Medullary reticulospinal tract mediating the generalized motor inhibition in cats: II. Functional organization within the medullary reticular formation with respect to postsynaptic inhibition of forelimb and hindlimb motoneurons

Distinct Contributions of Mesencephalic Locomotor Region Nuclei to Locomotor Control in the Freely Behaving Mouse

The mesencephalic locomotor region (MLR) has been initially identified as a supraspinal center capable of initiating and modulating locomotion. Whereas its functional contribution to locomotion has been widely documented throughout the phylogeny from the lamprey to humans, there is still debate about its exact organization. Combining kinematic and electrophysiological recordings in mouse genetics, our study reveals that glutamatergic neurons of the cuneiform nucleus initiate locomotion and induce running gaits, whereas glutamatergic and cholinergic neurons of the pedunculopontine nucleus modulate locomotor pattern and rhythm, contributing to slow-walking gaits. By initiating, modulating, and accelerating locomotion, our study identifies and characterizes distinct neuronal populations of this functional region important to locomotor command.

The anatomical, cellular and synaptic basis of motor atonia during rapid eye movement sleep

Rapid eye movement (REM) sleep is a recurring part of the sleep-wake cycle characterized by fast, desynchronized rhythms in the electroencephalogram (EEG), hippocampal theta activity, rapid eye movements, autonomic activation and loss of postural muscle tone (atonia). The brain circuitry governing REM sleep is located in the pontine and medullary brainstem and includes ascending and descending projections that regulate the EEG and motor components of REM sleep. The descending signal for postural muscle atonia during REM sleep is thought to originate from glutamatergic neurons of the sublaterodorsal nucleus (SLD), which in turn activate glycinergic pre-motor neurons in the spinal cord and/or ventromedial medulla to inhibit motor neurons. Despite work over the past two decades on many neurotransmitter systems that regulate the SLD, gaps remain in our knowledge of the synaptic basis by which SLD REM neurons are regulated and in turn produce REM sleep atonia. Elucidating the anatomical, cellular and synaptic basis of REM sleep atonia control is a critical step for treating many sleep-related disorders including obstructive sleep apnoea (apnea), REM sleep behaviour disorder (RBD) and narcolepsy with cataplexy.

Brainstem control of locomotion and muscle tone with special reference to the role of the mesopontine tegmentum and medullary reticulospinal systems

The lateral part of the mesopontine tegmentum contains functionally important structures involved in the control of posture and gait. Specifically, the mesencephalic locomotor region, which may consist of the cuneiform nucleus and pedunculopontine tegmental nucleus (PPN), occupies the interest with respect to the pathophysiology of posture-gait disorders. The purpose of this article is to review the mechanisms involved in the control of postural muscle tone and locomotion by the mesopontine tegmentum and the pontomedullary reticulospinal system. To make interpretation and discussion more robust, the above issue is considered largely based on our findings in the experiments using decerebrate cat preparations in addition to the results in animal experimentations and clinical investigations in other laboratories. Our investigations revealed the presence of functional topographical organizations with respect to the regulation of postural muscle tone and locomotion in both the mesopontine tegmentum and the pontomedullary reticulospinal system. These organizations were modified by neurotransmitter systems, particularly the cholinergic PPN projection to the pontine reticular formation. Because efferents from the forebrain structures as well as the cerebellum converge to the mesencephalic and pontomedullary reticular formation, changes in these organizations may be involved in the appropriate regulation of posture-gait synergy depending on the behavioral context. On the other hand, abnormal signals from the higher motor centers may produce dysfunction of the mesencephalic-reticulospinal system. Here we highlight the significance of elucidating the mechanisms of the mesencephalic-reticulospinal control of posture and locomotion so that thorough understanding of the pathophysiological mechanisms of posture-gait disorders can be made.

