Neuronal Pathways and Synaptic Connexions in the Abdominal Cord of the Crayfish

1. An investigation has been made into the function and distribution of nerve fibres in the abdominal ganglion chain and its roots in the crayfish, Procambarus clarkii, by leading off action potentials from small prepared bundles following sensory stimulation. 2 .The sensory fields belonging to the first and second roots of each abdominal ganglion were determined, and the antero-posterior pathway of sensory fibres within the cord noted. It was found that the primary sensory fibres of the dorsal muscle receptor organs, entering through the second root, send out an anterior branch to the brain and a posterior one to the last ganglion. For most other sensory fibres much shorter intracentral branches are indicated, though some of them extend for two ganglia in the anterior direction and for one posteriorly. All sensory fibres in the connectives run on the same side as they enter. 3. The segmental divisions of the external skeleton and of the nervous system do not coincide, the neural segment slants in a posterior dorsal direction with respect to the skeletal one. 4. For the majority of the interneurones which innervate more than two abdominal segments it has been proved that they synapse with primary sensory fibres in each of the ganglia that these enter. Depending on the segment stimulated with respect to the leading-off position, both ascending and descending impulses are obtained in such interneurones and collision of the impulses has been observed. Some consequences of this type of integration are discussed. 5. For interneurones responding to bilateral or heterolateral stimulation the course of the impulses proved to be of at least two types. In some, cutting the fibre prevents the arrival of impulses except those set up on the side of the cut from which the recording is made. In others, recording from either side of the cut fibre does not exclude any of the sensory fields to which the fibre normally responded. 6. At least one interneurone is present in which all primary sensory fibres from the different segments to whose activity it responds collect in one ganglion

Adaptive motor control in crayfish

International audienceThis article reviews the principles that rule the organization of motor commands that have been described over the past ®ve decades in cray®sh. 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 re¯ex) and movement (modulation of sensory feedback and reversal of the re¯ex into an assistance re¯ex). More complex integration is required to solve problems of coordination of joint movements in a pluri-segmental appendage, and coordination of dierent limbs and dierent motor systems. In addition, beyond the question of mechanical ®tting, 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 ecacy). Finally, the question of how``automatic'' local component networks are controlled by descending pathways, in order to achieve behaviors, is discussed.

Neural events underlying escape swimming behaviour in the squat lobster 'Galathea strigosa' (Crustacea, Anomura)

1. The anatomy and physiology of escape swimming behaviour in the squat lobster, Galathea strigosa, have been investigated and the results discussed in the context of comparative mechanisms of escape in rela~ed species. 2. In contrast to many other decapods, G. strigosa, does not possess a giant-fibre system which underlies escape. 3. In terms of the number, size and position of neuronal somata, the fast flexor motorneuron pools in Galathea and crayfish are homologous. 4. A neuron, homologous with the crayfish MeG, has been studied. Unlike the crayfish neuron, MoGH is a typical, unspecialized fast flexor motorneuron. 5. The anatomy of afferent and efferent neurons involved in abdominal extension has been investigated. The extensor motorneuron and accessory neuron pools in crayfish and Galathea are largely homologous. 6. A small degree of intersegmental and interspecific variation in abdominal flexor and extensor motorneuron pools is reported. 7. The anatomy and physiology of the abdominal MRO's has been examined. These are found to be homologous in structure and function with other decapod MRO's. 8. The considerable differences between the phasic and tonic MRO sensory dendrites may account for their different response characteristics. 9. The MRO's excite both extensor motorneurons and the flexor inhibitor motorneuron via an apparently monosynaptic pathway. Similar input properties have been described for the crayfish MHO's . 10. The MRO's, which are shown to fire in response to abdominal flexion, produce EPSP's in extensor motorneurons which both summate and facilitate. This feature has not been decribed previously and may be important in the reflex function of the MHO's during escape swimming behaviour. 11. The relative roles of proprioceptive and exteroceptive feedback on the generation of the swimming rhythm have been studied using a variety of preparations involving restraint and deafferenta- tion. 12. Sensory feedback both excites and inhibits swimming. It is deduced that proprioceptive feedback has excitatory effects and extero- ceptive feedback inhibits swimming behaviour. 13. It is suggested that the MHO's may playa role in exciting the neural circuits underlying swimming bewvbur via both direct connections with the thoracic nervous system and a restricted portion of the abdominal motorneuron pool. 14. A deafferented preparation has been used to analyse the motor programme underlying swimming behaviour. The ability to record swimming activity, identical with that recorded in the intact animal, in the absence of sensory feedback from the abdomen, suggests that swimming behaviour is controlled by a central pattern generator (CPG). 15. A method of inducing swimming activity by high frequency electrical- stimulation of abdominal sensori-motor roots is described. 16. The CPG for swimming behaviour is shown to be most likely to reside in the suboesophageal or thoracic ganglion. 17. The activity of flexor and extensor motorneurons in abdominal ganglia has been analysed at the cellular level using both extra- cellular and intracellular recording techniques. 18. Fast flexor motorneurons are driven by a combination of brief unitary synaptic potentials and a large underlying oscillatory slow wave depolarization. 19. Current injection into the somata of fast flexor motorneurons during swimming has dramatic effects on slow wave amplitude and suggests that motorneuron drive results from powerful periodic excitation via chemical synapses. 20. In contrast to the fast flexor motorneurons, fast extensor motorneurons are driven by only brief unitary synaptic potentials and not by an underlying slow wave depolarization. The contrasting mechanisms for excitation in antagonistic sets of motorneurons is documented and a possible explanation presented. 21. Among the fast extensor motorneurons there is an apparent gradation in spike thresholds which can be correlated with a gradation in soma diameter. The largest of the available pool of extensor motorneurons has the highest spike threshold. 22. The activity of the phasic inhibitors of the extensor and flexor muscles has been analysed. The extensor inhibitor, which fires in antiphase with other extensor motorneurons during the flexion phase of the swim cycle, appears to receive the same slow wave depolarization as fast flexor motorneurons. The extensor inibitor motorneuron burst is terminated by a high frequency barrage of IPSP's superimposed upon the membrane slow wave. The flexor inhibitor motorneuron receives complex excitation and inhibition during swimming, involving both unitary events and membrane waves. 23. The coordination of segmented limb structures during swimming has been investigated. The walking legs are physically protracted during flexion while the unmodified male swimmerets are flicked posteriorly. 24. Swimmeret retraction during swimming is controlled by the activity of a single swimmeret motorneuron which appears to be part of the swimming circuit and which may also be a primitive homologue of the Segmental Giant neuron in crayfish. 25. It is concluded that escape swimming behaviour is homologous with non-giant backwards swimming in crayfish and may also be homologous with swimming in certain sand crab species. The evolutionary relationships of a number of decapods is discussed on the basis of escape circuitry and it is suggested that Galathea may represent an ancestral type of swimming decapod

