Functional morphology of the pedicellariae of the asteroid Marthasterias glacialis (Echinodermata)

Summary Marthasterias glacialis bears two kinds of pedicellariae. The straight pedicellariae are single and occur everywhere on the asteroid body surface except in the ambulacral groove. The crossed pedicellariae are clustered on mobile structures (the rosettes) build around marginal and abactinal spines.Basically, each pedicellaria has a head and a stalk. A skeleton occurs only in the pedicellarial head. It consists of two valves and a basal piece. Muscular bundles are anchored on these skeletal ossicles. The straight pedicellariae have two pairs of adductor muscles (the inner and the outer adductors) and one pair of abductor muscles, these latter being weakly developed. Longitudinal muscle fibers occur all along the stalk of straight pedicellariae. The crossed pedicellariae have two pairs of adductor muscles (the distal and the proximal adductors) and two pairs of abductor muscles (the distal and the proximal abductors). The proximal adductors of crossed pedicellariae are homologous to the stalk muscles of straight pedicellariae.The pedicellariae are able to react to direct and indirect tactile stimuli. There is a great deal of individual variation among pedicellarial responses. Moreover, the reactions occur at random and lack coordination. The seemingly aberrant behavior of the pedicellariae is interpreted as a preventive activity that protects the asteroid body surface against unwanted materials and organisms.     

The venom apparatus of the globiferous pedicellariae of the toxopneustid Sphaerechinus granularis (Echinodermata,Echinoida): Fine structure and mechanism of venom discharge

Summary Globiferous pedicellariae of Sphaerechinus granularis are venomous defensive appendages consisting of a stalk bearing a head made of three movable jaws. Each jaw is supported by a calcareous valve ending with a terminal grooved tooth. A venom apparatus is located in each jaw and consists of a venom gland surrounded by a muscular envelope and terminating in a duct which completely encircles the terminal tooth of the valve. Contrary to previous statements, the duct does not lie inside the groove of the terminal tooth. In mature pedicellariae, the venom is stored in intracellular vacuoles of highly differentiated cells which are no longer active. The cells fill the whole space of the venom gland which is without a lumen; they are segregated into two types that occur in distinct regions and differ from each other by morphological and staining properties of their secretions. Upon contraction of the muscular envelope, the venom is released via a holocrine mechanism and infiltrates the predator's tissues through the wound inflicted by the three calcareous teeth of the valves. In no case is the venom emitted through the tooth groove.     

Histology of the pedicellariae of the Sand dollar, Dendraster excentricus (Echinodermata)

The pedicellariae in Dendraster excentricus are the bidentate type with a density of 80/cm². There are two distinct size groups; the larger ones measure 0.6 mm long while the small ones are only 0.14 mm. One interesting feature is the fluid of highly sulphated acid mucopolysaccharide in the lumen of the neck. This fluid functions as a hydroskeleton while the flexor muscle operates on it for complicated movement of the neck. The possibility that this fluid is also toxic and so acts as a defence mechanism is discussed. All the muscles involved in the movement of pedicellariae are smooth muscle fibres.     

Histology of the globiferous pedicellariae of Psammechinus miliaris (Echinodermata: Echinoidea)

The general morphology, function and histology of the globiferous pedicallariae of the regular sea-urchin, Psammechinus miliaris are described. There is a ganglion under the sensory epidermis on the inner surface of each jaw. This is the first discovery of a ganglion in any echinoid.
The regeneration of globiferous pedicellariae in laboratory conditions takes 25 to 40 days.     

Limbs vs. Jaws: Can They Be Compared?

Current experimental research on mammalian limb muscle structureand function is compared to that on mammalian jaw muscles. Twomajor areas of comparison are stressed: structural and functional.Comparisons of limbs and jaws are made from the point of viewof the impact of recent studies on simple mechanical modelsof limb/jaw muscle function. Limb muscle structure is comparedto jaw muscles at the level of muscle architecture, muscle histochemicaland motor unit properties, and the organization of motor unitsinto neuromuscular compartments. Such comparisons reveal thatalthough limb muscles and jaw muscles might be organized insimilar ways, fundamental differences exist, both in terms ofmuscle structure and the functional conclusions which have beenbased on studies of muscle structure. The comparisons also demonstratethat much recent evidence from structural studies have had littledirect impact on simple models of muscle function but a muchlarger influence on the assumptions of the models. Comparisonsof limb/jaw muscle function from kinematic and EMG studies,indicate that many masticatory strategies are used by differentmammals but the basic problems of posture and locomotion havebeen met with essentially similar solutions, even among diversemammalian groups. The results of such comparisons demonstratethat both limb and jaw muscle function are sufficiently complexthat new or re-vitalized models are needed if the relationshipbetween structure and function are ever to be understood.     

NEUROBIOLOGY OF ECHINODERMATA

During the past ten years much information has been added to our knowledge of nerve and muscle systems of echinoderms. 1. Electron-microscopy has shown that all the main nerve trunks consist of large numbers of small, parallel-running unmyelinated axons which are packed tightly together. Glial cells are generally absent. Discrete regions of neuropile are recognizable by the interweaving of axons, and the presence of vesicles. It has not yet been found possible to locate synapses with certainty in the nervous system, but it appears that they are chiefly confined to neuropile. The obvious nerve cords are massive accumulations of neurons which do not appear to interact locally. 2. Peripheral axons are difficult to distinguish because both interstitial and muscle cells have processes which often resemble axons. Ultrastructural analysis of this problem is aggravated by difficulties in fixation. However, the electron-microscope has shown that much of the echinoderm body wall contains a thick subepithelial plexus of processes from epithelial cells. Epithelial cells may thus act as sensory cells and supply axons to the plexus. 3. With the exception of striated muscles in some pedicellariae, all echinoderm muscles so far examined are of the smooth type. These muscles characteristically contain large filaments, and in this way do not resemble vertebrate smooth muscle. Some muscles are innervated by simple axonal contact, in others the muscles themselves send processes towards the nervous tissue. 4. Physiological studies of electrical activity in nerve and muscle systems have not added significantly to our knowledge of function. Several authors have demonstrated that massed electrical activity is conducted decrementally along the radial nerve cords, but this does not explain any known aspect of coordination. The only records of electrical activity from single neurons (Takahashi, 1964) have not been repeated. 5. There is strong evidence for two types of neurons in the central nervous system of echinoderms. One of these contains acetylcholine, the other dopamine and/or noradrenaline. Electron-microscopical histochemistry has given good indication that catecholamines are bound in echinoderm nerve tissue to particles similar to those reported in other invertebrate nervous tissues, and there is good evidence that acetylcholine is bound to synaptic vesicles which are morphologically identical to those present in the mammalian brain. The available data further indicate that acetylcholine is a transmitter in sensory and motor neurons, while dopamine and/or noradrenaline are transmitters in interneurons. Such interneurons may be involved in the coordination of the movement of the tube-feet. Other substances which have been implicated in neuro-effector mechanisms in other animal groups have not been found or are present in very small quantities. 6. Studies on the reproductive physiology of starfish have shown that several substances in the radial cords play important roles in its control. Such substances cannot at present be called neurosecretions because it is not known if they are derived from neurons. 7. Pharmacological studies on isolated muscle tissues have not added significantly to our knowledge of their control. The potency of ACh in causing contraction is well documented, and anticholinesterases are similar in effect. Catecholamines, although clearly very important in the nervous system, do not produce clear-cut effects. The published reports of relaxation to noradrenaline may well be due to direct effects on the muscle. No definite information has been obtained on the role of the adrenergic parts of the nervous systems of echinoderms, other than showing that they are not involved in motor responses. Extensive studies with a wide variety of drugs have produced inconsistent and largely negative results.     

The orientation of the force of the jaw muscles and the length of the mandible in mammals

Ungulates generally have large masseter and pterygoid muscles and a necessarily large angular process provides attachment surface on the mandible. The temporalis muscle tends to be small. It has been suggested that this is an adaptation for enhanced control of the lower jaw and reduction of forces at the jaw joint. I suggest an additional reason: because of the geometry of the jaw, the length of that segment of the lower jaw that spans the distance from the jaw joint to the most posterior tooth is significantly reduced when the masseler and pterygoid are the dominant muscles; this region is necessarily much longer when the temporalis is large.     

Morphological study of nerve endings in jaw muscles of post-hatching American alligators (Alligator mississippiensis)

Two months after hatching, the fibers of the jaw muscles of the American alligator are associated with three types of nerve terminals namely, plates, simple plates, and grape endings. Simple plate endings are mainly observed on the small muscle fibers. Grape-type endings are found on muscle fibers that resemble the tonic fibers of garter snakes (Hess, Am. J. Anat., '63). Most terminals are plate endings and account for 53.7–74.7% of terminals per muscle. Fibers with grape-type endings were found in all the jaw muscles studied; they lack well organized T-systems, M-lines, and post-junctional sarcolemmal folds, as well as irregularly distributed small of fibrils, and zigzag Z-lines. The properties of nerve endings of the American alligator indicate that M. depressor mandibulae, M. pseudotemporalis, and M. pterygoideus posterior have primary roles in jaw movements. M. pterygoideus anterior and M. intramandibularis contribute mainly to postural adjustments of the jaws. The multiplicity of nerve terminals in the jaw muscles of American alligators contrasts with the simple movements of their jaws. © 1994 Wiley-Liss, Inc.     

