Nakaakira Tsukahara

Properties of cerebello-precerebellar reverberating circuits

Intracellular recordings were made from neurons of the red nucleus (RN) in cats where the cerebellar cortical effects were removed by chronic ablation of the intermediate part of the anterior lobe of the cerebellum. A prolonged depolarization could be elicited by stimulating the nucleus interpositus (IP) of the cerebellum, nucleus reticularis tegmenti pontis (NRTP) and the nucleus reticularis paramedianus (PMRN). This prolonged depolarization was abolished after cooling the inferior and middle cerebellar peduncles and persisted after ablation of the cerebral sensorimotor cortex. The prolonged depolarization was also recorded intracellularly from IP neurons. It was concluded that the prolonged depolarization set up in RN neurons is due to the repetitive discharges of IP neurons which produces tonic bombardment onto RN cells. The mechanisms of the repetitive discharges of IP neurons are considered to be due to impulse reverberation via the IP. The dynamic properties of the reverberating circuits were characterized by regenerative behavior. Above and below threshold, there were two states, the excited state where many constituent neurons were active, and the resting state where all neurons were inactive. It was found that cats with chronically stimulated cerebral peduncle (CP), and tested in an acute experiment, showed sometimes effective for inducing the prolonged depolarization and repetitive discharges of RN neurons by stimulation of IP. The prolonged depolarization thus produced could be reversibly abolished by cooling the middle and inferior cerebellar peduncles. The possible constituent neurons of the reverberating circuits were investigated in light of previous physiological investigations of stimulating the NRTP, PMRN, nucleus reticularis lateralis (LRN), nucleus olivaris inferior (IO) and recording EPSPs in RN cells. The RN cells receive axon reflex activation from NRTP and PMRN, and disynaptic excitation from NRTP, PMRN, LRN and IO. Based on these and other available data, the components of the cerebello-precerebellar reverberating circuits are discussed.

Classical Conditioning Mediated by the Red Nucleus in the Cat

We have attempted to develop a behavioral and neuronal model for classical conditioning in the corticorubrospinal system. A conditioned stimulus (CS) was applied to the cerebral peduncle (CP) in cats which had lesions that interrupted the corticofugal fibers caudal to the red nucleus. The unconditioned stimulus (US) was an electric shock to the skin of the forelimb that produced flexion of the limb. After pairing of the CS and US in close temporal association, an initially ineffective stimulus to the cerebral peduncle was found to give rise to the flexion of the elbow. Extinction of the conditioned response was achieved by applying the CS alone or by reversing the sequence of the stimuli (US-CS: backward pairing). Furthermore, the US alone did not produce an increase in the effectiveness of the CS stimulus. Finally, pairing the fixed CS stimuli with the US at random intervals did not produce any increase in performance in response to the CS. In these respects, the observed behavioral modification has the features of associative conditioning. Because the thresholds for and the strength of elbow flexion induced by stimulation of the nucleus interpositus of the cerebellum were identical in the experimental and control animals, the interpositorubrospinal system cannot be the site of the plastic change. Since the conditioned response is most probably mediated by the corticorubrospinal system, it is likely that a modification of the corticorubral synapses underlies this behavioral change.

A quantitative study of synaptic reorganization in red nucleus neurons after lesion of the nucleus interpositus of the cat: an electron microscopic study involving intracellular injection of horseradish peroxidase

A quantitative electron microscopic analysis of the corticorubral projection was performed in the red nucleus (RN) of adult cats to determine morphological correlates of synaptic reorganization that occur following a lesion of the interpositus nucleus (IP). Corticorubral synaptic endings were identified by lesioning the sensorimotor cortex 2-6 days before electrophysiological experiments. Horseradish peroxidase (HRP) was injected into electrophysiologically identified RN neurons. Sagittal sections 100 micrometers thick were cut and reacted by diaminobenzidine. Sections containing HRP-positive neurons were selected and embedded in Epon. In normal cats, degenerating corticorubral terminals in the RN region frequently made contact with dendritic profiles, having small cross-sections, while a few made contact with somatic profiles. Similar results were obtained when degenerating terminals making contact with HRP-filled dendrites were analyzed. In the experimental animals, the cortical lesion was performed more than 8 weeks after lesion of the IP. In these animals, degenerating corticorubral terminals were frequently found on proximal dendrites and somata in RN region and HRP-positive neurons in contrast to the findings in normal cats. The results indicate that new corticorubral synapses were formed on proximal dendrites and somata of RN neurons as a consequence of IP lesions.

