What is the difference between dendrites and axons

Like dendrites, they are also protoplasmic projections of nerve cells, or neurons, and their primary purpose is to conduct electrochemical impulses away from the cell body of neurons. Most nerve cells only have a single axon. Axons extend from the soma to its terminal endings. Neural signals are transmitted through them after they have entered the soma of the neuron.

Larger axons are said to transmit information signals more quickly. Some axons are myelinated i. The myelin coverings are insulators, and with their presence, axons are said to transmit more quickly.

However, these assertions are in a general sense, as there are some exceptions. Other distinguishing physical characteristics of the axons and dendrites, besides the length and branching, are their shapes.

Dendrites tube-like shape usually tapers, while the radius of axons remains constant. Dendrites receive electrochemical impulses from other neurons, and carry them inwards and towards the soma, while axons carry the impulses away from the soma. Dendrites are short and heavily branched in appearance, while axons are much longer.

Generally, dendrites receive neuron signals, and axons transmit them. Cite APA 7 ,. Difference Between Axons and Dendrites. Difference Between Similar Terms and Objects. Cajal noticed that these dendrite-emanating axons were also present in many neuronal types in mammals e.

In light of these findings, Cajal had no choice but to reconsider the propagation of information in neurons and reformulate his law of dynamic polarization. The second version then stated that information flows towards the axis-cylinder axipetal away from the soma and dendrites somafugal and dendrofugal.

Therefore, Cajal acknowledged that the soma is not a central compartment in terms of information transfer, but merely in terms of cellular metabolism, due to the presence of the nucleus. For instance, some neurons faithfully propagate and sometimes initiate action potentials APs in their dendrites oriens-alveus interneurons, midbrain dopaminergic neurons, olfactory bulb mitral cells, Gonadotropin-Releasing Hormone, GnRH neurons, DRG neurons; Hausser et al.

Much like Cajal was urged to correct the dynamic polarization law, these examples oblige us to reconsider the respective functional contributions of dendrites and axons to neuronal excitability in a case by case manner.

Rather than providing an exhaustive view of these variations, we will attempt to highlight the differences in functional and morphological constraints that may explain the variety of dendritic and axonal properties observed.

Before describing the variations in the functional contribution of axons and dendrites in different neuronal types, it is important to remind what is meant when we define a given compartment as an axon or a dendrite. Dendrites and axons exhibit important differences in their anatomical, functional and structural properties.

Anatomically, axons are usually longer than dendrites and their diameter is more or less constant, even after collateral branching. In contrast, the dendritic diameter is known to taper off with distance from the soma Craig and Banker, Moreover, the dendrites of many, but not all, mammalian neurons are covered with specialized protrusions called dendritic spines, whereas axons are considered to be devoid of spines.

In addition some axons are ensheathed by myelin produced by Schwann cells or oligodendrocytes while dendrites are considered to be non-myelinated. Functionally, axons contain clusters of synaptic vesicles at release sites that confer them the role of the pre-synaptic compartment while dendrites, as post-synaptic compartments, generally contain essentially neurotransmitter receptors. The molecular composition of axons and dendrites also differ substantially. Dendrites essentially contain all the somatic organelles ribosomes, endoplasmic reticulum, Golgi apparatus while axons contain little, if any, of these components.

The ion channels expressed by both compartments can also differ substantially reviewed in Craig and Banker, ; Harris, ; Jan and Jan, One of the most important structural differences concerns the cytoskeleton composition observed in axons and dendrites: in particular, microtubules display different polarities in dendrites and axons, the latter containing only plus-end-out oriented microtubules Baas et al. This structural peculiarity is associated with differences in microtubule dynamics, protein trafficking and Microtubule-Associated Proteins MAP2 in dendrites, MAP1B and tau in axons and plays a major role in neuronal polarization reviewed in Conde and Caceres, ; Neukirchen and Bradke, The maintenance of neuronal polarity then depends on the establishment of the AIS in the proximal portion of the axon.

The AIS constitutes an axonal subdomain that serves as: i a barrier that controls the mobility and diffusion of dendritic proteins along the axolemma; and ii a cytoplasmic selectivity filter ensuring the differential trafficking between the somatodendritic SD and axonal compartments Winckler et al. Therefore, important anatomical, functional and structural differences exist between dendrites and axons, which should allow us to easily distinguish these two compartments.

