Ipsilateral Ventral Funiculus Research Articles

Projections from the oral pontine reticular nucleus to the spinal cord of the mouse

The present study investigated projections of the mouse oral pontine reticular nucleus (PnO) to the spinal cord by (a) injecting a retrograde tracer fluoro-gold (FG) to the lumbar cord and (b) an anterograde tracer biotinylated dextran amine (BDA) to PnO. We found that PnO projects to the entire spinal cord with an ipsilateral predominance. PnO fibers mainly travel in the ipsilateral ventral funiculus in the entire cord, terminating in laminae 7–10 with a lower density of fibers and boutons in lower segments. A small number of fibers travel in the contralateral ventral funiculus in the cervical cord with a similar terminating pattern to the ipsilateral counterpart. The present study is the first demonstration of PnO fiber terminals in the mouse spinal cord. This pathway might be responsible for muscle atonia during REM sleep, but needs physiological research to confirm this.

A Molecular Program for Contralateral Trajectory: Rig-1 Control by LIM Homeodomain Transcription Factors

Despite increasing evidence for transcriptional control of neural connectivity, how transcription factors regulate discrete steps in axon guidance remains obscure. Projection neurons in the dorsal spinal cord relay sensory signals to higher brain centers. Some projection neurons send their axons ipsilaterally, whereas others, commissural neurons, send axons contralaterally. We show that two closely related LIM homeodomain proteins, Lhx2 and Lhx9, are expressed by a set of commissural relay neurons (dI1c neurons) and are required for the dI1c axon projection. Midline crossing by dI1c axons is lost in Lhx2/9 double mutants, a defect that results from loss of expression of Rig-1 from dI1c axons. Lhx2 binds to a conserved motif in the Rig-1 gene, suggesting that Lhx2/9 regulate directly the expression of Rig-1. Our findings reveal a link between the transcriptional programs that define neuronal subtype identity and the expression of receptors that guide distinctive aspects of their trajectory.

Mechanism for Activation of Locomotor Centers in the Spinal Cord by Stimulation of the Mesencephalic Locomotor Region

The synaptic pathways of mesencephalic locomotor region (MLR)-evoked excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) recorded from lumbar motoneurons of unanesthetized decerebrate cats during fictive locomotion were analyzed prior to, during, and after cold block of the medial reticular formation (MedRF) or the low thoracic ventral funiculus (VF). As others have shown, electrical stimulation of the MLR typically evoked short-latency excitatory or mixed excitatory/inhibitory PSPs in flexor and extensor motoneurons. The bulbospinal conduction velocities averaged approximately 88 m/s (range: 62-145 m/s) and segmental latencies for EPSPs ranged from 1.2 to 10.9 ms. The histogram of segmental latencies showed three peaks, suggesting di-, tri-, and polysynaptic linkages. Segmental latencies for IPSPs suggested trisynaptic or polysynaptic transmission. Most EPSPs (69/77) were significantly larger during the depolarized phase of the intracellular locomotor drive potential (LDP), and most IPSPs (35/46) were larger during the corresponding hyperpolarized phase. Bilateral cooling of the MedRF reversibly abolished locomotion of both hindlimbs as measured from the electroneurogram (ENG) activity of muscle nerves and simultaneously abolished or diminished the motoneuron PSPs and LDPs. Unilateral cooling of the VF blocked locomotion ipsilaterally and diminished it contralaterally with concomitant loss or decrease the motoneuron PSPs and LDPs. Relative to the side of motoneuron recording, cooling of the ipsilateral VF sometimes uncovered longer-latency EPSPs, whereas cooling of the contralateral VF abolished longer-latency EPSPs. It is concluded that MLR stimulation activates a pathway that relays in the MedRF and descends bilaterally in the VF to contact spinal interneurons that project to motoneurons. Local segmental pathways that activate or inhibit motoneurons during MLR-evoked fictive locomotion appear to be both ipsilateral and contralateral.

