State-dependent responses of two motor systems in the crayfish, Pacifastacus leniusculus

The effects of electrical stunning on the nervous activity and physiological stress response of a commercially important decapod crustacean, the brown crab Cancer pagurus L

Increasing attention is being paid to the welfare of decapod crustaceans. Legislation exists for their humane slaughter in several countries and this is being debated in others. Electrical stunning may have potential for humane slaughter of crustaceans in some circumstances, although scientific data on the effectiveness of electrical stunning when applied to various species are limited. Assessment criteria for effective stunning have so far been based mainly on behavioural assessments, but these do not always reflect neural insensibility. In this study direct recordings of neural activity, both centrally and peripherally, have been used to provide more direct measures of the state of sensibility. We have also examined whether electrical stunning acts as a physiological stressor, using measures of haemolymph L-lactate. Experiments were performed on a commercially important decapod species, the brown crab Cancer pagurus L. Spontaneous activity within the CNS was arrested by electrical stunning, which is an indication of loss of sensibility. There were also specific effects on the peripheral nervous system, with loss of responsiveness to sensory stimulation, rendering the animals unresponsive to external stimuli, and a failure of motor activation. All these effects were apparent immediately after a 10s stun, and persisted for as long as tested (4h) indicating that the animals were also killed by the procedure. No autotomy of limbs occurred. Haemolymph L-lactate was found to be no greater following electrical stunning than after handling and sampling alone, and both were significantly lower than values reached in a range of environmental and commercial situations. For all these reasons we find that electrical stunning may meet criteria for humane slaughter of C. pagurus.

The swimmeret system of crayfish: a practical guide for the dissection of the nerve cord and extracellular recordings of the motor pattern

Here we demonstrate the dissection of the crayfish abdominal nerve cord. The preparation comprises the last two thoracic ganglia (T4, T5) and the chain of abdominal ganglia (A1 to A6). This chain of ganglia includes the part of the central nervous system (CNS) that drives coordinated locomotion of the pleopods (swimmerets): the swimmeret system. It is known for over five decades that in crayfish each swimmeret is driven by its own independent pattern generating kernel that generates rhythmic alternating activity . The motor neurons innervating the musculature of each swimmeret comprise two anatomically and functionally distinct populations. One is responsible for the retraction (power stroke, PS) of the swimmeret. The other drives the protraction (return stroke, RS) of the swimmeret. Motor neurons of the swimmeret system are able to produce spontaneously a fictive motor pattern, which is identical to the pattern recorded in vivo. The aim of this report is to introduce an interesting and convenient model system for studying rhythm generating networks and coordination of independent microcircuits for students' practical laboratory courses. The protocol provided includes step-by-step instructions for the dissection of the crayfish's abdominal nerve cord, pinning of the isolated chain of ganglia, desheathing the ganglia and recording the swimmerets fictive motor pattern extracellularly from the isolated nervous system. Additionally, we can monitor the activity of swimmeret neurons recorded intracellularly from dendrites. Here we also describe briefly these techniques and provide some examples. Furthermore, the morphology of swimmeret neurons can be assessed using various staining techniques. Here we provide examples of intracellular (by iontophoresis) dye filled neurons and backfills of pools of swimmeret motor neurons. In our lab we use this preparation to study basic functions of fictive locomotion, the effect of sensory feedback on the activity of the CNS, and coordination between microcircuits on a cellular level.

Neurobiology of the crustacean swimmeret system

The crustacean swimmeret system includes a distributed set of local circuits that individually control movements of one jointed limb. These modular local circuits occur in pairs in each segmental ganglion, and normally operate synchronously to produce smoothly coordinated cycles of limb movements on different body segments. The system presents exceptional opportunities for computational and experimental investigation of neural mechanisms of coordination because: (a) The system will express in vitro the periodic motor pattern that normally drives cycles of swimmeret movements during forward swimming. (b) The intersegmental neurons which encode information that is necessary and sufficient for normal coordination have been identified, and their activity can be recorded. (c) The local commissural neurons that integrate this coordinating information and tune the phase of each swimmeret are known. (d) The complete set of synaptic connections between coordinating neurons and these commissural neurons have been described. (e). The synaptic connections onto each local pattern-generating circuit through which coordinating information tunes the circuit's phase have been discovered. These factors make possible for the first time a detailed, comprehensive cellular and synaptic explanation of how this neural circuit produces an effective, behaviorally significant output. This paper is the first comprehensive review of the system's neuroanatomy and neurophysiology, its local and intersegmental circuitry, its transmitter pharmacology, its neuromodulatory control mechanisms, and its interactions with other motor systems. Each of these topics is covered in detail in an attempt to provide a complete review of the literature as a foundation for new research. The series of hypotheses that have been proposed to account for the system's properties are reviewed critically in the context of experimental tests of their validity.

