A kinematic model of swallowing inAplysia californicabased on radula/odontophore kinematics andin vivomagnetic resonance images

A computational neural model that incorporates both intrinsic dynamics and sensory feedback in the Aplysia feeding network

Studying the nervous system underlying animal motor control can shed light on how animals can adapt flexibly to a changing environment. We focus on the neural basis of feeding control in Aplysia californica. Using the Synthetic Nervous System framework, we developed a model of Aplysia feeding neural circuitry that balances neurophysiological plausibility and computational complexity. The circuitry includes neurons, synapses, and feedback pathways identified in existing literature. We organized the neurons into three layers and five subnetworks according to their functional roles. Simulation results demonstrate that the circuitry model can capture the intrinsic dynamics at neuronal and network levels. When combined with a simplified peripheral biomechanical model, it is sufficient to mediate three animal-like feeding behaviors (biting, swallowing, and rejection). The kinematic, dynamic, and neural responses of the model also share similar features with animal data. These results emphasize the functional roles of sensory feedback during feeding.

Wear patterns of radular teeth in Loligo vulgaris (Cephalopoda; Mollusca) are related to their structure and mechanical properties

Radular teeth have to cope with wear, when interacting with ingesta. In some molluscan taxa, wear-coping mechanisms, related to the incorporation of high contents of iron or silica, have been previously determined. For most species, particularly for those which possess radulae without such incorporations, wear-coping mechanisms are understudied. In the present study, we documented and characterized the wear on radular teeth in the model species Loligo vulgaris (Cephalopoda). By applying a range of methods, the elementary composition and mechanical properties of the teeth were described, to gain insight into mechanisms for coping with abrasion. It was found that the tooth regions that are prone to wear are harder and stiffer. Additionally, the surfaces interacting with the ingesta possessed a thin coating with high contents of silicon, probably reducing abrasion. The here presented data may serve as an example of systematic study of radular wear, in order to understand the relationship between the structure of radular teeth and their properties.

Ontogeny of the elemental composition and the biomechanics of radular teeth in the chiton Lepidochitona cinerea

BACKGROUND

The radula, a chitinous membrane with embedded teeth, is one important molluscan autapomorphy. In some taxa (Polyplacophora and Patellogastropoda) one tooth type (the dominant lateral tooth) was studied intensively in the last decades with regard to its mechanical properties, chemical and structural composition, and the relationship between these parameters. As the dominant lateral tooth is probably one of the best studied biological materials, it is surprising, that data on elements and mechanical properties of the other tooth types, present on a chiton radula, is lacking.

RESULTS

We provide data on the elemental distribution and mechanical properties (hardness and elasticity, i.e. Young's modulus) of all teeth from the Polyplacophora Lepidochitona cinerea (Linnaeus, 1767) [Chitonidae: Ischnochitonidae]. The ontogeny of elements, studied by energy-dispersive X-ray spectroscopy, and of the mechanical properties, determined by nanoindentation, was analysed in every individual tooth type. Additionally, we performed breaking stress experiments with teeth under dry and wet condition, highlighting the high influence of the water content on the mechanical behaviour of the radula. We thereby could determine the forces and stresses, teeth can resist, which were previously not studied in representatives of Polyplacophora. Overall, we were able to relate the mineral (iron, calcium) content with the mechanical parameters (hardness and Young's modulus) and the breaking force and stress in every tooth type. This led to a better understanding of the relationship between structure, material, and function in radular teeth. Further, we aimed at determining the role of calcium for the mechanical behaviour of the teeth: we decalcified radulae by ethylene diamine tetra acetic acid and performed afterwards elemental analyses, breaking stress experiments, and nanoindentation. Among other things, we detected that wet and decalcified radular teeth could resist highest forces, since teeth have a higher range of bending motion leading to a higher capability of teeth to gain mechanical support from the adjacent tooth row. This indicates, that the tooth material is the result of a compromise between failure reduction and the ability to transfer forces onto the ingesta.

CONCLUSION

We present novel data on the elemental composition, mechanical properties, and the mechanical behaviour of chiton teeth, which allows conclusions about tooth function. We could also relate the parameters mentioned, which contributes to our understanding on the origins of mechanical property gradients and the processes reducing structural failure in radular teeth. Additionally, we add more evidence, that the elemental composition of radular is probably species-specific and could be used as taxonomic character.

Collective effect of damage prevention in taenioglossan radular teeth is related to the ecological niche in Paludomidae (Gastropoda: Cerithioidea)

The molluscan radula, a thin membrane with embedded rows of teeth, is the structure for food processing and gathering. For proper functioning, radular failures must be either avoided or reduced when interacting with the preferred food, as this might be of high significance for the individual fitness. Thus, the analysis of structural failure in radular teeth could be included in studies on trophic specializations. Here, we tested the failure of non-mineralized, chitinous radular teeth from taxa, belonging to an African paludomid species flock from Lake Tanganyika and surrounding river systems. These species are of high interest for evolutionary biologists since they represent a potential result of an adaptive radiation including trophic specialisations to distinct substrates, the food is attached to. In a biomechanical experiment a shear load was applied to tooth cusps with a force transducer connected to a motorized stage until structural failure occurred. Subsequently broken areas were measured and breaking stress was calculated. As the experiments were carried out under dry and wet conditions, the high influence of the water content on the forces, teeth were capable to resist, could be documented. Wet teeth were able to resist higher forces, because of their increased flexibility and the flexibility of the embedding membrane, which enabled them either to slip away or to gain support from adjacent teeth. This mechanism can be understood as collective effect reducing structural failure without the mineralisation with wear-minimizing elements, as described for Polyplacophora and Patellogastropoda. Since the documented mechanical behaviour of radular teeth and the maximal forces, teeth resist, can directly be related to the gastropod ecological niche, both are here identified as an adaptation to preferred feeding substrates. STATEMENT OF SIGNIFICANCE: The radula, a chitinous membrane with teeth, is the molluscan feeding structure. Here we add onto existing knowledge about the relationship between tooth's mechanical properties and species' ecology by determining the tooth failure resistance. Six paludomid species (Gastropoda) of a prominent species flock from Lake Tanganyika, foraging on distinct feeding substrates, were tested. With a force transducer wet and dry teeth were broken, revealing the high influence of water content on mechanical behaviour and force resistance of teeth. Higher forces were needed to break wet radulae due to an increased flexibility of teeth and membrane, which resulted in an interlocking or twisting of teeth. Mechanical behaviour and force resistance were both identified as trophic adaptations to feeding substrate.

Radular force performance of stylommatophoran gastropods (Mollusca) with distinct body masses

The forces exerted by the animal's food processing structures can be important parameters when studying trophic specializations to specific food spectra. Even though molluscs represent the second largest animal phylum, exhibiting an incredible biodiversity accompanied by the establishment of distinct ecological niches including the foraging on a variety of ingesta types, only few studies focused on the biomechanical performance of their feeding organs. To lay a keystone for future research in this direction, we investigated the in vivo forces exerted by the molluscan food gathering and processing structure, the radula, for five stylommatophoran species (Gastropoda). The chosen species and individuals have a similar radular morphology and motion, but as they represent different body mass classes, we were enabled to relate the forces to body mass. Radular forces were measured along two axes using force transducers which allowed us to correlate forces with the distinct phases of radular motion. A radular force quotient, AFQ = mean Absolute Force/bodymass^0.67, of 4.3 could be determined which can be used further for the prediction of forces generated in Gastropoda. Additionally, some specimens were dissected and the radular musculature mass as well as the radular mass and dimensions were documented. Our results depict the positive correlation between body mass, radular musculature mass, and exerted force. Additionally, it was clearly observed that the radular motion phases, exerting the highest forces during feeding, changed with regard to the ingesta size: all smaller gastropods rather approached the food by a horizontal, sawing-like radular motion leading to the consumption of rather small food particles, whereas larger gastropods rather pulled the ingesta in vertical direction by radula and jaw resulting in the tearing of larger pieces.

Feeding experiments on Vittina turrita (Mollusca, Gastropoda, Neritidae) reveal tooth contact areas and bent radular shape during foraging

The radula is the food gathering and processing structure and one important autapomorphy of the Mollusca. It is composed of a chitinous membrane with small, embedded teeth representing the interface between the organism and its ingesta. In the past, various approaches aimed at connecting the tooth morphologies, which can be highly distinct even within single radulae, to their functionality. However, conclusions from the literature were mainly drawn from analyzing mounted radulae, even though the configuration of the radula during foraging is not necessarily the same as in mounted specimens. Thus, the truly interacting radular parts and teeth, including 3D architecture of this complex structure during foraging were not previously determined. Here we present an experimental approach on individuals of Vittina turrita (Neritidae, Gastropoda), which were fed with algae paste attached to different sandpaper types. By comparing these radulae to radulae from control group, sandpaper-induced tooth wear patterns were identified and both area and volume loss could be quantified. In addition to the exact contact area of each tooth, conclusions about the 3D position of teeth and radular bending during feeding motion could be drawn. Furthermore, hypotheses about specific tooth functions could be put forward. These feeding experiments under controlled conditions were introduced for stylommatophoran gastropods with isodont radulae and are now applied to heterodont and complex radulae, which may provide a good basis for future studies on radula functional morphology.

