![]() The axon is presynaptic and so contains clusters of synaptic vesicles at release sites, and the dendrite is postsynaptic and houses neurotransmitter receptors. ![]() Axons tend to be longer than dendrites and maintain the same diameter or caliber as they branch, while dendrites are shorter and taper as they branch ( Craig and Banker, 1994). This cell body gives rise to a single axon and multiple branched dendrites. The most familiar layout of a vertebrate neuron is that of a multipolar neuron with a central cell body, which houses the nucleus and most protein synthetic machinery. So, how do we know which neurites are axons and which are dendrites? This is the key to understanding in which direction information flows and how it is processed. In assembling the functional circuit maps that underlie behaviors, it is very helpful to know whether a particular neurite is an axon or a dendrite. This specialization is termed neuronal polarity. In vertebrates, directional signaling of neurons is most often accomplished by specialization of neuronal processes into axons and dendrites, with axons as the output side of the cell and dendrites the input side. In this review, we re-examine the question of whether neuronal polarity is indeed a fundamentally vertebrate innovation, or whether it evolved much earlier in the history of the nervous system. Thus, at least some aspects of the polar neuron are likely to have ancient origins. ![]() However, directional transfer of information between neurons is a key feature of the neuronal circuits that allow all bilaterian animals to move, find food, avoid enemies and perform a myriad of other activities. Indeed, in a highly cited review on neuronal polarity from 1994, invertebrate neurons are seen as ‘sufficiently different’ in terms of organization from vertebrate neurons that the polarized sorting mechanisms are suggested to also differ ( Craig and Banker, 1994). The polarized functional organization of neurons that facilitates assembly of complex circuits might be a good candidate for being associated specifically with vertebrates. Vertebrate-specific neuronal features, in contrast, must be studied in model systems such as zebrafish or mouse, for which time and expense often limit the scientific questions that can be asked. Another very practical reason for asking these questions is that it is helpful to know which features of neurons are shared with and can thus be studied in cheap, efficient invertebrate model systems. But are the neurons of the vertebrate nervous system themselves also unique or special in some way? Do they have vertebrate-only features that facilitate the assembly of large, complex nervous systems? Or is the vertebrate nervous system based on an ancient and highly adaptable neuronal cell type? Understanding the evolutionary history of neurons is critical to understanding how vertebrate nervous systems and behavior came to be. The developmental mechanisms that assemble the vertebrate nervous system and physical mapping of the circuits that underlie behavior are subjects of intense study. The elaborate behaviors of vertebrates are made possible by the evolution of nervous systems of unparalleled size and complexity. Introduction: are vertebrates special because of their neurons? As a next step, it will be extremely interesting to determine whether the nerve nets of cnidarians and ctenophores also contain polarized neurons with true axons and dendrites, or whether polarity evolved in concert with the more centralized nervous systems found in bilaterians. It thus seems likely that all bilaterians generate axons and dendrites in the same way. ![]() However, it is now becoming clear that two key cytoskeletal features that underlie polarized sorting, a specialized region at the base of the axon and polarized microtubules, are found in invertebrate neurons as well. It has been suggested that many of the distinct features of axons and dendrites, including the axon initial segment, are found only in vertebrates. In these neurons, dendrites typically receive signals and axons send signals. The most familiar neurons, those in vertebrates, have additional cellular features that allow them to send directional signals efficiently. All neurons have a central cell body with thin processes that extend from it to cover long distances, and they also all rely on voltage-gated ion channels to propagate signals along their length. The topic we address here is when the key aspects of neuronal polarity evolved. Polarized distribution of signaling molecules to axons and dendrites facilitates directional information flow in complex vertebrate nervous systems.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |