Shigemi Mori

Locomotor role of the corticoreticular–reticulospinal–spinal interneuronal system

In vertebrates, the descending reticulospinal pathway is the primary means of conveying locomotor command signals from higher motor centers to spinal interneuronal circuits, the latter including the central pattern generators for locomotion. The pathway is morphologically heterogeneous, being composed of various types of inparallel-descending axons, which terminate with different arborization patterns in the spinal cord. Such morphology suggests that this pathway and its target spinal interneurons comprise varying types of functional subunits, which have a wide variety of functional roles, as dictated by command signals from the higher motor centers. Corticoreticular fibers are one of the major output pathways from the motor cortex to the brainstem. They project widely and diffusely within the pontomedullary reticular formation. Such a diffuse projection pattern seems well suited to combining and integrating the function of the various types of reticulospinal neurons, which are widely scattered throughout the pontomedullary reticular formation. The corticoreticular-reticulospinal-spinal interneuronal connections appear to operate as a cohesive, yet flexible, control system for the elaboration of a wide variety of movements, including those that combine goal-directed locomotion with other motor actions.

Site-specific postural and locomotor changes evoked in awake, freely moving intact cats by stimulating the brainstem

Locomotor behaviors evoked by stimulating the hypothalamus and the brainstem were studied in freely moving, awake cats. To do this, stimulating microelectrodes were chronically implanted into the subthalamic locomotor region (SLR) in the lateral hypothalamic area (LHA), the mesencephalic locomotor region (MLR) corresponding to the nucleus cuneiformis, the dorsal tegmental field (DTF) and the ventral tegmental field (VTF) of caudal pons along its midline. After recovery from surgery (2-3 days), open field tests were performed to study stimulus effects upon posture and locomotor movements. The stimuli consisted of pulses of 0.2 ms duration of less than 80 microA delivered at 50 pulses/s for 5-20 s. DTF stimulation resulted in suppression of postural support by the hindlimbs. When the cat was in a standing posture, DTF stimulation simply resulted in a sequential alteration of posture to a squatting and then to a final lying posture. In contrast, VTF stimulation evoked an almost opposite series of postural changes to those induced by DTF stimulation. With VTF stimulation, the cat changed from a lying or a squatting position, and then started to walk during continuation of the stimulation. With MLR stimulation, the cat invariably exhibited fast walking and then running movements. It ran straight forward, avoiding collision with walls or other obstacles, and even tried to jump over a fence placed in front of it. With LHA stimulation, the cat started to walk slowly extending its head forward and looking around repeatedly. It tended to walk with a stoop and stealthy steps along the corners of the room. Induced postural and locomotor changes were always accompanied by behavioral arousal reactions.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrical and chemical stimulations of the pontine micturition center

In an acute decerebrate cat, electrical stimulation of the pontine micturition center (PMC) resulted in micturition. Carbachol injection into the PMC also resulted in micturition. The pattern of changes in bladder pressure and the sphincter activity observed during carbachol-induced micturition was almost identical to that observed during reflex micturition. The injection site corresponded to the nucleus locus coeruleus alpha (LC alpha). These results suggest that activation of cholinoceptive neurons in the LC alpha presumably becomes a trigger to recruit any one of a number of neuronal circuits involved in micturition.

Discharge properties of medullary reticulospinal neurons during postural changes induced by intrapontine injections of carbachol, atropine and serotonin, and their functional linkages to hindlimb motoneurons in cats