Dynamical consequences of sensory feedback in a half-center oscillator coupled to a simple motor system

We investigate a simple model for motor pattern generation that combines central pattern generator (CPG) dynamics with a sensory feedback (FB) mechanism. Our CPG comprises a half-center oscillator with conductance-based Morris-Lecar model neurons. Output from the CPG drives a push-pull motor system with biomechanics based on experimental data. A sensory feedback conductance from the muscles allows modulation of the CPG activity. We consider parameters under which the isolated CPG system has either "escape" or "release" dynamics, and we study both inhibitory and excitatory feedback conductances. We find that increasing the FB conductance relative to the CPG conductance makes the system more robust against external perturbations, but more susceptible to internal noise. Conversely, increasing the CPG conductance relative to the FB conductance has the opposite effects. We find that the "closed-loop" system, with sensory feedback in place, exhibits a richer repertoire of behaviors than the "open-loop" system, with motion determined entirely by the CPG dynamics. Moreover, we find that purely feedback-driven motor patterns, analogous to a chain reflex, occur only in the inhibition-mediated system. Finally, for pattern generation systems with inhibition-mediated sensory feedback, we find that the distinction between escape- and release-mediated CPG mechanisms is diminished in the presence of internal noise. Our observations support an anti-reductionist view of neuromotor physiology: Understanding mechanisms of robust motor control requires studying not only the central pattern generator circuit in isolation, but the intact closed-loop system as a whole.

Control for multifunctionality: bioinspired control based on feeding in Aplysia californica

Animals exhibit remarkable feats of behavioral flexibility and multifunctional control that remain challenging for robotic systems. The neural and morphological basis of multifunctionality in animals can provide a source of bioinspiration for robotic controllers. However, many existing approaches to modeling biological neural networks rely on computationally expensive models and tend to focus solely on the nervous system, often neglecting the biomechanics of the periphery. As a consequence, while these models are excellent tools for neuroscience, they fail to predict functional behavior in real time, which is a critical capability for robotic control. To meet the need for real-time multifunctional control, we have developed a hybrid Boolean model framework capable of modeling neural bursting activity and simple biomechanics at speeds faster than real time. Using this approach, we present a multifunctional model of Aplysia californica feeding that qualitatively reproduces three key feeding behaviors (biting, swallowing, and rejection), demonstrates behavioral switching in response to external sensory cues, and incorporates both known neural connectivity and a simple bioinspired mechanical model of the feeding apparatus. We demonstrate that the model can be used for formulating testable hypotheses and discuss the implications of this approach for robotic control and neuroscience.

Not just scratching the surface: distinct radular motion patterns in Mollusca

The radula is the organ for mechanical food processing and an important autapomorphy of Mollusca. Its chitinous membrane, embedding small radular teeth, is moved by the set of muscles resulting in an interaction with the ingesta, tearing it and collecting loosened particles. Radulae and their teeth can be quite distinct in their morphology and had been of high research interest, but only a few studies have examined the basic functional principles of this organ, the movement and motion during feeding action. Here, the radular motion of 20 representative species, belonging to four major gastropod lineages (Vetigastropoda, Neritimorpha, Caenogastropoda and Heterobranchia) and Polyplacophora, were recorded and classified. Comparisons of the video footage with the scanning electron microscope (SEM) images of the radula resulted in the recognition of functional tooth rows and the correct position of the teeth during feeding. We identified six different types of radular movements, including rotations and bending of the radula itself. In each movement type, different structures act as counter bearings enabling the animals to grab and tear food.

