Immunostaining of peripheral nerves and other tissues in whole mount preparations from hatchling cephalopods.

Newly hatched paralarvae ("hatchlings") or late-stage embryos of Loligo opalescens were dissected and pieces of tissue removed for immunostaining as flat whole mounts. The general layout of the peripheral nervous system in the mantle and gills was investigated using antisera for tubulin and FMRFamide. Primary sensory neurons are densely distributed in the outer mantle epidermis and show strong FMRFamide immunoreactivity. Their axons form a plexus in the underlying dermis, but do not appear to innervate the chromatophore muscles, which are well visualized with anti-tubulin. Some cross the muscle layer and enter the stellate ganglia via the stellar nerves. The stellate ganglion neuropil contains a rich FMRFamide-immunoreactive mass of axons. It is suggested that these axons originate in large part from sensory neurons in the skin and that the known modulatory effects of FMRFamide-related peptides on motor output of the stellate ganglion may be a reflection of this sensory input in normal life. FMRFamide-immunoreactive primary sensory neurons are also abundant in the gills, but unlike those in the mantle, these cells lack cilia or other external projections. Anti-tubulin staining reveals a network of interstitial cells in the mantle dermis. Such networks may have been mistaken for nerve nets in older accounts. Additional results with Octopus vulgaris hatchlings and immunostaining for serotonin (5HT), small cardioactive peptide (SCP), and gonadotropin releasing hormone (GnRH) are briefly reported.

The biology of glass sponges.

As the most ancient extant metazoans, glass sponges (Hexactinellida) have attracted recent attention in the areas of molecular evolution and the evolution of conduction systems but they are also interesting because of their unique histology: the greater part of their soft tissue consists of a single, multinucleate syncytium that ramifies throughout the sponge. This trabecular syncytium serves both for transport and as a pathway for propagation of action potentials that trigger flagellar arrests in the flagellated chambers. The present chapter is the first comprehensive modern account of this group and covers work going back to the earliest work dealing with taxonomy, gross morphology and histology as well as dealing with more recent studies. The structure of cellular and syncytial tissues and the formation of specialised intercellular junctions are described. Experimental work on reaggregation of dissociated tissues is also covered, a process during which histocompatibility, fusion and syncytialisation have been investigated, and where the role of the cytoskeleton in tissue architecture and transport processes has been studied in depth. The siliceous skeleton is given special attention, with an account of discrete spicules and fused silica networks, their diversity and distribution, their importance as taxonomic features and the process of silication. Studies on particle capture, transport of internalised food objects and disposal of indigestible wastes are reviewed, along with production and control of the feeding current. The electrophysiology of the conduction system coordinating flagellar arrests is described. The review covers salient features of hexactinellid ecology, including an account of habitats, distribution, abundance, growth, seasonal regression, predation, mortality, regeneration, recruitment and symbiotic associations with other organisms. Work on the recently discovered hexactinellid reefs of Canada's western continental shelf, analogues of long-extinct Jurassic sponge reefs, is given special attention. Reproductive biology is another area that has benefited from recent investigations. Seasonality, gametogenesis, embryogenesis, differentiation and larval biology are now understood in broad outline, at least for some species. The process whereby the cellular early larva becomes syncytial is described. A final section deals with the classification of recent and fossil glass sponges, phylogenetic relationships within the Hexactinellida and the phylogenetic position of the group within the Porifera. Palaeontological aspects are covered in so far as they are relevant to these topics.

Central circuitry in the jellyfish Aglantha digitale IV. Pathways coordinating feeding behaviour.

