Neurobiology of Stomotoca. II. Pacemakers and conduction pathways

Evidence is presented for separate conduction pathways for swimming and for tentacle coordination in the marginal nerves of the jellyfish Stomotoca. The effector muscles are fired through junctions sensitive to excess Mg⁺⁺, probably represented by the neuromuscular synapses observed by electron microscopy. The swimming effector (striated muscle) fires one-to-one with nerve input signals and myoid conduction occurs. Tentacle responses (smooth muscle contractions) involve facilitation, presumably at the neuro-effector junction; responses are graded and nonpropagating. Electrical correlates of two further conducting systems using the marginal nerves have been recorded. Their functions are unknown. One, the bridge system, extends up the four radii and encircles the peduncle; the other (ring system) is confined to the margin. A fifth conducting system is inferred in the case of the pointing response and its distribution is plotted. Signals have not been obtained from it. Pointing is accompanied by a burst of muscle potentials in the radial smooth muscles and is exhibited after a lengthy latency, indicating a local pacemaker. A sixth conducting pathway is the epithelial system, which mediates crumpling, a response involving the radial muscles without pacemaker intervention. Characteristic conduction velocities and wave forms are noted for the first four systems and for epithelial pulses. All systems, except perhaps the pointing conduction system, through-conduct under excess Mg⁺⁺. Spontaneous activity patterns are described for the swimming, tentacle pulse, and ring systems. Abrupt increases in light intensity inhibit spontaneous activity, sudden decreases augmenting it. In the absence of specialized photoreceptors, light is presumed to act directly on central neurons. Epithelial pulses inhibit swimming, apparently by blocking the generation or conduction of the primary nervous events. This observation, taken in conjunction with evidence of feedback inhibition of the primary swimming system by the cells it fires, is discussed in relation to possible mechanisms whereby the output of nerve cells might be altered by activity in the excitable epithelial cells which envelop them.     

Neurobiology of Stomotoca. I. Action systems

The layout of nerves, muscles, and conducting epithelia is described for the simple hydrozoan medusa Stomotoca. Comparisons are drawn with Sarsia and other recently studied forms. The major action systems are those responsible for swimming, crumpling (protective involution), tentacle posture, pointing (unilateral reciprocal flexions of the manubrium and margin), and visceral movements (barely mentioned). Crumpling is a simple summating response in this species. Crumpling and pointing are considered to use the same effectors but different conduction pathways. New histological results include the demonstration of a nerve plexus running through the endodermal canal system and a nerve plexus in the ectoderm encircling the peduncle. Special attention is given to the distribution of synapses and gap junctions, as possible trasmission pathways in behavioral responses. Some details are included on organization within the marginal nerve rings.     

Neurobiology of Stomotoca. I. Action systems.

The layout of nerves, muscles, and conducting epithelia is described for the simple hydrozoan medusa Stomotoca. Comparisons are drawn with Sarsia and other recently studied forms. The major action systems are those responsible for swimming, crumpling (protective involution), tentacle posture, pointing (unilateral reciprocal flexions of the manubrium and margin), and visceral movements (barely mentioned). Crumpling is a simple summating response in this species. Crumpling and pointing are considered to use the same effectors but different conduction pathways. New histological results include the demonstration of a nerve plexus running through the endodermal canal system and a nerve plexus in the ectoderm encircling the peduncle. Special attention is given to the distribution of synapses and gap junctions, as possible trasmission pathways in behavioral responses. Some details are included on organization within the marginal nerve rings.     

Neurobiology of Polyorchis. I. Function of effector systems.

The electrical correlates of activity in the effector systems responsible for swimming, crumpling and postural changes have been recorded in the anthomedusan Polyorchis penicillatus. Motor spikes (pre-swim pulses), that initiate swimming contractions, appear without delay at distant sites on the inner nerve-ring in unstimulated preparations. Levels of Mg++ anaesthesia which block the neuromuscular junctions between PSP giant neurons and swimming muscle do not affect PSP activity. Swimming muscle potentials can be recorded from subumbrella and velar muscle sheets using extra- and intracellular electrodes. These action potentials have a distinct plateau and are propagated in a myoid fashion. Resting potentials average -70 mV with spikes overshooting zero by some 62 mV. The effects of repetitive stimulation are described. Extracellular recordings indicate that neuronal pathways may play a major role in mediating crumpling, unlike many other species where epithelial pathways are more important. Endodermal spikes recorded intracellularly from the radial and ring canals have amplitudes of some 92 mV arising from resting potentials that average -55 mV. Repetitive stimulation causes a decrease in amplitude and increase in duration of epithelial action potentials. Tentacle length is controlled by a pacemaker system located in both nerve rings. The frequency of spikes (PTPs) generated by this system determines the length and tonus of tentacles. The neuromuscular junctions between the motor neurons and tentacle muscle are Mg++ sensitive and show facilitating properties.     