Absence of impairments or recovery mediated by the uncrossed pyramidal tract in the rat versus enduring deficits produced by the crossed pyramidal tract

The pyramidal tract of the rat consists of at least two components. A majority of the fibers cross in the lower medulla and descend through the spinal cord in the ventral portion of the dorsal funiculus. The remaining 5% of the corticospinal projection does not cross and descends in the ipsilateral ventral funiculus into the cervical spinal region where its projections terminate in the internuncial portions of the spinal gray matter. The anatomical origin and terminal distribution of the ipsilateral component suggests that it may be involved in the control of the ipsilateral limb, but the possible contribution of the ipsilateral corticospinal tract has not been systematically examined. To determine whether the ipsilateral corticospinal tract makes a contribution to skilled movement, the corticospinal tract was severed unilaterally at the medullary level rostral to the decussation, thus severing both the crossed component of the tract as well as the ipsilateral component. Performance of the ipsilateral and the contralateral limbs of rats were then evaluated on tests of limb posture, preference, placing, and use in two skilled reaching tasks. No impairments on any quantitative or qualitative measure of performance were detected in the use of the limb ipsilateral to the lesion but severe, enduring impairments on all qualitative and quantitative measures were obtained in use of the limb contralateral to the lesion. Thus, the study finds: (1) no evidence that the ipsilateral portion of the corticospinal tract makes a contribution to skilled movement of the kind made by the contralateral portion of the corticospinal tract, and (2) no evidence that the remaining uncrossed portion of the tract contributes to recovery of symptoms produced by severing the crossed portion of the tract.

Cells of origin, course, and termination patterns of the ventral, uncrossed component of the mature rat corticospinal tract.

The cells of origin, the course, and termination patterns of the ventral, uncrossed component of the rat corticospinal tract (CST) was investigated by using retrograde and anterograde tracing methods. Anterograde tracing with biotin dextran-amine (BDA) revealed the position and detailed morphology of CST fibers in the spinal cord. Cross sections on spinal levels C4, T8, and L4 showed labeled fibers in the ipsilateral ventral funiculus on all levels. Although ipsilateral ventral CST fibers run close to the midline in the cervical cord, they were found to disperse more in the ventromedial funiculus at lower spinal levels. To study the termination patterns of the ipsilateral ventral projection, a dorsal spinal cord hemisection was performed at level T8, severing the crossed dorsomedial and dorsolateral components but leaving ipsilateral ventral running fibers intact. These fibers were observed to have sometimes several collaterals with terminal arbors extending into different spinal segments, innervating mostly laminae III-VI. Structures closely resembling synaptic boutons were identified in these arbors. By retrograde tracing in animals with dorsal spinal cord hemisection, we found labeled cells equally distributed throughout the spinally projecting cortical areas corresponding to the level of tracer injection. Labeled cells were found in layer V. The diameter of the labeled cells was not significantly different from other spinally projecting cortical neurons. In summary, a neuroanatomically complete ipsilateral, ventral corticospinal projection down to low spinal levels was found. The large extension of the terminal arborizations in intermediate laminae of the spinal cord suggests a modulatory role of this CST component.

Morphology of single vestibulospinal collaterals in the upper cervical spinal cord of the cat: III collaterals originating from axons in the ventral funiculus ipsilateral to their cells of origin.

