Nerve net

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A nerve net consists of interconnected neurons lacking a brain or any form of cephalization. While organisms with bilateral body symmetry are normally associated with a condensation of neurons or, in more advanced forms, a central nervous system, organisms with radial symmetry are associated with nerve nets, and are found in members of the Ctenophora, Cnidaria, and Echinodermata phyla, all of which are found in marine environments. In the Xenacoelomorpha, a phylum of bilaterally symmetrical animals, members of the subphylum Xenoturbellida also possess a nerve net. [1] Nerve nets can provide animals with the ability to sense objects through the use of the sensory neurons within the nerve net.

Contents

It also exists in several other phyla, like chordates, annelids and flatworms, but then always alongside longitudinal nerve(s) and/or a brain. [2]

The nerve net is the simplest form of a nervous system found in multicellular organisms. Unlike central nervous systems, where neurons are typically grouped together, neurons found in nerve nets are spread apart. This nervous system allows cnidarians to respond to physical contact. They can detect food and other chemicals in a rudimentary way. While the nerve net allows the organism to respond to its environment, it does not serve as a means by which the organism can detect the source of the stimulus. For this reason, simple animals with nerve nets, such as Hydra, will typically produce the same motor output in response to contact with a stimulus regardless of the point of contact.

The anatomy and positioning of nerve nets can vary from organism to organism. 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.

Evolution

The emergence of true nervous tissue was once thought to have followed the divergence of last common ancestor of Porifera (sponges) and Cnidaria and Ctenophora. Recent taxonomic divisions, however, suggest that Ctenophora is sister to the other extant Metazoa. [3] [4] [5] [6]

Porifera is an extant phylum within the animal kingdom, and species belonging to this phylum do not have nervous systems. The placement of Ctenophora implies that either nervous systems were lost in the ancestor of Porifera, or they evolved independently in the ancestors of Ctenophora and ParaHoxozoa. Although Porifera do not form synapses and myofibrils which allow for neuromuscular transmission, they do differentiate a proto-neuronal system and contain homologs of several genes found in Cnidaria which are important in nerve formation. [7] Sponge cells have the ability to communicate with each other via calcium signaling or by other means. [8] Sponge larvae differentiate sensory cells which respond to stimuli including light, gravity, and water movement, all of which increase the fitness of the organism. In addition to sensory cells differentiated during development, adult Porifera display contractile activity. [9]

The emergence of nervous systems has been linked to the evolution of voltage-gated sodium (Nav) channels. The Nav channels allow for communication between cells over long distances through the propagation of action potentials, whereas voltage-gated (Cav) calcium channels allow for unmodulated intercellular signaling. It has been hypothesized that Nav channels differentiated from Cav channels either at the emergence of nervous systems or before the emergence of multicellular organisms, although the origin of Nav channels in history remains unknown. Porifera either came about as a result of the divergence with Cnidaria and Ctenophora or they lost the function of the gene encoding Nav channels. As a result, Porifera contain Cav channels which allows for intercellular signaling, but they lack Nav channels which provide for the conductance of action potentials in nerve nets. [10]

Nerve nets are found in species in the phyla Cnidaria (e.g. scyphozoa, box jellyfish, and sea anemones), Ctenophora, and Echinodermata. Cnidaria and Ctenophora both exhibit radial symmetry and are collectively known as coelenterates. Coelenterates diverged 570 million years ago, prior to the Cambrian explosion, and they are the first two phyla to possess nervous systems which differentiate during development and communicate by synaptic conduction. Most research on the evolution of nervous tissue concerning nerve nets has been conducted using cnidarians. The nervous systems of coelenterates allow for sensation, contraction, locomotion, and hunting/feeding behaviors. Coelenterates and bilaterians share common neurophysiological mechanisms; as such, coelenterates provide a model system for tracing the origins of neurogenesis. This is due to the first appearance of neurogenesis having occurred in eumetazoa, which was a common ancestor of coelenterates and bilaterians. A second wave of neurogenesis occurred after the divergence of coelenterata in the common ancestor of bilateria. [9] Although animals with nerve nets lack a true brain, they have the ability to display complex movements and behaviors. The presence of a nerve net allows an organism belonging to the aforementioned phyla of Cnidaria, Ctenophora, and Echinodermata to have increased fitness as a result of being able to respond to their environment.

Developmental neurogenesis

Developmental neurogenesis of nerve nets is conserved between phyla and has been mainly studied in cnidaria, especially in the model organism Hydra. The following discusses the development of the nerve net in Cnidaria, but the same mechanism for the differentiation of nervous tissue is seen in Ctenophora and Echinodermata.

