Neuroscience Blog

see a larger version of the animation in high quality here

The nerve rings found  in cnidarians  can be considered one  of the first attempts of centralisation and an adaptation to the radial form . An interesting type of neurons in Cnidarians are those with giant axons. Giant axons are distinguishable from normal axons by their large diameter and relatively high speed of  signal conduction.

This giant axons form the motor giants and the ring giants in many jellysh species. Regarding the circuitry, Aglantha has the most complicated nervous system (in the jellyfish group). Also it seems that neuroendocrinology has much to reveal in this animal group, showing an unexpected complexity. Also, the giant axons in A. digitale can conduct

two types of action potentials: Ca2+ spikes and Na + spikes. Ca2+ spikes are generated during 

slow swimming, while Na + spikes are generated during the escape behavior. Na+ spikes are generated by fast-rising excitatory postsynaptic potentials (PSPs) representing input from the ring giant (RG) axon, while Ca2+ spikes arise from slow PSPs representing input from the pacemaker (P) system. Also, Mackie and Meech (1995) demonstrated the existence of two sets of interneurons, the relay and carrier systems. They transfer information from the pacemaker neurons to the tentacle action system during slow swimming. There is also a rootlet system which was first described by Weber et al. (1982). Also there is a small axon bundle connecting the margin and manubrium and it is a possible pathway that mediates the feeding behaviour in some species. In A. digitale, between the margin and the manubrium, there is an E system . This E system - that lies in the immediate vicinity of the endodermal radial canals - mediates lip aring, while the F system lies in the ectoderm. The F system is overlying the radial canals and mediates pointing [1]. The endodermal epithelium itself is an excitable tissue. In P. penicillatus, in the endodermal epithelium the impulses are transmitted via gap junctions (King and Spencer, 1979). This may also be true in Aglantha. A bundle of small axons runs up the subumbrella from the margin. They are placed close to the motor giant axon. The small axon bundles follow the motor giant path, then their course continue around the apex, head down the peduncle and enter the manubrium . Axons in these bundles show FMRFamide-like immunoreactivity (FaIR).

Electron microscopy failed to show synapses between the axons in the small axon

bundle or between them and other cells. The canals of the jellysh may be involved

in impulses transmission [2].

The ring giant lies in parallel with the much smaller axons (up to 6 fim) of the outer

nerve-ring and receives synaptic contacts from some of these. The ring giant axon is

covered, at the exterior, by a single layer (4-5 m thick) of epithelial cells [3].

The second type of giant axon runs from the margin up the inside of the bell in the

myoepithelium. They parallel each of the eight radial digestive canals. The term

used to call this nerve tracts is motor giant axons . Each motor giant axon is accompanied

by a number of smaller axons, some of which contribute to a nerve plexus in

the subumbrellar myoepithelium.

In jellysh there are two nerve rings: the subumbrellar inner nerve ring and the

exumbrellar outer nerve ring. Both nerve rings are found at the junction of the swimming

bell and the velum, a narrow ap of muscular tissue.