Synaptic actions produced by individual ventrolateral tract fibres in frog lumbar motoneurones

Excitatory post-synaptic potentials (EPSPs) were evoked in lumbar motoneurones of the isolated frog spinal cord by impulses in single ventrolateral tract fibres. In a few cases after recording an EPSP the fibre and the motoneurone involved were both filled with horseradish peroxidase (HRP) and the synaptic connexion between them was studied histologically. Monosynaptic EPSPs produced by direct stimulation of supraspinal (mainly reticulospinal) or unidentified (presumably propriospinal) fibres are mediated via chemical and, less frequently, dual-action synapses. The shape indices of chemical single-fibre EPSPs varied considerably in different connexions being, as a whole, similar to those of chemical components of EPSPs at synapses between primary afferents and motoneurones. Quantal analysis of the single-fibre EPSPs yielded quantal unit amplitude 18-113 microV and mean quantum content ranging from 1.14 to 16.4, the applicability of both Poisson and binomial models to transmitter release was revealed. Descending fibres electrically coupled with lumbar motoneurones were found to generate a depolarizing response to dorsal root stimulation. They were also characterized by a larger depolarization to superfused glutamate. The presence of electrical junctions between descending axons and spinal motoneurones suggests that the depolarization seen in these axons in response to synaptic excitation and glutamate could be the result of passive flow of depolarizing current from motoneurones electrically coupled to them. gamma-aminobutyric acid (GABA) did not produce conspicuous actions in axons forming both chemical and dual-action synapses. Axons injected with HRP have been followed to their site of termination in the lateral motor column. Synaptic boutons and varicosities were found to form contacts predominantly with dendrites of target motoneurones.     

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.

     

Synaptic actions of single interneurones mediating reciprocal Ia inhibition of motoneurones

1. The investigation was aimed at defining the function of the interneurones which, according to indirect evidence, mediate the reciprocal Ia inhibition of motoneurones (Hultborn, Jankowska & Lindstrom, 1971 b) by studying their direct synaptic actions. These actions were tested by recording post-synaptic potentials in motoneurones following spike activity of single interneurones activated by iontophoretic application of glutamate. The interneurones were found to produce unitary monosynaptic IPSPs in those motoneurones in which disynaptic IPSPs are evoked by the group Ia afferents which monosynaptically excite the interneurones.2. Unitary IPSPs were found in more than 80% of the motoneurones impaled in the immediate vicinity of the axonal branches of the investigated Q interneurones in the PBSt motor nucleus. It is estimated that each interneurone might inhibit about every fifth PBSt motoneurone. The amplitudes of the unitary IPSPs ranged between 8 and 220 muV and were 10-200 times smaller than the maximal Ia IPSPs evoked in the same motoneurones.3. The synaptic delay in the generation of unitary IPSPs was measured in relation to the spike potentials recorded from the terminal branches of interneurones in the immediate vicinity of the impaled motoneurones. The synaptic delays ranged between 0.28 and 0.42 msec.4. From chloride reversal tests and an analysis of the time course of the unitary IPSPs it was concluded that the terminals of the investigated interneurones make synaptic contact predominantly on the soma and/or on the proximal parts of the dendrites of the motoneurones, their distribution being, however, not quite uniform.     

Input-output organization of reticulospinal neurones,with special reference to connexions with dorsal neck motoneurones in the cat

