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Incellular neuroscience, thesoma (pl.:somata orsomas; from Greek σῶμα (sôma) 'body'),neurocyton, orcell body is the bulbous, non-process portion of aneuron or otherbrain cell type, containing thecell nucleus. Although it is often used to refer to neurons, it can also refer to other cell types as well, includingastrocytes,[1]oligodendrocytes,[2] andmicroglia.[3] The part of the soma without the nucleus is calledperikaryon (pl.:perikarya).[4]
There are many different specialized types of neurons, and their sizes vary from as small as about 5micrometres to over 10 millimetres for some of the smallest and largest neurons ofinvertebrates, respectively.
The soma of a neuron (i.e., the main part of the neuron in which thedendrites branch off of) contains manyorganelles, including granules calledNissl granules, which are composed largely ofrough endoplasmic reticulum and freepolyribosomes.[5] The cell nucleus is a key feature of the soma. The nucleus is the source of most of theRNA that is produced in neurons. In general, mostproteins are produced frommRNAs that do not travel far from the cell nucleus. This creates a challenge for supplying new proteins toaxon endings that can be a meter or more away from the soma. Axons containmicrotubule-associatedmotor proteins that transport protein-containingvesicles between the soma and thesynapses at theaxon terminals. Such transport of molecules towards and away from the soma maintains critical cell functions. In case of neurons, the soma receives a large number of inhibitory synapses,[6] which can regulate the activity of these cells. It has also been shown that microglial processes constantly monitor neuronal functions through somatic junctions, and exert neuroprotection when needed.[7]
Theaxon hillock is a specialized domain of the neuronal cell body from which the axon originates. A high amount of protein synthesis occurs in this region, as it contains many Nissl granules (which are ribosomes wrapped inRER) and polyribosomes. Within the axon hillock, materials are sorted as either items that will enter the axon (like the components of the cytoskeletal architecture of the axon, mitochondria, etc.) or will remain in the soma. In addition, the axon hillock also has a specialized plasma membrane that contains large numbers of voltage-gated ion channels, since this is most often the site ofaction potential initiation and triggering.[5]
The survival of somesensory neurons depends on axon terminals making contact with sources of survival factors that preventapoptosis. The survival factors areneurotrophic factors, including molecules such asnerve growth factor (NGF). NGF interacts withreceptors at axon terminals, and this produces a signal that must be transported up the length of the axon to the nucleus. A current theory of how such survival signals are sent from axon endings to the soma includes the idea that NGF receptors areendocytosed from the surface of axon tips and that such endocytotic vesicles are transported up the axon.[8]
Intermediate filaments are abundant in both perikarya and axonal and dendritic processes and are calledneurofilaments. The neurofilaments become cross linked with certain fixatives and when impregnated with silver, they form neuro fibrils visible with the light microscope.[9]
