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What is a Nerve Net?

Instructor: Sarah Phenix
In this lesson we will explore what a nerve net is, how the body plan of an organism directly relates to its neural design, as well as the unique characteristics of this neural design.

What is a Nerve System?

Humans have a very complex nervous system composed of our 'supercomputer brains', the high-speed communication highway of our spinal cord, and an extensive network of nerves that spiderweb throughout our bodies. This complexity stems from the basic trait that we are cephalized organisms, meaning that our nervous tissue is concentrated in our cephalic (head) region. This may not seem important, but it's a trait that all cognitively advanced organisms such as dogs, whales, monkeys, birds and even octopi share. Now, this may make you wonder how more simplistic organisms such as the jellyfish function. If so, today is your lucky day, because that is the question we are here to answer!

Body Symmetry & The Nervous System

Body Plan Symmetry
Body plan symmetry

Body symmetry plays a big role in the distribution and organization of the nervous system. We, as well as all other cephalized organisms, are bilaterally symmetrical, meaning that our right and left sides are mirror images of each other. But this isn't the only body shape that exists. Many organisms, such as the group known as cnidarians (organisms with stinging cnidocyte cells such as sea anemones, corals, and jellyfish), are radially symmetrical, meaning their bodies are arranged circularly around a central gastrointestinal core.

Why does this matter? Bilateral symmetry means that both the right and left sides can easily be controlled by a single nerve cord centralized down the midline of the body and by a single brain or brain-like collection of nerve cells. Radial body plans, however, lack a centralized brain and require 360 degrees of nervous system control. Due to this fact, radially symmetrical organisms, like sea stars and jellyfish, have either radial nerves, which are nerves that radiate from a central ring and run down each appendage as sea stars do, or nerve nets, a network of nerves that drapes throughout the skin, almost like a fisherman's net or volleyball net were suspended in their tissue.

Jellyfish & Nerve Nets

Jellyfish Anatomy
True Jellyfish

What's important to remember is that radially symmetrical animals, like the jellyfish, don't have a central 'supercomputer' brain. Now, we humans take in sensory information from our environment that is then sent to our brains for interpretation via a unidirectional (one-way) pathway. Our brains interpret the information and then send out motor response via a totally different unidirectional pathway. Jellyfish, on the other hand, have only one pathway for both sensation and motor signals, which is bidirectional.

To get a better idea of how this bidirectional pathway works, imagine the ripple effect of a single droplet of water falling on a still pond. In the same way, a 'droplet', or cue from the jellyfish's environment, such as a chemical signal or motion in the surrounding waters, excites sensory receptors, sending out a ripple-like wave through the neural net. The jellyfish immediately respond by stimulating contractile and pacemaker cells in their bell that results in their characteristic pulsing, rhythmic swimming pattern either towards or away from the stimulus.

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