Some vestibulospinal pathways are composed of a homogeneous collection of axons with similar intraspinal collaterals. Other pathways contain axons whose collaterals vary in terms of shape, distribution, and complexity. The purpose of the present study was to extend the study of homogeneity versus heterogeneity of vestibulospinal axons to vestibulospinal axons that travel in the ventral funiculus ipsilateral to their cells of origin. Collaterals of these axons were stained following extracellular injections of Phaseolus vulgaris-leucoagglutinin in rostral parts of the medial and descending vestibular nuclei. All collaterals found in C2 and C3 were reconstructed. Collaterals arising from small diameter (0.5 to 2.9 microns) axons usually consisted of a single main branch with short side branches. The termination zones of most of these collaterals formed a narrow path in lamina VIII, but the location of this pathway was highly variable. Collaterals arising from large-diameter (3.0-6.1 microns) axons were usually more complex and consisted of many branches with en passant and terminal boutons that were located in motoneuron nuclei as well as laminae VIII and VII. Despite a relationship between termination zone and the position of the parent axon in the ventral funiculus, the variability in collaterals from large-diameter axons precluded a simple classification scheme. These results demonstrate that diversity, instead of homogeneity, is a characteristic feature of vestibulospinal axons that originate from the medial and descending vestibular nuclei and travel in the ipsilateral ventral funiculus. This pathway is therefore composed of multiple anatomical subunits that, as individuals, may selectively coordinate the activity of specific combinations of interneurons and motoneurons.

Lumbosacral spinal neurons in the cat that are candidates for being activated by collaterals from the spinocervical tract

Lumbosacral spinal neurons activated via the spinocervical tract were stained by intracellular injection of horseradish peroxidase in cats anaesthetized with chloralose and paralysed with gallamine triethiodide. The neurons were activated orthodromically by single shock stimulation of the ipsilateral dorsolateral funiculus at the second to third cervical segment, but not from the rostral part of the first cervical segment. Twenty nine cells were recovered from the histological material and subsequently reconstructed from transverse sections. Sixteen cells (55%) had axons that projected ipsilaterally to the lateral funiculus and their somata were located in two regions of the spinal cord, one group in the dorsal horn (laminae IV-V) and the other in the intermediate gray matter (laminae VI-VII). The axons of 10 of these cells gave off collaterals, and in seven of them the collaterals ramified in the grey matter deep to the cell body. The axons of five cells (17%) projected medially towards the central canal, four crossing the mid line in the ventral white commissure and ascending in the contralateral ventral funiculus. Only one of these cells had an axon collateral that crossed into the contralateral dorsal horn. Of the remaining eight cells, three had no obvious long axons but had many local axon collaterals, the axons of three cells were not stained, one had an axon projecting towards the ipsilateral ventral funiculus and one was a motoneuron and its axon projected into a ventral root. A feature of the dendritic trees of many cells was their wide spread in the mediolateral and/or the dorsoventral directions, although no dendrites reached dorsally into lamina II. Twenty-two cells (76%) were excited by moving hairs and by noxious pinch, three (10%) by hair movement alone, two (7%) by noxious pinch and pressure, and for two cells (7%) no receptive field could be found. It is concluded that not only postsynaptic dorsal column neurons receive input from the spinocervical tract but also other cells in the dorsal and ventral horns and the intermediate gray matter. Possible identities for these cells are discussed.

Pathfinding by growth cones of commissural interneurons in the chick embryo spinal cord: A light and electron microscopic study

To investigate putative axonal guidance mechanisms used by commissural interneurons in the chick embryo spinal cord, we have examined growth cone morphology, the microenvironment through which the growth cones advance, and interactions between growth cones and their surroundings. Growth cones of both early and late developing commissural interneurons were examined. The growth cones were visualized by injection of either horseradish peroxidase (HRP) or the fluorescent dye Di-I. Unlabelled growth cones as well as HRP-labelled growth cones were also examined by electron microscopy. The early developing growth cones project circumferentially without fasciculation until they reach the region of the longitudinal pathway in the contralateral ventral funiculus (CVF). In their trajectory towards the floor plate, axons exhibited elaborate growth cones with filopodia and lamellipodia. They projected between processes of neuroepithelial cells within abundant extracellular spaces. Upon arrival at the ipsilateral ventral funiculus, growth cones did not appear to contact preexisting longitudinal axons. Within the floor plate, the growth cones were less complex and lacked long filopodia and exhibited bulbous or varicose shapes with short processes. Electron microscopic observations of the floor plate at this stage revealed that there was only a small amount of extracellular space and that the basal portion of the floor plate cells were directionally oriented (polarized) in the transverse plane. It is of particular interest that contacts between growth cones and the basement membrane in the floor plate were often observed. When the growth cones reached the contralateral ventrolateral region, they again exhibited an elaborate morphology. Close contacts between growth cones and the preexisting contralateral longitudinal axons were observed. Growth cones advancing in the contralateral longitudinal pathway exhibited various shapes and were observed to contact other axons and processes of neuroepithelial cells. Most of the later developing growth cones of commissural cells exhibited lamellipodial shapes irrespective of their location along the circumferential trajectory. Electron microscopic observations revealed that these late developing growth cones always contacted or fasciculated with preexisting axons and that the cellular environment through which they grow is oriented in such a way that the growth cones appear to be guided in specific directions. Growth cones entering the CVF exhibited more elaborated shapes with ramified lamellipodia that made multiple contacts with preexisting longitudinal axons. The present results indicate that differential axonal guidance mechanisms may be employed along the pathway followed by spinal commissural interneurons and that axons and growth cones projecting along this pathway at different developmental stages employ different mechanisms for pathfinding and guidance.(ABSTRACT TRUNCATED AT 400 WORDS)