Development of the trigeminal motor neurons in parrots: Implications for the role of nervous tissue in the evolution of jaw muscle morphology

Vertebrates have succeeded to inhabit almost every ecological niche due in large part to the anatomical diversification of their jaw complex. As a component of the feeding apparatus, jaw muscles carry a vital role for determining the mode of feeding. Early patterning of the jaw muscles has been attributed to cranial neural crest‐derived mesenchyme, however, much remains to be understood about the role of nonneural crest tissues in the evolution and diversification of jaw muscle morphology. In this study, we describe the development of trigeminal motor neurons in a parrot species with the uniquely shaped jaw muscles and compare its developmental pattern to that in the quail with the standard jaw muscles to uncover potential roles of nervous tissue in the evolution of vertebrate jaw muscles. In parrot embryogenesis, the motor axon bundles are detectable within the muscular tissue only after the basic shape of the muscular tissue has been established. This supports the view that nervous tissue does not primarily determine the spatial pattern of jaw muscles. In contrast, the trigeminal motor nucleus, which is composed of somata of neurons that innervate major jaw muscles, of parrot is more developed compared to quail, even in embryonic stage where no remarkable interspecific difference in both jaw muscle morphology and motor nerve branching pattern is recognized. Our data suggest that although nervous tissue may not have a large influence on initial patterning of jaw muscles, it may play an important role in subsequent growth and maintenance of muscular tissue and alterations in cranial nervous tissue development may underlie diversification of jaw muscle morphology. J. Morphol. 275:191–205, 2014. © 2013 Wiley Periodicals, Inc.     

Functional significance of the coupling between head and jaw movements

When humans open or close the jaw they also move the head. Unintentionally, it rotates backwards when the jaw opens and returns upon jaw closure. We hypothesized that this mutual movement coupling is related to the muscles in the floor of the mouth. A biomechanical model was applied to comprehend the functional significance of this movement coupling. As the jaw opened the jaw opening muscles shortened and became less forceful. Meanwhile they had to stretch the jaw closing muscles. The simulations showed that a simultaneous head extension facilitated jaw opening. A possible functional significance for the coupling between head and jaw movements is that it can extend jaw gape. Head extension can contribute to a wider jaw gape by on the one hand a reduced shortening of the jaw opening muscles and on the other hand by a reorientation of these muscles so that they obtain a more favorable position for jaw opening.     

Neurobiology of the basal platyhelminth <Emphasis Type="Italic">Macrostomum lignano</Emphasis>: map and digital 3D model of the juvenile brain neuropile

We have analyzed brain structure in Macrostomum lignano, a representative of the basal platyhelminth taxon Macrostomida. Using confocal microscopy and digital 3D modeling software on specimens labeled with general markers for neurons (tyrTub), muscles (phalloidin), and nuclei (Sytox), an atlas and digital model of the juvenile Macrostomum brain was generated. The brain forms a ganglion with a central neuropile surrounded by a cortex of neuronal cell bodies. The neuropile contains a stereotypical array of compact axon bundles, as well as branched terminal axons and dendrites. Muscle fibers penetrate the flatworm brain horizontally and vertically at invariant positions. Beside the invariant pattern of neurite bundles, these “cerebral muscles” represent a convenient system of landmarks that help define discrete compartments in the juvenile brain. Commissural axon bundles define a dorsal and ventro-medial neuropile compartment, respectively. Longitudinal axons that enter the neuropile through an invariant set of anterior and posterior nerve roots define a ventro-basal and a central medial compartment in the neuropile. Flanking these “fibrous” compartments are neuropile domains that lack thick axon bundles and are composed of short collaterals and terminal arborizations of neurites. Two populations of neurons, visualized by antibodies against FMRFamide and serotonin, respectively, were mapped relative to compartment boundaries. This study will aid in the documentation and interpretation of patterns of gene expression, as well as functional studies, in the developing Macrostomum brain.     

