Vestibulospinal projections in the toad. An experimental electron microscopic study

An experimental electron microscopical study has been made on the mode of termination of the vestibulospinal projections in the toad. Degenerating fibers and boutons were observed in the ventromedial region of the spinal cord in which five types of terminals were identified in normal preparations: S-type, F-type, C-type, NfS-type and NfF-type. The latter two types represent the largest terminals in the spinal gray. In the medial region of the ventral horn they seem to belong to supraspinal fibers.Following electrolytic lesions of the dorsal and ventral nuclei nervi octavi, some of the NfF-type of boutons undergo filamentous degeneration which progresses into the dark type at longer survival periods (7–20 days). In two animals, a few NfS-type of boutons also display filamentous hypertrophy.Problems connected with the filamentous type of degeneration in amphibians and with the identification of bouton types belonging to the vestibulospinal pathway are discussed.Degenerating fibers and boutons are seen at all levels of the spinal cord, ipsilateral and also, in smaller number, contralateral to the lesions. The vestibulospinal fibers terminate predominantly on large and small dendrites, and only occasionally on the somata of large neurons on the ipsilateral side.     

Fine structure of normal and degenerating primary afferent boutons associated with characterized spinocervical tract neurons in the cat

Spinocervical tract neurons in the dorsal horn of the cat spinal cord were intracellularly stained with horseradish peroxidase. The neurons came from one intact animal and from animals with dorsal rhizotomies (L3-S2) 3, 5, 10, 28 and 42 days previously. The morphology of terminals associated with spinocervical tract neurons was examined in a combined light and electron microscopical study. Some terminals containing agranular, circular vesicles degenerated as a result of deafferentation; these are therefore the terminals forming monosynaptic inputs to the neurons from primary afferent fibres. Other terminals containing agranular circular vesicles and terminals containing ovoid agranular vesicles survived deafferentation; these boutons therefore do not originate from primary afferent fibres.     

Ultrastructure of supraspinal dorsal root projections in the toad. II. The cerebellar granular layer

Summary Following section of the left dorsal roots, degenerating fibres and boutons were observed in the granular layer of the ipsilateral cerebellum. The degenerating terminals were identified as largeen passant varicosities of mossy fibres contacting the dendrites of presumptive granule cells. They contained round synaptic vesicles and neurofilaments and established Gray type I contacts. The terminals initially underwent filamentous degeneration with neurofilamentous hypertrophy, swollen mitochondria and loss of synaptic vesicles. At later survival times (6–30 days) they acquired an electron-dense appearance due to an increase and clumping of the filamentous component.After injection of horseradish peroxidase into the left cerebellum, all ipsilateral spinal ganglia showed a few (2–3%) labelled cells, indicating that a primary afferent contribution to this pathway originated from each segment of the spinal cord.     

Effects from the sensorimotor cortex on the spinal cord in cats with transected pyramids

Summary Effects of stimulation of the sensorimotor cortex on activity of the lumbosacral cord were studied in pyramidotomized cats. The following actions initiated by corticofugal volleys were found: 1. postsynaptic effects on motoneurones, mainly excitatory in flexor motoneurones and inhibitory or excitatory in extensor motoneurones, 2. facilitation of spinal reflexes to motoneurones at an interneuronal level, 3. depolarization of presynaptic terminals of group Ib and cutaneous fibres. The latencies of the earliest cortical effects on motoneurones as indicated by modification of monosynaptic reflexes or PSPs were 9–12 msec. Experiments with lesions of different spinal tracts suggest that the effects on motoneurones are mediated mainly by pathways in the ventral part of the lateral funiculus (probably reticulospinal), the facilitation of reflex transmission by pathways in the dorsal part of the lateral funiculus (probably rubrospinal) and primary afferent depolarization by both the former and the latter pathways. The strongest cortical effects were evoked by stimulation of an area around the postcruciate dimple.JBRO-Fellow     

The effect of transection and cold block of the spinal cord on synaptic transmission between Ia afferents and motoneurones

Composite excitatory postsynaptic potentials were elicited in lumbar motoneurones by Ia afferents from muscles of the triceps surae group. These excitatory postsynaptic potentials were examined in the same cell before, during and after interruption of descending spinal pathways. After transection or cold block of the spinal cord at T12-T13, the amplitude of composite excitatory postsynaptic potentials showed no significant change for a period of up to seven hours after transection. However, there was a reduction in amplitude of the monosynaptic reflex in the extensor motoneurones which may be due to an observed hyperpolarization and reduction in membrane time constant in these neurones. The reduction in amplitude of the monosynaptic reflex observed in spinal shock can be attributed to the effects of these changes, rather than to a decrease in the size of the monosynaptic excitatory postsynaptic potential.     

大鼠脊髓边缘细胞初级传入特点的研究

将６只Ｗｉｓｔｅｒ大鼠脊髓Ｌ（１～３）段双侧后根切断，并将ＨＲＰ注入小脑内。术后分别在２、３、４天灌杀，取出Ｌ（１～３）脊髓节段，对ＨＲＰ标记的脊髓边缘细胞及其与溃变末梢的突触联系进行了电镜观察。结果表明，在脊髓边缘细胞的胞体或近侧树突表面发现有少量的溃变终末。根据溃变终末内小泡的形状，它们属于Ｓ型和Ｆ型即球形和扁平形小泡。根据溃变终末的外形又可分为圆型和细长型。按不同术后存活期，这些终末显示的溃变特点有所不同。术后２天溃变终末主要呈电子密度增高象；术后３天溃变终末中的线粒体肿胀、溶解以及部分突触小泡溶解；术后４天的溃变终末内则表现突触小泡和线粒体大部溶解消失。各类溃变末梢周围均可见到有胶质细胞突起包绕或充填。本研究进一步证明，作为脊小脑前束起始细胞的脊髓边缘细胞所接受的外周传入从性质和特点上均与脊小脑后束的起始细胞有所不同。这些结构的不同说明脊髓边缘细胞在向小脑输送信息中有其特殊功能。     

Different susceptibility of facilitatory and inhibitory spinal pathways to ischemia in the cat

The sensitivity of different excitatory and inhibitory segmental reflex pathways to ischemia was investigated by monosynaptic reflex testing in the spinal cat. Spinal cord ischemia was established by aortal snare occlusion of 1-10 min duration. Excitatory and inhibitory spinal pathways showed statistically significant different susceptibility to ischemic impact. In the period of decreasing responses after the onset of ischemia the transmission through oligo- or polysynaptic, facilitatory or inhibitory pathways was found to be depressed earlier than that of monosynaptic pathways. The period from the end of ischemia until the beginning of recovery of reflex effects was significantly longer for inhibitory effects, compared to the monosynaptic reflexes alone.The results indicated that interneurones of excitatory segmental pathways may be less sensitive to ischemia than motoneurones, and motoneurones seem to be less sensitive to ischemia than interneurones of inhibitory pathways. In high spinal animals, with a relatively high level of extensor inhibition, the enhanced excitability of inhibitory interneurones to GS motoneurones may be responsible for their sensitivity to ischemia, due to an increased rate of O(2) consumption and exhaustion of high-energy phosphate resources.     

Plasticity of lumbosacral monosynaptic reflexes after a ventral root transection injury in the adult cat

Injuries to spinal ventral roots may induce plastic changes in adjacent segmental reflex pathways. Earlier studies in the cat have demonstrated that a partial loss of target motoneurons, following a ventral root avulsion injury, induces a compensatory enhancement of monosynaptic reflexes in adjacent segments. Here, we studied electrophysiologically the effects of a primarily non-lethal motoneuron injury of lumbosacral ventral roots on monosynaptic reflexes in adjacent intact motoneurons in the adult cat. A unilateral L7 or a combined L7 and S1 ventral root transection was first performed. We next recorded bilaterally monosynaptic reflexes from the L6 and S1 ventral roots while stimulating the bilateral L6, L7 and S1 dorsal roots at 6 and 12 weeks postoperatively. We demonstrated a prominent strengthening of monosynaptic reflexes in the immediately adjacent spinal cord segments. The reflexes had almost doubled in size at 6 and 12 weeks postoperatively. Possible mechanisms and factors contributing to the reflex enhancement are discussed.     

