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In order to understand the nervous system, it is necessary to know the synaptic connections between the neurons, yet to date, only the wiring diagram of the adult hermaphrodite of the nematode Caenorhabditis elegans has been determined. Here, we present the wiring diagram of the posterior nervous system of the C. elegans adult male, reconstructed from serial electron micrograph sections. This region of the male nervous system contains the sexually dimorphic circuits for mating. The synaptic connections, both chemical and gap junctional, form a neural network with four striking features: multiple, parallel, short synaptic pathways directly connecting sensory neurons to end organs; recurrent and reciprocal connectivity among sensory neurons; modular substructure; and interneurons acting in feedforward loops. These features help to explain how the network robustly and rapidly selects and executes the steps of a behavioral program on the basis of the inputs from multiple sensory neurons.

Key Points There are two main modalities of synaptic transmission: chemical and electrical. Although chemical synapses are perceived to be structurally more complex and functionally dynamic than electrical synapses, emerging evidence indicates that electrical synapses might be similarly complex, functionally diverse and highly modifiable. Far from functioning independently and serving unrelated functions, these two modalities of synaptic transmission closely interact. Rather than conceiving synaptic transmission as either chemical or electrical, this article emphasizes the notion that synaptic transmission is both chemical and electrical, and that interactions between these two forms of interneuronal communication are required for normal brain development and function. The development of neural circuits in disparate nervous systems (both vertebrate and invertebrate) seems to rely critically on interactions between chemical and electrical synapses, which reciprocally and dynamically regulate the emergence of these two forms of transmission. During development, interactions between electrical synapses are crucial for the formation of neural circuits; however, such interactions in the adult brain result in dynamic reconfiguration of hardwired networks. The strength of electrical synapses is regulated by neuromodulaters such as dopamine and by glutamatergic synapses in an activity-dependent manner. Interactions between electrical and chemical synapses are also likely to have important pathological implications. Recapitulation of developmental interactions between chemical and electrical synapses has been observed after brain injury, and dysregulation of electrical synapses by neurotransmitters could contribute to cognitive impairment. Synaptic transmission occurs through two main modalities — namely, chemical and electrical transmission. In this Review, Pereda discusses the complex nature of electrical transmission and explores the mounting evidence that chemical and electrical synapses functionally interact both during development and in adulthood. Brain function relies on the ability of neurons to communicate with each other. Interneuronal communication primarily takes place at synapses, where information from one neuron is rapidly conveyed to a second neuron. There are two main modalities of synaptic transmission: chemical and electrical. Far from functioning independently and serving unrelated functions, mounting evidence indicates that these two modalities of synaptic transmission closely interact, both during development and in the adult brain. Rather than conceiving synaptic transmission as either chemical or electrical, this article emphasizes the notion that synaptic transmission is both chemical and electrical, and that interactions between these two forms of interneuronal communication might be required for normal brain development and function.

Key Points All currents in the brain superimpose to yield an 'electric field' at any given point in space. The current sources and sinks form dipoles or higher-order n-poles. Extracellular currents arise from many sources, including synaptic currents, fast action potentials and their afterpotentials, calcium spikes and voltage-dependent intrinsic currents. The magnitude of extracellular currents depends critically on two factors: the cytoarchitectural organization of a network and the temporal synchrony of the various current sinks and sources. Depending on the recording method, neuroscientists distinguish between electroencephalogram (EEG), electrocorticogram (ECoG) and local field potential (LFP; also known as micro-, depth or intracranial EEG), although all of these measures refer to the same biophysical process. The electric field is the force 'felt' by an electric charge, and can be transmitted through brain volume. The extent of volume conduction depends on the relationships between the current dipole and the features of the conductive medium. High-density sampling of the extracellular field with contemporary methods enables the calculation of current source density, and therefore the localization of current sinks and sources. The voltage gradients generated by highly synchronous activity of neuronal groups can affect the transmembrane potential of the member neurons and alter their excitability through ephaptic coupling. Synchronous spiking of nearby neurons is the main source of the high-frequency components of the local field. There is a discernable relationship between the temporal evolution of cell assemblies and the time-dependent changes of the spatially distributed currents. High-density, wide-band recordings of the local field can therefore provide access to both afferent inputs and the spiking output of neurons. Neuronal activity in the brain gives rise to transmembrane and extracellular electromagnetic fields that can be measured in the extracellular medium using several approaches. In this Review, Buzsáki and colleagues provide an overview of the mechanisms that underlie the generation of extracellular currents and fields. Neuronal activity in the brain gives rise to transmembrane currents that can be measured in the extracellular medium. Although the major contributor of the extracellular signal is the synaptic transmembrane current, other sources — including Na + and Ca 2+ spikes, ionic fluxes through voltage- and ligand-gated channels, and intrinsic membrane oscillations — can substantially shape the extracellular field. High-density recordings of field activity in animals and subdural grid recordings in humans, combined with recently developed data processing tools and computational modelling, can provide insight into the cooperative behaviour of neurons, their average synaptic input and their spiking output, and can increase our understanding of how these processes contribute to the extracellular signal.