From the perspective of the current review, we will rely mainly on structural arguments to define a given neurite as a dendrite or an axon. The canonical division of labor assumed for dendrites and axons is presented in Figure 1A and corresponds for instance to the behavior of cortical output neurons, such as neocortical and hippocampal pyramidal neurons or cerebellar Purkinje neurons.

Although some important differences exist between these neuronal types, synaptic inputs are received by the dendrites, travel more or less passively towards the soma, and are integrated at the level of a soma-juxtaposed AIS where a high density of voltage-gated sodium channels supports the triggering of an AP Figure 1A. In fact, a number of studies suggested that back-propagating APs might be involved in short-term and long-term synaptic plasticity mechanisms for review, see Waters et al.

Figure 1. Excitability and morphological constraints of neurons with a classical polarity. A Schematics representing the morphology and excitability process in a neocortical or hippocampal pyramidal neuron left and a cerebellar Purkinje neuron right. C Tuning of axon initial segment AIS geometry as a function of preferred frequency in auditory neurons of the nucleus laminaris of birds. B Top, adapted from Stuart and Sakmann ; bottom, adapted from Stuart and Hausser with permission.

C Reproduced from Kuba et al. The same general behavior is observed in cerebellar Purkinje neurons, albeit with significant differences in the distribution of sodium channels Stuart and Hausser, In Purkinje neurons, although the AP initiation site has long been debated Clark et al.

Interestingly, the very short distance between the AIS and the soma in this cell type Clark et al. To recapitulate, in pyramidal and Purkinje neurons, the AP is initiated in the distal AIS due to a high density of sodium channels and fails to back-propagate efficiently although to different extents in the two cell types due to a low density of dendritic sodium channels. Interestingly, an elegant theoretical study Vetter et al.

The densely ramified Purkinje dendrites are highly unfavorable to AP back-propagation, while the back-propagating AP linearly attenuates in the apical trunk but vanishes when entering the apical tuft of pyramidal cells Vetter et al. In some way, the density of sodium channels and the dendritic morphology have synergistic effects explaining why AP back-propagation is not faithful in these cell types.

Based on the strong differences in sodium channel density between the AIS and the dendrites in these cell types, AIS geometry distance from the soma and length has been postulated to have a major influence on excitability, as it may modify AP threshold or the threshold current needed to trigger an AP Grubb and Burrone, b ; Bender and Trussell, ; Kole and Brette, Indeed, several studies have shown that chronic changes in pyramidal neuron activity induced by KCl application, optogenetic stimulation or M-type current inhibition were associated with a displacement of the AIS away from the soma Grubb and Burrone, a ; Muir and Kittler, ; Wefelmeyer et al.

Along the same line, a series of beautiful studies performed on the nucleus laminaris of birds showed that the variations in AIS position and length are associated with the variation in the preferred frequency of these auditory neurons Kuba et al. Specifically, these authors demonstrated that high-frequency neurons displayed a significantly shorter and more distal AIS than low-frequency neurons, the middle-frequency neurons presenting an intermediate phenotype Figure 1C ; Kuba et al.

The resulting differences in AP initiation site location appear to be optimized to provide the lowest AP threshold for the preferred frequency, improving the discrimination of characteristic frequencies and the detection of interaural time differences, a critical property for determining sound location.

This cell-type-specific spatial tuning of the AIS is achieved in two phases during embryonic development Kuba et al. Interestingly, similar results have been obtained in pyramidal neurons of the mouse visual cortex, showing a developmental activity-dependent control of AIS geometry during the first post-natal weeks Gutzmann et al. In summary, in cell types with low SD excitability, AIS geometry and the resulting sodium channel distribution seems to play a predominant role in defining neuronal activity.

Consistently, changes in AIS geometry in these cell types are associated with variations in neuronal excitability. In contrast to the examples described above, some neuronal types display a highly excitable SD compartment, such that APs can be faithfully propagated or even initiated in dendrites in physiological conditions Hausser et al.

