Nerve net

Nerve net
Nettle Jelly

A nerve net is a type of simple nervous system that is found in members of the cnidaria, ctenophora, and echinodermata phyla. Nerve nets consist of interconnected neurons lacking a brain or any form of cephalization. This nervous system allows cnidarians to respond to physical contact. They may then detect food and other chemicals in a rudimentary way. Although the nerve net allows the animal to respond to its environment, it has trouble alerting the animal from where the stimulus is coming. For this reason, simple animals with nerve nets, such as hydra, will typically respond in the same way to contact with an object, regardless of where the contact occurs.

Hydra, which are cnidarians, have a nerve net throughout their body. On the other hand, Sea stars, which are echinoderms, have a nerve net in each arm, connected by a central radial nerve ring at the center. This is better suited to controlling more complex movements than a diffuse nerve net.

Contents

Physiology of Nerve Nets

A nerve net is a diffuse network of cells that can congregate to form ganglia in some organisms, but does not constitute a brain. Therefore, each sensory neuron responds to each stimulus, like odors or tactile stimuli. Signaling happens at synapses using chemical stimulants. The motor neurons communicate with cells via chemical synapse to produce a certain reaction to a certain stimuli. A stronger stimulus produces a stronger reaction from the organism and vice versa. In a typical unmyelinated axon, the action potential is conducted at a rate of about 5 meters per second, compared to a mylinated human neural fiber which conducts at around 120 meters per second.[1]

While nerve nets use hormones, the total physiology isn’t very well understood. Hormones normally found in vertebrates have been identified in nerve net tissues.[2] Whether they serve the same function as those found in vertebrates isn’t known and little research has been performed to solve the question. Hormones such as steroids, neuropeptides, indolamines, and other iodinated organic compounds have been seen in tissues of cnidarians. These hormones play a role in multiple pathways in vertebrae neurophysiology and endocrine system including reward and complex biochemical stimulation pathways for regulation of lipid synthesis or similar sex steroids.[2]

Since cnidarian cells are not organized into organ systems it is difficult to assume the role of the endocrine-nerve net system employed by these types of species. A nerve net is considered to be a separate structure in the cnidarians and is associated with signal molecules, it’s primarily considered a neurochemical pathway. Potential signal molecules have been noted in certain nerve net anatomy. How the signal molecules work is not known. It has been shown, however, that the nematocyst (stinging) response is not related to nerve activity.[3]

Nerve Net Evolution

Evolutionary ancestors of the cnidarians, such as sponges, do not have nervous tissue however do contain homologs of several genes which are important in nerve formation.[4] Sponge cells can communicate with each other via calcium signalling, or by other means; new research suggests that sponges express a group of proteins which conglomerate together to produce a partly synaptic formation.[1] It is possible that cnidarian nerve cells came from these formations and formed into a diffuse nerve net.[5] Nerve nets are found in species in the phylum Cnidaria (e.g. true jellys, box jellyfish, and sea anenomes), Echinodermata (Sea Stars), and Ctenophora (comb jellys).

Despite the lack of a true brain, animals with nerve nets can display quite complex behaviors. The cnidarian box jellyfish has a more complex nervous system than its cousin the true jelly. Box jellies actively hunt their prey, have a concentrated visual ganglia, and even display a sleeping pattern.[6]

See also

References

  1. ^ a b Jacobs DK1, Nakanishi N, Yuan D, et al. (2007). "Evolution of sensory structures in basal metazoa". Integr Comp Biol 47 (5): 712–723. doi:10.1093/icb/icm094. http://icb.oxfordjournals.org/cgi/content/full/47/5/712. 
  2. ^ a b Tarrant, Ann M., et al. (2005). "Endocrine-like Signaling in Cnidarians: Current Understanding and Implications for Ecophysiology". Integr. Comp. Biol. 45 (1): 201–214. 
  3. ^ Ruppert, E.E., Fox, R.S., and Barnes, R.D. (2004). Invertebrate Zoology (7 ed.). Brooks / Cole. pp. 76–97. ISBN 0030259827. 
  4. ^ Sakarya O, Armstrong KA, Adamska M, et al. (2007). Vosshall, Leslie. ed. "A post-synaptic scaffold at the origin of the animal kingdom". PLoS ONE 2 (6): e506. doi:10.1371/journal.pone.0000506. PMC 1876816. PMID 17551586. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1876816. 
  5. ^ Koizumi, O, et al. (2007). "Nerve Ring of the Hypostome in Hydra: Is It an Origin of the Central Nervous System of Bilaterian Animals?". Brain and behavior : proceedings of the ... Conference / [Conference on Brain and Behavior] American Institute of Biological Sciences 69 (2): 151. 
  6. ^ Seymour, Jamie E; Teresa J Carrette and Paul A Sutherland (2004). "Do box Jellyfish Sleep?". Medical Journal of Australia 181 (11/12): 707. http://www.mja.com.au/public/issues/181_11_061204/sey10757_fm.html. 
  • Kass-Simon G, Pierobon P. Cnidarian chemical neurotransmission, an updated overview. Comp Biochem Physiol A Mol Integr Physiol. 2007 Jan;146(1):9-25. Epub 2006 Sep 22. PMID 17101286
  • Miljkovic-Licina M, Gauchat D, Galliot B. Neuronal evolution: analysis of regulatory genes in a first-evolved nervous system, the hydra nervous system. Biosystems. 2004 Aug-Oct;76(1-3):75-87. PMID 15351132
  • Koizumi O, Mizumoto H, Sugiyama T, Bode HR. Nerve net formation in the primitive nervous system of Hydra—an overview. Neurosci Res Suppl. 1990;13:S165-70. PMID 2259484
  • Shimizu H. Feeding and wounding responses in Hydra suggest functional and structural polarization of the tentacle nervous system. Comp Biochem Physiol A Mol Integr Physiol. 2002 Mar;131(3):669-74. PMID 11867292
  • Sakaguchi M, Mizusina A, Kobayakawa Y. Structure, development, and maintenance of the nerve net of the body column in Hydra. J Comp Neurol. 1996 Sep 9;373(1):41-54. PMID 8876461
  • Gillis MA, Anctil M. Monoamine release by neurons of a primitive nervous system: an amperometric study. J Neurochem. 2001 Mar;76(6):1774-84. PMID 11259495
  • Grimmelikhuijzen CJ, Westfall JA. The nervous systems of cnidarians. EXS. 1995;72:7-24. PMID 7833621

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