In spinal motoneurons, activation of dendritically located depolarizing conductances can lead to amplification of synaptic inputs and the production of plateau potentials. Immunohistochemical and computational studies have implicated dendritic CaV1.3 channels in this amplification and suggest that CaV1.3 channels in spinal motoneurons may be organized in clusters in the dendritic tree. Our goal was to provide physiological evidence for the presence of multiple discrete clusters of voltage-gated calcium channels in spinal motoneurons and to explore the spatial arrangement of these clusters in the dendritic tree. We recorded voltage-gated calcium currents from spinal motoneurons in slices of mature mouse spinal cords. We demonstrate that single somatic voltage-clamp steps can elicit multiple inward currents with varying delays to onset, resulting in a current with a “staircase”-like appearance. Recordings from cultured dorsal root ganglion cells at different stages of neurite development provide evidence that these currents arise from the unclamped portions of the dendritic tree. Finally, both voltage- and current-clamp data were used to constrain computer models of a motoneuron. The resultant simulations impose two conditions on the spatial distribution of CaV channels in motoneuron dendrites: one of asymmetry relative to the soma and another of spatial separation between clusters of CaV channels. We propose that this compartmentalization would provide motoneurons with the ability to process multiple sources of input in parallel and integrate this processed information to produce appropriate trains of action potentials for the intended motor behavior.
PMID: 19386931 CAMSID: cams1856
To ensure alternation of flexor and extensor muscles during locomotion, the spinal locomotor network provides rhythmic inhibition to motoneurons. The source of this inhibition in mammals is incompletely defined. We have identified a population of GABAergic inter-neurons located in medial laminae V/VI that express green fluorescent protein (GFP) in glutamic acid decarboxylase-65::GFP transgenic mice. Immunohistochemical studies revealed GFP + terminals in apposition to motoneuronal somata, neurons in Clarke’s column, and in laminae V/VI where they apposed GFP + interneurons, thus forming putative reciprocal connections. Whole-cell patch-clamp recordings from GFP + interneurons in spinal cord slices revealed a range of electrophysiological profiles, including sag and postinhibitory rebound potentials. Most neurons fired tonically in response to depolarizing current injection. Calcium transients demonstrated by two-photon excitation microscopy in the hemisected spinal cord were recorded in response to low-threshold dorsal root stimulation, indicating these neurons receive primary afferent input. Following a locomotor task, the number of GFP + neurons expressing Fos increased, indicating that these neurons are active during locomotion. During fictive locomotion induced in the hemisected spinal cord, two-photon excitation imaging demonstrated rhythmic calcium activity in these interneurons, which correlated with the termination of ventral root bursts. We suggest that these dorsomedial GABAergic interneurons are involved in spinal locomotor networks, and may provide direct rhythmic inhibitory input to motoneurons during locomotion.
PMID: 20089922 CAMSID: cams1858
Despite significant advances in our understanding of pattern generation in invertebrates and lower vertebrates, there have been barriers to the application of the principles learned to the definition of networks underlying mammalian locomotion. Major difficulties have arisen in identifying spinal interneurones in preparations which allow study of neuronal intrinsic properties and the role of identified interneurones in locomotor networks. Recent genetic technologies in which selective expression of fluorescent proteins in specific populations of mouse spinal neurones have provided new avenues of investigation. In this review, we focus on the generation of locomotor rhythm and outline criteria that rhythm-generating neurones might be expected to fulfill. We then examine the extent to which a recently identified population of spinal interneurones, Hb9 interneurones, fulfill these criteria. Finally, we suggest that Hb9 interneurones could be involved in an asymmetric model of locomotor rhythmogenesis through projections of electrotonically coupled rhythm-generating modules to flexor pattern formation half-centres. The principles learned from studying this population of interneurones have led to strategies to systematically evaluate neurones that may be involved in locomotor rhythmogenesis.