The projection of locus coeruleus neurons to the spinal cord in the rat determined by anterograde tracing combined with immunocytochemistry

Pontospinal noradrenergic neurons located in the A5, A7 and locus coeruleus/subcoeruleus (LC/SC) nuclei are the major source of the noradrenergic innervation of the spinal cord. However, the specific terminations of spinally-projecting noradrenergic neurons located in these nuclei have not been clearly defined. The purpose of the experiments described in this report was to more precisely define the spinal terminations of neurons located in the LC/SC using the anterograde tracer phaseolus vulgaris-leucoagglutinin in combination with dopamine-β-hydroxylase (DβH) immunocytochemistry. In addition, the spinal cord regions in which LC/SC neurons terminate was assessed by measuring the reduction in the density of DβH-immunoreactive axon terminals in specific spinal cord regions after a unilateral electrolytic lesion that included LC/SC neurons. The results of these experiments indicate that the axons of LC neurons are located primarily in the ipsilateral ventral funiculus and terminate most heavily in the medial part of laminae VII and VIII, the motoneuron pool of lamina IX, and lamina X. LC neurons provide a moderately dense innervation of the ventral part of the dorsal horn, but only a very sparse innervation of the superficial dorsal horn. The SC projects ipsilaterally in the ventrolateral funiculus and terminates diffusely in the intermediate and ventral laminae of the spinal cord.

Medullary reticular neurons in the Japanese toad: morphologies and excitatory inputs from the optic tectum.

1. To elucidate the neural mechanisms that mediate visual responses of optic tectum (OT) to medullary and spinal motor systems, we analyzed medullary reticular neurons in paralyzed Japanese toads (Bufo japonicus). We examined their responses to electrical stimulation of OT, and stained some neurons intracellularly. Responses to stimulation of the glossopharyngeal nerve (IX) were also analyzed. 2. Extracellular single unit recording revealed excitatory responses of medullary neurons to OT and IX stimulation. Among 92 units encountered, 79 responded to OT stimuli, 10 to IX stimuli, and 3 to both. Some units responded to successive stimuli of short intervals with relatively stable lags. 3. Intracellular recording and staining experiments revealed morphologies of reticular neurons that received excitatory inputs from OT. Thirteen units were identified after complete reconstruction of somata and dendrites. Neurons in the nucleus reticularis medius received excitatory inputs from bilateral OT. They had wide dendrites in ventral, ventrolateral and lateral funiculi, and single axons descending in the ipsilateral ventral funiculus as far caudally as the cervical spinal cord. Some collaterals of these axons projected directly to the hypoglossal and spinal motor nuclei. Some neurons in other medullary nuclei (nuc. reticularis superior, pretrigeminal nucleus, nuc. reticularis inferior, and nuc. tractus spinalis nervi trigemini) also responded to the OT stimulation. 4. Activities in bilateral OT converge onto medullary reticular neurons, which may directly control medullary and spinal motor systems.

