Extensive fusion of mitochondria in spinal cord motor neurons

The relative roles played by trafficking, fission and fusion in the dynamics of mitochondria in neurons have not been fully elucidated. In the present study, a slow widespread redistribution of mitochondria within cultured spinal cord motor neurons was observed as a result of extensive organelle fusion. Mitochondria were labeled with a photoconvertible fluorescent protein (mitoKaede) that is red-shifted following brief irradiation with blue light. The behavior of these selectively labeled mitochondria was followed by live fluorescence imaging. Marking mitochondria within the cell soma revealed a complete mixing, within 18 hours, of these organelles with mitochondria coming from the surrounding neurites. Fusion of juxtaposed mitochondria was directly observed in neuritic processes at least 200 microns from the cell body. Within 24 hours, photoconverted mitoKaede was dispersed to all of the mitochondria in the portion of neurite under observation. When time lapse imaging over minutes was combined with long-term observation of marked mitochondria, moving organelles that traversed the field of view did not initially contain photoconverted protein, but after several hours organelles in motion contained both fluorescent proteins, coincident with widespread fusion of all of the mitochondria within the length of neurite under observation. These observations suggest that there is a widespread exchange of mitochondrial components throughout a neuron as a result of organelle fusion.     

Neuronal development: sorting out motor neurons

In the developing spinal cord, motor neurons become segregated into important functional units termed motor pools. Now it has been discovered that repertoires of cadherin surface molecules play key roles in motor pool sorting.     

Unpleasant visual stimuli increase the excitability of spinal motor neurons

Abstract

Purpose: In physical therapy for post-stroke patients, we often experience cases in which unpleasant emotions cause abnormal muscle tonus. Previously, we suggested that the magnitude of spinal motor neuron excitability was correlated with the grade of muscle tonus. Therefore, spinal motor neuron excitability was considered to be a useful index to evaluate the influence of unpleasant emotions on muscle tonus. In this study, we investigated whether unpleasant emotions evoked by visual stimuli affected the excitability of spinal motor neurons.

Materials and Methods: The F-waves, an indicator of spinal motor neuron excitability, were measured in 19 healthy adult volunteers. Firstly, for the rest trial, F-waves were measured during relaxation to determine the baseline of spinal motor neuron excitability. Following the rest trial, the unpleasant trial was conducted in which F-waves were measured while the subjects viewed an unpleasant picture for 1?min. After the unpleasant trial, F-waves were measured during relaxation. For the control condition, F-waves were measured while the subjects viewed a neutral picture instead of the unpleasant picture. The recorded F-wave data were analysed for persistence and the F/M amplitude ratio.

Results: Persistence and the F/M amplitude ratio were significantly greater during the unpleasant trial than during the rest trial. In the control condition, there was no significant difference in persistence and the F/M amplitude ratio compared with the three trials.

Conclusions: Our findings indicate that unpleasant emotions may affect spinal motor neuron excitability. Therefore, learning how to control emotions should be important aspect of physical therapy.     

Cell adhesion and migration in the organization of spinal motor neurons

Spinal motor neurons are critical to the ability of animals to move and thus essential to survival. Motor neurons that project axons to distinct limb-muscle targets are topographically organized such that central nervous system position reflects the location of the muscle in the limb. The central positioning of limb-projecting motor neurons arises during development through motor neuron migration followed by a period of coalescence into discrete groupings of motor neurons which project axons to an individual muscle. These so-called motor pools are a common feature of motor organization in higher vertebrates. Recent work has highlighted the critical role for armadillo family member catenin-dependent functions of the cadherin family of cell adhesion molecules in directing the organization of motor neurons. Cadherin function appears to be important for both the motor neuron migration and coalescence phases of the emergence of motor neuron topography. Here, I review this recent work in the context of our understanding of the general development of spinal motor neurons.     