Summary Dorsal neck motoneurones receive disynaptic tectal and pyramidal EPSPs via common reticulospinal neurones (RSNs). This study was aimed at identification of the RSNs projecting directly to neck motoneurones and mediating these EPSPs. 1. Stimulation of the tectum and the cerebral peduncle evoked monosynaptic descending volleys in the spinal cord, which were chiefly mediated by reticulospinal neurones in the pons and the medulla. Systematic tracking of the C3 and C7 segments was made to locate descending volleys in the spinal funiculi. The tectal monosynaptic volley was largest in the medial part of the ventral funiculus and decreased gradually as the recording electrode was moved to the lateral part of the ventral funiculus and the lateral funiculus. In contrast, the peduncle-evoked monosynaptic volley was distributed rather evenly in the ventral funiculus and the ventral half of the lateral funiculus. 2. Differences in funicular distribution of the two descending volleys suggest the existence of subgroups of RSNs which differed in strength of inputs from the two descending fibre systems and in the funicular location of descending axons. 3. The RSNs were classified into the following four groups; (1) mRSNs which descended in the medial part of the ventral funiculus, (2) in RSNs which descended in the ventrolateral funiculus, (3) 1RSNs which descended in the dorsal 2/3 of the lateral funiculus and (4) coRSNs which descended in the contralateral funiculi. The mRSNs were located in a fairly localized region corresponding to the nucleus reticularis pontis caudalis (N.r.p.c.), while inRSNs, 1RSNs and coRSNs were mainly in the nucleus reticularis gigantocellularis (N.r.g.), in the nucleus reticularis magnocellularis (N.r.m.) and in the nucleus reticularis ventralis (N.r.v.). RSNs were further divided into three types depending on the levels of projection. L-RSNs projected to the lumbar spinal segments. C-RSNs descended to the C6–C7 spinal segment but not to the lumbar segments. N-RSNs projected to the C3 but not to the C6–C7 segments. 4. Stimulation of the tectum and the cerebral peduncle produced monosynaptic negative field potentials in the medial two thirds of the reticular formation in the pons and medulla. Tectal field potentials were largest in the N.r.p.c. and the rostral part of the N.r.g., while pyramidal field potentials were largest in the N.r.g. Correspondingly, RSNs in the N.r.p.c. (mRSNs) received larger monosynaptic EPSPs from tectal than from pyramidal volleys, while RSNs in the N.r.g. (in-, 1- and coRSNs) received stronger input from the peduncle than from the tectum. 5. Stimulation of the C7 ventral but not the lateral funiculus evoked monosynaptic EPSPs on all the dorsal neck motoneurones tested. Stimulation of the L1 segment only produced monosynaptic EPSPs in 35% of the motoneurones. The L1 evoked EPSPs were much smaller than C7 evoked EPSPs. 6. The C7 evoked EPSPs (C7 EPSP) showed complete occlusion (collision) with the tectal or pyramidal disynaptic EPSPs. Similar results were obtained with L1 EPSPs. These results indicate that tectal and pyramidal disynaptic EPSPs in dorsal neck motoneurones were mediated chiefly by C-mRSNs and C-inRSNs and partly by L-RSNs.     

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.     

Monosynaptic excitatory connexions of reticulospinal neurones in the nucleus reticularis pontis caudalis with dorsal neck motoneurones in the cat

Summary 1. Projections of reticulospinal neurones (RSNs) in the nucleus reticularis pontis caudalis (N.r.p.c.) to dorsal neck motoneurones supplying splenius (SPL, lateral head flexor) and biventer cervicis and complexus (BCC, head elevator) muscles were studied in the cat anaesthetized with pentobarbiturate or -chloralose. 2. Threshold mapping for evoking antidromic spikes revealed that most of RSNs tested projecting down to brachial segments but not to lumbar segments (C-RSNs) gave off collaterals to the gray matter of the upper spinal cord in C2–C3 segments. 3. Spike triggered averaging showed that negative field potentials were evoked after firing of a single C-RSN (single fibre focal synaptic potentials, FSPs) in the region of C2–C3 where large antidromic field potentials from nerves supplying SPL or BCC muscles were evoked. The single fibre FSPs ranged between 1 and 10 V in amplitude and had latencies between 0.7 and 1.2 ms from the onset of the triggering spike. In most cases, a presynaptic spike preceded the negative potential by 0.3 ms. These results indicated that C-RSNs project to the SPL or BCC motor nucleus. 4. Spike triggered averaging of postsynaptic potentials revealed EPSPs (single fibre EPSPs) in 36 dorsal neck motoneurones, predominantly in SPL (25) and less in BCC (11) motoneurones, evoked from 15 C-RSNs. The amplitude of the single fibre EPSPs ranged from 5 to 310 V, and had latencies of 0.8–2.0 ms from the onset of the triggering spikes of C-RSNs, or 0.3–0.5 ms from the presynaptic spike when recorded. The results indicated monosynaptic excitatory connexions of C-RSNs to dorsal neck motoneurones. 5. Single fibre EPSPs from a C-RSN were usually recorded from either BCC or SPL motoneurones but not from both types of motoneurones, when tested in many motoneurones. This showed that connexions of C-RSNs with dorsal neck motoneurones were muscle specific. 6. RSNs projecting down to the lumbar segment (L-RSN) also showed branching in C2–C3 segments. Excitatory monosynaptic connexion of L-RSNs with neck motoneurones were demonstrated by recording single fibre postsynaptic population potentials (p.s.p.p.s.) from the C2 ventral root perfused with sucrose. The probability of evoking monosynaptic single fibre p.s.p.p.s. was less (19%) than for C-RSNs (59%).     