Anatomical evidence for red nucleus projections to motoneuronal cell groups in the spinal cord of the monkey

In 4 rhesus monkeys wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injections were made in the mesencephalic tegmentum. In 3 cases with injections involving the red nucleus (RN), rubrospinal fibers descended mainly contralaterally to terminate in laminae V, VI and dorsal VII of the spinal cord and in the lateral motoneuronal cell groups at the level of the cervical and lumbosacral enlargements. In all 4 cases the area of the interstitial nucleus of Cajal (INC) was injected, which resulted in labeled interstitiospinal fibers in the medial part of the ipsilateral ventral funiculus of the spinal cord. The results indicate that there is no major qualitative difference between the mesencephalic (RN and INC) and motor cortical projections to the spinal cord.

Axonal trajectories of posterior canal-activated secondary vestibular neurons and their coactivation of extraocular and neck flexor motoneurons in the cat.

Unit activities of 148 secondary vestibular neurons related to the posterior semicircular canal were recorded extracellularly in anesthetized cats. Axonal projections of these neurons were examined by their antidromic responses to stimulation of the excitatory target motoneurons of the contralateral (c-) inferior rectus muscle (IR) and bilateral (bi-) motoneuron pools of longus capitis muscles, neck flexors, in the C1 segment (C1LC). The neurons were classified into 4 groups according to their axonal projections. The first group of neurons, termed vestibulo-oculo-collic (VOC) neurons, sent axon collaterals both to the c-IR motoneuron pool and to the c-C1LC motoneuron pool. The majority of them (72%) were located in the descending nucleus. The second group of neurons were termed vestibuloocular (VO) neurons and sent their axons to the c-IR motoneuron pool but not to the cervical cord. Most of them (86%) were located in the medial nucleus. The third group of neurons, termed vestibulo-collic (contralateral) (VCc) neurons, sent axons to the c-C1LC motoneuron pool via the contralateral ventral funiculus but not to the oculomotor nuclei. They were mostly (75%) found in the descending nucleus. The last group of neurons were vestibulo-collic (ipsilateral) (VCi) neurons, which gave off axons to the ipsilateral (i-) C1LC motoneuron pool via the ipsilateral ventral funiculus but not to the oculomotor nuclei. One of them also sent an axon collateral to the c-C1LC motoneuron pool. The majority of them (74%) were located in the ventral part of the lateral nucleus. It was also observed in some of the VOC and VCi neurons that they produced unitary EPSPs in the c-C1LC and i-C1LC motoneurons, respectively. Their synaptic sites were estimated to be on the cell somata and/or proximal dendrites of the motoneurons.

Chapter 8 Descending cortical and tectal control of dorsal neck motoneurons via reticulospinal neurons in the cat

Dorsal neck motoneurons innervating SPL and BCC muscles received disynaptic exitatory effects after stimulation of the tectum and the pyramid in cats. These effects were mediated by RSNs in the NRPC and the NRG. While a majority of RSNs received monosynaptic excitatory inputs from the tectal and pyramidal volleys, RSNs in the NRPC and those in the NRG differed in the following points. The majority of RSNs in the NRPC received a larger input from the tectum than from the pyramid, and descended in the ipsilateral ventral funiculus in the C3 segment. RSNs in the NRG, in contrast, had a stronger input in general from the pyramid than from the tectum and descended in the ventrolateral or lateral funiculus of the ipsilateral side, or contralateral funiculi in the C3 segment. Studies of systematic threshold mapping for evoking antidromic spikes of RSNs in C2–C3 segments and intra-axonal staining of their descending axons in C1–C3 segments with horseradish peroxidase revealed the following. Among RSNs, those descending to brachial segments (C6–Thl) but not down to lumbar segments (C-RSNs) issued collaterals to the ventral horn of C1–C3 segments, while RSNs descending to the lumbar cord rarely gave off collaterals at C1–C3 segments. Direct monosynaptic excitatory connexions of C–RSNs in the NRPC and NRG with SPL and BCC motoneurons were demonstrated by the spike-triggered averaging method. Their connexions were muscle-specific. It was found that RSNs were lastorder premotor relay neurons mediating descending commands from the cerebral cortex and the superior colliculus to neck motoneurons.