The contribution of the SD compartment to AP waveform was already observed in the early intracellular recordings obtained from different vertebrate neurons in the s Coombs et al. However, the first evidence for faithful dendritic back-propagation of APs was only obtained in from rat substantia nigra pars compacta dopaminergic neurons Hausser et al.

This cell type has the particularity of spontaneously generating at a regular frequency pacemaking activity broad biphasic APs, suggesting a strong involvement of SD sodium channels Grace and Bunney, This faithful back-propagation, which appears highly reliable during spontaneous pacemaking Gentet and Williams, ; Blythe et al. Since the seminal observation of Hausser et al. Moreover, these three cell types present morphological peculiarities favoring AP back-propagation.

In mitral cells, the primary dendrite is mainly unbranched and of constant diameter Shepherd, , thus limiting low-safety points for current spread. The secondary dendrites have also few branching points and are therefore favorable to AP propagation Price and Powell, a ; Xiong and Chen, Along the same line, Vetter et al.

Interestingly, oriens-alveus interneurons exhibit a dendritic morphology very similar to dopaminergic neurons, with a large soma, short and seldom branched dendrites and a dendrite-emanating axon McBain et al. Therefore in these three cell types, dendritic morphology and SD sodium channel density may have synergistic effects favoring faithful back-propagation of the AP in the entire dendritic arborization. Figure 2. Neurons faithfully back-propagating APs and releasing neurotransmitters from their dendrites.

A Schematics corresponding to a dopaminergic neuron of the substantia nigra pars compacta left and a mitral cell of the olfactory bulb right. B AP back-propagation in substantia nigra dopaminergic neurons top and oriens-alveus interneurons bottom.

C Dendritic release of dopamine from substantia nigra dopaminergic neurons measured in paired recordings of dopaminergic neurons. The post-synaptic neuron shows a substantial hyperpolarization red trace in response to APs in the pre-synaptic neuron. D Dendritic release of glutamate from mitral cells of the olfactory bulb measured by calcium fluorescence in the post-synaptic cells. Mitral cells were filled with Alexa A1 panel , and regions of interest ROI on post-synaptic periglomerular cells A3 were used to measure post-synaptic responses around the apical tuft of the mitral cell Calcium transients were measured in all ROIs after spiking of the mitral cell top right and were blocked by antagonists of glutamate receptors bottom right.

C Reproduced from Vandecasteele et al. In addition to faithfully back-propagating APs, mitral cells and oriens-alveus interneurons are also able to initiate full APs from the dendrites in specific experimental conditions Chen et al. In mitral cells, AP initiation occurs in the distal apical dendrite when: i the soma is transiently inhibited by local interneurons Chen et al. This second mode of triggering of dendritic initiation is reminiscent of the oriens-alveus interneurons, where brief high-intensity stimulation has to be used to displace the initiation site from the AIS to the non-axon-bearing dendrite Martina et al.

What may be the role of dendritic initiation in these two cell types? In mitral cells, synaptic inputs are strongly compartmentalized, with excitatory olfactory nerve inputs impinging specifically on the distal tuft of the primary dendrite and local inhibitory inputs projecting onto secondary dendrites near the soma for review, see Schoppa and Urban, While AIS geometry has not been closely examined in this cell type, the axon seems to always arise from the soma, with a rather proximal AIS Lorincz and Nusser, The existence of a dendritic initiation site remote from the soma may ensure that responses to sensory inputs persist in the presence of strong inhibition of the soma and AIS by local interneurons Chen et al.

Such a clear segregation of inputs does not seem to be present in oriens-alveus interneurons, where the axon can arise either from a subiculum- or a CA3-oriented dendrite Martina et al. The sensitivity of dendritic initiation to strong excitatory inputs suggests that it may ensure fast and reliable activation of these neurons.

Alternatively, it could be important for the induction of long-term changes in synaptic efficacy Martina et al. In contrast to these two cell types, dendritic initiation of APs has so far not been observed in dopaminergic neurons. As suggested by several publications, the presence of a high density of sodium channels in the SD compartment may not only serve AP back-propagation in this cell type but also play a central role in the generation of regular spontaneous firing Wilson and Callaway, ; Tucker et al.

