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StatPearls . Sweetheart Island (FL): StatPearls Publishing; 2021 Jan-.


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Introduction

In 1860, the German anatomist rose oil Friedrich karl Deiters (1834-1863) defined the basic structure the the nerve cell and also identified two different protoplasmatic protrusions the the cell body that he termed as "axis cylinder," and "protoplasmatic processes," dong axons and dendrites.<1> Axons are the elongated section of the neuron located in the facility of the cell in between the soma and axon terminals. In size, the axon may represent over 95% that the total volume that the neuron. Functionally, that carries electrical impulses and projects to synapses v dendrites or cabinet bodies of other neurons or with non-neuronal targets such as muscle fibers.

Concerning length, the size of axons different according come the duty of the neuron. Considering the functional difference between estimate neurons and interneurons, cortical estimate neurons (CPNs), likewise termed as pyramidal neurons and also spinal cord forecast neurons (dorsal horn neurons), usually have long axons (from number of mm and also up to 1 m). In contrast, interneurons, that work within regional circuits, have a brief axonal terminal (up to number of mm). The longest axons of the person body space those that comprise the sciatic nerve whereby the length can exceed one meter. Furthermore, compared to projecting neurons, interneurons usually have smaller soma, fewer organelles, and also a diminished amount the cytoplasm (axoplasm).

Histological monitoring of axon mirrors a cylindrical structure, but recent 3D electron microscopy studies demonstrated that more than likely axon has not the shape of a perfect cylinder.<2> The diameter is variable as it ranges in between 1 and 25 micrometers. In squid, that reaches a diameter that 1 mm. The sports of the diameter has vital functional implications because the speed of propagation the the advertise (i.e., activity potential), as well as being dependency on the existence of the myelin sheath, is directly proportional come the diameter that the axon. Moreover, they have actually demonstrated far-ranging changes in the diameter follow me the solitary axon.<2>

The axon is just one of two varieties of protoplasmic protrusions that the neuronal soma. The various other protrusion is the dendrites. Axons are identified from dendrites by several characteristics including:


Structure. Substantial structural differences exist between dendrites and also axons. For example, just dendrites contain stormy endoplasmic reticulum and ribosomes, and also the structure of the cytoskeleton is different. Differences also impact the membrane together it contains mostly voltage-gated ion channels in axons, conversely, ligand-gated ion channels space present, specifically in dendrites.
Functions. Dendrites usually receive signals, while axons generally transmit them. However, every these rules have actually exceptions. Furthermore, axons generate and also transmit all-or-none activity potential, whereas dendrites create depolarizing (below the threshold the the activity potential) or hyperpolarizing (lowering the resting membrane potential) graded potentials.

Of note, although each neuron has only one axon, bifurcations that room branches the the main axon deserve to be present. A collateral branch is one axonal protrusion over10 micrometers in length.<3> these collaterals provide modulation and also regulation the the cabinet firing pattern and also represent a feedback mechanism for the neuronal activity. The terminal component of the axon and collaterals tapers progressively. These parts are called telodendron and continue v the synapse (synaptic knob or button) which to represent the specialized structure the comes into contact with another neuron (soma, axon or dendrite), or muscle fiber. Axon extension and growth of new telodendrons (and synapses) room guided by numerous factors, including the nerve expansion factor (NGF). The branching processes, in turn, pat a function of fundamental importance in neuroplasticity, for instance, in cognitive procedures such together memory and learning.

Anatomically and based ~ above the illustration of the protoplasmatic protrusions, neurons space classified right into three groups:


Multipolar neurons. They are the most usual neurons; Shape: a single axon and many dendrites extending from the cell body. Localization: central nervous device (CNS)
Unipolar (or pseudounipolar) neurons. Shape: a single short procedure that extends from the cabinet body and also then splits into two branches in opposite directions; one branch travels to the peripheral nervous device (PNS) because that the sensory reception, and also the other to the CNS (central process). This neurons have no dendrites as the branched axon serving both functions. Localization: dorsal root ganglion and sensory ganglia that cranes nerves, and also some mesencephalic nucleus
Bipolar neurons. Shape: one axon and one dendrite that expand from the cell human body in opposite directions. Localization: retinal cells and olfactory system

Two noteworthy features differentiate the axon from the soma (also described as perikaryon). First, no rough endoplasmic delusion extends into the axon; secondly, the composition of the axonic membrane (axolemma) is fundamentally various from that of the somatic membrane. These structural distinctions translate right into functional distinctions. In fact, because the lack of ribosomes does not enable protein synthesis, every axon protein originate in the soma. Furthermore, the certain structure the the membrane because of the presence of specific protein channels permits information to travel along the food of the axon. Again, depending on the ar within the body, these structures have the right to be extended in sheaths of an insulating material known as myelin. Based top top the visibility or absence of the myelin sheath, axons are distinguishable right into myelinated and non-myelinated axons. 

