Bidirectional flow of action potentials in axons drives activity dynamics in neuronal cultures

Objective: Recent technological advances are revealing the complex physiology of the axon and challenging long-standing assumptions. Namely, while most action potential (AP) initiation occurs at the axon initial segment in central nervous system neurons, initiation in distal parts of the axon has been reported to occur in both physiological and pathological conditions. The functional role of these ectopic APs, if exists, is still not clear, nor its impact on network activity dynamics. Approach: Using an electrophysiology platform specifically designed for assessing axonal conduction we show here for the first time regular and effective bidirectional axonal conduction in hippocampal and dorsal root ganglia cultures. We investigate and characterize this bidirectional propagation both in physiological conditions and after distal axotomy. Main results: A significant fraction of APs are not coming from the canonical synapse-dendrite-soma signal flow, but instead from signals originating at the distal axon. Importantly, antidromic APs may carry information and can have a functional impact on the neuron, as they consistently depolarize the soma. Thus, plasticity or gene transduction mechanisms triggered by soma depolarization can also be affected by these antidromic APs. Conduction velocity is asymmetrical, with antidromic conduction being slower than orthodromic. Significance: Altogether these findings have important implications for the study of neuronal function in vitro, reshaping our understanding on how information flows in neuronal cultures.

The Insula: A Stimulating Island of the Brain

Direct cortical stimulation (DCS) in epilepsy surgery patients has a long history of functional brain mapping and seizure triggering. Here, we review its findings when applied to the insula in order to map the insular functions, evaluate its local and distant connections, and trigger seizures. Clinical responses to insular DCS are frequent and diverse, showing a partial segregation with spatial overlap, including a posterior somatosensory, auditory, and vestibular part, a central olfactory-gustatory region, and an anterior visceral and cognitive-emotional portion. The study of cortico-cortical evoked potentials (CCEPs) has shown that the anterior (resp. posterior) insula has a higher connectivity rate with itself than with the posterior (resp. anterior) insula, and that both the anterior and posterior insula are closely connected, notably between the homologous insular subdivisions. All insular gyri show extensive and complex ipsilateral and contralateral extra-insular connections, more anteriorly for the anterior insula and more posteriorly for the posterior insula. As a rule, CCEPs propagate first and with a higher probability around the insular DCS site, then to the homologous region, and later to more distal regions with fast cortico-cortical axonal conduction delays. Seizures elicited by insular DCS have rarely been specifically studied, but their rate does not seem to differ from those of other DCS studies. They are mainly provoked from the insular seizure onset zone but can also be triggered by stimulating intra- and extra-insular early propagation zones. Overall, in line with the neuroimaging studies, insular DCS studies converge on the view that the insula is a multimodal functional hub with a fast propagation of information, whose organization helps understand where insular seizures start and how they propagate.

Learning to be on time: temporal coordination of neural dynamics by activity-dependent myelination

Activity-dependent myelination is the mechanism by which myelin changes as a function of neural activity, and plays a fundamental role in brain plasticity. Mediated by structural changes in glia, activity-dependent myelination regulates axonal conduction velocity. It remains unclear how neural activity impacts myelination to orchestrate the timing of neural signaling. We developed a model of spiking neurons enhanced with neuron-glia feedback. Inspired by experimental data and use-dependent synaptic plasticity, we introduced a learning rule, called the Activity-Dependent Myelination (ADM) rule, by which conduction velocity scales with firing rates. We found that the ADM rule implements a homeostatic control mechanism that promotes and preserves synchronization. ADM-mediated plasticity was found to optimize synchrony by compensating for variability in axonal lengths by scaling conduction velocity in an axon-specific way. This property was maintained even when the network structure is altered. We further explored how external stimuli interact with the ADM rule to trigger bidirectional and reversible changes in conduction delays. These results highlight the role played by activity-dependent myelination in synchronous neural communication and brain plasticity.