The hydromedusan jellyfish Aglantha digitale feeds on small planktonic organisms carried to the margin by tentacle flexions. During feeding, the manubrium bends across ("points") and seizes the prey with flared lips. In immobilized preparations, pointing to a source of electrical stimulation was accurate, 70% of the time, to within 15 degrees. Cutting experiments showed that the conduction pathways concerned with pointing and lip flaring are located in eight radial strands consisting of a radial canal, a giant nerve axon and a bundle of small axons with FMRFamide-like immunoreactivity. Application of food juices to sites on the margin and tentacles evoked trains of impulses in the axon bundles (F events; conduction velocity 15.5+/-3.7 cm s(-1)) and in the epithelium lining the radial canals (E events; conduction velocity 28.5+/-3.5 cm s(-1)). Impulses were conducted circularly in the outer nerve ring (F events) or in the ring canal (E events). Unilateral flexions of the manubrium during pointing arise from preferential excitation of one or more of eight longitudinal "muscle bands" in the wall of the manubrium and peduncle. Lip flaring represents symmetrical contraction of all eight bands. Cutting experiments revealed that F events mediate pointing; E events mediate lip flaring. Thus the endodermal radial canals, which in other hydromedusae mediate protective 'crumpling', provide the conduction pathway for manubrial lip flaring. Aglantha's alternative protective response--escape swimming--makes crumpling unnecessary, releasing the pathway for use in feeding. Trains of E events, generated in the manubrium during ingestion, propagate to the margin and inhibit rhythmic (slow) swimming with a duration that depended on their number and frequency. Inhibition of swimming appeared to facilitate transfer of food from the margin to the mouth, but how it comes about is unclear.

The capsular organ of Chelyosoma productum (Ascidiacea: Corellidae): a new tunicate hydrodynamic sense organ.

Chelyosoma has about 200 sense organs in the atrial wall of the branchial sac that show structural features suggestive of a role in hydrodynamic sensing. They consist of a fluid-filled capsule with an acellular diaphragm spanning an opening in the top. The floor of the capsule is a sensory macula, with 5-6 primary sensory neurons whose cilia project into the capsular cavity and whose axons go to the brain. Animals were tested for vibrational sensitivity using a loudspeaker probe. They responded by muscular contractions of the siphons and/or arrests of the branchial cilia, both of which caused changes in water flow velocity through the siphons, which were monitored non-invasively using a thermistor flow meter. The animals showed peak sensitivity at 240-260 Hz. Subsequent calibration of the loudspeaker probe indicated that the animals could detect 104 decibels re 1 microPa from a source 80 cm away. The majority of axons leaving the capsular organs go to the brain via the visceral nerve. Cutting this nerve abolished or greatly reduced responses. Electrophysiological recordings from the distal nerve stump showed bursts of electrical events following vibrational stimulation. No such records could be obtained from other major nerves and cutting them did not affect responsivity. Corella inflata, a close relative of Chelyosoma, lacks capsular organs and failed to show any responses when the source was more than 3.5 cm away. We conclude that Chelyosoma's vibration-sensing ability is due to its capsular organs and is adaptive in terms of detecting the movements of objects in the vicinity. The findings are discussed in relation to the evolution of hydrodynamic mechanoreceptors in tunicates, amphioxus and craniates.

Central circuitry in the jellyfish Aglantha digitale. III. The rootlet and pacemaker systems.

Tactile stimulation of the subumbrella of Aglantha digitale was found to evoke an escape swimming response similar to that evoked by stimulation of the outer surfaces of the margin but that does not involve the ring giant axon. Evidence is presented that conduction around the margin takes place via an interconnected system of rootlet interneurones. Confocal microscopy of carboxyfluorescein-filled axons showed that the rootlet neurones run out from the bases of the motor giant axons within the inner nerve ring and come into close contact with those of the neighbouring motor giant axons on either side. Transmission between the rootlet neurones has the properties of chemical synaptic transmission. A distinct type of fast excitatory postsynaptic potential (rootlet PSP) was recorded in motor giant axons following stimulation of nearby axons in 3-5 mmol l(-)(1) Mn(2+), which lowered the PSP below spike threshold. Immune labelling with anti-syntaxin 1 showed structures tentatively identified as synapses in the inner nerve ring, including some on the rootlet neurones. Neuromuscular junctions were not labelled. A secondary consequence of stimulating motor giant axons was the triggering of events in the pacemaker system. Triggering was blocked in 105 mmol l(-)(1) Mg(2+), indicating a synaptic link. Activity in the pacemaker system led indirectly to tentacle contractions (as described in earlier papers in this series), but the contractions were not as sudden or as violent as those seen when escape swimming was mediated by the ring giant axon. Events triggered in the pacemaker system fed back into the motor giants, producing postsynaptic potentials that appeared as humps in the spike after-potential. The conduction velocity of events propagating in the relay system was increased when the rootlet pathway was simultaneously excited (piggyback effect). With the addition of the rootlet pathway, the number of identified systems concerned with locomotion, feeding and tentacle contractions comes to fourteen, and the list is probably nearly complete.