Cell adhesion and migration in the organization of spinal motor neurons

Spinal motor neurons are critical to the ability of animals to move and thus essential to survival. Motor neurons that project axons to distinct limb-muscle targets are topographically organized such that central nervous system position reflects the location of the muscle in the limb. The central positioning of limb-projecting motor neurons arises during development through motor neuron migration followed by a period of coalescence into discrete groupings of motor neurons which project axons to an individual muscle. These so-called motor pools are a common feature of motor organization in higher vertebrates. Recent work has highlighted the critical role for armadillo family member catenin-dependent functions of the cadherin family of cell adhesion molecules in directing the organization of motor neurons. Cadherin function appears to be important for both the motor neuron migration and coalescence phases of the emergence of motor neuron topography. Here, I review this recent work in the context of our understanding of the general development of spinal motor neurons.     

Maturation of spinal motor neurons derived from human embryonic stem cells

Our understanding of motor neuron biology in humans is derived mainly from investigation of human postmortem tissue and more indirectly from live animal models such as rodents. Thus generation of motor neurons from human embryonic stem cells and human induced pluripotent stem cells is an important new approach to model motor neuron function. To be useful models of human motor neuron function, cells generated in vitro should develop mature properties that are the hallmarks of motor neurons in vivo such as elaborated neuronal processes and mature electrophysiological characteristics. Here we have investigated changes in morphological and electrophysiological properties associated with maturation of neurons differentiated from human embryonic stem cells expressing GFP driven by a motor neuron specific reporter (Hb9::GFP) in culture. We observed maturation in cellular morphology seen as more complex neurite outgrowth and increased soma area over time. Electrophysiological changes included decreasing input resistance and increasing action potential firing frequency over 13 days in vitro. Furthermore, these human embryonic stem cell derived motor neurons acquired two physiological characteristics that are thought to underpin motor neuron integrated function in motor circuits; spike frequency adaptation and rebound action potential firing. These findings show that human embryonic stem cell derived motor neurons develop functional characteristics typical of spinal motor neurons in vivo and suggest that they are a relevant and useful platform for studying motor neuron development and function and for modeling motor neuron diseases.     

Eph and ephrin signaling: lessons learned from spinal motor neurons

In nervous system assembly, Eph/ephrin signaling mediates many axon guidance events that shape the formation of precise neuronal connections. However, due to the complexity of interactions between Ephs and ephrins, the molecular logic of their action is still being unraveled. Considerable advances have been made by studying the innervation of the limb by spinal motor neurons, a series of events governed by Eph/ephrin signaling. Here, we discuss the contributions of different Eph/ephrin modes of interaction, downstream signaling and electrical activity, and how these systems may interact both with each other and with other guidance molecules in limb muscle innervation. This simple model system has emerged as a very powerful tool to study this set of molecules, and will continue to be so by virtue of its simplicity, accessibility and the wealth of pioneering cellular studies.     

Distributions of active spinal cord neurons during swimming and scratching motor patterns

The spinal cord can generate motor patterns underlying several kinds of limb movements. Many spinal interneurons are multifunctional, contributing to multiple limb movements, but others are specialized. It is unclear whether anatomical distributions of activated neurons differ for different limb movements. We examined distributions of activated neurons for locomotion and scratching using an activity-dependent dye. Adult turtles were stimulated to generate repeatedly forward swimming, rostral scratching, pocket scratching, or caudal scratching motor patterns, while sulforhodamine 101 was applied to the spinal cord. Sulforhodamine-labeled neurons were widely distributed rostrocaudally, dorsoventrally, and mediolaterally after each motor pattern, concentrated bilaterally in the deep dorsal horn, the lateral intermediate zone, and the dorsal to middle ventral horn. Labeled neurons were common in all hindlimb enlargement segments and the pre-enlargement segment following swimming and scratching, but a significantly higher percentage were in the rostral segments following swimming than rostral scratching. These findings suggest that largely the same spinal regions are activated during swimming and scratching, but there are some differences that may indicate locations of behaviorally specialized neurons. Finally, the substantial inter-animal variability following a single kind of motor pattern may indicate that essentially the same motor output is generated by anatomically variable networks.     