Lhx3-Chx10 reticulospinal neurons in locomotor circuits

Motor behaviors result from the interplay between the brain and the spinal cord. Reticulospinal neurons, situated between the supraspinal structures that initiate motor movements and the spinal cord that executes them, play key integrative roles in these behaviors. However, the molecular identities of mammalian reticular formation neurons that mediate motor behaviors have not yet been determined, thus limiting their study in health and disease. In the medullary reticular formation of the mouse, we identified neurons that express the transcription factors Lhx3 and/or Chx10, and demonstrate that these neurons form a significant component of glutamatergic reticulospinal pathways. Lhx3-positive medullary reticular formation neurons express Fos following a locomotor task in the adult, indicating that they are active during walking. Furthermore, they receive functional inputs from the mesencephalic locomotor region and have electrophysiological properties to support tonic repetitive firing, both of which are necessary for neurons that mediate the descending command for locomotion. Together, these results suggest that Lhx3/Chx10 medullary reticular formation neurons are involved in locomotion.

Subtle gait changes in patients with REM sleep behavior disorder

Many people with rapid eye movement (REM) sleep behavior disorder (RBD) have an underlying synucleinopathy, the most common of which is Lewy body disease. Identifying additional abnormal clinical features may help in identifying those at greater risk of evolving to a more severe syndrome. Because gait disorders are common in the synucleinopathies, early abnormalities in gait in those with RBD could help in identifying those at increased risk of developing overt parkinsonism and/or cognitive impairment. We identified 42 probable RBD subjects and 492 controls using the Mayo Sleep Questionnaire and assessed gait velocity, cadence, and stride dynamics with an automated gait analysis system. Cases and controls were similar in age (79.9 ± 4.7 and 80.1 ± 4.7, P = 0.74), Unified Parkinson's Disease Rating Scale Part III (UPDRS) score (3.3 ± 5.5 and 1.9 ± 4.1, P = 0.21) and Mini-Mental State Examination scores (27.2 ± 1.9 and 27.7 ± 1.6, P = 0.10). A diagnosis of probable RBD was associated with decreased velocity (-7.9 cm/s; 95% confidence interval [CI], -13.8 to -2.0; P < 0.01), cadence (-4.4 steps/min; 95% CI, -7.6 to -1.3; P < 0.01), significantly increased double limb support variability (30%; 95% CI, 6-60; P = 0.01), and greater stride time variability (29%; 95% CI, 2-63; P = 0.03) and swing time variability (46%; 95% CI, 15-84; P < 0.01). Probable RBD is associated with subtle gait changes prior to overt clinical parkinsonism. Diagnosis of probable RBD supplemented by gait analysis may help as a screening tool for disorders of α-synuclein.

Control of sleep and wakefulness

This review summarizes the brain mechanisms controlling sleep and wakefulness. Wakefulness promoting systems cause low-voltage, fast activity in the electroencephalogram (EEG). Multiple interacting neurotransmitter systems in the brain stem, hypothalamus, and basal forebrain converge onto common effector systems in the thalamus and cortex. Sleep results from the inhibition of wake-promoting systems by homeostatic sleep factors such as adenosine and nitric oxide and GABAergic neurons in the preoptic area of the hypothalamus, resulting in large-amplitude, slow EEG oscillations. Local, activity-dependent factors modulate the amplitude and frequency of cortical slow oscillations. Non-rapid-eye-movement (NREM) sleep results in conservation of brain energy and facilitates memory consolidation through the modulation of synaptic weights. Rapid-eye-movement (REM) sleep results from the interaction of brain stem cholinergic, aminergic, and GABAergic neurons which control the activity of glutamatergic reticular formation neurons leading to REM sleep phenomena such as muscle atonia, REMs, dreaming, and cortical activation. Strong activation of limbic regions during REM sleep suggests a role in regulation of emotion. Genetic studies suggest that brain mechanisms controlling waking and NREM sleep are strongly conserved throughout evolution, underscoring their enormous importance for brain function. Sleep disruption interferes with the normal restorative functions of NREM and REM sleep, resulting in disruptions of breathing and cardiovascular function, changes in emotional reactivity, and cognitive impairments in attention, memory, and decision making.