PMID: 17905441 CAMSID: cams1854
Post-inhibitory rebound; Locomotion; Conditional bursting; Central pattern generator; Electrotonic coupling
In spinal motoneurons, late spike frequency adaptation (SFA) is defined as the slowing of the firing rate over tens of seconds and can be seen during sustained or intermittent current injection. Although the function of late SFA is not known, it may result in a decrease in force production over time, or muscle fatigue. Because locomotion can persist for long periods of time without fatigue, late SFA was studied using intracellular recordings from adult cat motoneurons during fictive locomotion. Of eight lumbar motoneurons studied, all showed late adaptation during control conditions, but none demonstrated late adaptation during locomotor activity. The most consistent properties that correlated with the presence or absence of late SFA were those related to availability of fast, inactivating sodium channels, particularly action potential rate of rise. Evidence of the reversal of late SFA during locomotion was present for several minutes following locomotor trials, consistent with the suggestion that SFA is modulated through slow metabotropic pathways. The abolition of late adaptation in spinal motoneurons during fictive locomotion is an example of a state-dependent change in the “intrinsic” properties of mammalian motoneurons. This change contributes to increased excitability of motoneurons during locomotion and results in robust firing during sustained locomotion.
PMID: 21177992 CAMSID: cams1850
action potentials; neuromodulation; sodium conductance; walking
Since the early 20th century, it has been recognized that motoneurons must fire repetitive trains of action potentials to produce muscle contraction. In 1932, Sir John Eccles, together with Hebbel Hoff, found that action potential spike trains in motor axons were produced by “rhythmic centres”, which were within the motoneurons themselves. Two decades later, Eccles attended a Cold Spring Harbor Symposium in NY, USA entitled “The Neuron”. Two of the many notable presentations at this symposium were juxtaposed: one by Eccles from the University of Otago, Dunedin, NZL, and the other by J. Walter Woodbury and Harry Patton from the University of Washington, Seattle, USA. Both presentations included data obtained using sharp microelectrodes to study the intracellularly recorded potentials of cat motoneurons. In this review, I discuss some of the events leading up to and surrounding this jointly accomplished advance and proceed to discussion of subsequent studies over 5+ decades that have made use of intracellular recordings from motoneurons to study their repetitive firing behavior. This begins with early descriptions of primary and secondary range firing, and continues to the discovery of dendritic persistent inward currents and their relation to plateau potentials, synaptic amplification, and motoneuronal firing. Following a brief description of the possible mechanisms underlying spike frequency adaptation, I discuss the modulation of repetitive firing properties during various motor behaviors. It has become increasingly clear that the central nervous system has exquisite control of the repetitive firing of motoneurons. Eccles’ work laid the foundation for the present-day study of these processes.
PMID: 16725251 CAMSID: cams1862
History; Microelectrode; Motoneuron; Repetitive firing; Spinal cord; Electrophysiology
Intracellular (IC) recording of action potentials in neurons of the vertebrate central nervous system (CNS) was first reported by John Eccles and two colleagues, Walter Brock and John Coombs, in Dunedin, NZL in 1951/1952 and by Walter Woodbury and Harry Patton in Seattle, WA, USA in 1952. Both groups studied spinal cord neurons of the adult cat. In this review, we discuss the precedents to their notable achievement and reflect and speculate on some of the scientific and personal nuances of their work and its immediate and later impact. We then briefly discuss early achievements in IC recording in the study of CNS neurobiology in other laboratories around the world, and some of the methods that led to enhancement of CNS IC-recording techniques. Our modern understanding of CNS neurophysiology directly emanates from the pioneering endeavors of the five who wrote the seminal 1951/1952 articles.
PMID: 21782158 CAMSID: cams1860
Interneuron; Intracellular recording; Mammalian; Motoneuron; Non-mammalian
Nerve injuries resulting in prolonged periods of denervation result in poor recovery of motor function. We have previously shown that embryonic stem cell‐derived motoneurons transplanted at the time of transection into a peripheral nerve can functionally reinnervate muscle. For clinical relevance, we now focused on delaying transplantation to assess reinnervation after prolonged denervation.