Pharmacological blockade, dynamic-clamp experiments and computational modeling indeed suggest that SD sodium channels control pacemaking frequency Tucker et al. Interestingly, oriens-alveus interneurons have also been shown to generate a spontaneous pacemaking pattern of activity in vitro McBain et al.

In substantia nigra dopaminergic neurons, pacemaking in juvenile neurons has been postulated to be HCN and sodium channel-dependent Chan et al.

The expression of a high density of both types of channels in the dendrites of oriens-alveus interneurons Maccaferri and McBain, ; Martina et al. Knowing that these interneurons project a densely branched axon onto the apical dendrites of CA1 pyramidal cells McBain et al.

In summary, we provided examples demonstrating that, in cell types with a high density of SD sodium channels, APs can be faithfully propagated in the entire dendritic arborization and sometimes be initiated at the dendritic level. The SD release of dopamine DA by midbrain dopaminergic neurons was first demonstrated in the late 70s in both acute midbrain slices and in vivo Geffen et al. In parallel, dendro-dendritic synapses containing DA-filled vesicles were observed between neighboring DA neurons Wilson et al.

More recently electrophysiological measurements demonstrated that the SD release of DA was associated with hyperpolarization of the post-synaptic neuron Figure 2C ; Beckstead et al. Although many proofs of the SD release of DA between neighboring dopaminergic neurons have been gathered, the details about the release mechanisms are still debated for review, see Ludwig et al.

What is clearly admitted is that DA released at dendro-dendritic synapses binds to D2 receptors on the post-synaptic neuron, which triggers a hyperpolarization mainly due to GIRK channel activation Beckstead et al.

This release appears to depend at least partly on back-propagating APs Vandecasteele et al. Interestingly, a recent study suggested that bursts of APs may fail to faithfully back-propagate to the entire dendritic arborization Gentet and Williams, , suggesting that dendro-dendritic release of DA would not follow high-frequency discharge of APs, in contrast with the documented potentiation of axonal DA release in the striatum during bursting patterns of activity of midbrain dopaminergic neurons Gonon, ; Heien and Wightman, ; Zweifel et al.

This suggests that, at least in some conditions, axonal and SD release of DA might be dissociated. It is noteworthy that the axonal and dendritic release sites are separated by several millimeters in the rodent brain. In addition, the dendro-dendritic release of DA onto neighboring GABAergic neurons has also been functionally described and would involve D1 receptors coupled to TRPC3 ion channels, resulting in an increase of activity of the post-synaptic target Zhou et al.

Among these, the dendro-dendritic synapses formed by mitral cells onto other types of neurons are the best documented Schoppa and Urban, The synapses between mitral cells and granule cells were the first dendro-dendritic synapses described Hirata, , and their functional role in the processing of olfactory information was identified early on by the electrophysiological and computational studies of Rall et al.

These synapses are formed by the secondary dendrites of the glutamatergic mitral cells contacting specific dendritic structures gemmules of the GABAergic granule cells. Moreover, as APs are back-propagating faithfully along mitral cell secondary dendrites Xiong and Chen, and each granule cell contacts several mitral cells for review, see Shepherd et al. Interestingly, mitral cells also make dendro-dendritic synapses with another type of interneurons, the periglomerular cells, via their primary dendrite Figure 2D ; for review, see Schoppa and Urban, ; Nagayama et al.

These dendro-dendritic synapses made by mitral cells play an essential role in olfactory processing as they mediate interglomerular mitral-granule and intraglomerular inhibition mitral-periglomerular , respectively Schoppa and Urban, ; Nagayama et al.

The synchronous activation of mitral cells belonging to the same glomerulus is also important for olfactory processing, and evidence has been found that overlapping mitral cells can be coupled by reciprocal excitation Schoppa and Westbrook, ; Urban and Sakmann, Interestingly, this excitation would be mediated by glutamate released at the apical tuft of the primary dendrite and activating neighboring synapses by spillover Schoppa and Westbrook, It must be noted that tufted cells, another group of excitatory projection neurons located in a different layer than mitral cells, present very similar patterns of dendro-dendritic interactions with olfactory interneurons Schoppa and Urban, ; Nagayama et al.