Organization of functional synaptic connections between medullary reticulospinal neurons and lumbar descending commissural interneurons in the neonatal mouse

The medullary reticular formation (MRF) of the neonatal mouse is organized so that the medial and lateral MRF activate hindlimb and trunk motoneurons (MNs) with differential predominance. The goal of the present study was to investigate whether this activation is polysynaptic and mediated by commissural interneurons with descending axons (dCINs) in the lumbar spinal cord. To this end, we tested the polysynapticity of inputs from the MRF to MNs and tested for the presence of selective inputs from medial and lateral MRF to 574 individual dCINs in the L2 segment of the neonatal mouse. Reticulospinal-mediated postsynaptic Ca(2+) responses in MNs were reduced in the presence of mephenesin and after a midline lesion, suggesting the involvement of dCINs in mediating the responses. Consistent with this, stimulation of reticulospinal neurons in the medial or lateral MRF activated 51% and 57% of ipsilateral dCINs examined (255 and 352 dCINs, respectively) and 52% and 46% of contralateral dCINs examined (166 and 133 dCINs, respectively). The proportion of dCINs that responded specifically to stimulation of medial or lateral MRF was similar to the proportions of dCINs that responded to both MRF regions or to neither. The three responsive dCIN populations had largely overlapping spatial distributions. We demonstrate the existence of dCIN subpopulations sufficient to mediate responses in lumbar motoneurons from reticulospinal pathways originating from the medial and lateral MRF. Differential control of trunk and hindlimb muscles by the medullary reticulospinal system may therefore be mediated in part by identifiable dCIN populations.

[Motor control by the basal ganglia]

The cerebral cortex controls cognitive and voluntary process of movements. The brainstem and spinal cord are involved in the execution of innately acquired motor patterns such as postural reflexes, muscle tone regulation and locomotion. Cortico-reticular projections arising from the motor cortical areas contribute to the postural control that precedes the voluntary movement process. The basal ganglia cooperatively regulates the activities of the cerebral cortex and the brainstem-spinal cord by its strong inhibitory and dis-inhibitory effects upon these target structures so that goal-directed movements could be appropriately performed. We propose that basal ganglia disfunction, including the abnormality in the dopaminergic projection system, may disturb the cooperative regulation, resulting in motor deficiencies expressed in basal diseases.

Substrates for normal gait and pathophysiology of gait disturbances with respect to the basal ganglia dysfunction

In this review, we have tried to elucidate substrates for the execution of normal gait and to understand pathophysiological mechanisms of gait failure in basal ganglia dysfunctions. In Parkinson's disease, volitional and emotional expressions of movement processes are seriously affected in addition to the disturbance of automatic movement processes, such as adjustment of postural muscle tone before gait initiation and rhythmic limb movements during walking. These patients also suffer from muscle tone rigidity and postural instability, which may also cause reduced walking capabilities in adapting to various environments. Neurophysiological and clinical studies have suggested the importance of basal ganglia connections with the cerebral cortex and limbic system in the expression of volitional and emotional behaviors. Here we hypothesize a crucial role played by the basal ganglia-brainstem system in the integrative control of muscle tone and locomotion. The hypothetical model may provide a rational explanation for the role of the basal ganglia in the control of volitional and automatic aspects of movements. Moreover, it might also be beneficial for understanding pathophysiological mechanisms of basal ganglia movement disorders. A part of this hypothesis has been supported by studies utilizing a constructive simulation engineering technique that clearly shows that an appropriate level of postural muscle tone and proper acquisition and utilization of sensory information are essential to maintain adaptable bodily functions for the full execution of bipedal gait. In conclusion, we suggest that the major substrates for supporting bipedal posture and executing bipedal gait are 1) fine neural networks such as the cortico-basal ganglia loop and basal ganglia-brainstem system, 2) fine musculoskeletal structures with adequately developed (postural) muscle tone, and 3) proper sensory processing. It follows that any dysfunction of the above sensorimotor integration processes would result in gait disturbance.