Neurobiology of Polyorchis. II. Structure of effector systems

The gross and fine morphology of the major effector systems in the anthomedusan, Polyorchis penicillatus, is described and discussed in relation to the known physiological and behavioral properties of these systems. Swimming is controlled by an anastomosing network of giant neurons within the inner nerve ring and radial nerves. Although these neurons may be coupled by gap junctions it is likely that they form a syncytium. The photosensitivity of the “giants” is attributed to reflexive membranes within the cytoplasm. Giant neurons act as both the pre- and postsynaptic cell when forming synapses with other neurons of the inner nerve ring. Neuromuscular synapses between “giants” and the striated swimming muscle are found around the margin and along the radii. Swimming muscle cells are connected laterally by gap junctions and end-to-end by desmosomes which are sometimes elaborated with extra-thick filaments. Unstriated sphincter and radial muscles, the major muscles associated with crumpling, are both greatly folded over mesogloeal ridges and have processes that cross the mesogloea to contact the ring and radial canals, respectively. Synapses or other sites that might be responsible for exciting these muscles during crumpling have not been found. The ability of the endodermal lamella and canals to propagate action potentials can be accounted for by the numerous gap junctions that are seen in these tissues. The precise location where excitation is transferred to the nervous system to initiate crumpling is not known but epithelial bridges crossing the mesogloea are likely routes. Synapses between neurons originating in the outer nerve ring and tentacle longitudinal muscle can account for the control of tentacle length. Neurons of the outer nerve ring also synapse onto velar, radial fibers and the sphincter muscle. The inner and outer nerve rings have nervous connections. The organisation of the outer nerve ring and the arrangement of nerves within the endodermal plexus is described. A diagram showing the major connections and interactions of components of the effector systems is presented.     

Neurobiology of Polyorchis. I. Function of effector systems

The electrical correlates of activity in the effector systems responsible for swimming, crumpling and postural changes have been recorded in the anthomedusan Polyorchis penicillatus. Motor spikes (pre-swim pulses), that initiate swimming contractions, appear without delay at distant sites on the inner nerve-ring in unstimulated preparations. Levels of Mg⁺⁺ anaesthesia which block the neuromuscular junctions between PSP giant neurons and swimming muscle do not affect PSP activity. Swimming muscle potentials can be recorded from subumbrella and velar muscle sheets using extra- and intracellular electrodes. These action potentials have a distinct plateau and are propagated in a myoid fashion. Resting potentials average ?70 mV with spikes overshooting zero by some 62 mV. The effects of repetitive stimulation are described. Extracellular recordings indicate that neuronal pathways may play a major role in mediating crumpling, unlike many other species where epithelial pathways are more important. Endodermal spikes recorded intracellularly from the radial and ring canals have amplitudes of some 92 mV arising from resting potentials that average ?55 mV. Repetitive stimulation causes a decrease in amplitude and increase in duration of epithelial action potentials. Tentacle length is controlled by a pacemaker system located in both nerve rings. The frequency of spikes (PTPs) generated by this system determines the length and tonus of tentacles. The neuromuscular junctions between the motor neurons and tentacle muscle are Mg⁺⁺ sensitive and show facilitating properties.     

