Convergent chemical and electrical synaptic inputs from proprioceptive afferents onto an identified intersegmental interneuron in the crayfish

Local and Intersegmental Interactions of Coordinating Neurons and Local Circuits in the Swimmeret System

During forward swimming, periodic movements of swimmerets on different segments of the crayfish abdomen progress from back to front with the same period. Information encoded as bursts of spikes by coordinating neurons in each segmental ganglion is necessary for this coherent organization. This information is conducted to targets in other ganglia. When an individual coordinating neuron is stimulated at different phases in the system's cycle of activity, the timing of motor output from other ganglia may be altered. In models of this coordinating circuit, we assumed that each coordinating neuron encodes information about the state of the local pattern-generating circuit in its home ganglion but is not part of that local circuit. We tested this assumption by stimulating individual coordinating neurons of two kinds—ASCE and DSC—at different phases under two conditions: with the target ganglion functional, and with the target ganglion silenced. Blocking a DSC neuron's target ganglion did not alter its negligible influence on the output from its home ganglion; the phase-response curves (PRC) remained flat. Blocking an ASCE neuron's target ganglion significantly affected its influence on the output from its home ganglion. We had predicted that ASCE's modest phase-dependent influence would disappear with the target silenced, but instead the amplitude of the PRCs increased significantly. Thus we have two different results: DSC neurons conformed to prediction based on the models’ assumptions, but ASCE neurons showed an unexpected property, one that is partially masked when the bidirectional flow of information between neighboring ganglia is operating normally.

Representation of auditory signals in the M-cell: role of electrical synapses

The teleost Mauthner (M-) cell mediates a sound-evoked escape behavior. A major component of the auditory input is transmitted by large myelinated club endings of the posterior VIIIth nerve. Paradoxically, although nerve stimulations revealed these afferents have mixed electrical and glutamatergic synapses on the M-cell's distal lateral dendrite, paired pre- and postsynaptic recordings indicated most individual connections are chemically silent. To determine the sensory information encoded and the relative contributions of these two transmission modes, M-cell responses to acoustic stimuli in air were recorded intracellularly. Excitatory postsynaptic potentials (EPSPs) evoked by both short 100- to 900-Hz "pips" and longer-lasting amplitude- and frequency-modulated sounds were dominated by fast, repetitive EPSPs superimposed on an underlying slow depolarization. Fast EPSPs 1) have kinetics comparable to presynaptic action potentials, 2) are maximal on the distal lateral dendrite, and 3) are insensitive to GluR antagonists. They presumably are coupling potentials, and power spectral analysis indicated they constitute a high-pass signal that accurately tracks sound frequency and amplitude. The spatial profile of the slow EPSP suggests both proximal and distal dendritic sources, a result supported by predictions of a multicompartmental model and the effects of AMPAR antagonists, which preferentially reduced the proximal component. Thus a second class of afferents generates a portion of the slow EPSP that, with sound stimuli, demonstrate that the dominant mode of transmission at LMCE synapses is electrical. The slow EPSP is a dynamic, low-pass representation of stimulus strength. Accordingly, amplitude and phase information, which are segregated in other systems, are faithfully represented in the M-cell.

Long-lasting potentiation of excitatory synaptic signaling to the crayfish lateral giant neuron

The neural circuit that underlies the lateral giant fiber (LG)-mediated reflex escape in crayfish has provided findings relating synaptic change to nonassociative learning such as sensitization and habituation. The LGs receive sensory inputs from the primary sensory afferents and a group of mechanosensory interneurons (MSIs). An increase of excitability by suprathreshold repetitive excitation of this circuit, which is similar to Hebbian long-term potentiation (LTP), has been reported. This potentiation was previously thought to result from the enhancement of transmission at cholinergic synapses between primary afferents and MSIs but not the electrical synapses onto LG. In this study, we found that potentiation of synaptic signaling at the electrical synapse onto LG can also be induced when the synapse was activated with subthreshold repetitive pulses or with a few strong suprathreshold shocks. LG LTP was induced in the preparation which had received pulses at limited frequency range. Although whether this LTP is involved in the learning process of escape behavior in crayfish is not clear, the intensity and amount of sensory stimulation used here mimicked those that could easily be produced by a predator trying to catch a crayfish and could be of adaptive significance in life.

Differential dye coupling reveals lateral giant escape circuit in crayfish

The lateral giant (LG) escape circuit of crayfish mediates a coordinated escape triggered by strong attack to the abdomen. The LG circuit is one of the best understood of small systems, but models of the circuit have mostly been limited to simple ball-and-stick representations, which ignore anatomical details of contacts between circuit elements. Many of the these contacts are electrical; here we use differential dye coupling, a technique which could help reveal connection patterns in many neural circuits, to reveal in detail the circuit within the terminal abdominal ganglion. Sensory input from the tailfan forms a somatotopic map on the projecting LG dendrites, which together with interafferent coupling mediates a lateral excitatory network that selectively amplifies strong, phasic, converging input to LG. Mechanosensory interneurons contact LG at sites distinct from the primary afferents and so maximize their summated effect on LG. Motor neurons and premotor interneurons are excited near the initial segments of the LGs and innervate muscles for generating uropod flaring and telson flexion. Previous research has shown that spatial patterns of input are important for signal integration in LG; this map of electrical contact points will help us to understand synaptic processing in this system.