Mixtures of Octopamine and Serotonin Have Nonadditive Effects on the CNS of the Medicinal Leech

It is well established that neural networks respond to a wide variety of modulatory substances by which they can become reconfigured, yet few studies have examined the effects of neurotransmitter mixtures on such networks. In a previous study of the medicinal leech using triple intracellular recordings, we found that stimulation of identified mechanosensory neurons activated both the serotonergic cell 21 (a swimgating neuron) and the dorsal lateral octopamine (DLO) cell. Because these findings suggested that serotonin (5-HT) and octopamine (OA) may be released together, we investigated the effects of 5-HT and OA mixtures on isolated nerve cords of Hirudo medicinalis (which contained both head and tail brains). Fifty micromolar OA, 50 μM 5-HT, or a mixture of 50 μM OA and 50 μM 5-HT was bath applied to the nerve cord under constant perfusion conditions. Additional experiments were performed with combinations of either 25 or 100 μM OA and 5-HT. Neural activity was examined specifically in the segmentally repeated dorsal posterior (DP) nerve because it has been shown to contain identified swim motor units. Nonadditive effects of amine combinations were most apparent in their ability to decrease overall activity in the DP nerve and to alter patterned motor activity in the form of fictive swimming. Whereas swim burst activity has been previously shown to increase in nerve cords bathed in either 5-HT or OA solutions alone, we demonstrated that a mixture of the two amines resulted in a robust decrease in the number of swim bursts expressed and an inhibition of swim activity in preparations already swimming. Most compelling was the observation that when the amine mixture was replaced with normal saline, swim burst activity increased dramatically. We discuss that the effects of amine mixtures may be due to their interaction with descending interneurons known to trigger and inhibit swimming as the mixture-induced effects were not observed in nerve cords lacking the head and tail brains. Because the net effect of the two amines was not simply additive (i.e., 5-HT or OA is known to activate swimming, yet the mix inhibits swimming), this result reveals yet another layer of complexity inherent in “simpler” invertebrate nervous systems.

RCPH modulation of a multi-oscillator network: effects on the pyloric network of the spiny lobster

The neuropeptide red pigment concentrating hormone (RPCH), which we have previously shown to activate the cardiac sac motor pattern and lead to a conjoint gastric mill-cardiac sac pattern in the spiny lobster Panulirus, also activates and modulates the pyloric pattern. Like the activity of gastric mill neurons in RPCH, the pattern of activity in the pyloric neurons is considerably more complex than that seen in control saline. This reflects the influence of the cardiac sac motor pattern, and particularly the upstream inferior ventricular (IV) neurons, on many of the pyloric neurons. RPCH intensifies this interaction by increasing the strength of the synaptic connections between the IV neurons and their targets in the stomatogastric ganglion. At the same time, RPCH enhances postinhibitory rebound in the lateral pyloric (LP) neuron. Taken together, these factors largely explain the complex pyloric pattern recorded in RPCH in Panulirus.