The present study was aimed at elucidating the pontomedullary and spinal cord mechanisms of postural atonia induced by microinjection of carbachol and restored by microinjections of serotonin or atropine sulfate into the nucleus reticularis pontis oralis (NRPo). Medullary reticulospinal neurons (n=132) antidromically activated by stimulating the L1 spinal cord segment were recorded extracellularly. Seventy-eight of them were orthodromically activated with mono- or disynaptic latencies by stimulating the NRPo area at the site where carbachol injections effectively induced postural atonia. Most of these reticulospinal neurons (71 of 78) were located in the nucleus reticularis gigantocellularis (NRGc). Following carbachol injection into the NRPo, discharge rates of the NRGc reticulospinal neurons (29 of 34) increased, while the activity of soleus muscles decreased bilaterally. Serotonin or atropine injections into the same NRPo area resulted in a decrease in the discharge rates of the reticulospinal neurons with a concomitant increase in the levels of hindlimb muscle tone. Membrane potentials of hindlimb extensor and flexor alpha motoneurons (MNs) were hyperpolarized and depolarized by carbachol and serotonin or atropine injections, respectively. In all pairs of reticulospinal neurons and MNs (n=11), there was a high correlation between the increase in the discharge rates and the degree of membrane hyperpolarization of the MNs. Spike-triggered averaging during carbachol-induced atonia revealed that inhibitory postsynaptic potentials (IPSPs) were evoked in 15 MNs by the discharges of nine reticulospinal neurons. Four of them evoked IPSPs in more than one MN. The mean segmental delay and the mean time to the peak of IPSPs were 1.6 ms and 2.0 ms, respectively. Axonal trajectories of reticulospinal neurons (n=6), which evoked IPSPs in MNs, were investigated in the lumbosacral segments (L1-S1) by antidromic threshold mapping. The stem axons descended through the ventral (n=2) and ventrolateral (n=4) funiculi in the lumbar segments. All axons projected their collaterals to the intermediate region (laminae V, VI) and ventromedial part (laminae VII, VIII) of the gray matter. All these results suggest that the reticulospinal pathway originating from the NRGc is involved in postural atonia induced by pontine microinjection of carbachol, and that the pathway is inactivated during the postural restoration induced by subsequent injections of serotonin or atropine. It is further suggested that the pontine inhibitory effect is mediated via segmental inhibitory interneurons projecting to MNs.

Lumbar commissural interneurons with reticulospinal inputs in the cat: Morphology and discharge patterns during fictive locomotion

The purpose of this study was 1) to characterize the morphology of lumbar commissural neurons (CNs) with reticulospinal inputs and 2) to quantitate their activity during locomotor rhythm generation. Intraaxonal recordings at the L4–7 level of the spinal cord were obtained in 67 neurons in the decerebrate, paralyzed cat. Fourteen of them were subsequently nearly fully visualized following their intraaxonal injection with the tracer neurobiotin. All 14 were CNs with axons projecting across the midline of the spinal cord. Their somata were located mainly in lamina VIII and additionally in laminae VII–VI. Most of the lamina VIII CNs were excited monosynaptically from reticulospinal pathways. They were judged to be interneuronal CNs if they had no, or a short, rostral projection. These CNs commonly gave off multiple axon collaterals in and around their somatas segmental level. They projected mainly to laminae VIII–VII and some additionally to lamina IX. Some laminae VIII and the laminae VII–VI CNs were excited polysynaptically from reticulospinal pathways or were not excited. They were judged to be long propriospinal or ascending tract CNs because they had only an ascending axon. Most lamina VIII CNs discharged rhythmically during fictive locomotion evoked by stimulation of the mesencephalic locomotor region, exhibiting one peak per locomotor cycle. The peak was in phase with neurographic activity of either a left or a right hindlimb extensor nerve. These results suggested that lamina VIII CNs are reciprocally connected bilaterally at each segmental level. Such an arrangement suggests their participation in the generation and coordination of reciprocal and bilateral locomotor activity. J. Comp. Neurol. 474:546–561, 2004.