Myelin sheath

Myelin forms by the concentric wraps of the plasma membrane of neuroglia cells around the axon. These cells are the Schwann cells (or neurolemmocytes) in the PNS and also oligodendrocytes in the CNS. Together a basic rule, oligodendrocytes myelinate multiple surrounding axons, if Schwann cells myelinate only one axon. In structural terms, the myelin sheath wraps the axons discontinuously as it is interrupted at constant intervals referred to as Ranvier nodes (also termed as myelin sheath gaps), which stand for the an are between two consecutive Schwann cells and at i m sorry the axon is there is no of the sheath. In this way, employing the jump system from one Ranvier node to the next, the propagation that the electric signal is much faster than in the myelin sheathed axons. The cabinet membrane of Schwann cells is arranged around the axon, developing a twin membrane framework (mesaxon), i beg your pardon elongates and wraps itself in a spiral, in concentric layers, roughly the axon itself. Throughout this winding process, the cytoplasm of the Schwann cabinet is pushed towards the outside, when the surface of the contact membranes finish up condensing, creating the lamellae that the myelin sheath. As soon as the myelin sheath wraps roughly the axon, the mesaxon disappears by blend of the cytoplasmic membrane in contact, other than in correspondence with the innermost gyrus (internal mesaxon) and also the outermost gyrus (external mesaxon or neurilemma) where there is a rotate outermost wealthy in the cytoplasm. As soon as the myelin sheath creates by oligodendrocytes (in PNS), the outermost gyrus reduces come a tongue and, in turn, although over there is the internal mesaxon, the external one is no recognizable. Functionally, myelin to represent an electrical insulator, permitting an boosted speed the conduction in addition to an axon. The facilitates electric transmission via saltatory conduction. Structurally, myelin is written of roughly 80% the lipids (mostly cholesterol and also variable quantities of cerebrosides and also phospholipids) and 20% of proteins. However, depending on its location, myelin has a different composition together CNS myelin has actually more glycolipid and less phospholipid 보다 PNS myelin.


Issues that Concern

A far-reaching issue of issue with the histology that axons is the staining. In enhancement to the visibility of myelin (in myelinated axons) and also to the certain membrane structure, an additional feature differentiate the axon indigenous the soma. The axon is devoid of a stormy endoplasmic reticulum. Together a result, timeless cellular stains such as the Nissl staining method, i m sorry identifies the so-called Nissl bodies (granules of the stormy endoplasmic reticulum) is only able to stain the soma and dendrites but not the axon or axon hillock. The said, newer techniques of staining involving luminescence philosophies have created enlightening results. Retrograde stained tracers making use of fluorescent dyes have shown promise with greater resolution photo data.

Issues of concern of clinical interest are manifold as a huge number of practical and/or structure alterations the the axon are at the basis of neurological disorders that can involve the CNS or PNS. On this bases, the examine of the structure and functioning the the axon (i.e., axonology) is a field of research study of substantial interest. Together a consequence, immunocytochemical and also immuno-ultrastructural techniques have illustrated the structure of channels, exchangers, and pumps the are involved in the operative mechanisms of axons. Moreover, the advance of target strategies against axonal damage, or concentrated on axon regeneration, are of an essential importance because that the bulk of all neurological diseases.


Structure

Axon connects to the soma at a cone-like component of the structure well-known as the axon hillock. This component of the axon has considerable functional prestige since action potential originates here. In various other words, this region of the neurolemma processes the incoming signals from other neurons. It represents a cause zone wherein the summation of just arrived graded excitatory (excitatory postsynaptic potentials, EPSPs) and also inhibitory (inhibitory postsynaptic potentials, IPSPs) potentials space realized and, in turn, the action potential is realized or not. If the summation overcomes the threshold limit of the axon hillock, potential starts, and it will certainly betransfer , in a continuous or saltatory mode, along the axon towards the synapse. In structural terms, the axon hillock might contain fragments of Nissl substance. 