Soft-surface grasping: radular opening in Aplysia californica

Grasping soft, irregular material is challenging both for animals and robots. The feeding systems of many animals have adapted to this challenge. In particular, the feeding system of the marine mollusk Aplysia californica, a generalist herbivore, allows it to grasp and ingest seaweeds of varying shape, texture and toughness. On the surface of the grasper of A. californica is a structure known as the radula, a thin flexible cartilaginous sheet with fine teeth. Previous in vitro studies suggested that intrinsic muscles, I7, are responsible for opening the radula. Lesioning I7 in vivo does not prevent animals from grasping and ingesting food. New in vitro studies demonstrate that a set of fine muscle fibers on the ventral surface of the radula - the sub-radular fibers (SRFs) - mediate opening movements even if the I7 muscles are absent. Both in vitro and in vivo lesions demonstrate that removing the SRFs leads to profound deficits in radular opening, and significantly reduces feeding efficiency. A theoretical biomechanical analysis of the actions of the SRFs suggests that they induce the radular surface to open around a central crease in the radular surface and to arch the radular surface, allowing it to softly conform to irregular material. A three-dimensional model of the radular surface, based on in vivo observations and magnetic resonance imaging of intact animals, provides support for the biomechanical analysis. These results suggest how a soft grasper can work during feeding, and suggest novel designs for artificial soft graspers.

In slow motion: radula motion pattern and forces exerted to the substrate in the land snail Cornu aspersum (Mollusca, Gastropoda) during feeding

The radula is the anatomical structure used for feeding in most species of Mollusca. Previous studies have revealed that radulae can be adapted to the food or the substrate the food lies on, but the real, in vivo forces exerted by this organ on substrates and the stresses that are transmitted by the teeth are unknown. Here, we relate physical properties of the radular teeth of Cornu aspersum (Müller. 1774 Vermium terrestrium et fluviatilium, seu animalium infusoriorum, helminthicorum, et testaceorum, non marinorum, succincta historia. Volumen alterum. Heineck & Faber, Havniæ & Lipsiæ.), a large land snail, with experiments revealing their radula scratching force. The radula motion was recorded with high-speed video, and the contact area between tooth cusps and the substrate was calculated. Forces were measured in all directions; highest forces (106.91 mN) were exerted while scratching, second highest forces while pulling the radula upwards and pressing the food against its counter bearing, the jaw, because the main ingesta disaggregation takes place during those two processes. Nanoindentation revealed that the tooth hardness and elasticity in this species are comparable to wood. The teeth are softer than some of their ingesta, but since the small contact area of the tooth cusps (227 µm²) transmits high local pressure (4698.7 bar) on the ingesta surface, harder material can still be cut or pierced with abrasion. This method measuring the forces produced by the radula during feeding could be used in further experiments on gastropods for better understanding functions and adaptations of radulae to ingesta or substrate, and hence, gastropods speciation and evolution.

Successful and unsuccessful attempts to swallow in a reduced Aplysia preparation regulate feeding responses and produce memory at different neural sites

Sensory feedback shapes ongoing behavior and may produce learning and memory. Motor responses to edible or inedible food in a reduced Aplysia preparation were examined to test how sensory feedback affects behavior and memory. Feeding patterns were initiated by applying a cholinomimetic onto the cerebral ganglion. Feedback from buccal muscles increased the response variability and response rate. Repeated application of the cholinomimetic caused decreased responses, expressed in part by lengthening protractions. Swallowing strips of "edible" food, which in intact animals induces learning that enhances ingestion, increased the response rate, and shortened the protraction length, reflecting more swallowing. Testing memory by repeating the procedure prevented the decrease in response rate observed with the cholinomimetic alone, and shortened protractions. Training with "inedible" food that in intact animals produces learning expressed by decreased responses caused lengthened protractions. Testing memory by repeating the procedure did not cause decreased responses or lengthened protractions. After training and testing with edible or inedible food, all preparations were exposed to the cholinomimetic alone. Preparations previously trained with edible food displayed memory expressed as decreased protraction length. Preparations previously trained with inedible food showed decreases in many response parameters. Memory for inedible food may arise in part via a postsynaptic decrease in response to acetylcholine released by afferents sensing food. The lack of change in response number, and in the time that responses are maintained during the two training sessions preceding application of the cholinomimetic alone suggests that memory expression may differ from behavioral changes during training.