Impulse conduction in a sponge.

All-or-none propagated electrical impulses were recorded from the hexactinellid sponge Rhabdocalyptus dawsoni using suction electrodes attached to lumps of aggregated sponge tissue grafted onto the surface of pieces of the same sponge. Impulses were normally evoked by means of externally applied electrical shocks. Recorded externally using an a.c.-coupled amplifier, the electrical event was triphasic and lasted approximately 30 s; integration gave a diphasic waveform. A further integration to give the form of the membrane action potential produced a monophasic signal. Impulses propagated at 0.27+/-0.1 cm s-1 with an absolute refractory period of 29 s and a relative refractory period of approximately 150 s. Concurrent thermistor flow meter recordings confirmed that water flow through the sponge was arrested following the passage of an impulse, presumably as result of the cessation of beating of the flagella in the flagellated chambers. Tactile stimuli also evoked impulses, as did addition of particulate material to the incoming water stream. Impulses continued to propagate through the sponge during arrests, indicating that the conduction and effector systems were independent. Sponges lack nerves, and a variety of evidence indicates that the conducting tissues are the syncytial trabecular reticulum and pinacoderm layers. Na+-deficient solutions had little effect on the action potential, but propagation was blocked by 10 mmol l-1 Co2+, 1 mmol l-1 Mn2+ or 24 micromol l-1 nimodipine. Tetraethylammonium ions at 1-5 mmol l-1 also blocked propagation without prolonging the action potential. Impulse conduction in the sponge is discussed in relation to excitability and conduction in the protozoa and in plants and to non-nervous conduction in more advanced animals.

Gonadotropin-releasing hormone in mulberry cells of Saccoglossus and Ptychodera (Hemichordata: Enteropneusta).

Mulberry cells are epidermal gland cells bearing a long basal process resembling a neurite and are tentatively regarded as neurosecretory cells. They occur scattered through the ectoderm of the proboscis, collar, and anterior trunk regions of the acorn worms Saccoglossus, usually in association with concentrations of nervous tissue. They contain secretion granules that appear from electron micrographs to be released to the exterior. The granules are immunoreactive with antisera raised against mammalian and salmon gonadotropin-releasing hormone (GnRH). Similar results were obtained with another enteropneust, Ptychodera bahamensis, using antisera raised against tunicate-1 and mammalian GnRH. Mulberry cells were not found in either Cephalodiscus or Rhabdopleura (Hemichordata: Pterobranchia). Extracts of tissues from 4200 Saccoglossus contain an area of immunoreactive GnRH that is detected by an antiserum raised against lamprey GnRH when characterized by high-performance liquid chromatography and radioimmunoassay. This is the first report of the occurrence of GnRH in hemichordates, probably the most primitive group clearly belonging to the chordate lineage. The physiological function of GnRH in enteropneusts is unknown, but an exocrine function appears more likely than an endocrine or neurotransmitter role.

Sequence of two gonadotropin releasing hormones from tunicate suggest an important role of conformation in receptor activation.

The primary structure of two forms of gonadotropin releasing hormone (GnRH) from tunicate (Chelyosoma productum) have been determined based on mass spectrometric and chemical sequence analyses. The peptides, tunicate GnRH-I and -II, contain features unprecedented in vertebrate GnRH. Tunicate GnRH-I contains a putative salt bridge between Asp5 and Lys8. A GnRH analog containing a lactam bridge between Asp5 and Lys8 was found to increase release of estradiol compared with that of the native tunicate GnRH-I and -II. Tunicate GnRH-II contains a cysteine residue and was isolated as a dimeric peptide. These motifs suggest that the conformation plays an important role in receptor activation.