Anatomical evidence for an anticonvulsant relay in the rat ventromedial medulla

Pharmacological manipulation of the ventrolateral pontine reticular formation (vlPRF) of rats has an anticonvulsant effect in the maximal electroshock model of epilepsy. This study presents three anatomical experiments that determine the efferent projections from this region likely to mediate this anticonvulsant effect. In the first, the anterograde tracer biotinylated dextran amine (BDA) was injected into the vlPRF. A strong projection to the ventromedial medullary reticular formation (vmMRF) was revealed which continued only weakly to the spinal cord. In the second experiment, double-label procedures were used to indicate whether the BDA-labelled terminals from the vlPRF make contacts with neurons in vmMRF, retrogradely labelled with cholera-toxin B subunit from the lumbar spinal cord. Sections of the vmMRF were examined by: (i) light microscopy which showed significant overlap between terminals from vlPRF and retrogradely-labelled reticulospinal cells; (ii) confocal microscopy which showed labelled terminals in close association with reticulospinal cell bodies; and (iii) electron microscopy which showed vlPRF terminals making synaptic contact with reticulospinal neurons. Finally, immunohistochemical procedures in combination with anterograde tracing revealed that significant numbers of terminals labelled from vlPRF were also positive for markers of glutamatergic or GABAergic neurotransmission. This suggests that the projection from the vlPRF to the vmMRF is likely to include several different functional components. These connections could represent a final critical link of an anticonvulsant circuit that originates in the dorsal midbrain and projects via relays in the vlPRF and the vmMRF to interact with the low-level motor circuitry in the spinal cord.

Orexinergic projections to the cat midbrain mediate alternation of emotional behavioural states from locomotion to cataplexy

Orexinergic neurones in the perifornical lateral hypothalamus project to structures of the midbrain, including the substantia nigra and the mesopontine tegmentum. These areas contain the mesencephalic locomotor region (MLR), and the pedunculopontine and laterodorsal tegmental nuclei (PPN/LDT), which regulate atonia during rapid eye movement (REM) sleep. Deficiencies of the orexinergic system result in narcolepsy, suggesting that these projections are concerned with switching between locomotor movements and muscular atonia. The present study characterizes the role of these orexinergic projections to the midbrain. In decerebrate cats, injecting orexin-A (60 microm to 1.0 mm, 0.20-0.25 microl) into the MLR reduced the intensity of the electrical stimulation required to induce locomotion on a treadmill (4 cats) or even elicit locomotor movements without electrical stimulation (2 cats). On the other hand, when orexin was injected into either the PPN (8 cats) or the substantia nigra pars reticulata (SNr, 4 cats), an increased stimulus intensity at the PPN was required to induce muscle atonia. The effects of orexin on the PPN and the SNr were reversed by subsequently injecting bicuculline (5 mm, 0.20-0.25 microl), a GABA(A) receptor antagonist, into the PPN. These findings indicate that excitatory orexinergic drive could maintain a higher level of locomotor activity by increasing the excitability of neurones in the MLR, while enhancing GABAergic effects on presumably cholinergic PPN neurones, to suppress muscle atonia. We conclude that orexinergic projections from the hypothalamus to the midbrain play an important role in regulating motor behaviour and controlling postural muscle tone and locomotor movements when awake and during sleep. Furthermore, as the excitability is attenuated in the absence of orexin, signals to the midbrain may induce locomotor behaviour when the orexinergic system functions normally but elicit atonia or narcolepsy when the orexinergic function is disturbed.