Mathematical Relationships between Motoneuron Properties Derived by Empirical Data Analysis: Size Determines All Motoneuron Properties

Our understanding of the behaviour of motoneurons (MNs) in mammals partly relies on our knowledge of the relationships between MN membrane properties, such as MN size, resistance, rheobase, capacitance, time constant, axonal conduction velocity and afterhyperpolarization period. Based on scattered but converging evidence, current experimental studies and review papers qualitatively assumed that some of these MN properties are related. Here, we reprocessed the data from 27 experimental studies in cat and rat MN preparations to empirically demonstrate that all experimentally measured MN properties are associated to MN size. Moreover, we expanded this finding by deriving mathematical relationships between each pair of MN properties. These relationships were validated against independent experimental results not used to derive them. The obtained relationships support the classic description of a MN as a membrane equivalent electrical circuit and describe for the first time the association between MN size and MN membrane capacitance and time constant. The obtained relations indicate that motor units are recruited in order of increasing MN size, muscle unit size, MN rheobase, unit force recruitment thresholds and tetanic forces, but underlines that MN size and recruitment order may not be related to motor unit type.

Central nervous system hypomyelination disrupts axonal conduction and behaviour in larval zebrafish

Myelination is essential for central nervous system (CNS) formation, health and function. As a model organism, larval zebrafish have been extensively employed to investigate the molecular and cellular basis of CNS myelination, due to their genetic tractability and suitability for non-invasive live cell imaging. However, it has not been assessed to what extent CNS myelination affects neural circuit function in zebrafish larvae, prohibiting the integration of molecular and cellular analyses of myelination with concomitant network maturation. To test whether larval zebrafish might serve as a suitable platform with which to study the effects of CNS myelination and its dysregulation on circuit function, we generated zebrafish myelin regulatory factor (myrf) mutants with CNS-specific hypomyelination and investigated how this affected their axonal conduction properties and behaviour. We found that myrf mutant larvae exhibited increased latency to perform startle responses following defined acoustic stimuli. Furthermore, we found that hypomyelinated animals often selected an impaired response to acoustic stimuli, exhibiting a bias towards reorientation behaviour instead of the stimulus-appropriate startle response. To begin to study how myelination affected the underlying circuitry, we established electrophysiological protocols to assess various conduction properties along single axons. We found that the hypomyelinated myrf mutants exhibited reduced action potential conduction velocity and an impaired ability to sustain high frequency action potential firing. This study indicates that larval zebrafish can be used to bridge molecular and cellular investigation of CNS myelination with multiscale assessment of neural circuit function.

In vitro neuronal networks show bidirectional axonal conduction with antidromic action potentials effectively depolarizing the soma

ABSTRACTRecent technological advances are revealing the complex physiology of the axon and challenging long-standing assumptions. Namely, while most action potential (AP) initiation occurs at the axon initial segment in central nervous system neurons, initiation in distal parts of the axon has been shown to occur in both physiological and pathological conditions. However, such ectopic action potential (EAP) activity has not been reported yet in studies using in vitro neuronal networks and its functional role, if exists, is still not clear. Here, we report the spontaneous occurrence of EAPs and effective antidromic conduction in hippocampal neuronal cultures. We also observe a significant fraction of bidirection axonal conduction in dorsal root ganglia neuronal cultures. We set out to investigate and characterize this antidromic propagation via a combination of microelectrode arrays, microfluidics, advanced data analysis and in silico studies. We show that EAPs and antidromic conduction can occur spontaneously, and also after distal axotomy or physiological changes in the axon biochemical environment. Importantly, EAPs may carry information (as orthodromic action potentials do) and can have a functional impact on the neuron, as they consistently depolarize the soma. Plasticity or gene transduction mechanisms triggered by soma depolarization can, therefore, be also affected by these antidromic action potentials/EAPs. Finally, we show that this bidirectional axonal conduction is asymmetrical, with antidromic conduction being slower than orthodromic. Via computational modeling, we show that the experimental difference can be explained by axonal morphology. Altogether, these findings have important implications for the study of neuronal function in vitro, reshaping completely our understanding on how information flows in neuronal cultures.