Dopamine, glutamate, and GABA are not the only neurotransmitters to be released from dendrites. Dendritic release of the neuropeptides oxytocin and vasopressin by the magnocellular neurons of the supraoptic and paraventricular nuclei has been shown to play a critical role in the regulation of activity of this neuronal population Ludwig et al.

SD release is not strictly dependent on APs, suggesting that axonal release in the neurohypophysis and SD release might be sensitive to different stimuli and regulated separately, at least to some extent Ludwig, ; Wotjak et al.

Moreover, this release does not seem to occur at well-defined synapses Pow and Morris, The SD released neuropeptides would exert their effects mainly by autocrine and paracrine actions, which are allowed by the long-lasting half-lives of these peptides in the cerebrospinal fluid Mens et al. Oxytocin and vasopressin released from the dendrites of magnocellular neurons are thought to exert powerful self-regulatory actions, inhibiting or promoting the activity of the neurons releasing them on a long-term range Wotjak et al.

In summary, we provided examples showing that several types of neurons are releasing neurotransmitters from their dendritic compartment. In the cases discussed here, it seems that long-range projecting neurons rely on dendritic release to exert a local control on activity: dendritic dopamine release inhibits neighboring dopamine neurons and may also have an inhibitory autocrine effect on the releasing neuron , dendritic glutamate release by mitral cells produces short-range lateral inhibition via the activation of interneurons and dendritically released oxytocin and vasopressin exert a self-regulatory action on magnocellular neuron activity.

Interestingly, the differences in release mechanisms between the axon and the dendrites magnocellular neurons or the possibility to gate back-propagating APs dopamine neurons, mitral cells seem to provide these cell types with the possibility to control independently axonal and dendritic release of neurotransmitter, hence considerably expanding the computational repertoire of these cell types.

In contrast, we will see now that the functional impact of some interneurons can be restricted to very local actions by the total absence of an axon. The first observation of axonless cells was made by Camillo Golgi in the mids on the mammalian olfactory bulb: Golgi identified small cells in the mitral cell body layer exhibiting long branching dendrites but apparently lacking an axon for review, see Shepherd et al.

Golgi was one of the main supporters of the reticular theory stating that nerve cells are all connected via a continuous network of axon collaterals. Despite the early observation that neurons without an axon do exist in the mammalian brain, little work was done on these peculiar cells and their physiology until the late s. Since then, several other axonless neurons have been identified in vertebrate Price and Powell, b , c ; Toida et al.

Although axonless neurons in invertebrates appear to be mainly non-spiking neurons Laurent et al. In the olfactory bulb, three axonless neuronal types have been identified: the granule cells Figure 3A , the parvalbumine interneurons of the external plexiform layer EPL and the juxtaglomerular tyrosine hydroxylase interneurons Rall et al.

Unlike what Cajal initially suggested, granule cells of the olfactory bulb are in fact inhibitory interneurons, releasing GABA on mitral cell secondary dendrites through reciprocal dendro-dendritic synapses Rall et al. Granule cell dendrites express Nav1. Interestingly, in the case of the juxtaglomerular tyrosine hydroxylase interneurons, only a fraction of the cells seems to be deprived of an axon, even though stainings against specific axonal markers were not performed to ascertain the total absence of an axon Kosaka and Kosaka, b ; Chand et al.

The axonic and axonless subpopulations can be distinguished: i morphologically, as the axonless neurons exhibit a smaller soma and a shorter dendritic arborization than their axonic counterpart but also ii functionally, as axonic interneurons appear to be more excitable and generate biphasic APs while axonless interneurons are less excitable and fire monophasic APs Figure 3C , Chand et al. Figure 3. Axon-less neurons.

A Schematic representing the morphology of an axonless granule cell of the olfactory bulb. B Dendritic APs emitted by granule cells of the olfactory bulb. C APs recorded at the soma of axonless blue traces and axonic green traces juxtaglomerular tyrosine hydroxylase interneurons.

E A single cluster of Nav1. B Reproduced from Nunes and Kuner C Reproduced from Chand et al. D Reproduced from Kosaka et al.

E Reproduced from Wu et al. In the retina, the main type of axonless neurons seems to be the AII amacrine cells, a class of interneurons mediating day and night vision by transferring information from the rod bipolar cells to the ON and OFF cone pathways via electrical synapses and dendritic release of glycine, respectively Strettoi et al.

