Encoding and decoding of reticulospinal commands

In the lamprey, the reticulospinal (RS) system is the main descending system transmitting commands to the spinal cord. We investigated these commands and their effect on the spinal mechanisms. The RS commands were studied by recording responses of RS neurons to sensory stimuli eliciting different motor behaviors. Initiation of locomotion was associated with symmetrical bilateral massive activation of RS neurons, whereas turns in different planes were associated with asymmetrical activation of corresponding neuronal groups. The sub-populations of RS neurons causing different motor behaviors partly overlap. We suggest that commands for initiation of locomotion and regulation of its vigour, encoded as the value of bilateral RS activity, are decoded in the spinal cord by integrating all RS signals arriving at the segmental locomotor networks. Commands for turns in different planes, encoded as an asymmetry in the activities of specific groups of RS neurons, are decoded by comparing the activities of those groups. This hypothesis was supported by the experiments on a neuro-mechanical model, where the difference between the activities in the left and right RS pathways was used to control a motor rotating the animal in the roll plane. Transformation of the descending commands into the motor responses was investigated by recording the effects of individual RS neurons on the motor output. Twenty patterns of influences have been found. This great diversity of the patterns allows the RS system to evoke body flexion in any plane. Since most neurons have asymmetrical projections we suggest that, for rectilinear swimming, RS neurons with opposite asymmetrical effects are co-activated.     

Putative spinal interneurons mediating postural limb reflexes provide a basis for postural control in different planes

The dorsal‐side‐up trunk orientation in standing quadrupeds is maintained by the postural system driven mainly by somatosensory inputs from the limbs. Postural limb reflexes (PLRs) represent a substantial component of this system. Earlier we described spinal neurons presumably contributing to the generation of PLRs. The first aim of the present study was to reveal trends in the distribution of neurons with different parameters of PLR‐related activity across the gray matter of the spinal cord. The second aim was to estimate the contribution of PLR‐related neurons with different patterns of convergence of sensory inputs from the limbs to stabilization of body orientation in different planes. For this purpose, the head and vertebral column of the decerebrate rabbit were fixed and the hindlimbs were positioned on a platform. Activity of individual neurons from L5 to L6 was recorded during PLRs evoked by lateral tilts of the platform. In addition, the neurons were tested by tilts of the platform under only the ipsilateral or only the contralateral limb, as well as during in‐phase tilts of the platforms under both limbs. We found that, across the spinal gray matter, strength of PLR‐related neuronal activity and sensory input from the ipsilateral limb decreased in the dorsoventral direction, while strength of the input from the contralateral limb increased. A near linear summation of tilt‐related sensory inputs from different limbs was found. Functional roles were proposed for individual neurons. The obtained data present the first characterization of posture‐related spinal neurons, forming a basis for studies of postural networks impaired by injury.     

Rostro-caudal distribution of reticulospinal projections from different brainstem nuclei in the lamprey

The reticulospinal (RS) system in the lamprey is responsible for the control of locomotion, postural corrections and steering. To perform these functions, the RS system has to affect different muscular compartments along the body axis selectively. In this study, the possibility that RS neurones in different nuclei may project to different parts of the spinal cord, was investigated. The rostro-caudal extent of single RS axons was defined by stimulating them antidromically while recording from their cell body. All recorded mesencephalic RS neurones projected to the caudal tip of the spinal cord. Of the rhombencephalic RS neurones, 26% of the recorded neurones did not reach the caudalmost fourth of the spinal cord and this proportion varied between the anterior (18%), middle (17%) and posterior (36%) rhombencephalic reticular nuclei. For these RS axons, the level of termination covered the whole rostro-caudal extent of the spinal cord. No correlation was found between the length of an axon and its conduction velocity or between the length of an axon and the rostro-caudal position of its cell body in the nuclei.     

The role of spinal cord inputs in modulating the activity of reticulospinal neurons during fictive locomotion in the lamprey

Lamprey reticulospinal neurons are rhythmically modulated during fictive swimming. The present study examines the possibility that this modulation may originate from the spinal cord locomotor networks rather than from the brainstem. To test this, the in vitro preparation of the lamprey brainstem-spinal cord was separated into two compartments which could be exposed to different chemical environments. Locomotor activity was induced pharmacologically in the caudal spinal cord compartment and reticulospinal (RS) neurons from the posterior rhombencephalic reticular nucleus (PRRN) were recorded intracellularly in the rostral compartment containing normal lamprey Ringer. Under these conditions, the membrane potential of RS neurons showed clear rhythmic oscillations which are correlated with the ongoing locomotor activity in the caudal spinal cord bath, although no locomotor discharges were present in the ventral roots of the rostral bath. Such oscillations were not present in the absence of locomotion. These results indicate that the spinal cord locomotor networks can contribute to the rhythmic oscillations which occur in RS neurons during fictive locomotion. Moreover, the latter oscillations of membrane potential are due to both phasic excitation and Cl- -dependent inhibition in the opposite phase.     