Premotor interneurones contributing to actions of feline pyramidal tract neurones on ipsilateral hindlimb motoneurones

The aim of the study was to analyse the potential contribution of excitatory and inhibitory premotor interneurones in reflex pathways from muscle afferents to actions of pyramidal tract (PT) neurones on ipsilateral hindlimb motoneurones. Disynaptic EPSPs and IPSPs evoked in motoneurones in deeply anaesthetized cats by group Ia, Ib and II muscle afferents were found to be facilitated by stimulation of the ipsilateral, as well as of contralateral, PT. The ipsilateral actions were evoked by either uncrossed or double-crossed pathways. The results show that interneurones mediating reflex actions of muscle afferents may be activated strongly enough by PT stimulation to contribute to movements initiated by ipsilateral PT neurones and that PT actions relayed by them might be enhanced by muscle stretches and/or contractions. However, in some motoneurones disynaptic IPSPs and EPSPs evoked from group Ib or II afferents were depressed by PT stimulation. In order to analyse the basis of this depression, the transmitter content in terminals of 11 intracellularly labelled interneurones excited by PT stimulation was defined immunohistochemically and their axonal projections were reconstructed. The interneurones included 9 glycinergic and 2 glutamatergic neurones. All but one of these neurones were mono- or disynaptically excited by group I and/or II afferents. Several projected to motor nuclei and formed contacts with motoneurones. However, all had terminal projections to areas outside the motor nuclei. Therefore both inhibitory and excitatory interneurones could modulate responses of other premotor interneurones in parallel with direct actions on motoneurones.     

Cortical and long spinal actions on lumbosacral motoneurones in the cat.

1. The effects of stimulating forelimb afferents on various ipsilateral motoneurones of the hind limb have been compared with those of volleys set up in the contralateral pericruciate cortex in cats anaesthetized with chloralose. 2. With intact neuraxis, brachial plexus volleys evoke discharge of flexor and extensor motoneurones; short cortical tetani also elicit discharge mainly of flexor motoneurons. After a pyramid-sparing brainstem lesion, little or no firing is evoked by either input. 3. Monosynaptic reflex testing and intracellular recording reveal subthreshold actions on hind-limb motoneurones, inhibition of FDHL and later facilitation of extensors and flexors by forelimb volleys, facilitation of flexors and extensors together with inconstant inhibition of the latter, by cortical stimulation. 4. Interruption of medullary extrapyramidal paths greatly reduces intensity and duration of facilitation from the forelimb, and largely removes cortically evoked extensor facilitation. Inhibition of FDHL from forelimb and cortex is unchanged; cortical volleys continue to facilitate flexors, and have mainly inhibitory action on extensors in these 'pyramidal' preparations. 5. Hyperpolarization of FDHL motoneurones occurs in response to forelimb and cortical volleys, of time course corresponding to depression of test reflexes. Spinal pathways responsible for the two inhibitory actions are independent, and unless each is very strong, their separate actions summate when elicited together. 6. Receptive field for FDHL inhibition from the forelimb is located distally in the forepaw, and its receptors are largely served by cutaneous fibres of low threshold; some Group II fibres in distal muscle nerves also contribute. Receptive field for facilitation embraces the whole limb, and the executant afferent fibres are of higher threshold. 7. Natural stimulation of the forelimb can evoke the long spinal actions, vibration or light pressure on the forepaw eliciting FDHL inhibition, and strong pinching evoking the more general facilitation. Possible functional roles of these actions in the intact animal are discussed.     