Two new forms of gonadotropin-releasing hormone in a protochordate and the evolutionary implications.

The neuropeptide gonadotropin-releasing hormone (GnRH) is the major regulator of reproduction in vertebrates. Our goal was to determine whether GnRH could be isolated and identified by primary structure in a protochordate and to examine its location by immunocytochemistry. The primary structure of two novel decapeptides from the tunicate Chelyosoma productum (class Ascidiacea) was determined. Both show significant identity with vertebrate GnRH. Tunicate GnRH-I (pGlu-His-Trp-Ser-Asp-Tyr-Phe-Lys-Pro-Gly-NH2) has 60% of its residues conserved, compared with mammalian GnRH, whereas tunicate GnRH-II (pGlu-His-Trp-Ser-Leu-Cys-His-Ala-Pro-Gly-NH2) is unusual in that it was isolated as a disulfide-linked dimer. Numerous immunoreactive GnRH neurons lie within blood sinuses close to the gonoducts and gonads in both juveniles and adults, implying that the neuropeptide is released into the bloodstream. It is suggested that in ancestral chordates, before the evolution of the pituitary, the hormone was released into the bloodstream and acted directly on the gonads.

Synaptic potentials and threshold currents underlying spike production in motor giant axons of Aglantha digitale.

1. Motor giant axons that excite swimming muscles in the jelly-fish Aglantha digitale interface with units of the inner and outer nerve rings in the margin at the base of the bell. External recording electrodes were used to monitor electrical activity at different sites within the nerve ring while events in the motor giant axon were recorded with intracellular micropipettes placed within 100 microns of the synaptic area. In some experiments, 4- to 6-micron-diam patch pipettes were used to record in situ from ion channel clusters at different locations along the axon. 2. Independently propagating calcium and sodium spikes in the motor giant axon were found to arise from different excitatory postsynaptic potentials (EPSPs). Two separate inputs were identified; one EPSP class represented an input from the pacemaker system in the inner nerve ring, whereas another represented an input from the giant axon in the outer nerve ring. EPSPs from the two nerve rings had significantly different time courses and amplitudes. EPSPs from the ring giant axon reached a peak in little more than 1 ms, whereas EPSPs from the pacemaker system reached a maximum in approximately 7 ms. These slower EPSPs may be compound events composed of postsynaptic potentials from multiple synapses excited in series by the passage of the pacemaker neuron signal. 3. The threshold for the production of calcium spikes by the slow EPSPs of the pacemaker system (-51 +/- 2.2 mV, mean +/- SD; n = 5) corresponded well with the voltage at which a net inward "T"-type calcium current first appeared in recordings from axon membrane patches (-55 to -50 mV); the threshold for the initiation of the sodium spike by the fast EPSPs of the ring giant system (-32 +/- 1.2 mV, mean +/- SD; n = 6) corresponded well with the voltage at which a net inward sodium current first appeared (-35 to -30 mV). 4. Inward currents were rarely observed in membrane patches formed using pipettes with tips of < 1 micron OD. Even with 4-micron pipettes, patches of membrane were sometimes obtained with a channel population consisting exclusively of potassium channels; calcium and sodium currents were found in highly discrete areas ("hot spots"). Preliminary findings on the undersurface of the axon, which makes synaptic contact with the myoepithelium, are consistent with a similar distribution. 5. The pathway by which the ring giant excites the motor giant axon is not definitely known. The synaptic delay between the peak of the ring giant action potential (monitored externally) and the initial rise of the fast EPSP (1.64 +/- 0.15 ms, mean +/- SD; n = 21) would allow for transmission at two synapses, because single synaptic delays at neuromuscular junctions in Aglantha are approximately 0.7 ms at 12 degrees C. The mean synaptic delay at the slow EPSP synapse was 0.88 +/- 0.09 (SD) ms (n = 12). 6. The delay between the impulse in the ring giant axon and the subsequent excitation of the motor giant axon may permit the animal to withdraw its tentacles and so lower the drag that would otherwise reduce the effectiveness of any escape swim and might induce tentacle autotomy.