Sprouting of cortico-rubral synapses in red nucleus neurones after destruction of the nucleus interpositus of the cerebellum

Après destruction chronique de la projection rubrale (somatique) interpositionnelle, on constate par examen électrophysiologique que le système corticorubral dendritique est de toute évidence capable, par bourgeonnement, de rétablir des contacts synaptiques avec la région “dénervée” des cellules RN.

Electrical constants of neurons of the red nucleus

SummaryMembrane electrical constants have been studied in neurons of the red nucleus (RN) of the cat which were identified antidromically from the spinal cord. For each cell, the input resistance was determined from the membrane potential changes to current steps and was found to be 2.5 ± 0.9 MΩ in twenty five RN cells studied. In addition, linear summation of the membrane responses induced by two current pulses was demonstrated.2.From the membrane voltage transients to current steps, the first membrane time constant, τ0, and second time contant, τ1, were determined as 5.6 ±1.0 msec and 0.6 ± 0.2 msec, respectively. The ratio of the amplitudes of two exponential functions, E1/E0, was 0.18 ± 0.05. A linear relation was found between the ratio of these amplitudes of exponential functions and that of the two time constants.3.The cable parameter (electrotonic length, L) of the combined soma and dendrites of the RN neurons was estimated as 1.1 from membrane transient data using the relation developed by Rall (1969).4.By using this parameter, an attempt was made to estimate the location of the two excitatory inputs on the soma-dendritic membrane of RN cells.

Formation of functional synapses in the adult cat red nucleus from the cerebrum following cross-innervation of forelimb flexor and extensor nerves: I. Appearance of new synaptic potentials

We investigated the effects of cross-innervating the peripheral forelimb flexor and extensor nerves of adult cats on the time course of corticorubral EPSPs. Red nucleus neurons were identified by antidromic invasion from C1 or L1 spinal segments as innervating the upper spinal segments (C-cells) or sending axons to the lumbosacral cord (L-cells). In C-cells, a fast-rising component, superimposed on the slow-rising corticorubral EPSPs induced by the cerebral sensorimotor cortex or the cerebral peduncle (CP) stimulation, was noted. The mean time-to-peak of this component in cross-innervated cats operated more than two months earlier was 1.9 +/- 0.9 ms (n = 160), shorter than in normal cats (3.6 +/- 1.4 ms, n = 100). The same value in cats cross-innervated less than two months before was 2.7 +/- 1.0 ms (n = 53). The mean time-to-peak of CP-EPSPs from L-cells was 2.9 +/- 0.9 ms (n = 115). The fast-rising component had a latency of 0.96 +/- 0.19 ms (n = 122), and it was mediated by fibers with conduction velocities of less than 20 m/s. The projective area of the fast-rising component is organized somatotopically. Since it is more sensitive to membrane hyperpolarization than slow rising corticorubral EPSPs, it is mediated by synapses located more proximally than the corticorubral synapses of normal cats. The time course of facilitation by preceding cerebral peduncle stimulation of the nucleus interpositus (IP)-induced RN population responses was measured. It was characterized by a rapid, followed by a slower, rise time in the RN region where C-cells are concentrated. In contrast, the L-cell region was characterized by a slow rise time. In cats subjected to self-union of the peripheral flexor and extensor nerves, the majority of C-cells had CP-EPSPs with a time-to-peak within the normal range. Our results suggest that after cross-innervation sprouting and formation of functional synapses occur on the proximal portion of the soma-dendritic membrane of red nucleus neurons.

SYNAPTIC PLASTICITY IN THE RED NUCLEUS

Publisher Summary The morphological discovery that synaptic reorganization takes place after the partial deafferentation of septal nucleus in adult rats indicates that the neuronal connection in the mammalian central nervous system is not as rigid as has long been considered. Red nucleus (RN) represents a suitable preparation to determine whether new, functionally effective synaptic connections are formed. Cortico-rubral fibers terminate on the distal dendrites and fibers from the contralateral nucleus interpositus (IP) of the cerebellum make synaptic contact on the soma. The cortico-rubral dendritic EPSPs are characterized with slow-rising time course, whereas the somatic IP-excitatory postsynaptic potentials (EPSPs) are characterized with fast-rising time course. EPSPs from medial lemniscus have rise time between those of IP-rubral and cortico-rubral EPSPs. Sprouting and synaptic reorganization are not limited to the cases of removal of the direct synaptic inputs of RN neurons. A major goal of the study of neuronal plasticity is to provide a neuronal basis for behavioral plasticity, such as learning and memory. These studies range from the examination of simple behavioral phenomena to classical conditioning phenomena.