Cnidaria develop from two layers of tissue, the ectoderm and the endoderm, and are thus termed diploblasts. The ectoderm and the endoderm are separated by an extra-cellular matrix layer called the mesoglea. Cnidaria begin to differentiate their nervous systems in the late gastrula. [9] In Hydrozoa and Anthozoa, interstitial stem cells from the endoderm generate neuroblasts and nematoblasts which migrate to the ectoderm and provide for the formation of the nervous system along the anterior-posterior axis. Non-hydrozoa lack interstitial stem cells, and the neurons arise from epithelial cells, which are most likely differentiated from the ectoderm as occurs in vertebrates. Differentiation occurs near the aboral pore and this is where most neurons remain. [11]

In Cnidaria larvae, neurons are not distributed homogenously along the anterior-posterior axis; Cnidaria demonstrate anatomical polarities during the differentiation of a nervous system. There are two main hypotheses that attempt to explain neuronal cell differentiation. The zootype hypothesis says that regulatory genes define an anterior-posterior axis and the urbilateria hypothesis says that genes specify a dorsal-ventral axis. Experiments suggest that developmental neurogenesis is controlled along the anterior-posterior axis. The mechanism by which this occurs is similar to that concerning the anterior to posterior patterning of the central nervous systems in bilaterians. The conservation of the development of neuronal tissue along the anterior-posterior axis provides insight into the evolutionary divergence of coelenterates and bilaterians. [11]

Neurogenesis occurs in Cnidaria not only during developmental stages, but also in adults. Hydra, a genus belonging to Cnidaria, is used as a model organism to study nerve nets. In the body column of Hydra, there is continuous division of epithelial cells occurring while the size of the Hydra remains constant. The movement of individual neurons is coupled to the movement of epithelial cells. Experiments have provided evidence that once neurons are differentiated, epithelial cell division drives their insertion into the nerve net. As neurogenesis occurs, a density gradient of neuronal cells appears in the body. The nerve net of each cnidarian species has a unique composition and the distribution of neurons throughout the body occurs by a density gradient along the proximal-distal axis. The density gradient goes from high to low from the proximal to the distal end of the Hydra. The highest concentration of neurons is in the basal disk and the lowest (if neurons are even present) is in the tentacles. During development of Hydra, the amount of neurons gradually increases to a certain level, and this density is maintained for the duration of the organism's life-span, even following an amputation event. After amputation, regeneration occurs and the neuron density gradient is reestablished along the Hydra. [12]

Anatomy

A nerve net is a diffuse network of cells that can congregate to form ganglia in some organisms, but does not constitute a brain. In terms of studying nerve nets, Hydra are an ideal class of Cnidaria to research and on which to run tests. Reasons why they are popular model organisms include the following: their nerve nets have a simple pattern to follow, they have a high rate of regeneration, and they are easy to manipulate in experimental procedures.

There are two categories of nerve cells that are found in the nerve nets of Hydra: ganglion and sensory. While ganglion cells are normally found near the basal ends of the epithelial cells, sensory cells generally extend in an apical direction from the muscle processes of the basal ends. While Ganglia generally provide intermediary connections between different neurological structures within a nervous system, sensory cells serve in detecting different stimuli which could include light, sound, touch or temperature. [13]

There are many subsets of neurons within a nerve net and their placement is highly position specific. Every subset of a nerve net has a constant and regional distribution. In a Hydra, cell bodies of epidermal sensory cells are usually found around the mouth at the hypostome's apical tip, neurites are usually directed down the sides of the hypostome in a radial direction, and ganglion cells are found in the hypostome's basal region (in between tentacles and just below the head). [13] Nerve nets contain intermediate neurons which allow for modulation of neural activity which occurs between the sensation of the stimulus and motor output. [14]

Physiology

Each sensory neuron within a nerve net responds to each stimulus, like odors or tactile stimuli. The motor neurons communicate with cells via chemical synapse to produce a certain reaction to a given stimulus, therefore a stronger stimulus produces a stronger reaction from the organism. If a particular stimulus is larger than another, then more receptors of the sensory cells (which detect stimuli) will be stimulated which will ultimately trigger a larger response. In a typical unmyelinated axon, the action potential is conducted at a rate of about 5 meters per second, compared to a myelinated human neural fiber which conducts at around 120 meters per second. [8]