Potassium channel family in giant motor axons of Aglantha digitale.

1. The simplicity of the jellyfish nervous system makes it an ideal preparation to assess the contributions of different ion channels to behavior. In the giant motor axons of the jellyfish Aglantha digitale, low-threshold spikes elicit slow swimming, whereas escape swimming depends on a higher-threshold, overshooting sodium-dependent action potential. At least three kinetically distinct transient potassium channels (fast, intermediate, and slow) are concerned with spike management in this preparation. 2. In situ recording with patch-clamp micropipettes from clusters of potassium channels provides a means of studying their properties in isolation. The three classes of ion channel were identified in ensemble current averages by their kinetics, their response to a conditioning prepulse and their voltage dependence. All three were highly selective for potassium, and the reversal potential of their unitary currents depended on the level of potassium used to fill the patch pipette. 3. A single potassium permeability coefficient (PK) calculated from the Goldman, Hodgkin, Katz "constant field" equation was used to fit unitary current data from all three channels in concentrations of external potassium < or = 500 mM. 4. Data from ensemble tail currents in seawater indicated that the sodium permeability coefficient (PNa) of channels with either intermediate or slow kinetics was < or = 0.015 PK; preliminary data from channels with fast kinetics suggested that they too had a PNa/PK selectivity of approximately 0.01. 5. We propose that spike management in the giant motor axons of Aglantha depends on three members of a family of potassium-selective ion channels that seem likely to be structurally related.

Ionic currents in giant motor axons of the jellyfish, Aglantha digitale.

1. In the motor system of the jellyfish, Aglantha digitale, there are eight giant axons connected by chemical synapses to a muscle epithelium. The simplicity of this structure makes it possible to assess the contribution of different ion conductances in the axon membrane to the two forms of swimming that provide the behavioral output of the system. In situ recordings from large clusters of ion channels provide a means of studying these membrane conductances in isolation so that the features that permit them to perform their behavioral function may be identified. 2. In Aglantha motor axons, low-amplitude, low-threshold spikes are associated with slow swimming, whereas escape swimming depends on a higher-threshold, overshooting action potential. The action potential was abolished by a sodium-free (choline-containing) bathing medium but was resistant to tetrodotoxin (0.09 mM; 3 x 10(-5) g/ml). It was prolonged by tetraethylammonium (TEA) ions (50 mM) but little affected by changes in holding potential in the range of -51 to -82 mV. The low-threshold spikes were unaffected by sodium-free saline containing TEA (30 mM). They were inactivated by holding the membrane potential at -51 mV. Average axon resting potentials were -63 +/- 6 (SD) mV (n = 17). 3. Shortened axons studied with the two-electrode voltage-clamp technique had a transient inward current with a low threshold for activation (about -60 mV). The inward current was fully inactivated at -51 mV; it was present in sodium-free saline and abolished by Mg2+ (120 mM) just like the low-threshold spike. 4. Calcium-dependent low-threshold spikes and sodium action potentials coexist in the same axons but may be elicited separately because an outward current limits the peak of the low-threshold spike to a level below the threshold of the action potential (about -20 mV). 5. Analysis of ensemble currents showed that axon-attached membrane patches contained clusters of different voltage-dependent potassium channels. Three channel classes were distinguished by prepulse inactivation experiments. All three channels were found to inactivate, but they had different voltage-dependencies and different inactivation kinetics (fast, intermediate, or slow). Recovery from inactivation was slow in each case (time constant 2-10 s). 6. All axon-attached membrane patches were found to contain one or two of the three classes of potassium channel. Channels with intermediate kinetics were found less frequently and may have been present at lower density.(ABSTRACT TRUNCATED AT 400 WORDS)

Giant Axons and Escape Swimming in Euplokamis dunlapae (Ctenophora: Cydippida).