The lateral cervical nucleus in the cat III. An electron microscopical study after transection of spinal afferents

Summary The ultrastructure of terminal degeneration within the lateral cervical nucleus (LCN) after transection of its spinal afferent fibers 2 days–2 years earlier is described. The degeneration after 2 days was of both the neurofilamentous and dense type. The highest number of degenerating terminals, about 15%, was found after 4–5 days. Then most of the degenerating boutons were of the dense type. The degenerating terminals had synaptic contact with cell bodies and dendrites of LCN-neurons. Removal of the degenerating boutons seemed to be effected by a phagocytic cell present in increased number compared to the normal LCN. In cases with long survival times an increase in the number of astroglial filaments was observed. In an animal where the spinal afferents to the LCN had been cut 2 years earlier a decrease in medium size of the neurons was observed. The amount of dendritic spines was also considerably smaller than normally.     

Effect of naloxone on transmission in excitatory and inhibitory spinal reflex pathways.

The effect of systemic administration of naloxone on transmission in hindlimb reflex pathways was investigated in acute low spinal cats by conditioning monosynaptic reflexes. A marked enhancement of excitatory effects from cutaneous, joint, group II and III muscle afferents was observed in posterior biceps and semitendinosus motoneurones in 4 out of 6 experiments. In contrast, inhibitory synaptic effects in gastrocnemius and soleus motoneurones were not enhanced except weakly in one experiment. The effects of naloxone are different from those observed after spinal cord lesions interrupting axons of a previously described group of upper lumbar propriospinal neurones, which tonically suppress reflex transmission in the acute low spinal state. It is postulated that the suppression exerted by this group of neurones is not dependent on endogenous opioid peptides.     

The Origin of a Descending Pathway with Monosynaptic Action on Flexor Motoneurones

The postsynaptic effects evoked in lumbar motoneurones were studied following electrical stimulation of the brain stem in the cat. The spinal cord was transected at the lower thoracic level leaving only the ipsilateral ventral quadrant intact. With a stereotactical method a low threshold focus was found in the medial brain stem from which monosynaptic EPSPs could be evoked in flexor motoneurones. It is concluded that this effect is mediated by fibres descending in the ipsilateral medial longitudinal fascicle and it is tentatively suggested that these fibres originate from the ipsilateral upper medullary or lower pontine reticular formation. Monosynaptic EPSPs were also evoked in some extensor motoneurones from this medial brain stem region and at such a strength of stimulation that stimulus escape to the lateral vestibulospinal tract is excluded.     

Sensitivity of monosynaptic test reflexes to facilitation and inhibition as a function of the test reflex size: a study in man and the cat

Summary In parallel experiments on humans and in the cat it was investigated how the sensitivity of monosynaptic test reflexes to facilitation and inhibition varies as a function of the size of the control test reflex itself. In man the monosynaptic reflex (the Hoffmann reflex) was evoked in either the soleus muscle (by stimulation of the tibial nerve) or the quadriceps muscle (by stimulation of the femoral nerve). In the decerebrate cat monosynaptic reflexes were recorded from the nerves to soleus and medial gastrocnemius muscles; they were evoked by stimulation of the proximal ends of the sectioned L7 and S1 dorsal roots. Various excitatory and inhibitory spinal reflex pathways were used for conditioning the test reflexes (e.g. monosynaptic Ia excitation, disynaptic reciprocal inhibition, cutaneous inhibition, recurrent inhibition, presynaptic inhibition of the Ia fibres mediating the test reflex). It was shown that the additional number of motoneurones recruited in a monosynaptic test reflex by a constant excitatory conditioning stimulus was very much dependent on the size of the test reflex itself. This dependency had the same characteristic pattern whatever the conditioning stimulus. With increasing size of the test reflex the number of additionally recruited motoneurones first increased, then reached a peak (or plateau) and finally decreased. A similar relation was also seen with inhibitory conditioning stimuli. The basic physiological factors responsible for these findings are discussed. Finally, the implications for the interpretation of experiments in man with the H-reflex technique are considered.     