Static and dynamic responses of slowly adapting joint receptors

1. (1) A systems analysis approach was used to study slowly adapting knee joint receptors. Transfer functions were obtained from the receptor response to sinusoidal inputs of knee joint angle over a range of frequencies from 0.01 to 7 Hz.2. (2) The Bode plots obtained in the dynamic studies revealed a high pass filter characteristic with a magnitude slope on the order of 5 dB/decade and a phase lead on the order of 22.5[deg]. The gain of the transfer function was found to be influenced by both the input angle excursion and the static bias angle.3. (3) The static angle sensitivity curves of the receptors were found to be `bell shaped' in some cases and to increase or decrease monotonically with joint angle in other cases.4. (4) The static sensitivity curve produced by a series of positive increments of joint angle was not the same as the static sensitivity curve produced by a series of negative increments of joint angle. Tentatively this finding is attributed to a non-Newtonian viscoelastic effect and is felt to limit the information available from a single, peripheral receptor.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22368/1/0000815.pd

There and back again: a stretch receptor's tale

Mechanotransduction is fundamental to many sensory processes, including balance, hearing and motor co-ordination. However, for such an essential feature, the mechanism(s) that underlie it are poorly understood. The mechanotransducing stretch receptors that relay information on the tonicity and length of skeletal muscles have been well-defined, particularly at the gross anatomical level, in a wide variety of species, encompassing both vertebrates and invertebrates. To date, there exists a wealth of data describing them, anatomically, as well as good electrophysiological data from stretch receptors of some larger organisms. However, comparatively few studies have succeeded in identifying putative mechanotransducing molecules in such systems. Nonetheless, this class of sensory mechanotransducers perhaps offer the best means of identifying molecules that permit the stretch-sensitivity of such endings, revealing new information about the underlying mechanisms of stretch receptors, and mechanoreceptors more generally. However, a different approach is clearly needed; a theoretical approach, utilising mathematical modelling, offers a powerful means of pooling the current wealth of knowledge on the reported electrophysiological behaviour of muscle stretch receptors. This study, therefore, develops an extended theoretical model of a stretch receptor system in order to reproduce, in silico, the reported behaviour of both vertebrate and invertebrate stretch receptors, within the same modelling environment, thus enabling the first quantitative framework for comparing these data, and moreover, making predictions of the likely roles of specific molecular entities within a stretch receptor system. Subsequently, this study utilises a model in vivo system to test these theoretical predictions. The genetic toolbox of D. melanogaster offers a wide range of tools that are extremely suitable for identifying mechanotransducing molecules in stretch receptors. However, very little is currently known about such endings in this organism. This study, therefore, firstly characterises a putative stretch receptor organ in larval Drosophila, the dbd neuron, via a novel experimental approach. It is shown that this neuron exhibits known properties of stretch receptors, as previously observed in other, similar organs. Furthermore, these observations bear out the predictions of the mathematical model. Having defined the dbd neuron as a muscle stretch receptor, pharmacological and genetic assays in this system, combined with predictions from the mathematical model, identify a key role for the recently-discovered DmPiezo protein as an amiloride-sensitive, mechanically-gated sodium channel (MNaC) in dbd neurons, with TRPA1 also acting in this system in a supporting role. These data confirm the essential role of an MNaC in mechanosensory systems, but also supply important evidence that, whilst the electrophysiological mechanisms in stretch receptors are remarkably similar across taxa, different species likely employ various molecular mechanisms to achieve this

Neutral coding - A report based on an NRP work session

Neural coding by impulses and trains on single and multiple channels, and representation of information in nonimpulse carrier