Movement suppression during anesthesia: Neural projections from the mesopontine tegmentum to areas involved in motor control

Microinjection of pentobarbital and GABA(A)-receptor agonists into a brainstem region we have called the mesopontine tegmental anesthesia area (MPTA; Devor and Zalkind [2001] Pain 94:101-112) induces a general anesthesia-like state. As in systemic general anesthesia, rats show loss of the righting reflex, atonia, nonresponsiveness to noxious stimuli, and apparent loss of consciousness. GABA(A) agonist anesthetics acting on the MPTA might suppress movement by engaging endogenous motor regulatory systems previously identified in research on decerebrate rigidity and REM sleep atonia. Anterograde and retrograde tracing revealed that the MPTA has multiple descending projections to pontine and medullary areas known to be associated with motor control and atonia. Prominent among these are the dorsal pontine reticular formation and components of the rostral ventromedial medulla (RVM). The MPTA also has direct projections to the intermediate gray matter and ventral horn of the spinal cord via the lateral and anterior funiculi. These projections show a rostrocaudal topography: neurons in the rostral MPTA project to the RVM, but only minimally to the spinal cord, while those in the caudal MPTA project to both targets. Finally, the MPTA has ascending projections to motor control areas including the substantia nigra, subthalamic nucleus, and the caudate-putamen. Projections are bilateral with an ipsilateral predominance. We propose that GABA(A) agonist anesthetics induce immobility at least in part by acting on these endogenous motor control pathways via the MPTA. Analysis of MPTA connectivity has the potential for furthering our understanding of the neural circuitry responsible for the various functional components of general anesthesia.

Role of basal ganglia–brainstem pathways in the control of motor behaviors

Here we review a role of a basal ganglia-brainstem (BG-BS) system throughout the mesopontine tegmentum in the control of various types of behavioral expression. First the basal ganglia-brainstem system may contribute to an automatic control of movements, such as rhythmic limb movements and adjustment of postural muscle tone during locomotion, which occurs in conjunction with voluntary control processes. Second, the basal ganglia-brainstem system can be involved in the regulation of awake-sleep states. We further propose the possibility that the basal ganglia-brainstem system is responsible for the integration of volitionally-guided and emotionally-triggered expression of motor behaviors. It can be proposed that dysfunction of the basal ganglia-brainstem system together with that of cortico-basal ganglia loop underlies the pathogenesis of behavioral disturbances expressed in basal ganglia dysfunction.

Changes in the excitability of hindlimb motoneurons during muscular atonia induced by stimulating the pedunculopontine tegmental nucleus in cats

We have previously reported that electrical stimulation delivered to the ventral part of the pedunculopontine tegmental nucleus (PPN) produced postural atonia in acutely decerebrated cats [Neuroscience 119 (2003) 293]. The present study was designed to elucidate synaptic mechanisms acting on motoneurons during postural atonia induced by PPN stimulation. Intracellular recording was performed from 72 hindlimb motoneurons innervating extensor and flexor muscles, and the changes in excitability of the motoneurons following the PPN stimulation were examined. Repetitive electrical stimulation (20-50 microA, 50 Hz, 5-10 s) of the PPN hyperpolarized the membrane potentials of both the extensor and flexor motoneurons by 2.0-12 mV (6.0 +/- 2.3 mV, n = 72). The membrane hyperpolarization persisted for 10-20 s even after termination of the stimulation. During the PPN stimulation, the membrane hyperpolarization was associated with decreases in the firing capability (n = 28) and input resistance (28.5 +/- 6.7%, n = 14) of the motoneurons. Moreover the amplitude of Ia excitatory postsynaptic potentials was also reduced (44.1 +/- 13.4%, n = 14). After the PPN stimulation, these parameters immediately returned despite that the membrane hyperpolarization persisted. Iontophoretic injections of chloride ions into the motoneurons reversed the polarity of the membrane hyperpolarization during the PPN stimulation. The polarity of the outlasting hyperpolarization however was not reversed. These findings suggest that a postsynaptic inhibitory mechanism, which was mediated by chloride ions, was acting on hindlimb motoneurons during PPN-induced postural atonia. However the outlasting motoneuron hyperpolarization was not due to the postsynaptic inhibition but it could be due to a decrease in the activity of descending excitatory systems. The functional role of the PPN in the regulation of postural muscle tone is discussed with respect to the control of behavioral states of animals.