Avian Jaw Function: Adaptation of the Seven–Muscle System and a Review

Avian jaw function is the most interesting part of the feeding apparatus, and essential in the life of birds. The usual seven jaw muscles in birds are highly adapted for diverse food-getting devices through muscular modifications as well as changes in kinesis of the skeletal components of the skull. In the first part I have described from an introspection of my earlier works, the functional morphology of the seven jaw muscles in different birds in four functional groups such as, adductors of the lower jaw, depressor of the lower jaw, protractors of the upper jaw and retractors-cum-adductors of the upper and lower jaws. Emphasis has been laid on the differential force production by these muscles, depending on the nature of their connective tissue attachments on the skeletal parts and changes in the kinesis of the skeletal parts. The contraction of the muscles and movements of the skeletal parts are rhythmically synchronized in such a way that their concerted action performs adaptively in different feeding adaptations. The differential force production by the one-joint and two-joint muscles in terms of ‘torque’ analysis is important in jaw kinesis. The second part of the text is a historical review of some notable works centred around the avian jaw muscles, jaw kinesis, tongue muscles, synchronization with the movements of the tongue apparatus and adaptational as well as evolutionary significance of the feeding apparatus in different feeding strategies.     

The retro-articular process, streptostyly and the caecilian jaw closing system

Caecilians have two functionally separate sets of jaw closing muscles. The jaw adductor muscles are parallel fibered muscles positioned close to the jaw joint and their lever mechanics suggests they are well suited to rapidly closing the jaws. A second set of muscles, the hypaxial interhyoideus posterior (IHP), levers the jaws closed by pulling on the retroarticular process (RA) of the lower jaw. Models of the lower jaw point out that the angle and length of the RA has a profound effect on the closure force exerted by the IHP. The caecilian skull is streptostylic – the quadrate-squamosal apparatus (QSA) moves relative to the rest of the skull, a condition that seems at odds with a well-ossified cranium. Modeling the contribution of this streptostylic suspension of the lower jaw shows that rotational freedom of the QSA amplifies the force of the IHP by redirecting force applied along the low axis of the lower jaw. Measurements from several species and life stages of preserved caecilians reveal a large variation in predicted bite force (as a multiple of IHP force) with age and phylogeny.     

Further evaluation of an EMG technique for assessment of the deep cervical flexor muscles

A novel surface electromyographic (EMG) technique was recently described for the detection of deep cervical flexor muscle activity. Further investigation of this technique is warranted to ensure EMG activity from neighbouring muscles is not markedly influencing the signals recorded. This study compared deep cervical flexor (DCF) muscle activity with the activity of surrounding neck and jaw muscles during various anatomical movements of the neck and jaw in 10 volunteer subjects. DCF EMG activity was recorded with custom electrodes inserted via the nose and fixed by suction to the posterior mucosa of the oropharynx. Surface electrodes were placed over the sternocleidomastoid, anterior scalene, masseter and suprahyoid muscles. Positioned in supine, subjects performed isometric cranio-cervical flexion, cervical flexion, right and left cervical rotation, jaw clench and resisted jaw opening. Across all movements examined, EMG amplitude of the DCF muscles was greatest during neck movements that would require activity of the DCF muscles, particularly during cranio-cervical flexion, their primary anatomical action. The actions of jaw clench and resisted jaw opening demonstrated significantly less DCF EMG activity than the cranio-cervical flexion action (p < 0.05). Across all other movements, the neighbouring neck and jaw muscles demonstrated greatest EMG amplitude during their respective primary anatomical actions, which occurred in the absence of increased EMG amplitude recorded from the DCF muscles. The finding of substantial EMG activity of the DCF muscles only during neck actions that would require their activity, particularly cranio-cervical flexion, and not during actions involving the jaw, provide further assurance that the majority of myoelectric signals detected from the nasopharyngeal electrode are from the DCF muscles.     

Elastic instability model of rapid beak closure in hummingbirds

The hummingbird beak, specialized for feeding on floral nectars, is also uniquely adapted to eating flying insects. During insect capture the beak often appears to close at a rate that cannot be explained by direct muscular action alone. Here we show that the lower jaw of hummingbirds has a shape and compliance that allows for a controlled elastic snap. Furthermore, hummingbirds have the musculature needed to independently bend and twist the sides of the lower jaw. According to both our simple physical model and our elastic instability calculation, the jaw can be smoothly opened and then snapped closed through an appropriate sequence of bending and twisting actions by the muscles of the lower jaw.     