Motor neuronal activity varies least among individuals when it matters most for behavior

How does motor neuronal variability affect behavior? To explore this question, we quantified activity of multiple individual identified motor neurons mediating biting and swallowing in intact, behaving Aplysia californica by recording from the protractor muscle and the three nerves containing the majority of motor neurons controlling the feeding musculature. We measured multiple motor components: duration of the activity of identified motor neurons as well as their relative timing. At the same time, we measured behavioral efficacy: amplitude of grasping movement during biting and amplitude of net inward food movement during swallowing. We observed that the total duration of the behaviors varied: Within animals, biting duration shortened from the first to the second and third bites; between animals, biting and swallowing durations varied. To study other sources of variation, motor components were divided by behavior duration (i.e., normalized). Even after normalization, distributions of motor component durations could distinguish animals as unique individuals. However, the degree to which a motor component varied among individuals depended on the role of that motor component in a behavior. Motor neuronal activity that was essential for the expression of biting or swallowing was similar among animals, whereas motor neuronal activity that was not essential for that behavior varied more from individual to individual. These results suggest that motor neuronal activity that matters most for the expression of a particular behavior may vary least from individual to individual. Shaping individual variability to ensure behavioral efficacy may be a general principle for the operation of motor systems.

Differential activation of an identified motor neuron and neuromodulation provide Aplysia's retractor muscle an additional function

To survive, animals must use the same peripheral structures to perform a variety of tasks. How does a nervous system employ one muscle to perform multiple functions? We addressed this question through work on the I3 jaw muscle of the marine mollusk Aplysia californica's feeding system. This muscle mediates retraction of Aplysia's food grasper in multiple feeding responses and is innervated by a pool of identified neurons that activate different muscle regions. One I3 motor neuron, B38, is active in the protraction phase, rather than the retraction phase, suggesting the muscle has an additional function. We used intracellular, extracellular, and muscle force recordings in several in vitro preparations as well as recordings of nerve and muscle activity from intact, behaving animals to characterize B38's activation of the muscle and its activity in different behavior types. We show that B38 specifically activates the anterior region of I3 and is specifically recruited during one behavior, swallowing. The function of this protraction-phase jaw muscle contraction is to hold food; thus the I3 muscle has an additional function beyond mediating retraction. We additionally show that B38's typical activity during in vivo swallowing is insufficient to generate force in an unmodulated muscle and that intrinsic and extrinsic modulation shift the force-frequency relationship to allow contraction. Using methods that traverse levels from individual neuron to muscle to intact animal, we show how regional muscle activation, differential motor neuron recruitment, and neuromodulation are key components in Aplysia's generation of multifunctionality.

Octopamine promotes rhythmicity but not synchrony in a bilateral pair of bursting motor neurons in the feeding circuit of Aplysia

Octopamine-like immunoreactivity was localized to a limited number (<40) of neurons in the Aplysia central nervous system, including three neurons in the paired buccal ganglia (BG) that control feeding movements. Application of octopamine (OA) to the BG circuit produced concentration-dependent (10(-8)-10(-4) mol l(-1)) modulatory actions on the spontaneous burst activity of the bilaterally paired B67 pharyngeal motor neurons (MNs). OA increased B67's burst duration and the number of impulses per burst. These effects reflected actions of OA on the intrinsic tetrodotoxin-resistant driver potential (DP) that underlies B67 bursting. In addition to its effects on B67's burst parameters, OA also increased the rate and regularity of burst timing. Although the bilaterally paired B67 MNs both exhibited rhythmic bursting in the presence of OA, they did not become synchronized. In this respect, the response to OA differed from that of dopamine, another modulator of the feeding motor network, which produces both rhythmicity and synchrony of bursting in the paired B67 neurons. It is proposed that modulators can regulate burst synchrony of MNs by exerting a dual control over their intrinsic rhythmicity and their reciprocal capacity to generate membrane potential perturbations. In this simple system, dopaminergic and octopaminergic modulation could influence whether pharyngeal contractions occur in a bilaterally synchronous or asynchronous fashion.

Localization of biogenic amines in the foregut of Aplysia californica: catecholaminergic and serotonergic innervation

This study examined the catecholaminergic and serotonergic innervation of the foregut of Aplysia californica, a model system in which the control of feeding behaviors can be investigated at the cellular level. Similar numbers (15-25) of serotonin-like-immunoreactive (5HTli) and tyrosine hydroxylase-like-immunoreactive (THli) fibers were present in each (bilateral) esophageal nerve (En), the major source of pregastric neural innervation in this system. The majority of En 5HTli and THli fibers originated from the anterior branch (En(2)), which innervates the pharynx and the anterior esophagus. Fewer fibers were present in the posterior branch (En(1)), which innervates the majority of the esophagus and the crop. Backfills of the two En branches toward the central nervous system (CNS) labeled a single, centrifugally projecting serotonergic fiber, originating from the metacerebral cell (MCC). The MCC fiber projected only to En(2). No central THli neurons were found to project to the En. Surveys of the pharynx and esophagus revealed major differences between their patterns of catecholaminergic (CA) and serotonergic innervation. Whereas THli fibers and cell bodies were distributed throughout the foregut, 5HTli fibers were present in restricted plexi, and no 5HTli somata were detected. Double-labeling experiments in the periphery revealed THli neurons projecting toward the buccal ganglion via En(2). Other afferents received dense perisomatic serotonergic innervation. Finally, qualitative and quantitative differences were observed between the buccal motor programs (BMPs) produced by stimulation of the two En branches. These observations increase our understanding of aminergic contributions to the pregastric regulation of Aplysia feeding behaviors.