Embryonic stem cell‐derived motoneurons were transplanted into the distal segments of transected tibial nerves in adult mice after prolonged denervation of 1–8 weeks. Twitch and tetanic forces were measured ex vivo 3 months posttransplantation. Tissue was harvested from the transplants for culture and immunohistochemical analysis.
In this delayed reinnervation model, teratocarcinomas developed in about one half of transplants. A residual multipotent cell population (~ 6% of cells) was found despite neural differentiation. Exposure to the alkylating drug mitomycin C eliminated this multipotent population in vitro while preserving motoneurons. Treating neural differentiated stem cells prior to delayed transplantation prevented tumor formation and resulted in twitch and tetanic forces similar to those in animals transplanted acutely after denervation.
Despite a neural differentiation protocol, embryonic stem cell‐derived motoneurons still carry a risk of tumorigenicity. Pretreating with an antimitotic agent leads to survival and functional muscle reinnervation if performed within 4 weeks of denervation in the mouse.
Accurate motor performance depends on the integration in spinal microcircuits of sensory feedback information. Hand grasp is a skilled motor behavior known to require cutaneous sensory feedback, but spinal microcircuits that process and relay this feedback to the motor system have not been defined. We sought to define classes of spinal interneurons involved in the cutaneous control of hand grasp in mice, and show that dI3 interneurons, a class of dorsal spinal interneurons marked by expression of Isl1, convey input from low threshold cutaneous afferents to motoneurons. Mice in which the output of dI3 interneurons has been inactivated exhibit deficits in motor tasks that rely on cutaneous afferent input. Most strikingly, the ability to maintain grip strength in response to increasing load is lost following genetic silencing of dI3 interneuron output. Thus, spinal microcircuits that integrate cutaneous feedback crucial for paw grip rely on the intermediary role of dI3 interneurons.
As the nexus between the nervous system and the skeletomuscular system, motoneurons effect all behaviour. As such, motoneuron activity must be well-regulated so as to generate appropriately timed and graded muscular contractions. Accordingly, motoneurons receive a large number of both excitatory and inhibitory synaptic inputs from various peripheral and central sources. Many of these synaptic contacts arise from spinal interneurons, some of which belong to spinal networks responsible for the generation of locomotor activity. Although the complete definition of these networks remains elusive, it is known that the neural machinery necessary to generate the basic rhythm and pattern of locomotion is contained within the spinal cord. One approach to gaining insights into spinal locomotor networks is to describe those spinal interneurons that directly control the activity of motoneurons, so called “last-order” interneurons. In this review, we briefly survey the different populations of last-order interneurons that have been identified using anatomical, physiological, and genetic methodologies. We discuss the possible roles of these identified last-order interneurons in generating locomotor activity, and in the process, identify particular criteria that may be useful in identifying putative last-order interneurons belonging to spinal locomotor networks.
PMID: 21111202 CAMSID: cams1859
Mammalian motor programs are controlled by networks of spinal interneurons that set the rhythm and intensity of motor neuron firing. Motor neurons have long been known to receive prominent ‘C-bouton’ cholinergic inputs from spinal interneurons, but the source and function of these synaptic inputs have remained obscure. We show here that the transcription factor Pitx2 marks a small cluster of spinal cholinergic interneurons, V0C neurons, that represents the sole source of C-bouton inputs to motor neurons. The activity of these cholinergic interneurons is tightly phase-locked with motor neuron bursting during fictive locomotor activity, suggesting a role in the modulation of motor neuron firing frequency. Genetic inactivation of the output of these neurons impairs a locomotor task-dependent increase in motor neuron firing and muscle activation. Thus V0C interneurons represent a defined class of spinal cholinergic interneurons with an intrinsic neuromodulatory role in the control of locomotor behavior.
cholinergic interneurons; synapses; locomotor activity; neuromodulation