Passive properties of swimmeret motor neurons

Four different functional types of motor neurons innervate each swimmeret: return-stroke excitors (RSEs), power-stroke excitors (PSEs), return-stroke inhibitors (RSIs), and power-stroke inhibitors (PSIs). We studied the structures and passive electrical properties of these neurons, and tested the hypothesis that different types of motor neurons would have different passive properties that influenced generation of the swimmeret motor pattern. Cell bodies of neurons innervating one swimmeret were clustered in two anatomic groups in the same ganglion. The shapes of motor neurons in both groups were similar, despite the differences in locations of their cell bodies and in their functions. Diameters of their axons in the swimmeret nerve ranged from <2 to approximately 35 microm. Resting membrane potentials, input resistances, and membrane time constants were recorded with microelectrodes in the processes of swimmeret motor neurons in isolated abdominal nerve cord preparations. Membrane potentials had a median of -59 mV, with 25th and 75th percentiles of -66.0 and -53 mV. The median input resistance was 6.4 M omega, with 25th and 75th percentiles of 3.4 and 13.7 M omega. Membrane time constants had a median of 9.3 ms, with 25th and 75th percentiles of 5.7 and 15.0 ms. Excitatory and inhibitory motor neurons had similar passive properties. RSE motor neurons were typically more depolarized than the other types, but the passive properties of RSE, PSE, RSI, and PSI neurons were not significantly different. Membrane time constants measured from cell bodies were briefer than those measured from neuropil processes, but membrane potentials and input resistances were not significantly different. The relative sizes of different motor neurons were measured from the sizes of their impulses recorded extracellularly from the swimmeret nerve. Smaller motor neurons had lower membrane potentials and were more likely to be active in the motor pattern than were large motor neurons. Motor neurons of different sizes had similar input resistances and membrane time constants. Motor neurons that were either oscillating or oscillating and firing in phase with the swimmeret motor pattern had lower average membrane potentials and longer time constants than those that were not oscillating. When the state of the swimmeret system changed from quiescence to continuous production of the motor pattern, the resting potentials, input resistances, and membrane time constants of individual swimmeret motor neurons changed only slightly. On average, both input resistance and membrane time constant increased. These similarities are considered in light of the functional task each motor neuron performs, and a hypothesis is developed that links the brief time constants of these neurons and graded synaptic transmission by premotor interneurons to control of the swimmeret muscles and the performance of the swimmeret system.

Neurotransmitter interactions in the stomatogastric system of the spiny lobster: one peptide alters the response of a central pattern generator to a second peptide

Two of the peptides found in the stomatogastric nervous system of the spiny lobster, Panulirus interruptus, interacted to modulate the activity of the cardiac sac motor pattern. In the isolated stomatogastric ganglion, red-pigment-concentrating hormone (RPCH), but not proctolin, activated the bursting activity in the inferior ventricular (IV) neurons that drives the cardiac sac pattern. The cardiac sac pattern normally ceased within 15 min after the end of RPCH superfusion. However, when proctolin was applied within a few minutes of that time, it was likewise able to induce cardiac sac activity. Similarly, proctolin applied together with subthreshold RPCH induced cardiac sac bursting. The amplitude of the excitatory postsynaptic potentials from the IV neurons to the cardiac sac dilator neuron CD2 (1 of the 2 major motor neurons in the cardiac sac system) was potentiated in the presence of both proctolin and RPCH. The potentiation in RPCH was much greater than in proctolin alone. However, the potentiation in proctolin after RPCH was equivalent to that recorded in RPCH alone. Although we do not yet understand the mechanisms for these interactions of the two modulators, this study provides an example of one factor that can determine the "state" of the system that is critical in determining the effect of a modulator that is "state dependent," and it provides evidence for yet another level of flexibility in the motor output of this system.

Interactions among neural networks for behavior

In recent years, as our understanding of the pattern-generating networks responsible for a variety of behaviors has increased, the interactions of multiple neural networks have been examined in a number of systems. These studies have shown that functionally related pattern generators can interact extensively, and that the extent to which two or more of these networks interact is not fixed, but can be altered by neuromodulators. Furthermore, a number of studies have begun to elucidate the mechanisms responsible for those interactions. In the crustacean stomatogastric system, for example, neurons can switch between different pattern generators, and whole networks can fuse into single patterns. In addition, several networks can be dismantled, and their components used to generate a new network. The mechanisms responsible for these changes are the same as those involved in other circuit re-configurations, namely alterations of both intrinsic membrane properties of component neurons and alterations in the strength of synapses within the networks.

Interneurons involved in the control of multiple motor centers in crayfish

A number of studies have suggested that abdominal positioning interneurons (APIs) in the crayfish Procambarus clarkii can influence activity in multiple motor centers. Previous work on this population of neurons has demonstrated that they control the activity of tonic abdominal motor neurons (MNs) which generate postural movements of the abdomen and, to some extent, the activity of swimmeret MNs. This investigation demonstrates that many identified APIs also affect the activity of two populations of MNs which open and close the uropods and, in some cases, that of the swimmeret MNs as well. The majority (64%) of APIs examined in this study have an excitatory effect on both populations of uropod MNs. A smaller number (23%) increase the level of activity in one population of uropod MNs, and suppress, or have no effect on, the activity of the other population of uropod MNs. Approximately 25% of the APIs which were examined influence the output of swimmeret MNs, in addition to affecting the activity of uropod MNs. There are also indications that previous estimates of the number of APIs may have been too low. This is based on the observation that many APIs possess what appear to be similar morphologies but generate different patterns of motor output. Taken together, these findings support the idea that APIs influence the output of multiple motor centers which play a role in the control of general body posture and balance in crayfish.