Swallowing and dysphagia rehabilitation: translating principles of neural plasticity into clinically oriented evidence

PURPOSE

This review presents the state of swallowing rehabilitation science as it relates to evidence for neural plastic changes in the brain. The case is made for essential collaboration between clinical and basic scientists to expand the positive influences of dysphagia rehabilitation in synergy with growth in technology and knowledge. The intent is to stimulate thought and propose potential research directions.

METHOD

A working group of experts in swallowing and dysphagia reviews 10 principles of neural plasticity and integrates these advancing neural plastic concepts with swallowing and clinical dysphagia literature for translation into treatment paradigms. In this context, dysphagia refers to disordered swallowing associated with central and peripheral sensorimotor deficits associated with stroke, neurodegenerative disease, tumors of the head and neck, infection, or trauma.

RESULTS AND CONCLUSIONS

The optimal treatment parameters emerging from increased understanding of neural plastic principles and concepts will contribute to evidence-based practice. Integrating these principles will improve dysphagia rehabilitation directions, strategies, and outcomes. A strategic plan is discussed, including several experimental paradigms for the translation of these principles and concepts of neural plasticity into the clinical science of rehabilitation for oropharyngeal swallowing disorders, ultimately providing the evidence to substantiate their translation into clinical practice.

The kinematics of multifunctionality: comparisons of biting and swallowing in Aplysia californica

What are the mechanisms of multifunctionality, i.e. the use of the same peripheral structures for multiple behaviors? We studied this question using the multifunctional feeding apparatus of the marine mollusk Aplysia californica, in which the same muscles mediate biting (an attempt to grasp food) and swallowing (ingestion of food). Biting and swallowing responses were compared using magnetic resonance imaging of intact, behaving animals and a three-dimensional kinematic model. Biting is associated with larger amplitude protractions of the grasper (radula/odontophore) than swallowing, and smaller retractions. Larger biting protractions than in swallowing appear to be due to a more anterior position of the grasper as the behavior begins, a larger amplitude contraction of protractor muscle I2, and contraction of the posterior portion of the I1/I3/jaw complex. The posterior I1/I3/jaw complex may be context-dependent, i.e. its mechanical context changes the direction of the force it exerts. Thus, the posterior of I1/I3 may aid protraction near the peak of biting, whereas the entire I1/I3/jaw complex acts as a retractor during swallowing. In addition, larger amplitude closure of the grasper during swallowing allows an animal to exert more force as it ingests food. These results demonstrate that differential deployment of the periphery can mediate multifunctionality.

Mechanical reconfiguration mediates swallowing and rejection in Aplysia californica

Muscular hydrostats, such as tongues, trunks or tentacles, have fewer constraints on their degrees of freedom than musculoskeletal systems, so changes in a structure's shape may alter the positions and lengths of other components (i.e., induce mechanical reconfiguration). We studied mechanical reconfiguration during rejection and swallowing in the marine mollusk Aplysia californica. During rejection, inedible material is pushed out of an animal's buccal cavity. The grasper (radula/odontophore) closes on inedible material, and then a posterior muscle, I2, pushes the grasper toward the jaws (protracts it). After the material is released, an anterior muscle complex (the I1/I3/jaw complex) pushes the grasper toward the esophagus (retracts it). During swallowing, the grasper is protracted open, and then retracts closed, pulling in food. Grasper closure changes its shape. Magnetic resonance images show that grasper closure lengthens I2. A kinetic model quantified the changes in the ability of I2 and I1/I3 to exert force as grasper shape changed. Grasper closure increases I2's ability to protract during rejection, and increases I1/I3's ability to retract during swallowing. Motor neurons controlling radular closure may therefore affect the behavioral outputs of I2's and I1/I3's motor neurons. Thus, motor neurons may modulate the outputs of other motor neurons through mechanical reconfiguration.