While nerve nets use hormones, the total physiology is not very well understood. Hormones normally found in vertebrates have been identified in nerve net tissues. [15] Whether they serve the same function as those found in vertebrates is not 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. [8]

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 is 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. [16]

See also

Related Research Articles

<span class="mw-page-title-main">Cnidaria</span> Aquatic animal phylum having cnydocytes

Cnidaria, is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, including jellyfish, hydroids, sea anemones, corals and some of the smallest marine parasites. Their distinguishing features are a decentralized nervous system distributed throughout a gelatinous body and the presence of cnidocytes or cnidoblasts, specialized cells with ejectable flagella used mainly for envenomation and capturing prey. Their bodies consist of mesoglea, a non-living, jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick. Cnidarians are also some of the only animals that can reproduce both sexually and asexually.

<i>Hydra</i> (genus) Genus of cnidarians

Hydra is a genus of small freshwater hydrozoans of the phylum Cnidaria. They are native to the temperate and tropical regions. The genus was named by Linnaeus in 1758 after the Hydra, which was the many-headed beast of myth defeated by Heracles, as when the animal has a part severed, it will regenerate much like the mythical hydra’s heads. Biologists are especially interested in Hydra because of their regenerative ability; they do not appear to die of old age, or to age at all.

<span class="mw-page-title-main">Neuron</span> Electrically excitable cell found in the nervous system of animals

Within a nervous system, a neuron, neurone, or nerve cell is an electrically excitable cell that fires electric signals called action potentials across a neural network. Neurons communicate with other cells via synapses, which are specialized connections that commonly use minute amounts of chemical neurotransmitters to pass the electric signal from the presynaptic neuron to the target cell through the synaptic gap.

<span class="mw-page-title-main">Nerve</span> Enclosed, cable-like bundle of axons in the peripheral nervous system

A nerve is an enclosed, cable-like bundle of nerve fibers in the peripheral nervous system.

<span class="mw-page-title-main">Nervous system</span> Part of an animal that coordinates actions and senses

In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes that impact the body, then works in tandem with the endocrine system to respond to such events. Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago. In vertebrates, it consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord. The PNS consists mainly of nerves, which are enclosed bundles of the long fibers, or axons, that connect the CNS to every other part of the body. Nerves that transmit signals from the brain are called motor nerves or efferent nerves, while those nerves that transmit information from the body to the CNS are called sensory nerves or afferent. Spinal nerves are mixed nerves that serve both functions. The PNS is divided into three separate subsystems, the somatic, autonomic, and enteric nervous systems. Somatic nerves mediate voluntary movement. The autonomic nervous system is further subdivided into the sympathetic and the parasympathetic nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. The enteric nervous system functions to control the gastrointestinal system. Both autonomic and enteric nervous systems function involuntarily. Nerves that exit from the cranium are called cranial nerves while those exiting from the spinal cord are called spinal nerves.

<span class="mw-page-title-main">Placozoa</span> Basal form of free-living invertebrate

Placozoa is a phylum of marine and free-living (non-parasitic) animals. They are simple blob-like animals without any body part or organ, and are merely aggregates of cells. Moving in water by ciliary motion, eating food by engulfment, reproducing by fission or budding, placozoans are described as "the simplest animals on Earth." Structural and molecular analyses have supported them as among the most basal animals, thus, constituting the most primitive metazoan phylum.

<span class="mw-page-title-main">Ctenophora</span> Phylum of gelatinous marine animals

Ctenophora comprise a phylum of marine invertebrates, commonly known as comb jellies, that inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming, and they are the largest animals to swim with the help of cilia.

<span class="mw-page-title-main">Nociceptor</span> Sensory neuron that detects pain

A nociceptor is a sensory neuron that responds to damaging or potentially damaging stimuli by sending "possible threat" signals to the spinal cord and the brain. The brain creates the sensation of pain to direct attention to the body part, so the threat can be mitigated; this process is called nociception.

<span class="mw-page-title-main">Rhopalium</span>

Rhopalia are small sensory structures of certain Scyphozoan and Cubozoan species.

<span class="mw-page-title-main">Neurotrophin</span>

Neurotrophins are a family of proteins that induce the survival, development, and function of neurons.

A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans produce two or three primary germ layers. Some animals, like cnidarians, produce two germ layers making them diploblastic. Other animals such as bilaterians produce a third layer between these two layers, making them triploblastic. Germ layers eventually give rise to all of an animal's tissues and organs through the process of organogenesis.