Specificity of the Newly-formed Corticorubral Synapses in the Kitten Red Nucleus

SummaryWe have examined the formation of new synapses that occurs in the kitten red nucleus (RN) following lesions of the cerebral cortex or cerebellum. In normal 2–3 month old kittens, stimulation of the cerebral cortex or cerebral peduncle (CP) produces slow-rising monosynaptic EPSPs in ipsilateral RN cells; while fast-rising EPSPs are produced by stimulating the contralateral nucleus interpositus (IP) of the cerebellum. In these normal animals stimulation of the contralateral cerebral cortex, or CP, or the ipsilateral IP never produces detectable postsynaptic potentials.In contrast, in kittens with chronic lesions of the ipsilateral sensorimotor cortex performed less than 2 months after birth, it was found that (1) stimulation of the contralateral sensorimotor cortex or CP produced slow-rising EPSPs. (2) Upon stimulation of the contralateral IP a slow-rising component appeared superimposed on the fast-rising IP-EPSPs. (3) In some cells, stimulation of the ipsilateral IP produced slow-rising EPSPs. These results indicate that new synapses were formed on the dendrites of RN cells by neurons from the contralateral CP, contralateral IP and ipsilateral IP. The majority of denervated RN neurons were found to receive new synapses from only one of these three inputs, those from the contralateral cerebral cortex being most prominent.The conduction velocities of corticorubral fibers responsible for the EPSPs recorded in these lesioned animals are similar to those of slow conducting pyramidal tract fibers. Sometimes IPSPs were also produced by a train of CP stimuli. The IPSPs are probably mediated by fast conducting pyramidal tract fibers because their threshold is lower than that of the EPSPs. These findings suggest that newly formed crossed corticorubral projections have organizational specificity with respect to excitatory vs. inhibitory connections which is similar to that of normal ipsilateral corticorubral projections.Somatotopical organization was found in the newly-formed corticorubral excitatory projections. The forelimb cortical area was found to project to contralateral RN neurons innervating the forelimb spinal segments, while the hindlimb cortical area was found to project to RN neurons innervating the lumbar segments. However, after chronic ablation of the forelimb area of the contralateral sensorimotor cortex, in addition to the ipsilateral cerebral cortex, a new connection was formed from the hindlimb area of the contralateral cerebral cortex to the RN neurons innervating the spinal forelimb segments. This indicates that the specificity of the topographical organization is subject to reorganization.In chronically hemicerebellectomized kittens, ipsilateral IP stimulation produced fast-rising monosynaptic EPSPs in some cases. Thus, it was concluded that new synapses were formed on the somatic or proximal dendritic portion of RN cells from the ipsilateral IP after ablation of the contralateral IP.

The cerebellar control of accomodation of the eye in the cat

The effect of cerebellar stimulation on the accommodation of the lens was examined in anesthetized cats. An infrared optometer was used to measure the refractive power of the lens during stimulation of the cerebellum. The area giving responses within latencies shorter than 160 msec and amplitudes larger than 0.15 diopters is localized in the contralateral interpositus and fastigial nuclei and the ipsilateral interpositus nucleus. No responses could be evoked by stimulating the bilateral lateral nuclei. Accommodation responses were also evoked by cerebellar cortex stimulation. Accommodation responses evoked by stimulating the cerebellar nuclei were inhibited by preceding cerebellar cortical stimulation.

Cortical neurons in and around the Clare-Bishop area related with lens accommodation in the cat

Single-unit discharges in the cat Clare-Bishop area were correlated with spontaneous accommodation responses. No appreciable change was found in accommodation responses evoked by stimulating the Clare-Bishop area, when cerebellar outflow was blocked reversibly by cooling the superior cerebellar peduncle. It is suggested, therefore, that the Clare-Bishop area plays an important role in the lens accommodation system through a pathway independent from that of the cerebellum.