The morphology of physiologically identified deep spinothalamic tract cells in the lumbar spinal cord of the cat.

1. The morphology of eleven physiologically identified, deep, spinothalmic tract (s.t.t.) cells in the seventh lumbar segment of the cat were studied after being intracellularly injected with horseradish peroxidase (HRP).2. Four of these cells, prepared for combined light and electron microscopy, did not appear to be as well filled with HRP as the other cells which were prepared solely for the light microscope. In these four cells the axons were not stained and significantly fewer distal dendrites were stained.3. The axons of the other seven cells projected medially, crossed the mid line in the ventral white commissure and ascended in the contralateral ventral funiculus. No axon collaterals were found.4. The dendrites of eight cells could be divided into three groups. The first group projected laterally across lamina VII, usually passing through the ventrolateral portion of lamina VI before entering the lateral funiculus. The second group projected ventrally, ventromedially and ventrolaterally through the ventral parts of laminae VII and VIII and into the ipsilateral ventral funiculus. The third group projected towards the central canal, the ventral border of the dorsal columns and the dorsal parts of the ipsilateral ventral funiculus. This group usually branched profusely and projected into lamina X.5. Of the remaining three cells, one was located more deeply than any other cell, one was probably incompletely filled, and one was located more laterally. The first two cells had dendritic trees which bore strong similarities to the other eight cells described above.6. In all, 12.3-37.5% of dendritic tips were found in the white matter of the ventral or lateral funiculi.7. Dendrites never entered the dorsal columns but they could often be traced to points very close to the dorsal columns before they turned abruptly and ran parallel to the grey-white border.8. These data are discussed in relation to the excitatory and inhibitory responses of s.t.t. cells to somatic stimuli and the axonal projections of physiologically identified primary afferents and spinal interneurones.

Properties of a new vestibulospinal projection, the caudal vestibulospinal tract

Neurons in the caudal portions of the medial and descending vestibular nuclei and in vestibular cell group f that project to the cervical or lumbar spinal cord were located by antidromic spinal stimulation. These caudal vestibulospinal tract (CVST) neurons have a median conduction velocity of 12 m/sec, which is well below the conduction velocities of typical lateral or medial vestibulospinal tract (LVST, MVST) axons. The descending fiber trajectories of CVST neurons, determined by comparing thresholds for activation of each neuron from six points in the spinal white matter, were remarkably diverse. Unlike LVST and MVST axons, which are located in the ipsilateral ventral funiculi, CVST axons can be found in both the ventral and dorsolateral funiculi on both sides of the spinal cord. The CVST system is thus both anatomically and physiologically different from the LVST and MVST.

Relative strength of synaptic input from short-latency pathways to motor units of defined type in cat medial gastrocnemius

1. Intracellular recording and stimulation techniques were used in anesthetized cats to study the interrelations between amplitudes of PSPs produced by electrical stimulation of several short-latency pathways to MG alpha motoneurons and the mechanical properties of muscle units innervated by the same cells. Motor-unit types were identified by muscle-unit properties.2. The maximum amplitudes of monosynaptic EPSPs produced in MG motoneurons by activation of homonymous (MG) and heteronymous (LGS) group Ia afferents were clearly related to motor-unit type, being, on the average, largest in type S units, somewhat smaller in type FR and F(int) units, and smallest in type FF units. Correspondingly, group Ia EPSP amplitudes were inversely correlated with muscle-unit tension production and directly correlated with resistance to fatigue. The same input distribution was true for disynaptic IPSPs produced by group Ia afferents from antagonist ankle flexors.3. The amplitudes of monosynaptic EPSPs produced by fibers descending in the ipsilateral ventral funiculi of the low thoracic cord were not clearly related to MG motor-unit type or (therefore) to muscle-unit properties.4. A quantitative input-output model of the MG motor-unit pool, based in part on the present results, suggests that overall characteristics of MG motor units, and their relative numbers in the MG pool, reflect functional specializations determined by specific mechanical demands placed on the MG muscle by the usual motor behavior of the animal.