Epithelial Conduction in Hydromedusae

Sarsia, Euphysa, and other hydromedusae have been studied by electrophysiological techniques and are found to have nonnervous conducting epithelia resembling those described earlier for siphonophores. Simple, non-muscular epithelia fire singly or repetitively following brief electrical stimuli. The pulses recorded with suction electrodes are biphasic, initially positive, and show amplitudes of 0.75–2.0 mv, durations of 5–15 msec, and velocities of 15–35 cm/sec with short refractory periods. In the swimming muscle (myoepithelium) 2.0–4.0 mv composite events lasting 150–300 msec are associated with contraction waves. Propagation in nonnervous epithelia is typically all-or-none, nondecremental, and unpolarized. The subumbrellar endoderm lamella conducts independently of the adjacent ectoderm. The lower regions of the tentacles do not show propagated epithelial events. The spread of excitation in conducting epithelia and associated effector responses are described. Examples are given of interaction between events seemingly conducted in the nervous system and those in nonnervous epithelia. Either system may excite the other. Spontaneous activity, however, appears to originate in the nervous system. Conduction in nonnervous tissues is unaffected by excess Mg⁺⁺ in concentrations suppressing presumed nervous activity, although this may not be a wholly adequate criterion for distinguishing components of the two systems. Evidence from old work by Romanes is considered in the light of these findings and the general significance of epithelial conduction is discussed.     

Immunocytochemistry of the neuromuscular systems of Loxosomella vivipara and L. parguerensis (Entoprocta: Loxosomatidae)

Little detailed information exists on the anatomy of the nervous system and the musculature of Entoprocta. Herein we describe the distribution of the neurotransmitters RFamide and serotonin as well as the myo-anatomy of adults and asexually produced budding stages of the solitary entoproct species Loxosomella vivipara and L. parguerensis using immunocytochemistry and epifluorescence as well as confocal microscopy. The development of the RFamidergic and serotonergic nervous system starts in early budding stages. In the adults, RFamide is present in the bilateral symmetric cerebral ganglion, a pair of oral nerves that innervate two pairs of nerve cell clusters in the heel of the foot, a pair of aboral nerves, the paired lateral nerves, the calyx nerves, the atrial ring nerve, the tentacle nerves, the stomach nerves, and the rectal nerves. Serotonin is only found in the cerebral ganglion, the oral nerves, and in the tentacle nerves. Some differences in the distribution of both neurotransmitters were found between L. vivipara and L. parguerensis and are most obvious in the differing number of large serotonergic perikarya associated with the oral nerves. Nerves arising from the cerebral ganglion and running in a ventral direction have not been described for Entoprocta before, and the homology of these to the ventral nerve cords of other Spiralia is considered possible. The body musculature of both Loxosomella species comprises longitudinal and diagonal muscles in the foot, the stalk, and the calyx. We found several circular muscles in the calyx. The stalk and parts of the foot and the calyx are surrounded by a fine outer layer of ring muscles. In addition to the congruent details regarding the myo-anatomy of both species, species-specific muscle structures could be revealed. The comparison of our data with recent findings of the myo-anatomy of two Loxosoma species indicates that longitudinal and diagonal body muscles, atrial ring muscles, tentacle muscles, esophageal and rectal ring muscles, as well as intestinal and anal sphincters are probably part of the ancestral entoproct muscle bauplan.     

Ultrastructure of the neuromuscular system of the polyp of Aurelia aurita L., 1758 (Cnidaria,scyphozoa)

Fine structural study indicates that the neuromuscular system of stage I polyps of Aurelia aurita is exclusively ectodermal. The three major muscle fields are the radial muscles of the oral disc, the longitudinal muscles of the tentacles, and the muscle cords of the septae and the column; the muscle fields are in physical continuity at the peristomial pits and share a common innervation and type of myofibril. The myofibril is striated in the tentacle base, in the outer oral disc, and in the upper part of the muscle cord; it grades into a smooth muscle toward the tentacle tip, the mouth, and the lower part of the cord. There is a fourth field of longitudinal smooth muscle in the pharynx. The nervous system consists of an epithelial sensory cell in the tentacle and a single type of neuron found in the subepithelial layer of the tentacle, oral disc, and muscle cord. The lack of gap junctions suggests that there is no nonnervous conduction system. The subepithelial layer also contains three types of fibers and a type of soma which cannot be characterized as neuronal. The soma is identified as the “neurosecretory cell” described in Chrysaora. The absence of neuromuscular elements in the column and stolon distinguishes the Aurelia aurita collected from Washington, USA, from English polyps previously described.     

