Nerve net

thumb|Nettle jelly

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. Nerve nets can provide animals with the ability to sense objects through the use of the sensory neurons within the nerve net.

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

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 a nerve net allows an organism to respond to its environment, it does not enable the organism to 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 may be sister to the other extant Metazoa.

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. Sponge cells have the ability to communicate with each other via calcium signaling or by other means. 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.

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.

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.

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.

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.

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).

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. 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.

Evolution

Anatomy

Physiology

See also

References