Modulation of synaptic effectiveness of Ia and descending fibers in cat spinal cord.

1. In the unanesthetized spinal cord, conditioning stimulation of low-threshold afferents (below 1.3 times threshold strength) in the biceps semitendinosus (BST) nerve often reduced the peak amplitude of the monosynaptic Ia EPSPs evoked in gastrocnemius motoneurons without affecting the monosynaptic component of the EPSPs evoked by stimulation of the ipsilateral ventral funiculus (VF) in the thoracic cord. 2. Volleys to the BST nerve comprising higher threshold afferents (usually above 1.4 times threshold strength) reduced the peak amplitude of the monosynaptic Ia and VF EPSPs and shortened their falling phase. 3. Conditioning volleys to low-threshold cutaneous afferents often increased the Ia-EPSP peak amplitude, sometimes without affecting the monosynaptic component of the VF EPSP. 4. In most cases the Ia nd VF monosynaptic EPSPs elicited in a given motoneuron summated nonlinearly. The amount of nonlinear summation between Ia and VF monosynaptic EPSPs was often reduced by low-threshold BST conditioning volleys. These observations suggest that in many instances, both species of fibers end in "electrotonically close" synaptic loci over the motoneuron surface. Therefore, amplitude changes of monosynaptic Ia EPSPs produced by conditioning afferent volleys without concomitant changes of monosynaptic VF EPSPs do not appear to result from postsynaptic remote conductance changes and may be attributed to a presynaptic mechanism. 5. At the time of occurrence of the Ia and VF monosynaptic EPSP the variance of the motoneuron membrane potential may be increased above base-line levels with a time course approximately matching the EPSP itself. Conditioning stimulation of BST afferents usually reduced Ia EPSP variance, often without affecting or even increasing the variance of the monosynaptic VF EPSPs. These observations provide additional evidence that Ia EPSP variability is introduced, at least in part, through the segmental pathways mediating primary afferent depolarization. 6. The possibility of a differential control of the information flow transmitted through two independent channels converging on a given cell ensemble is discussed.

Reticulospinal fibers of the opossum, Didelphis virginiana. II. Course, caudal extent and distribution.

AbstractLesions were placed in the deep midbrain tegmentum and within areas of the pontine and medullary reticular formation which were determined by the retrograde degeneration method to give rise to reticulospinal fibers. The location of each lesion was verified histologically and the spinal cords processed by either the Nauta or Fink‐Heimer method. Control material for the origin of degenerated fibers was provided by previous studies on corticospinal, tectospinal and rubrospinal fibers in the same form.Fibers degenerating as a result of mesencephalic tegmental lesions followed the ipsilateral ventral funiculus and could be traced as far caudal as midthoracic levels. They ended in Rexed's laminae VII and VIII.Lesions of the pontine reticular formation resulted in the degeneration of fibers within the ventral funiculus of the spinal cord. The great majority of these fibers followed an ipsilateral course. Such pontospinal fibers were present throughout the length of the spinal cord; their terminal distribution appears to take place mainly within laminae VII and VIII. At certain levels, however, such fibers could be traced into lamina IX.Lesions in the medial medullary reticular formation resulted in fiber degeneration chiefly within the ipsilateral ventral and lateral funiculi. However, a few fibers were present on the side opposite the ablation. This fiber system extends throughout the length of the spinal cord and distributes itself in laminae VII and VIII and, at certain levels, also in lamina IX. Lesions isolated to the lateral, parvocellular portion of the medullary reticular formation resulted in fiber degeneration located within the propriospinal bundles of the lateral funiculus which could be traced to rostral cervical levels. Such lesions also interrupted the rubrospinal tract in the its medullary trajectory.