Synaptic linkage of masseter muscle afferents to masseteric motoneurones in the cat

The synaptic linkage of masseter muscle afferents to masseteric motoneurones was investigated under blockage of soma-dendritic invasion of antidromic spikes by passing constant inward current across the cell membrane. A monosynaptic latency for excitatory postsynaptic potentials produced by group Ia afferents was measured as 1.3 ms and no group lb component was obtained. Inhibitory postsynaptic potentials with latencies of 5.5 ms were produced at a stimulus strength of 4.5 times the threshold of group Ia fibers. On the basis of stimulus strength, muscle afferents activated at 4.5 times the threshold and producing inhibitory postsynaptic potentials in masseteric motoneurones are probably group II afferents. The same reversal point was obtained in both the lingually induced and the group II IPSPs, indicating that the group II inhibitory postsynaptic potential is dependent on an increased permeability to Cl ions. The inhibitory postsynaptic potentials produced by stimulation of the high threshold muscle afferents were the composite of a strychnine-sensitive and strychnine-insensitive inhibitory postsynaptic potential. The latency of the inhibitory postsynaptic potentials caused by the high threshold muscle afferents was about 10 ms.     

Uncrossed actions of feline corticospinal tract neurones on hindlimb motoneurones evoked via ipsilaterally descending pathways

Despite numerous investigations on the corticospinal system there is only scant information on neuronal networks mediating actions of corticospinal neurones on ipsilateral motoneurones. We have previously demonstrated double crossed pathways through which pyramidal tract neurones can influence ipsilateral motoneurones, via contralaterally descending reticulospinal neurones and spinal commissural interneurones. The aim of the present study was to examine the effects of stimulation of pyramidal tract (PT) fibres mediated via ipsilaterally descending pathways and to find out which neurones relay these effects. This was done by using intracellular recordings from 96 lumbar motoneurones in deeply anaesthetized cats. To eliminate actions of fibres descending on the side contralateral to the location of the motoneurones, the spinal cords were hemisected on this side at a low-thoracic level. Stimuli that selectively activated ipsilateral PT fibres evoked EPSPs and/or IPSPs in 34/47 motoneurones tested. These PSPs were evoked at latencies indicating that the most direct coupling between PT neurones and motoneurones in uncrossed pathways is disynaptic. Occlusion and spatial facilitation between actions evoked by stimulation of ipsilateral PT and of reticulospinal tract fibres in the ipsilateral medial longitudinal fascicle (MLF) indicated that PT actions are mediated by reticulospinal neurones with axons in the MLF. However, after transection of the MLF in the caudal medulla, stimulation of the ipsilateral PT continued to evoke EPSPs and IPSPs with characteristics similar to when the MLF was intact (in 15/49 motoneurones) suggesting the existence of parallel disynaptic pathways via other relay neurones.     

L-glutamate and N-methyl-D-asparatate actions on membrane potential and conductance of cat abducens motoneurones

The actions of L-glutamate and N-methyl-D-aspartate (NMDA) were studied with intracellular recordings from cat abducens motoneurones. Amino acids were electrophoresed extracellularly from the same 7-barreled electrode types as those used in the spinal cord. Depolarization development, conductance changes and firing pattern induced by amino acids were very similar to those described for spinal motoneurones. The shape of NMDA depolarization suggests a uniform distribution of the involved receptors on the membrane of the motoneurone.     

Synaptic actions of the ventral root afferents on cat hindlimb motoneurons

Postsynaptic potentials (PSPs) with long latencies were evoked in cat hindlimb motoneurons by stimulation of the distal stump of a cut ventral root. Measurements of their latency and the threshold in the responsible afferent fibers showed that they were produced mainly by the activity of the unmyelinated fibers in the ventral root that enter the spinal cord through the dorsal root. Patterns of PSPs evoked in flexor and extensor motoneurons by ventral root stimulation were similar to those observed in the flexion reflex.     