Axonal regeneration of descending brain neurons in larval lamprey demonstrated by retrograde double labeling

In larval lamprey, the large, identified descending brain neurons (Müller and Mauthner cells) are capable of axonal regeneration. However, smaller, unidentified descending brain neurons, such as many of the reticulospinal (RS) neurons, probably initiate locomotion, and it is not known whether the majority of these neurons regenerate their axons after spinal cord transection. In the present study, this issue was addressed by using double labeling of descending brain neurons. In double-label control animals, in which Fluoro-Gold (FG) was applied to the spinal cord at 40% body length (BL; measured from anterior to posterior from tip of head) and Texas red dextran amine (TRDA) was applied later to the spinal cord at 20% BL, an average of 98% of descending brain neurons were double labeled. In double-label experimental animals, FG was applied to the spinal cord at 40% BL; two weeks later the spinal cord was transected at 10% BL; and, eight weeks or 16 weeks after spinal cord transection, TRDA was applied to the spinal cord at 20% BL. At eight weeks and 16 weeks after spinal cord transection, an average of 49% and 68%, respectively, of descending brain neurons, including many unidentified RS neurons, were double labeled. These results in larval lamprey are the first to demonstrate that the majority of descending brain neurons, including small, unidentified RS neurons, regenerate their axons after spinal cord transection. Therefore, in spinal cord-transected lamprey, axonal regeneration of descending brain neurons probably contributes significantly to the recovery of locomotor function.     

Intraspinal stretch receptor neurons mediate different motor responses along the body in lamprey

In lampreys, stretch receptor neurons (SRNs) are located at the margins of the spinal cord and activated by longitudinal stretch in that area caused by body bending. The aim of this study was a comprehensive analysis of motor responses to bending of the lamprey body in different planes and at different rostrocaudal levels. For this purpose, in vitro preparation of the spinal cord isolated together with notochord was used, and responses to bending were recorded from SRNs, as well as from motoneurons innervating the dorsal (dMNs) and ventral (vMNs) parts of a myotome. It was found that SRNs were activated on the convex (stretched) side of the preparation during bending both in the yaw and in the pitch plane. By contrast, responses of motoneurons depended on the site and plane of bending. In the yaw plane, concave responses to bending of rostral segments and convex responses to bending of mid‐body segments prevailed. In the pitch plane, convex responses in dMNs and concave responses in vMNs to bending in mid‐body segments prevailed. These spinal reflexes could contribute to feedback regulation of locomotor body undulations and to the control of body configuration during locomotion. After a longitudinal split of the spinal cord, only convex responses in motoneurons were present, suggesting an important role of contralateral networks in determining the type of motor response. Stimulation of the brainstem changed the type of motor response to bending, suggesting that these spinal reflexes can be modified by supraspinal signals in accordance with different motor behaviors. J. Comp. Neurol. 521:3847–3862, 2013. © 2013 Wiley Periodicals, Inc.     

Visual Potentiation of Vestibular Responses in Lamprey Reticulospinal Neurons

The lamprey normally swims with the dorsal side up. Illumination of one eye shifts the set-point of the vestibular roll control system, however, so that the animal swims with a roll tilt towards the source of light (the dorsal light response). A tilted orientation is often maintained for up to 1 min after the stimulation. In the present study, the basis for this behaviour was investigated at the neuronal level. The middle rhombencephalic reticular nucleus (MRRN) is considered a main nucleus for the control of roll orientation in lampreys. Practically all MRRN neurons receive vestibular and visual input and project to the spinal cord. Earlier extracellular experiments had shown that optic nerve stimulation potentiates the response to vestibular stimulation in the ipsilateral MRRN. This most likely represents a neural correlate of the dorsal light response. Experiments were carried out in vitro on the isolated brainstem of the silver lamprey (Ichthyomyzon unicuspis). MRRN cells were recorded intracellularly, and the overall activity of descending systems was monitored with bilateral extracellular electrodes. The responses to 10 Hz optic nerve stimulation and 1 Hz vestibular nerve stimulation, and the influence of optic nerve stimulation on the vestibular responses, were investigated. In most preparations, optic nerve stimulation excited practically all ipsilateral MRRN cells. After stimulation, the cell was typically depolarized and showed an increased level of synaptic noise for up to 80 s. In contralateral MRRN neurons, optic nerve stimulation usually evoked hyperpolarization or no response. Vestibular nerve stimulation evoked compound excitatory postsynaptic potentials (EPSPs) or spikes in -90% of the cells, both ipsilaterally and contralaterally. A smaller subpopulation of MRRN cells (-10%) received vestibular inhibition. In 26 of 48 recorded MRRN cells, the response to vestibular stimulation was potentiated after ipsilateral optic nerve stimulation. The potentiation was seen in cells receiving either excitatory or inhibitory vestibular input as an increase in EPSP amplitudelspiking (85%) and a decrease in inhibitory postsynaptic potential amplitude (15%) respectively. In most cases the vestibular responses did not return to control levels during the testing period (10–30 min), and thus the visual stimulation most likely induced long-lasting changes in the functional connectivity of the roll control network, in addition to the short-lasting afteractivity. In four of the 11 cells recorded contralateral to the stimulated optic nerve, a depression of the vestibular response could be seen. In potentiated cells, single vestibular pulses often evoked longer episodes of large synaptic noise and sometimes spiking. In the latter case, the action potentials appeared with highly variable latency after each stimulation pulse. This indicates that an important mechanism underlying the potentiation may be a long-lasting increase in excitability in a pool of unidentified interneurons located either upstream of the MRRN cells, relaying vestibular and visual inputs, or downstream, providing positive feedback.     