Animal behaviors are reinforced by subsequent rewards following within a narrow time window. Such reward signals are primarily coded by dopamine, which modulates the synaptic connections of medium spiny neurons in the striatum. The mechanisms of the narrow timing detection, however, remain unknown. Here, we optically stimulated dopaminergic and glutamatergic inputs separately and found that dopamine promoted spine enlargement only during a narrow time window (0.3 to 2 seconds) after the glutamatergic inputs. The temporal contingency was detected by rapid regulation of adenosine 3′,5′-cyclic monophosphate in thin distal dendrites, in which protein-kinase A was activated only within the time window because of a high phosphodiesterase activity. Thus, we describe a molecular basis of reinforcement plasticity at the level of single dendritic spines.

Spatial coexistence of coherent and incoherent dynamics in network of coupled oscillators is called a chimera state. We study such chimera states in a network of neurons without any direct interactions but connected through another medium of neurons, forming a multilayer structure. The upper layer is thus made up of uncoupled neurons and the lower layer plays the role of a medium through which the neurons in the upper layer share information among each other. Hindmarsh-Rose neurons with square wave bursting dynamics are considered as nodes in both layers. In addition, we also discuss the existence of chimera states in presence of inter layer heterogeneity. The neurons in the bottom layer are globally connected through electrical synapses, while across the two layers chemical synapses are formed. According to our research, the competing effects of these two types of synapses can lead to chimera states in the upper layer of uncoupled neurons. Remarkably, we find a density-dependent threshold for the emergence of chimera states in uncoupled neurons, similar to the quorum sensing transition to a synchronized state. Finally, we examine the impact of both homogeneous and heterogeneous inter-layer information transmission delays on the observed chimera states over a wide parameter space.

Electrical synapses are found in vertebrate and invertebrate nervous systems. The cellular basis of these synapses is the gap junction, a group of intercellular channels that mediate direct communication between adjacent neurons. Similar to chemical synapses, electrical connections are modifiable and their variations in strength provide a mechanism for reconfiguring neural circuits. In addition, electrical synapses dynamically regulate neural circuits through properties without equivalence in chemical transmission. Because of their continuous nature and bidirectionality, electrical synapses allow electrical currents underlying changes in membrane potential to leak to ‘coupled’ partners, dampening neuronal excitability and altering their integrative properties. Remarkably, this effect can be transiently alleviated when comparable changes in membrane potential simultaneously occur in each of the coupled neurons, a phenomenon that is dynamically dictated by the timing of arriving signals such as synaptic potentials. By way of this mechanism, electrical synapses influence synaptic integration and action potential generation, imparting an additional layer of dynamic complexity to neural circuits.Electrical synapses comprise intercellular channels termed gap junctions and are found in vertebrate and invertebrate nervous systems. In this Review, Pepe Alcamí and Alberto Pereda examine the properties of electrical synapses that influence neural circuit dynamics without modifying gap junction conductance.

Two-terminal memory elements, or memelements, capable of co-locating signal processing and memory via history-dependent reconfigurability at the nanoscale are vital for next-generation computing materials striving to match the brain’s efficiency and flexible cognitive capabilities. While memory resistors, or memristors, have been widely reported, other types of memelements remain underexplored or undiscovered. Here we report the first example of a volatile, voltage-controlled memcapacitor in which capacitive memory arises from reversible and hysteretic geometrical changes in a lipid bilayer that mimics the composition and structure of biomembranes. We demonstrate that the nonlinear dynamics and memory are governed by two implicitly-coupled, voltage-dependent state variables—membrane radius and thickness. Further, our system is capable of tuneable signal processing and learning via synapse-like, short-term capacitive plasticity. These findings will accelerate the development of low-energy, biomolecular neuromorphic memelements, which, in turn, could also serve as models to study capacitive memory and signal processing in neuronal membranes. Two-terminal memory elements hold promise to store and process information via history-dependent material configurations at low-energy cost. Here, Najem et al . show a voltage-controlled capacitive memory due to reversible geometrical changes in a lipid bilayer capable of learning via synapse-like plasticity.