MULTI-SEGMENTAL INNERVATION OF SINGLE PONTINE RETICULOSPINAL AXONS IN THE CERVICO-THORACIC REGION OF THE CAT : ANTEROGRADE PHA-L TRACING STUDY

To characterize the fine morphology of individual reticulospinal axons at multiple spinal segments, localized injections of the anterograde neural tracer, Phaseolus vulgaris leucoagglutinin (PHA‐L), were made into the nucleus reticularis pontis oralis (NRPo) of the cat. Following survival periods of 6–8 weeks, labelled axons, between 1 and 8 μm in diameter, were found throughout the cervical and upper thoracic segments. Thick axons (diameter ≥ 3 μm) were found to descend beyond the upper thoracic spinal cord, while most thin axons (diameter < 3 μm) ended in the upper cervical cord. From serial transverse sections (50 μm) of segments C3 to T2, in four cats, the trajectories of 23 single, thick reticulospinal axons were traced in continuity over distances of between 21.8 and 59.4 mm, corresponding to 3 and 8 segments, respectively. Most axons gave off at least one, and as many as four collaterals per segment, some preferentially in the cervical enlargement. The remainder gave off collaterals at most but not all segments. Detailed reconstruction of the collateralization and arborization in the spinal gray matter showed two major termination types, one where terminals remained ipsilateral to the stem axon, the other where additional collaterals extended across the midline from the ipsilateral gray matter to terminate in the contralateral gray matter. Axons tended to have collaterals of one type or the other, irrespective of the rostrocaudal level. Both ipsilateral and bilateral projections terminated mainly in laminae VII and VIII although the branching patterns varied from axon to axon. Individual stem axons, in general, showed similar termination patterns at each level. J. Comp. Neurol. 377:234–250, 1997.

Morphology of single pontine reticulospinal axons in the lumbar enlargement of the cat: a study using the anterograde tracer PHA-L.

The fine morphology of single pontine reticulospinal axons in the lumbar enlargement was investigated by using an anterograde Phaseolus vulgaris–leucoagglutinin (PHA‐L) tracing technique. Localized injections of PHA‐L were made into the nuclei reticularis pontis oralis and caudalis in four cats. Following survival periods of 8–9 weeks, PHA‐L‐labeled axons were found throughout the lumbar enlargement from segments L4 to S2, in which the diameter of labeled axons was 0.6–2.5 μm. From serial transverse sections (50 μm), trajectories of 21 single pontine reticulospinal axons were traced in continuity over distances of 18.9–36.3 mm, corresponding to three to six segments, respectively. All the identified axons gave off multiple (two to nine) axon collaterals along their courses, with mean intercollateral distances of approximately 5–6 mm. Detailed reconstruction of the collateral arborization in the lumbar enlargement showed a high degree of similarity to that of single axons in the cervical enlargement previously reported (Matsuyama et al. [1997] J. Comp. Neurol. 377:234–250). First, axon collaterals arising from a majority (n = 18) of identified axons innervated the gray matter unilaterally, ipsilateral to the parent axons, whereas those from the remaining three axons innervated the gray matter bilaterally. Second, collateral projections terminated mainly in laminae VIII and VII, with the arborization field confined to a narrow rostrocaudal extent (<1 mm). Third, the termination fields of axon collaterals arising from a given reticulospinal axon were similar at each segmental level and differed from one stem axon to another. These results suggest that the long descending pontine reticulospinal pathway is composed of different types of axons that may innervate the cervical and lumbar enlargements in continuity in a similar manner. J. Comp. Neurol. 410:413–430, 1999.

Generation of spontaneous respiratory rhythm in high spinal cats.