The generation and diversification of spinal motor neurons: signals and responses

Motor neurons are probably the best characterised neuronal class in the vertebrate central nervous system and have become a paradigm for understanding the mechanisms that control the development of vertebrate neurons. For many investigators working on this problem the chick embryo is the model system of choice and from these studies a picture of the steps involved in motor neuron generation has begun to emerge. These findings suggest that motor neuron generation is shaped by extracellular signals that regulate intrinsic, cell-autonomous determinants at sequential steps during development. The chick embryo has played a prominent role in identifying the sources of these signals, defining their molecular identities and determining the cell intrinsic programs they regulate.     

Neuronal development: putting motor neurons in their place

In the developing spinal cord, motor neurons occupy discrete columns with different identities and axon projections. This organisation has now been shown to depend crucially on sequential phases of expression of Hox-c proteins, generated in response to fibroblast growth factor signals.     

Experience-dependent structural plasticity of pyramidal neurons in the developing sensory cortices

Sensory experience regulates the structural and functional wiring of neuronal circuits, during development and throughout adulthood. Here, we review current knowledge of how experience affects structural plasticity of pyramidal neurons in the sensory cortices. We discuss the pros and cons of existing labeling approaches, as well as what structural parameters are most plastic. We further discuss how recent advances in sparse labeling of specific neuronal subtypes, as well as development of techniques that allow fast, high resolution imaging in large fields, would enable future studies to address currently unanswered questions in the field of structural plasticity.     

Control of respiratory motor pattern by sensory neurons in spinal cord of lamprey

Summary Extracellular stimulation over the dorsal funiculus in the spinal cord of lampreys was found to selectively activate prolonged episodes of fictive arousal respiration (Figs. 1, 3). The induced episodes showed comparable increases in cycle frequency and motoneuron burst duration to the spontaneous arousal pattern observed in isolated brain preparations (Fig. 2). Intracellular stimulation of primary sensory neurons with axons in the dorsal funiculus, called dorsal cells, also elicited the arousal pattern (Fig. 4). Mechanoreceptive dorsal cells respond to cutaneous stimulation. When mechanical stimuli were applied to the skin of intact lampreys (Fig. 6) or to lampreys with ipsilateral vagotomy, arousal respiration was induced (Figs. 7, 8). Bilateral, but not unilateral, trigeminal lesion blocked dorsal cell induction of the arousal response (Fig. 5). Spontaneous arousal respiration was recorded from intact, unrestrained lampreys (Fig. 9). These results suggest that fictive arousal respiration is the in vitro correlate of natural arousal respiration in lampreys, and that one mechanism leading to arousal respiration may be the activity of sensory dorsal cells. A model for respiratory motor pattern switching in lamprey is proposed. The model suggests that the normal and arousal patterns are produced by separately engaging rostral or caudal pattern generators in the medulla, rather than by modifying one pattern generator (Fig. 10).     

Effect of fenibut on the GABA B receptors of the spinal motor neurons

It has been established in experiments on the isolated spinal cord of 7-14-day-old rats that the GABAB-mimetic phenibut (10(-5)--10(-4) M) elicits a slow-developing depolarization of motoneurons, suppression of spontaneous activity and polysynaptic reflex discharges of motoneurons, recorded from the ventral roots. Administered under the same conditions GABA produces de- and hyperpolarization of motoneurons. The depolarization of motoneurons elicited by phenibut and GABA is not reversed by picrotoxin in contradistinction to the GABA-induced hyperpolarization of motoneurons, being associated with a direct action of the GABA-mimetics on postsynaptic GABAB receptors of motoneurons. Diazepam (10(-9)--10(-6) M) potentiates the effects of phenibut supposedly via benzodiazepine receptors bound with GABAA receptors (an independent interaction).