Activity of Reticulospinal Neurons During Locomotion

Cross references:   Reticulospinal Transmission    Lamprey Locomotion      
Lamprey Nervous System
   Lamprey Neurotransmitters          Lamprey GABA        
 Lamprey Motor Nerves    
Lamprey Striatum       Lamprey Nucleus Accumbens

Initiation of Locomotion in Lampreys     Lamprey Muscles    Lamprey Fast-Slow Twitch   
Amphioxus Locomotion      
Salamander Locomotion     

Phasic modulation of reticulospinal neurones during fictive locomotion and other types of spinal motor activity in lamprey
    "The intracellular activity of different types of reticulospinal neurones was studied during fictive locomotion and other types of spinal motor activity in an in vitro preparation of the lamprey brainstem-spinal cord. The examined neurones included large Müller cells of the rhombencephalic and mesencephalic reticular formation, the Mauthner cell, and neurones in the posterior rhombencephalic reticular nucleus with different sizes and conduction velocities. During bouts of fictive swimming initiated spontaneously or by stimulation of the trigeminal nerve or spinal cord, the Müller cells were depolarized and fired action potentials. Bulbar Müller cells in addition showed a phasic modulation of membrane potential with excitation in phase with ipsilateral motoneurones of the rostral spinal cord. The Mauthner cell was depolarized in phase with contralateral motoneurones. Many neurones in the posterior rhombencephalic reticular nucleus showed modulation in phase with ipsilateral motoneurones during fictive swimming. Such oscillations were observed in both fast-conducting neurones, located mainly in the medial part of the nucleus, and slower conducting cells with a more lateral distribution. All examined reticulospinal neurones showed a strong coupling also with other types of spinal motor activity, such as slow alternating bursting and synchronous bilateral ventral root bursts, but the reticulospinal activity had no correlation with respiratory activity recorded from the Xth nerve. The consequences of a phasic reticulospinal activity during locomotion are discussed."
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Phasic modulation of transmission from vestibular inputs to reticulospinal neurons during fictive locomotion in lampreys.  
    "The aim of this study was to determine whether the transmission from sensory inputs to reticulospinal neurons is modulated during fictive locomotion in lampreys. Reticulospinal neurons play a key role in the control of locomotion; modulation of sensory transmission to these neurons might be of importance for the adaptation of the control they exert during locomotion.  
    In this series of experiments, intracellular synaptic responses of reticulospinal neurons of the posterior rhombencephalic reticular nucleus elicited by electrical stimulation of vestibular nerves on each side were studied during fictive locomotion induced by 50 microM N-methyl-D-aspartate (NMDA).  
    Interestingly, shortly after NMDA had reached the bath and much before locomotor discharges were apparent in the recorded ventral roots, there was a significant depression of the synaptic transmission from vestibular nerves. The effect was reversed by washing out the NMDA and persisted in the isolated brainstem after spinal transection at the first segmental level.  
    As locomotor discharges appeared in the ventral roots, synaptic responses elicited by vestibular nerve stimulation showed a clear phasic modulation of their amplitude during the locomotor cycle. Responses to stimulation of the ipsilateral vestibular nerve were smaller during the ipsilateral burst discharge than during the contralateral activity, whilst responses to stimulation of the contralateral vestibular nerve were minimal during contralateral activity and maximal during ipsilateral activity.  
    This opposite pattern of modulation observed in the same reticulospinal neuron suggests that the phasic modulation of vestibular transmission is not due to changes in the membrane properties of the reticulospinal cell but is produced at a pre-reticular level."
My comments:   
fictive locomotion induced by 50 microM N-methyl-D-aspartate (NMDA)" Glutamate applied in the bath, therefore neuromodulator rather than neurotransmitter.  Metabotropic rather than ionotropic?    
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See PubMed abstract page.    


