Nerve net
Encyclopedia
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 neuron
s 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.
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. 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.
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.
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.
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
that is found in members of the cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...
, ctenophora, and echinodermata phyla. Nerve nets consist of interconnected neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
s lacking a brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
or any form of cephalization
Cephalization
Cephalization is considered an evolutionary trend, whereby nervous tissue, over many generations, becomes concentrated toward one end of an organism. This process eventually produces a head region with sensory organs....
. 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
Stimulus (physiology)
In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....
is coming. For this reason, simple animals with nerve nets, such as hydra
Hydra (genus)
Hydra is a genus of simple fresh-water animal possessing radial symmetry. Hydras are predatory animals belonging to the phylum Cnidaria and the class Hydrozoa. They can be found in most unpolluted fresh-water ponds, lakes, and streams in the temperate and tropical regions and can be found by...
, will typically respond in the same way to contact with an object, regardless of where the contact occurs.
Hydra
Hydra (genus)
Hydra is a genus of simple fresh-water animal possessing radial symmetry. Hydras are predatory animals belonging to the phylum Cnidaria and the class Hydrozoa. They can be found in most unpolluted fresh-water ponds, lakes, and streams in the temperate and tropical regions and can be found by...
, 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.
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.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. 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
Indolamines
Indolamines are a family of neurotransmitters that share a common molecular structure . A common example of an indolamine is serotonin,a neurotransmitter involved in mood and sleep...
, 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.
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.
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. 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. It is possible that cnidarian nerve cells came from these formations and formed into a diffuse nerve net.Nerve nets are found in species in the phylum Cnidaria (e.g. true jellys
Scyphozoa
Scyphozoa is a class within the phylum Cnidaria, sometimes referred to as the "true jellyfish".The class name Scyphozoa comes from the Greek word skyphos , denoting a kind of drinking cup and alluding to the cup shape of the organism....
, box jellyfish
Box jellyfish
Box jellyfish are cnidarian invertebrates distinguished by their cube-shaped medusae. Box jellyfish are known for the extremely potent venom produced by some species: Chironex fleckeri, Carukia barnesi and Malo kingi are among the most venomous creatures in the world...
, 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.