Cnidarians have a diffused nervous system that is not remarkably complex. It has a low morphological complexity and is probably homologous to the bilaterian nervous system. Despite its low complexity, it has some fascinating properties and structures. This provides us with a unique opportunity to study the evolution of more complex nervous systems.
Nematostella cnidarians have diffused nervous system
The central nervous system of cnidarians is a complex structure containing several distinct types of neurons. The neuronal network in cnidarians consists of diffusely distributed nerve cells, with ectodermal sensory cells and endodermal multipolar ganglion cells forming distinct neural territories along the oral-aboral axis. Researchers have found that these cells express many conserved bilaterian genes.
The network-like organization of neural tracts within cnidarians reflects an evolutionaryally primitive state of neural regionalization. It also suggests that the organization of neural tracts in cnidarians is related to the morphology of the central nervous system of bilaterians.
Although cnidarian nervous systems may be simple and diffuse, recent research suggests that they contain many specialized cells. In the freshwater polyp Hydra, for example, the nerve cells of this cnidarian are located at the base of the epithelial cells. Moreover, the neurons extend to the epithelial cell muscle layer.
Cnidarian nerve cells and neurites are highly regionalized in many species, including medusozoans, anthozoans, and cnidarians. This property is important for studying the evolution of the nervous system. Because of its unique morphology, the nervous system of cnidarians allows researchers to compare the development of the nervous system with that of bilaterian and ancestral animals.
Neuropeptides are released from synaptic sites of the cnidarian nervous system. They serve as synaptic transmitters and contribute to the neuronal function in cnidarians. As the cnidarians have a highly developed central nervous system, it is important to determine the exact role of neuropeptides in cnidarian neural development.
The central nervous system of cnidarians can coordinate complex macro-behaviours. Recent genomic studies and genome projects have revealed the molecular complexity of the cnidarian nervous system. This system is genetically accessible and may provide clues to the evolution of the centralized nervous system in bilaterians.
The neural system of cnidarians is a complex network that includes genes responsible for the development of axon guidance. This network has been conserved among several cnidarian species, including protostomes and deuterostomes. This is consistent with the fact that the cnidarian nervous system has been evolving long before the evolution of Bilateria. Nevertheless, more research is needed to identify the full regulatory network involved.
It may be homologous to bilaterian nervous system
The cnidarian nervous system shows some similarities to the bilaterian nervous system. The cnidarian nervous system exhibits regionalization of nerve cells and neurites, whereas bilaterians exhibit centralization. In addition, they display elaborate nerve tracts that may be homologous to bilateric nervous systems.
The Cnidarian nervous system also exhibits a high level of neuropeptide content. Neuropeptides that are short amidated in bilaterians and cnidarians have deep evolutionary roots in their common ancestor. These peptides are specific markers for mature neuronal cells in the cnidarian CNS.
The b-catenin-mediated differentiation of the cnidarian and bilaterian nervous systems has been suggested to be a shared ancestral mode. Furthermore, the cnidarian and a bilaterian ancestor share a common genetic signature.
It is unclear whether cnidarians have centralized nervous systems, or if they have a diffused nervous system. Although the evolutionary relationships between the chordate CNS and the bilaterian nervous system are unclear, some evidence supports that the bilaterian nervous system is a bipartite, uncentralized nervous system.
The earliest neural inducer in the bilaterian nervous system is enigmatic, and the molecular nature of the first neural inducer is unknown. This information will allow scientists to decipher the genetic programs of ancestral nervous systems.
The cnidarian nervous system is incomplete, but researchers have identified a number of neuropeptides that are found in cnidarian neurons. In addition, comparative genomics has revealed a complete set of neurogenic transcription factors and signalling molecules in cnidarians. Moreover, the expression of neurogenic transcription factors and signalling molecules is position dependent. Despite the lack of a complete nervous system, cnidarians are considered valuable model organisms for comparative molecular studies.
Some cnidarians have a specialized apical organ called the apical organ. This is the primary sensory integrative structure located at the aboral pole of the animal and is responsible for coordination of locomotion and other behaviors. Molluscs and chordates have well-defined central nervous systems, and seven other phyla of bilaterians have a similar gross anatomical arrangement.
The distribution of neural cells in the cnidarian nervous system is thought to be homologous to that in vertebrates. However, this finding has not been supported by other model animals. Therefore, it is necessary to decipher neural signaling pathways in cnidarians.
The asymmetric pattern of neurons along the directive axis is a feature of the Cnidarian nervous system. Specifically, NvArp6 and GLWa+ neurons express asymmetrically along the directive axis. This asymmetric pattern may be analogous to the evolutionarily conserved function of Bmp signalling in bilaterian nervous system.
Neurogenesis first appeared in eumetazoa, the common ancestor of coelenterates and bilateria. A second wave of neurogenesis occurred after the divergence of coelenterata and bilaterians. Once neurogenesis took place, animals with nerve nets began to exhibit complex behaviors and movements.
It may operate using a “chemical wiring diagram”
The Cnidarian nervous system is a model organism for understanding the general principles of neural circuit function, including the generation of dynamical attractors. Current views of the nervous system primarily focus on information-exchange functions, but there are many other functions that nervous systems may perform. These functions could include regeneration, development of innervated tissues, and bidirectional communication with commensal organisms. Moreover, the genomes of these animals support the notion that Cnidaria are members of the sister clade of Bilateria. This means that Cnidarian nerve nets are simple, yet complex, allowing researchers to study the structure and function of this nervous system.
Cnidarians’ nervous systems display remarkable neurophysiological specializations, including bidirectional chemical synapses, peptide-gated channels, and nematocyst discharge. They also exhibit neuronal integration and standard synaptic transmission mechanisms, whereas the bilaterian nervous system has an integrated network of neurons.
Although the structure and function of the cnidarian nervous system remain unclear, genomes of cnidarians provide insights into the molecular complexity of nerve nets. Their specialized nerve nets contain a range of synaptic proteins and small neurotransmitters that are similar to those of chordates. Furthermore, recent advances in gene manipulation and imaging have enabled functional analysis of cnidarian nervous systems. These findings may ultimately lead to a better understanding of the evolution of the nervous systems and how they operate.
Neuroscience in cnidarians is an exciting area of study. Their simple nervous systems, coupled with the small size of their neurons, provide a rich playground for researchers. Calcium imaging is a great tool for exploring the neural basis of behavior in these animals.
A diffused nervous system may be a result of stochastic gene loss in early vertebrate evolution. In cnidarians, a diffused nervous system may function by communicating between epithelial cells.
Cnidarians are also model organisms for studying whole-body regeneration. Abraham Trembley, in 1744, performed bisection experiments on cnidarians. Thomas Morgan and Ethel Brown, both of whom performed developmental organizer transplant experiments, found that cnidarians regenerate by integrating stem cell derived precursor cells into the nervous system. As a result, the Cnidarian nervous system is a potential model for understanding how the brain works and how complex behaviour is developed.