During fictive locomotion, graded synaptic currents drive bursts of impulses in swimmeret motor neurons

During forward swimming, motor neurons that innervate each crayfish swimmeret fire periodic coordinated bursts of impulses. These bursts occur simultaneously in neurons that are functional synergists but alternate with bursts in their antagonists. These impulses ride on periodic oscillations of membrane potential that occur simultaneously in neurons of each type. A model of the local circuit that generates this motor pattern has been proposed. In this model, each motor neuron is driven alternately by excitatory and inhibitory synaptic currents from nonspiking local interneurons. I tested this model by perturbing individual interneurons and recording synaptic currents and changes in input resistance from each class of motor neuron. I also simulated the synaptic currents that would be observed in a cell subject to different patterns of presynaptic input. When the CNS was actively expressing the swimming motor pattern, changes in the membrane potential of individual local interneurons controlled firing of whole sets of motor neurons. Membrane currents in these motor neurons oscillated in phase with the motor output from their own local circuit. The phases of these oscillations differed in different functional classes of motor neurons. In neurons that could be clamped at the reversal potential of their outward currents, the model predicted that large periodic inward currents would be recorded. I observed no signs of periodic inward currents, even when the outward currents clearly had reversed. These results permit a simplification of the cellular model. They are discussed in the context of neural control of locomotion in crustacea and insects.

A lateral excitatory network in the escape circuit of crayfish

A phasic stimulus directed to the rear of a crayfish (Procambarus clarkii) creates mechanosensory input to the lateral giant (LG) interneuron, a command neuron for escape. A single LG spike is necessary and sufficient to produce a highly stereotyped tail flip that thrusts the animal away from the source of stimulation. Here we describe a lateral excitatory network among primary afferent axons in the last abdominal ganglion of crayfish that produces nonlinear amplification of the sensory input to the command circuitry for escape. The lateral excitation is mediated by electrical synapses between central terminals of primary mechanosensory afferents. The network enables stimulated afferents to recruit unstimulated afferents that contribute additional input to LG and to mechanosensory interneurons that also converge on LG. When depolarized, the LG neuron increases its own inputs from primary afferents and primary interneurons by facilitating the recruitment of both. Conversely, hyperpolarization of LG reduces the excitability of primary afferents and primary interneurons. The crayfish's decision to escape, previously thought to lie exclusively in the synaptic integrative properties of LG, is now seen to depend on the interactions between LG dendritic postsynaptic potentials and the responses of primary afferent terminals in the lateral excitatory network.

Adaptive motor control in crayfish

This article reviews the principles that rule the organization of motor commands that have been described over the past five decades in crayfish. The adaptation of motor behaviors requires the integration of sensory cues into the motor command. The respective roles of central neural networks and sensory feedback are presented in the order of increasing complexity. The simplest circuits described are those involved in the control of a single joint during posture (negative feedback-resistance reflex) and movement (modulation of sensory feedback and reversal of the reflex into an assistance reflex). More complex integration is required to solve problems of coordination of joint movements in a pluri-segmental appendage, and coordination of different limbs and different motor systems. In addition, beyond the question of mechanical fitting, the motor command must be appropriate to the behavioral context. Therefore, sensory information is used also to select adequate motor programs. A last aspect of adaptability concerns the possibility of neural networks to change their properties either temporarily (such on-line modulation exerted, for example, by presynaptic mechanisms) or more permanently (such as plastic changes that modify the synaptic efficacy). Finally, the question of how "automatic" local component networks are controlled by descending pathways, in order to achieve behaviors, is discussed.

Processing of proprioceptive signals by ascending interneurones in the terminal abdominal ganglion of the crayfish

Intersegmental interneurones are crucial for the appropriate coordination of the activity of local circuits located in different body segments. We have analysed the synaptic inputs to ascending intersegmental interneurones from a proprioceptor in the tailfan of the crayfish. Twenty identified interneurones responded during stimulation of the exopodite-endopodite chordotonal organ. Of these 20 interneurones, three were excited phaso-tonically, nine were excited phasically and eight were inhibited. All received convergent exteroceptive inputs from water-motion- or touch-sensitive hairs on the uropods. The effects of simultaneous exteroceptive and proprioceptive stimulation depended upon the identity of an interneurone. For interneurones that were inhibited by proprioceptive stimulation, suprathreshold exteroceptive responses were reduced to a subthreshold level by simultaneous proprioceptive stimulation. In contrast, for interneurones that were excited by proprioceptive stimulation, the simultaneous application of subthreshold proprioceptive and exteroceptive stimulation elicited action potentials. Two of the interneurones that receive proprioceptive input (NE-1 and RC-8) are known to be presynaptic to giant interneurones that mediate and coordinate the tail-flip. Many of the other interneurones that receive proprioceptive inputs in the tailfan are known to excite abdominal extensor motor neurones. Thus, proprioceptive input to these intersegmental interneurones could serve two roles: first, to extend the abdomen during postural movements or prior to escape and, second, to drive the tail-flip escape response.