Alterations of synaptic action in chromatolysed motoneurones of the cat

1. Monosynaptic EPSPs in lumbosacral motoneurones undergoing chromatolysis were studied by intracellular recording from 7 to 20 days after section of the appropriate ventral roots of the cat.2. The maximum monosynaptic EPSPs evoked in chromatolysed motoneurones by afferent volleys from the biceps-semitendinosus or the triceps surae muscles ranged from 1.0 to 9.5 mV in amplitude. The time-to-peak of these EPSPs was 1.7 msec on the average. These values were significantly smaller and longer, respectively, than the amplitude and the time-to-peak of monosynaptic EPSPs observed in normal motoneurones. The long time-to-peak of EPSPs in chromatolysed motoneurones could not be accounted for by asynchronous transmitter release.3. The mean number of unit EPSPs responding to a single afferent impulse (m) in chromatolysed motoneurones was comparable to that found in normal motoneurones.4. The amplitude of unit EPSPs estimated from the mean EPSP amplitude and the m value following stimulation of a single afferent fibre was significantly smaller in chromatolysed motoneurones than in normal motoneurones. This difference was attributed to a difference in synaptic location.5. The shape of monosynaptic EPSPs evoked in chromatolysed motoneurones by stimulation of single afferent fibres was analysed on the basis of Rall's compartment model. The analysis suggested that there is a lack of the excitatory synaptic input to the cell body in chromatolysed motoneurones.6. Similar alterations were also found in IPSPs. The degree of change in synaptic responses evoked by stimulation of various pathways appears to depend on the synaptic location.7. Following the study of the interaction of several inputs on the motoneurone and of their dependence on membrane potential, a tentative model of the synaptic distribution of different pathways is proposed.     

Post-activation depression in various group I spinal pathways in humans

This investigation was designed to study the effects of post-activation depression in different spinal pathways fed by group I afferents available to investigation in human subjects. It was precipitated by a recent investigation in the cat showing that—contrary to the general assumption—post-activation depression is not a widespread phenomenon in the spinal cord. In 24 healthy subjects comparison was made between the effects of low and high-test stimulus rates on the monosynaptic Ia excitation, known to be subject to post-activation depression, and on oligosynaptic pathways fed by group I afferents. Both the amplitude of monosynaptic H reflexes and the amount of heteronymous monosynaptic Ia facilitation were significantly smaller at high than at low-test stimulus rates (1–2 s compared with 6–8 s between two consecutive stimuli). So was the amount of reciprocal Ia inhibition of tibialis anterior motoneurones. In contrast, the amount of other non-monosynaptic group I effects directed to the same motor nuclei (peroneal-induced excitation of quadriceps motoneurones, disynaptic non-reciprocal group I inhibition of flexor carpi radialis motoneurones, and D1 inhibition of flexor carpi radialis and soleus H reflexes) were enhanced at high stimulus rates. Results in humans confirm that post-activation depression depends on the type of group I afferents, and/or on the target neurones. The functional significance of the discrepancy between post-activation depression in pure Ia pathways and in other group I pathways is discussed with regard to the fusimotor-driven servo-assistance from Ia afferent discharges.     

Catecholaminergic innervation of the spinal dorsal horn: a correlated light and electron microscopic analysis of tyrosine hydroxylase-immunoreactive fibres in the cat.

The ultrastructural organization of presumed catecholamine-containing boutons, in the dorsal horn of the cat lumbosacral spinal cord, was examined in an immunocytochemical study using an antiserum against tyrosine hydroxylase. The study was restricted to the first four laminae of Rexed. Light microscopic inspection revealed numerous, varicose, tyrosine hydroxylase-immunoreactive axons throughout this region of the spinal cord. Within laminae I and II the fibres exhibited a prominent rostrocaudal orientation, while in laminae III and IV they were organized predominantly dorsoventrally. Correlated ultrastructural analysis confirmed that these varicosities were synaptic boutons. Forty-five of these structures were examined through serial sections and they were found to form symmetrical (Gray type II) synaptic junctions with dendrites (95%) and somata (5%). Immunoreactive boutons were not observed to be either presynaptic or postsynaptic to axon terminals. These findings suggest that catecholamines within the spinal dorsal horn act through a postsynaptic action upon dorsal horn neurons.     

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.     

Permanence of postsynaptic specializations in the frog sympathetic ganglion cells after denervation

Summary The junctional complex of the axosomatic synapses in the frog sympathetic ganglion is formed by active zones and attachment plates. 16% of the active zones present a dense band or subsynaptic formation on the postsynaptic side. Seven days after the preganglionic fibers have been cut, most of the axon terminals in the ganglion undergo degeneration. The junctional complex is broken by glial cytoplasm, which separates the axon terminals from the neuronal perikaryon. Two distinct morphological types of axonal degeneration are found at this stage: 1. dark and shrunken boutons with a honeycomb appearance, and 2. clear and swollen boutons. These two morphological varieties are interpreted as different aspects of the same degenerative process of the spiral apparatus. Ten days after transection of the preganglionic fibers, most of the degenerating axon terminals have disappeared, and only dark bodies, remnants of such endings, are seen in the glial cytoplasm. Twelve days after the experimental lesion, almost all the axon terminals have disappeared. In all three stages of survival the subsynaptic structures, postsynaptic differentiation and subsynaptic formation, remain unchanged. It is suggested that these structures are not intimately related to the functional integrity of the axon terminals.To the memory of Prof. Fernando de Castro     