Euplokamis dunlapae responds to anterior stimulation by reversing the beat direction of its comb plate cilia and swimming rapidly backwards. It responds to posterior stimulation by swimming forwards at an accelerated rate. Video playback and laser monitoring were used to analyze changes in the pattern of ciliary beating, while electrical activity was recorded extracellularly. Escape responses occur with latencies of less than 150 ms and involve greatly increased ciliary beat frequencies. Giant axons run longitudinally along each of the eight comb rows, as shown by optical and electron microscopy. They form chains of overlapping neurons, with diameters of about 12 µm in life, and conducting at over 50 cm · s-1 as recorded with an extracellular electrode placed directly over the chain. The giant neurons are synaptically linked with smaller neurites of the general ectodermal nerve plexus, with each other, and with the ciliated cells of the comb plates. They appear to constitute a single system mediating rapid conduction of signals in either direction, but a full analysis was not attempted for lack of sufficient material. Electro-physiological examination of two other ctenophores (Pleurobrachia and Beroe) gives no indication of rapid conduction pathways, and these forms probably lack giant axons.

Development of Giant Motor Axons and Neural Control of Escape Responses in Squid Embryos and Hatchlings.

Anatomical development of the third-order giant axons was studied in conjunction with ontogeny of the escape response and the underlying neural control. Stimulated escape jetting appears at stage 26 (Segawa et al., 1988); such responses are driven solely by a small axon motor system. Giant axons become morphologically identifiable in the more posterior stellar nerves that effect jetting by stage 28, and electrical activity in the stellate ganglia associated with the giant axons is first recordable at this time. Maturation of the giant axons is accompanied by a marked improvement in temporal aspects of escape behavior up to the time of hatching. In embryonic and hatchling Loligo, all escape responses, regardless of the mode of stimulation, are fast-start responses with latencies less than the minimum value displayed by adults (50 ms). Giant axon activity recorded in the stellate ganglion always precedes small axon motor activity; this is not true for adults which display two distinct modes of giant axon use. Both giant and non-giant motor systems are thus functional in embryonic and hatchling squid, and both contribute to escape jetting. However, these animals do not yet display the concerted interplay of the two motor systems characteristic of adults.

On the nervous system of Velella (hydrozoa: Chondrophora).

Silver impregnations, immunofluorescence microscopy, and electron microscopy of the nervous system of Velella confirm previous reports that there are two nerve nets, one composed of small and the other of "giant" neurites. Only one of these systems, the small-fibered open one, shows FMRFamide-like immunoreactivity. It appears to be primarily a sensory network. Despite presence of a neuropeptide in these neurons, they did not contain dense-cored vesicles. The "giant" nerve net (closed system) shows many connections that appear syncytial in the silver preparations. While it is confirmed that gap junctions are present between some neurites in the closed system, it is likely that fusion of neurites also occurs and that the system is a partial syncytium. Membrane complexes with gap junctions are abundant in the cytoplasm. It is suggested that fusion occurs by the engulfment of small neurons by large, resulting in an excess of cell membrane, which is internalized with gap junctions still intact. These internalized membranes appear to break up into vesicles eventually. A similar process may occur in the "giant" swimming motor neuron net of the medusa Polyorchis.

IMPULSE PROPAGATION AND CONTRACTION IN THE TUNIC OF A COMPOUND ASCIDIAN.

Diplosoma listerianum and D. macdonaldi (Fam. Didemnidae) have a network of cells ("monocytes") in the tunic which contain high concentrations of microfilaments and react positively with NBD-phallacidin, indicating the presence of F-actin. The tunic is contractile, especially in the areas around the cloacal apertures, which can be closed completely. Myocytes are concentrated in sphincter-like bundles around these openings, but also are found throughout the tunic. Electrophysiological recordings reveal a diffuse conduction system in the tunic propagating all-or-none impulses ("tunic potentials," TPs) through all parts with a conduction velocity of < 1.5 cm · s-1, and a refractory period of 1.6 s. TPs correlate one-for-one with contractions. The system is excitable to the touch, but is also spontaneously active, showing steady patterns of potentials as well as regular, `parabolic' bursts. The evidence suggests that the myocyte net itself conducts the impulses triggering the contractions. In the absence of conventional nerves and muscles, the system provides the colony with a way of regulating the effluent water current and hence the volume of a common cloacal space. The TP system is not `wired in' to the ascidiozooids either as a sensory or as a motor pathway. The tunic acts as an independent behavioral entity.

