Fast axonal transport by neurons from the jellyfish Cyanea capillata.

Neurons of the motor nerve net of Cyanea capillata were examined using video-enhanced DIC optics. A variety of organelles were visible within the axons and many were mobile. To quantify the movement organelles were divided into three classes (large, medium, and small) and the rates, direction, and types of movement displayed by the different particle types examined. The overall behavior and rates of movement of transported particles were comparable with those in axons from other species. The largest particles, mainly mitochondria were the slowest moving but were the only particles to reverse their direction of movement or to undergo interactions with other particles. The fastest movement was by the small particles, but both they and medium sized particles were transported continuously. In addition, the linear elements in these axons underwent considerable lateral movement.

Action potential in neurons of motor nerve net of Cyanea (Coelenterata).

Neurons of the motor nerve net of the jellyfish Cyanea were impaled with microelectrodes for intracellular recordings. The cells have conventional, negative resting potentials and produce variable-amplitude action potentials with complex waveforms. The variability and complexity of these spikes is due to the superimposition of two classes of Ca2+-dependent potentials on an otherwise fast, clean action potential. Repetitive stimulation and ionic manipulation reveal that most superimposed potentials are chemically induced excitatory postsynaptic potentials (EPSPs). These account for the complexity and variability of the action potential. The remaining potential is interpreted as a Ca2+ component of the action potential. The action potential is a Na+-dependent but tetrodotoxin- (TTX) insensitive event. Repolarization is achieved by two pharmacologically distinct mechanisms: a tetraethylammonium- (TEA) and 4-amino-pyridine- (4-AP) sensitive K+ efflux and a delayed, Ca2+-activated, K+ efflux. The latter is responsible for the afterhyperpolarization that follows the action potential. The results indicated that these neurons are physiologically conventional. This is interesting in view of the phylogenetic primitiveness of the preparation and important, since it means that this preparation can provide generally useful information on chemical synaptic physiology.

Lability of conduction velocity during repetitive activation of an excitable epithelium.

Conduction velocity lability was studied in the electrically excitable epithelium of Euphysa japonica by means of intracellular recordings. Three classes of response latency change were identified in response to bursts of stimuli: an initial jump, uniform drift and abrupt jumps in latency. In each case an increase in stimulus frequency produced an increase in latency. The initial jump in latency, which occurred between the first and second response of a series, was related to the afterpotential of the first response. The increased latency of the second response appears to result from the drop in membrane resistance during the hyperpolarizing afterpotential. The uniform drift in latency remains unexplained but may be the result of ion accumulation within the tissue, progressive inactivation of the ionic channels involved in producing the action potential, or junctional phenomena. The abrupt jumps in latency, which often preceded failure to respond, were found to be impulse initiation phenomena.

Electrical properties of an excitable epithelium.

The exumbrellar epithelium of the hydromedusa, Euphysa japonica, is composed of a single layer of broad (70 micrometers), thin (1--2 micrometers) cells which are joined by gap junctions and simple appositions. Although the epithelium lacks nerves, it is excitable; electrically stimulating the epithelium initiates a propagated action potential. The average resting potential of the epithelial cells is -46 mV. The action potential, recorded with an intracellular electrode, is an all-or-nothing, positive, overshooting spike. The epithelial cells are electrically coupled. The passive electrical properties of the epithelium were determined from the decrement in membrane hyperpolarization with distance from an intracellular, positive current source. The two-dimensional space constant of the epithelium is 1.3 mm, the internal longitudinal resistivity of the cytoplasm and intercellular junctions is 196 omega cm, and the resistivity of both apical and basal cell membranes is greater than 23 k omega cm2. Although the membrane resistivity is high, the transverse resistivity of the epithelium is quite low (7.5 omega cm2), indicating that the epithelium is leaky with a large, transverse, paracellular shunt.

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.

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.