Pallidosubthalamic projection in the Cat

Summary The degenerative changes within the cat's subthalamic nucleus (Sth) following lesions of the external pallidum were studied by electron microscopy.Four to five days following pallidal lesions a great number of terminals undergoing degenerative changes were encountered in the ipsilateral Sth. The contralateral Sth was free of degeneration. The degenerating terminals show predominantly the light degenerative type, less frequently the dark degenerative pattern, and occasionally exhibit signs of filamentous hyperplasia. The degenerated boutons usually insert on perikarya of the large Sth neurons, on proximal dendrites, and more rarely contact dendritic spines. They were observed neither to perform synaptic contacts with the perikarya of the small Sth neurons nor with other vesicle-containing profiles. On the basis of the ultrastructural aspect of the degenerating terminals, they were identified as F1 terminals, discriminated in a previous study (Romansky et al., 1978). The normal appearance, the synaptic relationships, and the degenerative features of the F1 terminals in the Sth closely resemble the entopeduncular terminals in the thalamus described by Rinvik and Grofová (1974a), and Grofová and Rinvik (1974).The possible contribution of the interrupted passing fibers to the observed degeneration is discussed. The present findings corroborate the relevant morphological, physiological, neurochemical, and neuropharmacological data in the literature.     

Morphology of the Mauthner axon inhibitory system in tench (Tinca tinca L.) spinal cord

Using ultrastructural features as a natural tracer we identified a commissural interneuron in the tench (Tinca tinca L.) spinal cord with the following characteristics: (1) the unipolar cell soma is located 100-150 micron dorsal from the central canal, the frequency of its occurrence being one cell on each side per spinal cord segment; (2) the first node of Ranvier of its axon is connected to a presynaptic branch of the ipsilateral Mauthner axon via gap junctions; (3) its postsynaptic targets are contralateral moto- and interneurons; (4) its terminal boutons contain f-type vesicles and form Gray-type-2 synapses abut on the initial segment or node of Ranvier of these target neurons distal to the input site of monosynaptic excitatory links from the contralateral Mauthner axon. Thus, it appears that this segmental interneuron may provide the structural basis for the well-known mutual crossed inhibition within the spinal circuit of Mauthner axons.     

Origin of corticospinal neurones evoking monosynaptic excitation in C3--C4 propriospinal neurones in the cat

Intracellular recording was made from propriospinal neurones (PNs) in the C3-C4 spinal cord segments in the cat (alpha-chloralose anaesthesia). The effect of electrical stimulation of corticospinal neurones (CSNs) in the cortex was investigated. Short C3-C4 PNs were identified by antidromic activation of their axons in the ventral horn in C6/C7 and in the lateral reticular nucleus. Long PNs were antidromically identified from Th12-13. In short PNs, monosynaptic excitory postsynoptic potentials (EPSPs) were elicited from the rostral part of the lateral sigmoid gyrus, the lateral part of the anterior sigmoid gyrus in area 4 gamma and in the adjacent area 6. Two subtypes of short PNs were identified. PNs of type I received monosynaptic EPSPs from the rostral part of the lateral sigmoid gyrus, the lateral part of the anterior sigmoid gyrus in area 4 gamma, which is from the same region as disynaptic cortical EPSPs were evoked in forelimb motoneurones. PNs of type II received monosynaptic EPSPs from regions slightly more rostrally in the anterior sigmoid gyrus in area 4 gamma and in the adjacent area 6, which is outside the region from which disynaptic EPSPs could be evoked in forelimb motoneurones. Long PNs received monosynaptic EPSPs, like the short PNs, by stimulation in the rostral part of the lateral sigmoid gyrus, the lateral part of the anterior sigmoid gyrus in area 4 gamma and in the adjacent area 6. In contrast, the long PNs also received monosynaptic EPSPs from area 3b near the border of area 1. The present results show segregation of the cortical control to functionally different premotoneuronal systems and suggest that this control could in part be separated for subtypes of short C3-C4 PNs.