Neuronal background of positioning of the posterior tentacles in the snail Helix pomatia

The location of cerebral neurons innervating the three recently described flexor muscles involved in the orientation of the posterior tentacles was investigated by applying parallel retrograde Co- and Ni-lysine tracing via the olfactory and the peritentacular nerves. Their innervation patterns in the flexor muscles were studied by applying anterograde neurobiotin tracings via these nerves. The labeled neurons are clustered in eight groups in the cerebral ganglion. They send both common and distinct innervation pathways to the flexor and the tegumental muscles and to the tentacular retractor muscle. The common pathway reaches the muscles via the olfactory nerve, whereas the distinct pathways innervate via the internal and external peritentacular nerves. The three anchoring points of the three flexor muscles at the base of the tentacle outline the directions of three force vectors generated by the contraction of the muscles and enable the protracted tentacle to bend around a basal pivot. In the light of earlier physiological and the present anatomical findings, we suggest that the common innervation pathway to the muscles is required for tentacle withdrawal and the retractor mechanism, whereas the distinct pathways primarily serve the bending of the protracted posterior tentacles during foraging.     

Central neural circuitry in the jellyfish Aglantha: a model 'simple nervous system'

Like other hydrozoan medusae, Aglantha lacks a brain, but the two marginal nerve rings function together as a central nervous system. Twelve neuronal and two excitable epithelial conduction systems are described and their interactions summarized. Aglantha differs from most medusae in having giant axons. It can swim and contract its tentacles in two distinct ways (escape and slow). Escape responses are mediated primarily by giant axons but conventional interneurons are also involved in transmission of information within the nerve rings during one form of escape behavior. Surprisingly, giant axons provide the motor pathway to the swim muscles in both escape and slow swimming. This is possible because these axons can conduct calcium spikes as well as sodium spikes and do so on an either/or basis without overlap. The synaptic and ionic bases for these responses are reviewed. During feeding, the manubrium performs highly accurate flexions to points at the margin. At the same time, the oral lips flare open. The directional flexions are conducted by FMRFamide immunoreactive nerves, the lip flaring by an excitable epithelium lining the radial canals. Inhibition of swimming during feeding is due to impulses propagated centrifugally in the same epithelium. Aglantha probably evolved from an ancestor possessing a relatively simple wiring plan, as seen in other hydromedusae. Acquisition of giant axons resulted in considerable modification of this basic plan, and required novel solutions to the problems of integrating escape with non-escape circuitry.     

The ontogeny of swimming behavior in the scyphozoan, Aurelia aurita. II. The effects of ions and drugs.

1. The responses of Aurelia medusae to pharmacological agents and ionic variation were classified into four response types: Type I, no response; Type II, inhibition of pacemaker activity; Type III, inhibition of both pacemakers and swimming muscles; and Type IV, increase in pacemaker output. 2. The swimming pacemakers of Aurelia medusae become hyperactive in Mg+2-free solutions (Type IV). This response appears to be general in swimming scyphozoa. 3. The response pattern to pharmacologically-active compounds indicates that the coelenterate neuromuscular system is quite different than those in other phyla. In fact, the response spectrum is not consistent within the Cnidaria. 4. Similarly, the responses of adult medusae to ionic variation show no consistent pattern within various scyphomedusae. 5. Test solutions from each response type established with medusae were selected and tested on the scyphistoma and strobila stages. The comparison of the responses to the test solutions between the medusa, scyphistoma, and strobila showed that the neuromuscular systems are physiologically different. The strobila, specificially the ephyra, is a mixture of both polypoid and medusoid response types. The strobila, therefore, is physiologically an intermediate stage in the development of the adult medusa.     

The ontogeny of swimming behavior in the scyphozoan, Aurelia aurita. I. Electrophysiological analysis.

1. Electrical correlates of behavioral activity were observed in the lip and tentacles of the polyp, but none were detected during column contraction. The tentacles are the most electrically active tissue, and the potentials are conducted along the length of the tentacle, but conduction to other parts of the animal were not observed. 2. Although the tentacles of the polyp and the rhopalia of the medusa are probably homologous, the development of pacemaker activity during strobilation is not a smooth transition from tentacle contraction potentials (TCPs) to marginal ganglion potentials (MGPs). This result indicates that each pacemaker activity develops de novo. 3. Two types of behavior were observed in the polyp: local responses, and coordinated activity which involved integrated responses in several body parts. The coordinated responses indicate that neurological coordination can take place in the polyp. Furthermore, feeding and spasm in the ephyra are similar to feeding and the protective response in the polyp. This similarity suggests that both coordinated responses in the polyp are coordinated by interneural facilitation in the diffuse nerve net (DNN) as in the ephyra. 4. Swimming in the ephyra is a medusoid behavior but feeding and spasm are coordinated by the DNN and are polypoid responses. Therefore, the ephyra is a mixture of polypoid and medusoid behaviors. As the ephyra matures into an adult medusa both polypoid responses are lost, but the DNN remains to modulate pacemaker output and control marginal tentacle contractions. As development proceeds from polyp, to ephyra, to medusa, each subsequent stage acquires some new behavior while retaining some aspect from the previous stage.     