Synaptic potentials induced by postganglionic stimulations in cat bladder parasympathetic neurones

Intracellular recording techniques were used to examine and compare synaptic potentials evoked by stimulating pre- and postganglionic nerve trunks in cat bladder parasympathetic ganglia. In the 76 ganglion cells exammed, two types of responses were recorded on stimulating the postganglionic nerve: an antidromic action potential (type Post NS1;n=30) or a fast excitatory postsynaptic potential (f-EPSP; type PostNS2;n=46) which resulted in an orthodromic-like action potential. In some of the cells exhibiting a PostNS1 response (n=19), a fast depolarization was superimposed on the antidromic spike. This depolarization was due to the synaptic activation of nicotinic receptors. In many of the cells exhibiting either PostNS1 or PostNS2 responses, repetitive stimulation of the postganglionic nerve induced a slow hyperpolarization. Applying nicotinic (hexamethonium, methonium, 0.5–1 mM), muscarinic (atropine, 1 M), alpha-adrenergic (phentolamine, 1 M) and purinergic (caffeine, 0.5–1 mM) receptor antagonists completely inhibited the tetanus-induced slow hyperpolarization in some cells (n=5). In other cells (n=15), a slow hyperpolarization persisted in the presence of these antagonists. These results indicate that stimulation of the postganglionic nerve trunk of cat bladder parasympathetic ganglia can elicit not only an antidromic action potential, but also synaptic potentials which are mediated by the activation of cholinergic (nicotinic and muscarinic), noradrenergic and purinergic receptors, as well as a non-cholinergic, non-alpha-adrenergic and non-purinergic synaptic potential.     

Synaptic actions of the superior colliculus on medial rectus motoneurons in the cat

Synaptic effects of superior colliculus stimulation on medial rectus motoneurons were studied in encéphale isolé cats. Excitatory postsynaptic potentials were observed in all medial rectus motoneurons located on the side of stimulation, whereas contralateral motoneurons received mainly inhibition. The latencies of stimulus-locked excitatory and inhibitory postsynaptic potentials were in the ranges of 1.3–2.6 and 2.0–3.5 ms. respectively, i.e. on the average longer than in abducens motoneurons. Acute lesions of paramedian structures at bulbar levels did not affect the excitatory responses. Pontine transection at the level of the abducens nucleus reduced the mass response of medial rectus motoneurons, but failed to abolish short latency excitatory potentials in motoneurons studied intracellularly.The present data suggest that the shortest excitatory pathway from the superior colliculus to medial rectus motoneurons is disynaptic. The inhibitory pathway appears to contain at least one additional interneuron. The reciprocal pattern of synaptic action on antagonistic (left and right) medial rectus motoneurons indicates that collicular stimulation activated connections responsible for conjugate contraversive eye movements. According to the results of transection experiments. bulbar structures cannot be regarded as the main relay site of tectofugal effects on ocular motoneurons. Although the exact location of relay neurons could not he at present established. the observed timing of synaptic events is not inconsistent with the idea that tectal influences on medial rectus and abducens motoneurons are mediated by common internuncial cells in the parabducens region.     

Candidate premotor neurones of skin reflex pathways to T1 forelimb motoneurones of the cat

This study explored the locations and input output properties of a large population of putative premotor neurones of skin reflex pathways in the cat. These neurones, interneurones excited by forelimb skin afferents and antidromically from the T1 motor nucleus (MN) and/or the lateral funiculus (LF, C8/T1 border), termed antidromic cells, were extracellularly recorded at C6-8. Selection of this site was based on data showing that cells retrogradely HRP labelled from the T1 MN were most numerous in C6-8 and the observation that transection of LF at the C8/T1 border abolished most skinevoked postsynaptic potentials of T1 motoneurones. Antidromic cells were located in laminae IV–V, VI and VII. The latencies of antidromic excitation ranged from 0.4 to 1.8 ms, with a tendency for laminae IV–V cells to show longer latencies than laminae VI and VII cells. Latency of skinevoked excitation ranged from 0.6 ms (IV–V cells), 0.8 ms (VI) and 1.4 ms (VII) to greater than 5 ms. The sum of the ortho and antidromic latencies (estimated central latency) of individual cells explained the central latencies of skinevoked postsynaptic potentials in T1 motoneurones. Skin-evoked firing responses (average of eight to ten cells) were earliest and largest in laminae IV–V antidromic cells, and latest and smallest in lamina VII cells. The antidromic cells also received inputs from muscle afferents and descending tracts. The following three results support the suggestion that the sampled antidromic cells are mostly premotor neurones. (1) Projection to the T1 MN via LF was verified in six laminae IV–VII antidromic cells, as tested with threshold mapping for antidromic excitation. (2) Three skinexcited axons of the middle LF projected to T1 MN, as revealed by intra-axonal staining (HRP). (3) PHA-L injection in laminae I–V of C8 anterogradely labelled terminals in lamina IX and LF axons at T1. It is suggested that last order neurones of skin reflex pathways to T1 motoneurones are widely distributed in laminae IV–VII of C6-8 and consist of a variety of neurones with different locations and input patterns.     