<span class="mw-page-title-main">Symmetry in biology</span> Geometric symmetry in living beings

Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, the face of a human being has a plane of symmetry down its centre, or a pine cone displays a clear symmetrical spiral pattern. Internal features can also show symmetry, for example the tubes in the human body which are cylindrical and have several planes of symmetry.

<span class="mw-page-title-main">Coelenterata</span> Term encompassing animal phyla Cnidaria and Ctenophora

Coelenterata is a term encompassing the animal phyla Cnidaria and Ctenophora. The name comes from Ancient Greek κοῖλος (koîlos) 'hollow', and ἔντερον (énteron) 'intestine', referring to the hollow body cavity common to these two phyla. They have very simple tissue organization, with only two layers of cells, and radial symmetry. Some examples are corals, which are typically colonial, and hydrae, jellyfish, and sea anemones, which are solitary. Coelenterata lack a specialized circulatory system relying instead on diffusion across the tissue layers.

Mesoglea refers to the extracellular matrix found in cnidarians like coral or jellyfish that functions as a hydrostatic skeleton. It is related to but distinct from mesohyl, which generally refers to extracellular material found in sponges.

<span class="mw-page-title-main">Ventral nerve cord</span>

The ventral nerve cord is a major structure of the invertebrate central nervous system. It is the functional equivalent of the vertebrate spinal cord. The ventral nerve cord coordinates neural signaling from the brain to the body and vice versa, integrating sensory input and locomotor output. Because arthropods have an open circulatory system, decapitated insects can still walk, groom, and mate—illustrating that the circuitry of the ventral nerve cord is sufficient to perform complex motor programs without brain input.

The evolution of nervous systems dates back to the first development of nervous systems in animals. Neurons developed as specialized electrical signaling cells in multicellular animals, adapting the mechanism of action potentials present in motile single-celled and colonial eukaryotes. Primitive systems, like those found in protists, use chemical signalling for movement and sensitivity; data suggests these were precursors to modern neural cell types and their synapses. When some animals started living a mobile lifestyle and eating larger food particles externally, they developed ciliated epithelia, contractile muscles and coordinating & sensitive neurons for it in their outer layer.

<span class="mw-page-title-main">Xenacoelomorpha</span> A deep-branching bilaterian clade of animals with a simple body plan

Xenacoelomorpha is a small phylum of bilaterian invertebrate animals, consisting of two sister groups: xenoturbellids and acoelomorphs. This new phylum was named in February 2011 and suggested based on morphological synapomorphies, which was then confirmed by phylogenomic analyses of molecular data.

<span class="mw-page-title-main">ParaHoxozoa</span> Clade of all animals except sponges and comb jellies

ParaHoxozoa is a clade of animals that consists of Bilateria, Placozoa, and Cnidaria. The relationship of this clade relative to the two other animal lineages Ctenophora and Porifera is debated. Some phylogenomic studies have presented evidence supporting Ctenophora as the sister to Parahoxozoa and Porifera as the sister group to the rest of animals. Other studies have presented evidence supporting Porifera as the sister to Parahoxozoa and Ctenophora as the sister group to the rest of animals, finding that nervous systems either evolved independently in ctenophores and parahoxozoans, or were secondarily lost in poriferans. If ctenophores are taken to have diverged first, Eumetazoa is sometimes used as a synonym for ParaHoxozoa.

Collocyte is a term variously applied in botany and zoology to cells that produce gluey substances, or that bind or capture prey or assorted objects by securing them with gluey materials and structures, or that simply look smooth and gelatinous. Literally the word means "glue cell", and it has a number of poorly distinguished synonyms, such as colloblast.

Neurogenesis is the process by which nervous system cells, the neurons, are produced by neural stem cells (NSCs). In short, it is brain growth in relation to its organization. This occurs in all species of animals except the porifera (sponges) and placozoans. Types of NSCs include neuroepithelial cells (NECs), radial glial cells (RGCs), basal progenitors (BPs), intermediate neuronal precursors (INPs), subventricular zone astrocytes, and subgranular zone radial astrocytes, among others.

References

  1. ↑ Perea-Atienza, Elena; Gavilán, Brenda; Chiodin, Marta; Abril, Josep F.; Hoff, Katharina J.; Poustka, Albert J.; Martinez, Pedro (2015). "The nervous system of Xenacoelomorpha: A genomic perspective". Journal of Experimental Biology. 218 (4): 618–628. doi: 10.1242/jeb.110379 . hdl: 2445/192702 . PMID   25696825.
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