The axon hillock proceeds with the early stage segment of the axone, located around 30 to 40 micrometers native the perikaryon and close come the very first myelinated element. Structurally, in this axonal segment, the assorted axoplasmic elements begin to align longitudinally. Neurofilaments and mitochondria room present. Microtubules are also present, arranged right into fascicles interconnected through sidearms. Furthermore, residual  Nissl substance have the right to persist. Functionally, in this region, axo-axonic synapses might occur. Of note, the axolemma of the axonal region where action potential originates and starts that run mirrors a dense granular layer comparable to that demonstrated at the nodes the Ranvier. Recent studies proved that action potentials can originate not just at the axon hillock but additionally in this early axonal segment. In part neurons, the activity potential may also originate from the first Ranvier node. 

Axons are both structurally donate of a neuron and the facilitators of communication both intra- and also interneuronally. The diameter the a single axon is incredibly small and, therefore, frequently measured in micrometers v the median diameter of one axon being about 1 micrometer. A cluster of axons together creates a nerve. The axonal membrane is a phospholipid double layer that has actually proteins installed inside it. These voltage-gated ion networks facilitate the activity of ions in and also out the the membrane and an important to the neuronal transmission. Many neurons space myelinated, meaning they save on computer sectioned covers wrapping about the axon known as the myelin sheath.<4> Axons that room thicker and an ext densely myelinated will be faster and also useful for reflex-based circuits and also other somatic nervous device functions. Contrarily, pain, and hunger receptor from the autonomic nervous system have actually lightly or unmyelinated slim neurons and also produce slow transmissions.

An crucial axonal structural facet is the style of the cytoskeleton. Interestingly, this style is partly various in composition and organization from the of the dendrite. Although the cytoskeletal protein are greatly synthesized in the cabinet body and also then transported come their various cellular sites, the dendritic microtubule-associated protein 2 (MAP2) is synthesized straight in the dendritic compartment. Top top the contrary, due to the absence of a protein synthetic apparatus, the axonal microtubules room synthesized and organized in the soma (by the microtubule-organizing center, MTOC) and then come the axon compartment. Furthermore, MAP2 is not existing in the axonal compartment. Another difference concerns the microtubule polarity. If dendritic microtubules display a blended polarity v both plus and minus end distal to the cell body, in the axon microtubules, have actually the plus end distal come the cabinet body. Again, research has actually demonstrated the axonal and also dendritic microtubules different in the various quantity that tau proteins and the different degrees of phosphorylation. Finally, back neurofilaments are plentiful in the axons, they room scarce in the dendritic site. 

The cytoskeleton dram a basic role in the processes of axon growth and guidance. Since these procedures are the basis of the formation of neural networks, there is considerable interest in this facet of neuroscience. Alterations during the ingredient phases that the circuits, indeed, are attached to the pathogenesis of neurodevelopmental obstacle such as intellectual disability and autism spectrum disorder.

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<5> ~ above the various other hand, in neurodegenerative pathologies, over there is progressive destruction of this circuits. Axon growth and guidance space regulated by genes and originate indigenous the so-called growth cone, a region described for the an initial time through the Spanish histologist Santiago Ramón y Cajal (1852-1934), in 1890. The ingredient of the environment about the axon guides the growth procedure of the nerve terminal and the accomplishment of its target. Eco-friendly attractively or repulsively acting molecules interact with receptors located on the expansion of cone and also induce, in turn, modifications of the cytoskeleton. In particular, the cytoskeleton produces fine expansions like microspikes referred to as filopodia the contain bundles of actin filaments (F-actin). Interposed between these an additional cytoplasmic projections are flat regions of thick actin. These regions that have a veil-like" appearance room termed as lamellopodia. In short, after ~ the interaction in between environmental factors (e.g., netrin, ephrins, and semaphorins), exterior to the axon and also present at a distance, through receptors located on the expansion cone, microtubules polymerize right into the growth cone and guide the elongation procedure through filopodia and also lamellopodia.<6> The development of axonal collaterals, the mechanisms of axonal regeneration after injury, and the development of dendrites also administer activation the the expansion cone and processes involving filopodia and lamellopodia.