Colonial conduction systems in the Anthozoa: Octocorallia.

1. The octocorals Alcyonium digitatum, Pennatula phosphorea and Virgularia mirabilis each have a through-conducting nerve net. The nerve net demonstrated electrophysiologically may well be the same as that previously shown by the use of histological techniques. 2. It exhibits both facilitation and defacilitation in the rate of conduction of pulses. 3. The distance of spread of nerve net activity is not limited by the number of stimuli applied. 4. The nerve net controls fast muscle contractions; the frequency of pulses is important in determining which muscles contract and in which sequence. 5. The nerve net is 'spontaneously' active. 6. A previously undescirbed slow system has been identified in Pennatula. It has many of the properties of slow systems in sea anemones and may well be ectodermal. It is suggested that multiple conduction systems are of common occurrence in the Anthozoa.     

Myoanatomy and serotonergic nervous system of plumatellid and fredericellid phylactolaemata (lophotrochozoa,ectoprocta)

The phylogenetic position of the Ectoprocta within the Lophotrochozoa is discussed controversially. For gaining more insight into ectoproct relationships and comparing it with other potentially related phyla, we analysed the myoanatomy and serotonergic nervous system of adult representatives of the Phylactolaemata (Plumatella emarginata, Plumatellavaihiriae, Plumatella fungosa, Fredericella sultana). The bodywall contains a mesh of circular and longitudinal muscles. On its distal end, the orifice possesses a prominent sphincter and continues into the vestibular wall, which has longitudinal and circular musculature. The tentacle sheath carries mostly longitudinal muscle fibres in Plumatella sp., whereas F. sultana also possesses regular circular muscle fibres. Three groups of muscles are associated with the lophophore: 1) Lophophoral arm muscles (missing in Fredericella), 2) epistome musculature and 3) tentacle musculature. The epistome flap is encompassed by smooth muscle fibres. A few fibres extend medially over the ganglion to its proximal floor. Abfrontal tentacle muscles have diagonally arranged muscle fibres in their proximal region, whereas the distal region is formed by a stack of muscles that resemble an inverted ‘V’. Frontal tentacle muscles show more variation and either possess one or two bases. The digestive tract possesses circular musculature which is striated except at the intestine where it is composed of smooth muscle fibres. The serotonergic nervous system is concentrated in the cerebral ganglion. From the latter a serotonergic nerve extends to each tentacle base. In Plumatella the inner row of tentacles at the lophophoral concavity lacks serotonergic nerves. Bodywall musculature is a common feature in many lophotrochozoan phyla, but among other filter feeders like the Ectoprocta is only present in the ‘lophophorate’ Phoronida. The longitudinal tentacle musculature is reminiscent of the condition found in phoronids and brachiopods, but differs to entoproct tentacles. Although this study shows some support for the ‘Lophophorata’, more comparative analyses of possibly related phyla are required. J. Morphol., 2011. © 2011 Wiley Periodicals, Inc.     

Delayed initiation of SS1 pulses in the sea anemone Calliactis parasitica: evidence for a fourth conducting system.