The spatial organization of the vestibulospinal neurons in the guinea pig]

Quantitative investigation of the spatial organization of vestibulospinal neurons has been performed on a guinea pig using the horseradish peroxidase retrograde transport method. After unilateral injection in the upper cervical and low thoracic spinal cord the labelled neurons were found unilaterally in the lateral vestibular nucleus and bilaterally in descending and medial vestibular nuclei. Distribution of vestibulospinal neurons within the brainstem has been analyzed. It was revealed that projections of some neurons of medial and descending vestibular nuclei coincided with spinal projections of cortical and rubral descending pathways. Functional significance of the vestibulospinal system in the supraspinal motor control was discussed.     

Axonal regeneration of descending and ascending spinal projection neurons in spinal cord-transected larval lamprey

The distributions of descending and ascending spinal projection neurons (i.e., spinal neurons with moderate to long axons) were compared in normal larval lamprey and in animals that had recovered for 8 weeks following a complete spinal cord transection at 50% body length (BL, normalized distance from the anterior head). In normal animals, application of HRP to the spinal cord at 60% BL (40% BL) labeled an average of 713.8 +/- 143.2 descending spinal projection neurons (718.4 +/- 108.0 ascending spinal projection neurons) along the rostral (caudal) spinal cord, most of which were unidentified neurons. Some of these neurons project for at least approximately 50-60 spinal cord segments (approximately 36-47 mm in animals with an average length of approximately 90 mm used in the present study). At 8 weeks posttransection, the numbers of HRP-labeled descending or ascending spinal neurons that extended their axons through the transection were about 40% of those in similar areas of the spinal cord in normal animals. Thus, in larval lamprey, axonal regeneration of descending and ascending spinal projection neurons is incomplete, similar to that found for descending brain neurons. The majority of restored projections were from unidentified spinal neurons that have not been documented previously. In contrast to results from several other lower vertebrates, in the lamprey ascending spinal neurons exhibited substantial axonal regeneration. Identified descending spinal neurons, such as lateral interneurons and crossed contralateral interneurons, and identified ascending spinal neurons, such as giant interneurons and edge cells, regenerated their axons at least 9 mm beyond the transection site in animals with an average length of approximately 90 mm, which is appreciably farther than previously reported. In contrast, most dorsal cells, which are centrally located sensory neurons, exhibited very little axonal regeneration.     

Postural responses to combined vestibular and hip proprioceptive stimulation in man

Vestibular-proprioceptive interaction in human postural control in the frontal plane was studied by analysing the lateral body sway evoked in a standing subject by a weak, near-threshold galvanic vestibular stimulation combined with a balanced, bilateral vibration of the medial gluteus muscles. The intensities of the stimuli were adjusted so that none of them produced a consistent postural response when delivered alone. The pattern of the lateral body sway evoked by the combined stimulation was compared with postural responses to suprathreshold vestibular stimulation and asymmetric (unilateral) vibration of the hip abductors. During the vestibular stimulation alone the head movement started earlier and was larger than movement of the hip. During unilateral vibration the head movement was delayed with respect to the hip movement and the amplitude of head deviation was less than that of the hip. The pattern of postural response to combined vestibular stimulation and balanced vibration resembled that observed under unbalanced, unilateral vibration in terms of both the latencies and amplitudes of deviation of the body segments from their respective baseline positions. It is suggested that the asymmetric vestibular signal provided by galvanic stimulation of the labyrinth introduces a bias into the reference frame for central interpretation of proprioceptive signals so that a symmetric proprioceptive input gives rise to a lateral body sway when referenced to an asymmetric vestibular input.