the albino rat consists of three distinct neuronal populations. Neurons with coarse Nissl bodies occupy the caudal end of the red nucleus and extend in diminishing number to the rostral tip. Neurons with finely granular Nissl material are the predominant cell type at the rostral tip of the red nucleus and interdigitate with the coarse neurons except at the caudal end of the nucleus. Coarse neurons, in contrast to fine neurons, are multiangular in contour and tend to be larger, although the two populations overlap in size. A population of interneurons, almost entirely smaller than the other cell types and less numerous, is ubiquitous within the red nucleus. Injections of horseradish peroxidase (HRP) at different levels of the spinal cord established that the coarse neurons on the contralateral side are the source of the rubrospinal tract and are topographically organized. The dorsal-medial part of the red nucleus emits axons which project to the cervical cord and the ventral-ventrolateral part of the nucleus to the lumbar cord. The thoracic cord receives projections from rubral neurons at intermediate positions. Further, coarse neurons from the entire rostrocaudal axis of the red nucleus contribute fibers to the rubrospinal tract.     

Excitatory connections between neurons of the central cervical nucleus and vestibular neurons in the cat

 The central cervical nucleus (CCN) of the cat receives input from upper cervical muscle afferents, particularly primary spindle afferents. Its axons cross in the spinal cord, and while in the contralateral restiform body give off collaterals to the vestibular nuclei. In order to study the connections between CCN axons and vestibular neurons, we stimulated the area of the CCN in decerebrate cats while recording intra- or extracellularly from neurons in the contralateral vestibular nuclei. CCN stimulation evoked excitatory postsynaptic potentials (EPSPs) or extracellularly recorded firing in the lateral, medial and descending vestibular nuclei. The latency of EPSPs (mean 1.6 ms) was on average 0.4 ms longer than the latency of antidromic spikes evoked in the CCN by stimulation of the contralateral vestibular nuclei (mean 1.2 ms), demonstrating that the excitation was typically monosynaptic. The results provide further evidence that the CCN is an important excitatory relay between upper cervical muscle afferents and neurons in the contralateral vestibular nuclei. Received: 1 August 1996 / Accepted: 16 December 1996     

Multiple axon collaterals of single corticospinal axons in the cat spinal cord

To investigate intraspinal branching patterns of single corticospinal neurons (CSNs), we recorded extracellular spike activities from cell bodies of 408 CSNs in the motor cortex in anesthetized cats and mapped the distribution of effective stimulating sites for antidromic activation of their terminal branches in the spinal gray matter. To search for all spinal axon branches belonging to single CSNs in the "forelimb area" of the motor cortex, we microstimulated the gray matter from the dorsal to the ventral border at 100-micron intervals at an intensity of 150-250 microA and systematically mapped effective stimulating penetrations at 1-mm intervals rostrocaudally from C3 to the most caudal level of their axons. From the depth-threshold curves, the comparison of the antidromic latencies of spikes evoked from the gray matter and the lateral funiculus, and the calculated conduction times of the collaterals, we could ascertain that axon collaterals were stimulated in the gray matter rather than stem axons in the corticospinal tract due to current spread. Virtually all CSNs examined in the forelimb area of the motor cortex had three to seven branches at widely separated segments of the cervical and the higher thoracic cord. In addition to terminating at the brachial segments, they had one to three collaterals to the upper cervical cord (C3-C4), where the propriospinal neurons projecting to forelimb motoneurons are located. About three quarters of these CSNs had two to four collaterals in C6-T1. This finding held true for both fast and slow CSNs. About one third of the CSNs in the forelimb area of the motor cortex projected to the thoracic cord below T3. These CSNs also sent axon collaterals to the cervical spinal cord. CSNs in the "hindlimb area" of the motor cortex had three to five axon branches in the lumbosacral cord. These branches were mainly observed at L4 and the lower lumbosacral cord. None of these CSNs had axon collaterals in the cervical cord. CSNs terminating at different segments of the cervical and the thoracic cord were distributed in a wide area of the motor cortex and were intermingled. To determine the detailed trajectory of single axon branches, microstimulation was made at a matrix of points of 100 or 200 micron at the maximum intensity of 30 microA, and their axonal trajectory was reconstructed on the basis of the location of low-threshold foci and the latency of antidromic spikes.(ABSTRACT TRUNCATED AT 400 WORDS)