1. Single electrical shocks to the column sometimes elicit a series of 1-6 pulses in the SS1 (ectodermal slow system) but the first pulse does not appear until 5-28 s after stimulation. These pulses occur in addition to the early SS1 pulse which follows every shock and which has a conduction delay of less than 1 s. 2. The threshold of the delayed SS1 response is different from the thresholds of the three known conducting systems (through-conducting nerve net, SS1, and SS2). 3. In the case of stimulation of the column, the delayed SS1 pulses do not arise at the point of stimulation but probably originate in the tentacles or upper column. The pulse origin can shift during a single burst. 4. The pathway from the point of stimulation to the site of origin of delayed SS1 pulses is endodermal. We propose that this pathway represents a fourth conducting system (Delayed Initiation System--DIS). The DIS must connect, across the mesogloea, with the ectodermal SS1. The long pulse delay and repetitive firing may derive from pacemaker activity in the DIS. The DIS pacemakers closely resemble the pacemakers connected to the through-conducting nerve net. The DIS may be neuronal. 5. Delayed SS1 pulse bursts from unattached anemones showed an earlier onset, and more pulses/burst, than those from attached anemones. 6. Delayed SS1 pulses can also be evoked by electrical, and in some cases mechanical, stimulation of the pedal disc, tentacles, and pharynx, but there are regional differences in the number of pulses evoked, in their delay, and in their site of origin.     

Ultrastructural evidence for two-cell and three-cell neural pathways in the tentacle epidermis of the sea anemone Aiptasia pallida.

Sensory and ganglion cells in the tentacle epidermis of the sea anemone Aiptasia pallida were traced in serial transmission electron micrographs to their synaptic contacts on other cells. Sensory cell synapses were found on spirocytes, muscle cells, and ganglion cells. Ganglion cells, in turn, synapsed on sensory cells, spirocytes, muscle cells, and other neurons and formed en passant axo-axonal synapses. Axonal synapses on nematocytes and gland cells were not traced to their cells of origin, i.e., identified sensory or ganglion cells. Direct synaptic contacts of sensory cells with spirocytes and sensory cells with muscle cells suggest a local two-cell pathway for spirocyst discharge and muscle cell contraction, whereas interjection of a ganglion cell between the sensory and effector cells creates a local three-cell pathway. The network of ganglion cells and their processes allows for a through-conduction system that is interconnected by chemical synapses. Although the sea anemone nervous system is more complex than that of Hydra, it has similar two-cell and three-cell effector pathways that may function in local responses to tentacle contact with food.     

Control of swimming in the hydrozoan jellyfish Aequorea aequorea: direct activation of the subumbrella

The epithelial cells that overlie the inner nerve ring of the hydrozoan jellyfish Aequorea aequorea were investigated ultrastructurally and electrophysiologically. The structurally unspecialized epithelial cells are interconnected by gap junctions and are electrically active during swimming as a single, long-duration action potential was recorded during each swim contraction. Intercellular electrical- and dye-coupling was demonstrated within the epithelial region extending into the velum and subumbrellar regions. Excitatory post-synaptic potentials were recorded from epithelial cells following swim motorneuron spikes with a short latency. Psps were up to 60 mV in amplitude and, when triggered in bursts, showed summation provided the interpulse interval was less than 25-35 ms. The initial gap in each of a series of bursts showed facilitation with the first few swim contractions following a period of inactivity. In actively swimming medusae, psp amplitude was relatively constant. The reversal potential for epithelial psp was estimated at between 0 and +20 mV. Spontaneous psps spread throughout the epithelial region electronically, but the amplitude decrease with conducting distance was less than that for current pulses injected into individual epithelial cells. This presumably represents the effect of widespread synaptic activation of epithelial cells via multiple input sites throughout the inner nerve ring as opposed to point-source input in current injection experiments. During a radial response, action potential amplitude was decreased and rise time increased due to decremental conduction through the inhibited region. It is postulated that conduction of a full action potential requires that electrotonic current spread from adjacent, active epithelial cells occur in synchrony with synaptic input from swim motoneurons.     

The Strategies of Behavioral Control in a Coelenterate

Although the cellular substrates of behavioral coordinationare uncertain in the hydroid Tubularia, much is known aboutthe strategies of behavioral control. The picture which emergesis that of an animal with diffusely distributed sites capableof initiating spontaneous activity, regional coordination ofpotential pacemaker loci to form pacemaker systems, and a loosehierarchical organization of pacemaker systems. At least onepacemaker system shows a short term increase in excitabilityand a longer duration depression of excitability following firing.The short-term excitability increase gives a tendency to firein bursts, the long-term depression acts as an intrinsic inhibitoryfeedback to terminate bursts and to control output frequency.All the known interactions between pacemaker systems are excitatory.Exogenous stimuli can excite a conducting system which inhibitsmost of the pacemaker systems, and one pacemaker system specificallyinhibits one set of muscles.