Activity of reticulospinal neurons during locomotion in the freely behaving lamprey.       Free Article
The reticulospinal (RS) system is the main descending system transmitting commands from the brain to the spinal cord in the lamprey. It is responsible for initiation of locomotion, steering, and equilibrium control.  
    In the present study, we characterize the commands that are sent by the brain to the spinal cord in intact animals via the reticulospinal pathways during locomotion. We have developed a method for recording the activity of larger RS axons in the spinal cord in freely behaving lampreys by means of chronically implanted macroelectrodes. In this paper, the mass activity in the right and left RS pathways is described and the correlations of this activity with different aspects of locomotion are discussed.  
    In quiescent animals, the RS neurons had a low level of activity. A mild activation of RS neurons occurred in response to different sensory stimuli. Unilateral eye illumination evoked activation of the ipsilateral RS neurons. Unilateral illumination of the tail dermal photoreceptors evoked bilateral activation of RS neurons. Water vibration also evoked bilateral activation of RS neurons. Roll tilt evoked activation of the contralateral RS neurons.  
    With longer or more intense sensory stimulation of any modality and laterality, a sharp, massive bilateral activation of the RS system occurred, and the animal started to swim. This high activity of RS neurons and swimming could last for many seconds after termination of the stimulus.  
    There was a positive correlation between the level of activity of RS system and the intensity of locomotion. An asymmetry in the mass activity on the left and right sides occurred during lateral turns with a 30% prevalence (on average) for the ipsilateral side. Rhythmic modulation of the activity in RS pathways, related to the locomotor cycle, often was observed, with its peak coinciding with the electromyographic (EMG) burst in the ipsilateral rostral myotomes. The pattern of vestibular response of RS neurons observed in the quiescent state, that is, activation with contralateral roll tilt, was preserved during locomotion. In addition, an inhibition of their activity with ipsilateral tilt was clearly seen. In the cases when the activity of individual neurons could be traced during swimming, it was found that rhythmic modulation of their firing rate was superimposed on their tonic firing or on their vestibular responses.  
    In conclusion, different aspects of locomotor activity-initiation and termination, vigor of locomotion, steering and equilibrium control-are well reflected in the mass activity of the larger RS neurons."   
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Differential Contribution of Reticulospinal Cells to the Control of Locomotion Induced By the Mesencephalic Locomotor Region
    Full length HTML and PDF available online for free.  
from the Abstract:   
    "In lampreys as in other vertebrates, the reticulospinal (RS) system relays inputs from the mesencephalic locomotor region (MLR) to the spinal locomotor networks. Semi-intact preparations of larval sea lamprey were used to determine the relative contribution of the middle (MRRN) and the posterior (PRRN) rhombencephalic reticular nuclei to swimming controlled by the MLR. Intracellular recordings were performed to examine the inputs from the MLR to RS neurons.  
    Stimulation of the MLR elicited monosynaptic excitatory responses of a higher magnitude in the MRRN than in the PRRN. This differential effect was not attributed to intrinsic properties of RS neurons. Paired recordings showed that at threshold intensity for swimming, spiking activity was primarily elicited in RS cells of the MRRN. Interestingly, cells of the PRRN began to discharge at higher stimulation intensities only when MRRN cells had reached their maximal discharge rate.  
    Glutamate antagonists were ejected in either nucleus to reduce their activity. Ejections over the MRRN increased the stimulation threshold for evoking locomotion and resulted in a marked decrease in the swimming frequency and the strength of the muscle contractions. Ejections over the PRRN decreased the frequency of swimming.  
    This study provides support for the concept that RS cells show a specific recruitment pattern during MLR-induced locomotion. RS cells in the MRRN are primarily involved in initiation and maintenance of low-intensity swimming. At higher frequency locomotor rhythm, RS cells in both the MRRN and the PRRN are recruited."  

My comment
    Is this relevant to Lamprey Fast-Slow Twitch ?             

Activity of individual reticulospinal neurons during different forms of locomotion in the lamprey
    "Lamprey (a lower vertebrate) can employ different modes of locomotion, i.e. swimming in open water and crawling in tight places. Swimming is due to the periodic waves of lateral undulations with reciprocal activity of right and left muscles. In contrast, crawling (forward and backward) is based on single waves with coactivation of muscles on two sides. Basic mechanisms of swimming and, most likely, crawling reside in the spinal cord, and are activated by supraspinal commands. The main source of these commands is the reticulospinal (RS) system. The goal of the present experiments was to characterize the activity of individual RS neurons during swimming and during crawling in a U-shaped tunnel. The activity was recorded by means of chronically implanted electrodes in freely behaving animals. All recorded RS neurons were active during swimming but silent in quiescent animals. Many of them (61%) showed phasic modulation of their firing rate approximately in phase with the activity of ipsilateral rostral muscles. The majority of the neurons (80%) were also active during crawling. Many of them either increased or decreased their activity during crawling as compared to the background activity. These changes were better correlated with the direction of progression (forward or backward) than with the direction of turning in the tunnel (right or left). No correlation of the activity of RS neurons during locomotion and their sensory inputs was found. The results of this study suggest that different modes of locomotion in lampreys can be caused by considerably overlapping groups of RS neurons."
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