Development of the next generation of bio- and nano-electronics is inseparably connected to the innovative concept of emulation and reproduction of biological sensorimotor systems and artificial neurobotics. Here, we report for the first time principally new artificial bioinspired optoelectronic sensorimotor system for the controlable immitation of opto-genetically engineered neurons in the biological motor system. The device is based on inorganic optical synapse (In-doped TiO 2 nanofilm) assembled into a liquid metal (galinstan) actuator. The optoelectronic synapse generates polarised excitatory and inhibitory postsynaptic potentials to trigger the liquid metal droplet to vibrate and then mimic the expansion and contraction of biological fibre muscle. The low-energy consumption and precise modulation of electrical and mechanical outputs are the distinguished characteristics of fabricated sensorimotor system. This work is the underlying significant step towards the development of next generation of low-energy the internet of things for bioinspired neurorobotic and bioelectronic system. The internet of things technologies relies on the development of sensorimotor systems. Here, Karbalaei Akbari and Zhuiykov show a bioinspired sensorimotor system based on an integration of an artificial optical synapse and a liquid metal actuator, which mimics the expansion and contraction of biological muscles.

Sensory regions of the brain integrate environmental cues with copies of motor-related signals important for imminent and ongoing movements. In mammals, signals propagating from the motor cortex to the auditory cortex are thought to have a critical role in normal hearing and behaviour, yet the synaptic and circuit mechanisms by which these motor-related signals influence auditory cortical activity remain poorly understood. Using in vivo intracellular recordings in behaving mice, we find that excitatory neurons in the auditory cortex are suppressed before and during movement, owing in part to increased activity of local parvalbumin-positive interneurons. Electrophysiology and optogenetic gain- and loss-of-function experiments reveal that motor-related changes in auditory cortical dynamics are driven by a subset of neurons in the secondary motor cortex that innervate the auditory cortex and are active during movement. These findings provide a synaptic and circuit basis for the motor-related corollary discharge hypothesized to facilitate hearing and auditory-guided behaviours. Here auditory cortex excitatory neurons are shown to decrease their activity during locomotion, grooming and vocalization, and this decrease was paralleled by increased activity in inhibitory interneurons; these findings provide a circuit basis for how self-motion and external sensory signals can be integrated to potentially facilitate hearing. Making sense of the world To make sense of one's environment, the sensory regions of the brain must process not just external stimuli but also internally generated neural signals such as motor commands. Richard Mooney and colleagues characterized the synaptic and circuit mechanisms by which motor signals influence auditory cortical activity in freely behaving mice. During locomotion, grooming and vocalization, auditory cortex excitatory neurons decreased their activity, and this decrease was paralleled by increased activity in inhibitory interneurons. Optogenetic modulation of neurons in the secondary motor cortex, which make long-range projections on to the auditory cortex, was sufficient to alter sensory-evoked activity in the auditory cortex. These findings provide a circuit basis for how self-motion and external sensory signals can be integrated to potentially facilitate hearing.

Synapse coupling can benefit signal exchange between neurons and information encoding for neurons, and the collective behaviors such as synchronization and pattern selection in neuronal network are often discussed under chemical or electric synapse coupling. Electromagnetic induction is considered at molecular level when ion currents flow across the membrane and the ion concentration is fluctuated. Magnetic flux describes the effect of time-varying electromagnetic field, and memristor bridges the membrane potential and magnetic flux according to the dimensionalization requirement. Indeed, field coupling can contribute to the signal exchange between neurons by triggering superposition of electric field when synapse coupling is not available. A chain network is designed to investigate the modulation of field coupling on the collective behaviors in neuronal network connected by electric synapse between adjacent neurons. In the chain network, the contribution of field coupling from each neuron is described by introducing appropriate weight dependent on the position distance between two neurons. Statistical factor of synchronization is calculated by changing the external stimulus and weight of field coupling. It is found that the synchronization degree is dependent on the coupling intensity and weight, the synchronization, pattern selection of network connected with gap junction can be modulated by field coupling.