Dynamic Model of the Octopus Arm. I. Biomechanics of the Octopus Reaching Movement

The octopus arm requires special motor control schemes because it consists almost entirely of muscles and lacks a rigid skeletal support. Here we present a 2D dynamic model of the octopus arm to explore possible strategies of movement control in this muscular hydrostat. The arm is modeled as a multisegment structure, each segment containing longitudinal and transverse muscles and maintaining a constant volume, a prominent feature of muscular hydrostats. The input to the model is the degree of activation of each of its muscles. The model includes the external forces of gravity, buoyancy, and water drag forces (experimentally estimated here). It also includes the internal forces generated by the arm muscles and the forces responsible for maintaining a constant volume. Using this dynamic model to investigate the octopus reaching movement and to explore the mechanisms of bend propagation that characterize this movement, we found the following. 1) A simple command producing a wave of muscle activation moving at a constant velocity is sufficient to replicate the natural reaching movements with similar kinematic features. 2) The biomechanical mechanism that produces the reaching movement is a stiffening wave of muscle contraction that pushes a bend forward along the arm. 3) The perpendicular drag coefficient for an octopus arm is nearly 50 times larger than the tangential drag coefficient. During a reaching movement, only a small portion of the arm is oriented perpendicular to the direction of movement, thus minimizing the drag force.

Variability of swallowing performance in intact, freely feeding aplysia

Variability in nervous systems is often taken to be merely "noise." Yet in some cases it may play a positive, active role in the production of behavior. The central pattern generator (CPG) that drives the consummatory feeding behaviors of Aplysia generates large, quasi-random variability in the parameters of the feeding motor programs from one cycle to the next; the variability then propagates through the firing patterns of the motor neurons to the contractions of the feeding muscles. We have proposed that, when the animal is faced with a new, imperfectly known feeding task in each cycle, the variability implements a trial-and-error search through the space of possible feeding movements. Although this strategy will not be successful in every cycle, over many cycles it may be the optimal strategy for feeding in an uncertain and changing environment. To play this role, however, the variability must actually appear in the feeding movements and, presumably, in the functional performance of the feeding behavior. Here we have tested this critical prediction. We have developed a technique to measure, in intact, freely feeding animals, the performance of Aplysia swallowing behavior, by continuously recording with a length transducer the movement of the seaweed strip being swallowed. Simultaneously, we have recorded with implanted electrodes activity at each of the internal levels, the CPG, motor neurons, and muscles, of the feeding neuromusculature. Statistical analysis of a large data set of these recordings suggests that functional performance is not determined strongly by one or a few parameters of the internal activity, but weakly by many. Most important, the internal variability does emerge in the behavior and its functional performance. Even when the animal is swallowing a long, perfectly regular seaweed strip, remarkably, the length swallowed from cycle to cycle is extremely variable, as variable as the parameters of the activity of the CPG, motor neurons, and muscles.

Tight or loose coupling between components of the feeding neuromusculature of Aplysia?

Like other complex behaviors, the cyclical, rhythmic consummatory feeding behaviors of Aplysia-biting, swallowing, and rejection of unsuitable food-are produced by a complex neuromuscular system: the animal's buccal mass, with numerous pairs of antagonistic muscles, controlled by the firing of numerous motor neurons, all driven by the motor programs of a central pattern generator (CPG) in the buccal ganglia. In such a complex neuromuscular system, it has always been assumed that the activities of the various components must necessarily be tightly coupled and coordinated if successful functional behavior is to be produced. However, we have recently found that the CPG generates extremely variable motor programs from one cycle to the next, and so very variable motor neuron firing patterns and contractions of individual muscles. Here we show that this variability extends even to higher-level parameters of the operation of the neuromuscular system such as the coordination between entire antagonistic subsystems within the buccal neuromusculature. In motor programs elicited by stimulation of the esophageal nerve, we have studied the relationship between the contractions of the accessory radula closer (ARC) muscle, and the firing patterns of its motor neurons B15 and B16, with those of its antagonist, the radula opener (I7) muscle, and its motor neuron B48. There are two separate B15/B16-ARC subsystems, one on each side of the animal, and these are indeed very tightly coupled. Tight coupling can, therefore, be achieved in this neuromuscular system where required. Yet there is essentially no coupling at all between the contractions of the ARC muscles and those of the antagonistic radula opener muscle. We interpret this result in terms of a hypothesis that ascribes a higher-order benefit to such loose coupling in the neuromusculature. The variability, emerging in the successive feeding movements made by the animal, diversifies the range of movements and thereby implements a trial-and-error search through the space of movements that might be successful, an optimal strategy for the animal in an unknown, rapidly changing feeding environment.