NEURONAL CONTROL OF CILIARY LOCOMOTION IN A GASTROPOD VELIGER (CALLIOSTOMA).

Intracellular recordings from pre-oral ciliated cells of competent Calliostoma ligatum veligers were used to demonstrate the mechanisms of neuronal control of ciliary locomotion. During normal ciliary beating at 5-7 Hz, the membrane potential shows no oscillations or spiking activity. It remains at a resting potential of about -60 mV. Depolarization from resting potential is due to excitatory input from the CNS and, depending upon the kind of input, veligers appear to show two types of locomotory behavior. In one type, normal ciliary beating is periodically interrupted by rapid, velum-wide ciliary arrests. These arrests are caused by a propagated, Ca++-dependent action potential in the pre-oral ciliated cells. The second type is characterized by either a velum-wide or local slowing of normal ciliary beating, and appears to result from a slow depolarization of the ciliated cell membrane. Pre-oral ciliated cells are electrically coupled to each other. This property may ensure the synchrony of velum-wide ciliary arrests or differential velar slowing of ciliary beating. These findings demonstrate some of the mechanisms ofthe fine control veligers possess over their locomotory and feeding behavior.

Neuromuscular transmission in the jellyfish Aglantha digitale.

In the jellyfish Aglantha digitale escape swimming is mediated by the nearly synchronous activity of eight giant motor axons which make direct synaptic contact with contractile myoepithelial cells on the under-surface of the body wall. The delay in transmission at these synapses was 0.7 +/- 0.1 ms (+/- S.D.;N = 6) at 12 degrees C as measured from intracellular records. Transmission depended on the presence of Ca2+ in the bathing medium. It was not blocked by increasing the level of Mg2+ to 127 mmol l-1. The myoepithelium is a thin sheet of electrically coupled cells and injection of current at one point was found to depolarize the surrounding cells. The potential change declined with distance from the current source as expected for two-dimensional current spread. The two-dimensional space constant (lambda) was 770 micron for current flow in the circular direction and 177 micron for radial flow. The internal resistance of the epithelium (178-201 omega cm) and the membrane time constant (5-10 ms) were direction independent. No propagated epithelial action potentials were observed. Spontaneous miniature synaptic potentials of similar amplitude and rise-time were recorded intracellularly at distances of up to 1 mm from the motor giant axon. Ultrastructural evidence confirms that neuro-myoepithelial synapses also occur away from the giant axons. It is likely that synaptic sites are widespread in the myoepithelium, probably associated with the lateral motor neurones as well as the giant axons. Local stimulation of lateral motor neurones generally produced contraction in distinct fields. We suppose that stimulation of a single motor giant axon excites a whole population of lateral motor neurones and hence a broad area of the myoepithelium.

Separate sodium and calcium spikes in the same axon.

Aglantha digitale is a jellyfish (order Hydromedusae) capable of two distinct kinds of locomotion; 'slow' swimming which is generated endogenously and is used in fishing behaviour, and 'fast' swimming which is evoked by predators and serves for escape. Both forms of swimming are produced by contraction of the bell-shaped body wall and expulsion of a jet of water from an opening at the base of the animal. During slow swimming, the contractions are weak and the animal moves about 15 mm, roughly one body length, but during a fast swim there is a more violent contraction which can propel the animal five times as far. Both forms of contraction depend on impulses in the eight giant motor axons that synapse directly with the muscle sheet making up the inner surface of the body wall. We report here that the giant motor axons are able to mediate both kinds of activity because they can conduct two different sorts of impulse. Fast swimming requires a rapidly conducted Na+-dependent action potential whereas slow swimming depends on a low amplitude Ca2+ 'spike'. This is the first report of an axon capable of two kinds of impulse propagation and it provides a physiological function for low potential Ca2+ activation.