Medullary reticulospinal tract mediating a generalized motor inhibition in cats: iii. functional organization of spinal interneurons in the lower lumbar segments

The previous report of intracellular recording of hindlimb motoneurons in decerebrate cats [ 511] has suggested that the following mechanisms are involved in a generalized motor inhibition induced by stimulating the medullary reticular formation. First, the motor inhibition, which was prominent in the late latency (30-80 ms), can be ascribed to the inhibitory effects in parallel to motoneurons and to interneuronal transmission in reflex pathways. Second, both a group of interneurons receiving inhibition from flexor reflex afferents and a group of Ib interneurons mediate the late inhibitory effects upon the motoneurons. To substantiate the above mechanisms of motor inhibition we examined the medullary stimulus effects upon intracellular (n=55) and extracellular (n=136) activity of spinal interneurons recorded from the lower lumbar segments (L6-L7). Single pulses or stimulus trains (1-3) pulses, with a duration of 0.2 ms and intensity of 20-50 microA) applied to the medullary nucleus reticularis gigantocellularis evoked a mixture of excitatory and inhibitory effects with early (<20 ms) and late (>30 ms) latencies. The medullary stimulation excited 55 interneurons (28.8%) with a late latency. Thirty-nine of the cells, which included 10 Ib interneurons, were inhibited by volleys in flexor reflex afferents (FRAs). These cells were mainly located in lamina VII of Rexed. On the other hand, the late inhibitory effects were observed in 67 interneurons (35.1%), which included cells mediating reciprocal Ia inhibition, non-reciprocal group I (Ib) inhibition, recurrent inhibition and flexion reflexes. Intracellular recording revealed that the late inhibitory effects were due to inhibitory postsynaptic potentials with a peak latency of about 50 ms and a duration of 50-60 ms. The inhibitory effects were attenuated by volleys in FRAs. Neither excitatory nor inhibitory effects with a late latency were observed in 69 (36.1%) cells which were located in the intermediate region and dorsal horn. These results suggest the presence of a functional organization of the spinal cord with respect to the production of the generalized motor inhibition. Lamina VII interneurons that receive inhibition from volleys in FRAs possibly mediate the postsynaptic inhibition from the medullary reticular formation in parallel to motoneurons and to interneurons in reflex pathways.

Neuronal basis of crossed actions from the reticular formation on feline hindlimb motoneurons

Pathways through which reticulospinal neurons can influence contralateral limb movements were investigated by recording from motoneurons innervating hindlimb muscles. Reticulospinal tract fibers were stimulated within the brainstem or in the lateral funiculus of the thoracic spinal cord contralateral to the motoneurons. Effects evoked by ipsilaterally descending reticulospinal tract fibers were eliminated by a spinal hemisection at an upper lumbar level. Stimuli applied in the brainstem evoked EPSPs, IPSPs, or both at latencies of 1.42 +/- 0.03 and 1.53 +/- 0.04 msec, respectively, from the first components of the descending volleys and with properties indicating a disynaptic linkage, in most contralateral motoneurons: EPSPs in 76% and IPSPs in 26%. EPSPs with characteristics of monosynaptically evoked responses, attributable to direct actions of crossed axon collaterals of reticulospinal fibers, were found in a small proportion of the motoneurons, whether evoked from the brainstem (9%) or from the thoracic cord (12.5%). Commissural neurons, which might mediate the crossed disynaptic actions (i.e., were antidromically activated from contralateral motor nuclei and monosynaptically excited from the ipsilateral reticular formation), were found in Rexed's lamina VIII in the midlumbar segments (L3-L5). The results reveal that although direct actions of reticulospinal fibers are much more potent on ipsilateral motoneurons, interneuronally mediated actions are as potent contralaterally as ipsilaterally, and midlumbar commissural neurons are likely to contribute to them. They indicate a close coupling between the spinal interneuronal systems used by the reticulospinal neurons to coordinate muscle contractions ipsilaterally and contralaterally.