Cycle-to-Cycle Variability of Neuromuscular Activity inAplysiaFeeding Behavior

Aplysia consummatory feeding behavior, a rhythmic cycling of biting, swallowing, and rejection movements, is often said to be stereotyped. Yet closer examination shows that cycles of the behavior are very variable. Here we have quantified and analyzed the variability at several complementary levels in the neuromuscular system. In reduced preparations, we recorded the motor programs produced by the central pattern generator, firing of the motor neurons B15 and B16, and contractions of the accessory radula closer (ARC) muscle while repetitive programs were elicited by stimulation of the esophageal nerve. In other similar experiments, we recorded firing of motor neuron B48 and contractions of the radula opener muscle. In intact animals, we implanted electrodes to record nerve or ARC muscle activity while the animals swallowed controlled strips of seaweed or fed freely. In all cases, we found large variability in all parameters examined. Some of this variability reflected systematic, slow, history-dependent changes in the character of the central motor programs. Even when these trends were factored out, however, by focusing only on the differences between successive cycles, considerable variability remained. This variability was apparently random. Nevertheless, it too was the product of central history dependency because regularizing merely the high-level timing of the programs also regularized many of the downstream neuromuscular parameters. Central motor program variability thus appears directly in the behavior. With regard to the production of functional behavior in any one cycle, the large variability may indicate broad tolerances in the operation of the neuromuscular system. Alternatively, some cycles of the behavior may be dysfunctional. Overall, the variability may be part of an optimal strategy of trial, error, and stabilization that the CNS adopts in an uncertain environment.

Neuromuscular modulation in Aplysia. II. Modulation of the neuromuscular transform in behavior

In this work we use mathematical modeling and complementary experiments to study the dynamics of modulation in the accessory radula closer (ARC) neuromuscular system of Aplysia. Here we join a dynamic model of the modulation from the preceding paper to a model of the basal neuromuscular transform (NMT). The resulting complete model of the NMT allows us to predict, test, and analyze the actual modulated contraction shapes in different types of feeding behavior, through entire quasi-realistic meals. The model reproduces a variety of published and new experimental observations. We find that components of the modulatory network act in interdependency and mutual complementarity, one or another playing a key role depending on the behavior and its past history. The history is remembered by slow dynamical components whose persistence prepares the system for future behavior of the same kind. The persistence becomes counterproductive, however, when the behavior suddenly changes. Superposition of fast dynamical components alleviates the problem under most, but not all, circumstances. In the quasi-realistic meals, the modulation improves functional performance on average, but degrades it after certain behavioral switches, when the model predicts sharp contraction transients. These are indeed seen in the real muscle. We propose that the real system does not switch the underlying motor neuron firing patterns abruptly, but relaxes them gradually, matching the relaxation of the peripheral modulatory state, through such behavioral transitions. We model food-induced arousal, a known phenomenon of this kind. The peripheral dynamics of the modulated NMT thus constrain the motor commands of the CNS.

Radula-centric and odontophore-centric kinematic models of swallowing inAplysia californica

Two kinematic models of the radula/odontophore of the marine mollusc Aplysia californica were created to characterize the movement of structures inside the buccal mass during the feeding cycle in vivo. Both models produce a continuous range of three-dimensional shape changes in the radula/odontophore, but they are fundamentally different in construction. The radulacentric model treats the radular halves as rigid bodies that can pitch, yaw and roll relative to a fixed radular stalk, thus creating a three-dimensional shape. The odontophore-centric model creates a globally convex solid representation of the radula/odontophore directly, which then constrains the positions and shapes of internal structures. Both radula/odontophore models are placed into a pre-existing kinematic model of the I1/I3 and I2 muscles to generate three-dimensional representations of the entire buccal mass. High-temporal-resolution, mid-sagittal magnetic resonance(MR) images of swallowing adults in vivo are used to provide non-invasive, artifact-free shape and position parameter inputs for the models. These images allow structures inside the buccal mass to be visualized directly, including the radula, radular stalk and lumen of the I1/I3 cavity. Both radula-centric and odontophore-centric models were able to reproduce two-dimensional, mid-sagittal radula/odontophore and buccal mass kinematics,but the odontophore-centric model's predictions of I1/I3, I2 and I7 muscle dimensions more accurately matched data from MR-imaged adults and transilluminated juveniles.