Scaling of chew cycle duration in primates

The biomechanical determinants of the scaling of chew cycle duration are important components of models of primate feeding systems at all levels, from the neuromechanical to the ecological. Chew cycle durations were estimated in 35 species of primates and analyzed in conjunction with data on morphological variables of the feeding system estimating moment of inertia of the mandible and force production capacity of the chewing muscles. Data on scaling of primate chew cycle duration were compared with the predictions of simple pendulum and forced mass-spring system models of the feeding system. The gravity-driven pendulum model best predicts the observed cycle duration scaling but is rejected as biomechanically unrealistic. The forced mass-spring model predicts larger increases in chew cycle duration with size than observed, but provides reasonable predictions of cycle duration scaling. We hypothesize that intrinsic properties of the muscles predict spring-like behavior of the jaw elevator muscles during opening and fast close phases of the jaw cycle and that modulation of stiffness by the central nervous system leads to spring-like properties during the slow close/power stroke phase. Strepsirrhines show no predictable relationship between chew cycle duration and jaw length. Anthropoids have longer chew cycle durations than nonprimate mammals with similar mandible lengths, possibly due to their enlarged symphyses, which increase the moment of inertia of the mandible. Deviations from general scaling trends suggest that both scaling of the jaw muscles and the inertial properties of the mandible are important in determining the scaling of chew cycle duration in primates.     

A dynamic model of mouth closing movements in clariid catfishes: the role of enlarged jaw adductors

Some species of Clariidae (air breathing catfishes) have extremely large (hypertrophied) jaw closure muscles. Besides producing higher bite forces, the enlarged muscles may also cause higher accelerations of the lower jaw during rapid mouth closure. Thus, jaw adductor hypertrophy could potentially also enable faster mouth closure. In this study, a forward dynamic model of jaw closing is developed to evaluate the importance of jaw adductor hypertrophy on the speed of mouth closure. The model includes inertia, pressure, tissue resistance and hydrodynamic drag forces on the lower jaw, which is modelled as a rotating half-ellipse. Simulations are run for four clariid species showing a gradual increase in jaw adductor hypertrophy (Clarias gariepinus, Clariallabes longicauda, Gymnallabes typus and Channallabes apus). The model was validated using data from high-speed videos of prey captures in these species. In general, the kinematic profiles of the fastest mouth closure from each species are reasonably well predicted by the model. The model was also used to compare the four species during standardized mouth closures (same initial gape angle, travel distance and cranial size). These simulations suggest that the species with enlarged jaw adductors have an increased speed of jaw closure (in comparison with the non-hypertrophied C. gariepinus) for short lower jaw rotations and when feeding at high gape angles. Consequently, the jaw system in these species seems well equipped to capture relatively large, evasive prey. For prey captures during which the lower jaw rotates freely over a larger distance before impacting the prey, the higher kinematic efficiency of the C. gariepinus jaw system results in the fastest jaw closures. In all cases, the model predicts that an increase in the physiological cross-sectional area of the jaw muscles does indeed contribute to the speed of jaw closure in clariid fish.     

Observations on the Activity of Echinoid Pedicellariae

The tridentate, ophiocephalous and globiferous pedicellariae of sea‐urchins react to specific stimuli of the test so that they become alert. Their jaws open, exposing their special receptors, and their stems move them towards or away from the stimulus. Stem movements are coordinated so that the jaw reflexes may alleviate the stimulus. When the stimulus ceases the animal usually settles down so that most of the pedicellarial jaws close and the stems lower. This quiescence reduces the chances of self attack, and may allow commensals to move freely. These stem movements parallel those of spines in covergence and divergence responses even on isolated test fragments, and it would appear that they are coordinated by the basi‐epithelial nerve plexus of the test. Experimental lesions and photography have been used to analyse this coordination and a theoretical form for it is proposed.     

The growth of the skull and jaw muscles and its functional consequences in the New Zealand rabbit (Oryctolagus cuniculus)

Between weaning and adulthood, the length and height of the facial skull of the New Zealand rabbit (Oryctolagus cuniculus) double, whereas much less growth occurs in the width of the face and in the neurocranium. There is a five-fold increase in mass of the masticatory muscles, caused mainly by growth in cross-sectional area. The share of the superficial masseter in the total mass increases at the cost of the jaw openers. There are changes in the direction of the working lines of a few muscles. A 3-dimensional mechanical model was used to predict bite forces at different mandibular positions. It shows that young rabbits are able to generate large bite forces at a wider range of mandibular positions than adults and that the forces are directed more vertically. In young and adult animals, the masticatory muscles differ from each other with respect to the degree of gape at which optimum sarcomere length is reached. Consequently, bite force can be maintained over a range of gapes, larger than predicted on basis of individual length-tension curves. Despite the considerable changes in skull shape and concurrent changes in the jaw muscles, the direction of the resultant force of the closing muscles and its mechanical advantage remain stable during growth. Observed phenomena suggest that during development the possibilities for generation of large bite forces are increased at the cost of a restriction of the range of jaw excursion.