Spontaneous respiratory neuronal activities within cervical spinal cord were investigated in two groups of 36 adult cats: cervical spinalized and non-spinalized preparations. In the first group of 18 animals, spontaneous breathing was abolished after total spinal transection at C1. However, spontaneous rhythmic breathing reappeared within 2 h after transection in 13 animals. In the other 6 animals spinalized at C3 level, we could not induce spontaneous breathing. The spinal respiratory movements were found to be mainly due to rhythmic diaphragmatic contraction. Such spinal respiratory activity continued for 30 min-1 h with a steady rate of 19-24/min and then they steadily deteriorated. Spinal respiratory activity developed usually without hindlimb muscle activity and even when hindlimb stepping rhythm was seen simultaneously, it was not locked to respiratory rhythmicity. During spinal respiration, phrenic motoneuron discharges were recorded from the C5-C6 ventral horn. The burst discharges containing 4.8-40.0 spikes were all in synchrony with the inspiratory phase of respiratory cycles. Even after breathing movements were temporarily paralyzed by gallamine, the rhythmic bursts still persisted for an appreciable time. In the second group of 12 lightly anesthetized cats, microelectrode explorations of the upper cervical cord were made in an attempt to record neuronal activities associated with respiratory movements. A total of 24 burst discharges of inspiratory type units which represent presumed cell body activities was recorded. The recording sites were histologically located in the intermediate zone of the spinal gray matter of C1-C2 cervical cord. These results suggested the possible existence of some intrinsic respiratory rhythm generators within the cervical cord. Possible neuronal mechanisms for generation of spontaneous respiratory rhythm were discussed.

Fastigiofugal projection to the brainstem nuclei in the cat: an anterograde PHA-L tracing study

Fastigial projections to brainstem nuclei were studied using an anterograde neural tracer, Phaseolus vulgaris leucoagglutinin (PHA-L). Microinjections of PHA-L were made into the rostral pole, and middle and caudal parts of the left fastigial nucleus in cat. In addition to fastigioreticular and fastigiovestibular projections, fastigiofugal projections to cranial motor nuclei (IV, VI and VII) and those nuclei involved in autonomic control were identified. At the medullary level, a topographic arrangement of fastigioreticular projection was observed. Rostral and caudal parts of the fastigial nucleus projected to the ventral and dorsal parts of the medial reticular formation, respectively. Fastigiofugal fibers which originated from the rostral part of the fastigial nucleus innervated heavily the nucleus reticularis gigantocellularis (NRGc), nucleus reticularis magnocellularis (NRMc) and the ventral paramedian reticular nucleus (PRN). Those fibers from the middle part innervated heavily the ventrolateral vestibular nucleus (VLV), NRGc, NRMc, ventral and dorsal PRN and parasolitary tract nucleus. From the caudal part of the fastigial nucleus, projections to the cranial motor nuclei (IV, VI and VII), VLV and inferior vestibular nucleus were observed.

Cerebellar-induced Locomotion: Reticulospinal Control of Spinal Rhythm Generating Mechanism in Catsa

Abstract: In a decerebrate cat (locomotor preparation), stimulation of a restricted region along the midline cerebellar white matter has been found to evoke generalized augmentation of postural muscle tone on a stationary surface (Asanome et al. 1998. Neurosci. Res. 30: 257‐269) and “controlled” locomotion on the surface of a moving treadmill. Characteristics of cerebellar‐evoked locomotion were similar to those of mesencephalic locomotor region‐evoked “controlled” locomotion on the same animal. Microinjection of a neural tracer (CTb‐HRP) into the lesioned stimulus site of the cerebellar white matter resulted in both retrograde labeling of cells in the fastigial nuclei, bilaterally, and anterograde labeling of fibers descending to the brain stem. These results indicated that the effective cerebellar stimulus site (cerebellar locomotor region) corresponded to the midline region of the hook bundle of Russell (Rasmussen, A. T., 1933. J. Comp. Neurol. 57: 165‐197), through which crossed fastigioreticular, fastigiovestibular, and fastigiospinal fibers pass. In this study, contribution of reticulospinal systems to the control of cerebellar‐evoked locomotion was extensively studied. By stimulating the cerebellar locomotor region and the MLR in the same animal, a majority of antidromically identified pontomedullary reticulospinal cells were synaptically activated. The results of the present study demonstrated that fastigial cells with crossed fastigioreticular fibers and reticulospinal fibers play a crucial role in the control of posture and locomotion in the locomotor preparation.