Origin of corticospinal neurones evoking disynaptic excitation in forelimb motoneurones mediated via C3-C4 propriospinal neurones in the cat

Intracellular recording was made from forelimb motoneurones in the cat (alpha-chloralose anaesthesia) during electrical stimulation of corticospinal neurones (CSNs) and their afferents in the contralateral cortex. Axons of the CSNs were stimulated in the contralateral pyramid. The corticospinal tract was transected at the C5/C6 segmental border in order to restrict transmission through the C3-C4 propriospinal neurones (C3-C4 PNs). Di- and trisynaptic cortical EPSPs could be evoked after transection of the corticospinal fibres in C5/C6 but not after a corresponding transection in C2/C3. Pyramidal stimulation elicited disynaptic EPSPs that were abolished after a C2/C3 transection. Disynaptic pyramidal EPSPs, mediated via C3-C4 propriospinal neurones could be facilitated by a single cortical stimulation. It is concluded that di- and trisynaptic cortical EPSPs and disynaptic pyramidal EPSPs are mediated via the same C3-C4 PNs. Cortical surface stimulation showed that di- and trisynaptic cortical EPSPs could be evoked from distinct spots in the lateral part of the anterior sigmoid gyrus (Sig. a) and/or in the rostral part of the lateral sigmoid gyrus (Sig. l). No cortical EPSPs or facilitation of pyramidal disynaptic EPSPs was evoked from the posterior part of the Sig. l, posterior sigmoid gyrus, coronal gyrus, lateral gyrus, suprasylvian gyrus and ectosylvian gyrus. It is concluded that the CSNs, which issue the command for visually guided target reaching with the forelimb via the C3-C4 PNs, originate in the lateral part of the Sig. a and in the rostral part of the Sig. l. A dual representation of the forelimb in the primary motor cortex of the cat has previously been proposed. The present results show that with respect to one identified interneuronal system like the C3-C4 propriospinal system, the CSNs may have their origin restricted to one region of the primary motor cortex.     

Synaptic actions of peripheral nerve impulses upon Deiters neurones via the mossy fibre afferents

1. The cerebellar integration of sensory inputs to Deiters neurones was investigated in decerebrate cats. In some preparations decerebration was combined with transection of the olivocerebellar fibres.2. In the latter preparations peripheral nerve impulses generally produced a response consisting of a sequence of the following post-synaptic potentials: (i) an initial e.p.s.p. (d(1)), (ii) early i.p.s.p. (h(1)), (iii) later i.p.s.p. (h(2)).3. The mean latencies of d(1), h(1) and h(2) were 5.7, 7.3 and 9.8 msec from the forelimb nerves, and 7.5, 9.0 and 13.4 msec from the hind limb nerves, respectively.4. The stimulus intensity-response relation indicates that the Group I muscle afferents as well as the low threshold cutaneous afferents contribute to the response.5. In the preparations with the intact inferior olive there were additional components of the post-synaptic potentials: a later e.p.s.p. (d(2)) and another later i.p.s.p. (h(3)), their mean latencies being 15.3 and 19.7 msec from the forelimb nerves, and 18.0 and 21.3 msec from the hind limb nerves, respectively.6. The d(1) and h(2) components were attributed to the mossy fibre afferents and d(2) and h(3) to the climbing fibres; d(1) and d(2) were due to excitation through the collaterals of the mossy and climbing fibres, and h(2) and h(3) to inhibition from Purkyne cells activated by the mossy and climbing fibres, respectively. h(1) was too early to be produced through the cerebellum, and was probably mediated by inhibitory neurones in the reticular formation.