Supplement to Module 2

The Cnidarian Nervous System

Those weird little jellyfish.

The Cnidarian nervous system is very unique in its organization and function. Why? What is so different about a jellyfish from say an octopus, an earthworm or a human?

The nerve net is the name applied to the cnidarian nervous system. Picture a fishing net with the ropes intertwined with each other to produce a web-like sheet. This is how the cnidarian nervous system is organized. Whenever a neuron crosses another neuron the two communicate = en passant synapses. This is atypical compared to the nervous system organization seen in other groups of organisms. For example, in human brains the neurons are so dense that millions are crossing over each other millions of times. But the neurons do not communicate indiscriminately at these cross-over points, they just simply lie one ontop another with no communication. Communication between human neurons occurs at very specific areas and only between certain neurons. In the Cnidaria all neurons can communicate with all other neurons whenever they cross. So how on earth can this seemingly random organization function effectively? Well, we don't completely understand it even yet, but it does. There appear to be at least 3 specific pathways that signals can take within the nerve net, each pathway has a characteristic conduction speed associated with it and seems to control different behavioral responses to a given stimulus.

		Sea Anemone		Jellyfish		Hydra

Another strange and unique feature you would notice if you were studying the nerve net is that a stimulus at any point on the organism triggers an impulse that radiates out away from the stimulus site in every direction. There doesn't seem to be a preferred direction or pathway that the impulses take. This is because Cnidarian neurons are bipolar in that they can propagate an AP in either direction. This entails that at each en passant synapse, either neuron may be the pre-synaptic neuron and release neurotransmitters or the post-synaptic neuron and bind neurotransmitters depending on which way the stimulus was initiated.

It is this bipolar nature of Cnidarian neurons that leads to the "echo effect" seen in the jellyfish Cyanea. Think of the way an echo works, you are standing at the edge of the Grand Canyon and you yell, "HELLO!". About a second later you here your own voice echoing back at you. What has happened is the sound waves from you initial "HELLO" hit the wall of the Grand Canyon and were bounced back at you, so you heard your own voice but after a short delay. This is what's happening in an en passant synapse in the jellyfish. In the figure below neuron #2 is being stimulated with progressively increasingly larger stimuli (record A-D). You can see the epsp that results from this subthreshold stimulus each time. I know it's subthreshold, because otherwise an AP would result. Finally in record D the stimulus was made large enough to surpass threshold and trigger an AP in neuron #2. It is at this point that synaptic transmission with neuron #1 occurs. Due to sufficiently large synaptic activity from neuron #2, neuron #1 also surpasses threshold and fires an AP (this is all seen in record D). Records E-G are replications of the situation that produced record D. The echo effect begins to appear in E (see arrowhead), but is quite clear in record G. Because both neuron #1 and #2 are bipolar and can transmit APs in both directions, sufficiently large stimuli produce an echo of the initial stimuli back in the direction it propagated.

The bioluminescence web page (our lab's web site) has some really good pictures of jellyfish. If you want to see some color photos of other jellies, visit the site.