These unipolar neurons exhibit two levels of dendritic branching reaching proximally the ON-sublamina and more distally the OFF-sublamina. Rod information is transmitted to AII amacrine cells via electrical synapses Tsukamoto et al. These sodium spikelets could act as a threshold mechanism to selectively amplify rod signals Smith and Vardi, ; Tian et al.

This last finding suggests that, in specific cases, compact axonless vertebrate neurons may be able to transmit information in the absence of APs, similar to what is known for invertebrate axonless neurons Laurent et al. One question that arises from the observation of axonless neurons is whether the AP is still generated from a preferred site in the dendritic tree?

Although the mechanisms of AP initiation in most of these cell types are poorly understood, some evidence suggests the presence of one or more AIS-like compartments in the dendrites of parvalbumine interneurons and potentially granule cells of the olfactory bulb Kosaka and Kosaka, a ; Kosaka et al.

Indeed, immunohistochemical studies by Kosaka et al. Although they did not study olfactory bulb granule cells directly, the authors also mentioned that multiple AIS-like hotspots could also be observed on their dendrites Kosaka et al. The idea of multiple sites for AP initiation in granule cells is also supported by the variability in amplitude of somatically recorded APs, which could be a consequence of the morphological characteristics of the specific branch it was generated from, as suggested by Zelles et al.

Unlike olfactory bulb axonless neurons, AII amacrine cells exhibit a single hotspot containing Nav1. Moreover, disrupting the AIS-targeting motif of Nav1. Interestingly, in several brain regions olfactory bulb, neocortex, striatum , neurons produced during adulthood adult neurogenesis are devoid of an axon Kosaka and Kosaka, ; Le Magueresse et al.

For instance, calretinin-positive interneurons with a granule cell-like morphology and a single primary dendrite are produced in the striatum by post-natal neurogenesis Inta et al. In the olfactory bulb, while both axonic and axonless tyrosine hydroxylase interneurons are produced by embryonic and perinatal neurogenesis, adult neurogenesis only produces axonless neurons Kosaka and Kosaka, ; Galliano et al.

In summary, some neurons involved in local treatment of information in sensory systems seem to have evolved to carry out their function without the need of a specific output compartment, the axon. GnRH neurons lie in the medial septum, rostral pre-optic area and anterior hypothalamic area and project to the median eminence where the release of GnRH regulates luteinizing and follicle-stimulating hormone release from the anterior pituitary.

Interestingly, the lack of immunostaining against classical axonal proteins Herde et al. Since no axon could be found in these neurons, several groups wondered whether these dendrites were capable of initiating and propagating APs Roberts et al.

Indeed, using double soma-dendrite recordings, Roberts et al. Since the targets of the GnRH axon are currently unknown, while dendrons projecting onto the median eminence have been clearly identified Herde et al. Figure 4. Gonadotropin-releasing hormone GnRH neurons display a dendron with mixed axonal and dendritic properties. A Schematic representing the simplified morphology of a GnRH neuron. The dendron appears on the right as a thicker dendrite. Expanded view of the dendron of the neuron shown in the top image, illustrating the presence of ankyrin-G staining purple, middle image colocalized with the GFP staining green, bottom image.

B Modified from Iremonger and Herbison C Adapted from Herde et al. In summary, although recent evidence suggested that GnRH neurons can possess an axon, the functional output of these neurons seems to substantially rely on a long-range projecting spiny dendrite that can initiate and faithfully propagate APs, due to the presence of ankyrin-G and clustering of sodium channels, and has therefore been considered to be neither an axon nor a dendrite, but a dendron.

Unipolar neurons are defined as having a single neurite arising from the soma. At least one type of neurons in mammals is unipolar: the DRG neurons Cajal, The DRG neurons constitute the first step of sensory pathways, conveying information about pain, temperature, proprioception, and mechanoreception. From an anatomical point of view, these neurons are unipolar, with a stem axon leaving the soma and two axonal branches projecting to the periphery and to the spinal cord, respectively Figure 5A , for review, see Nascimento et al.

Both branches are myelinated for myelinated DRG neurons , the AP being initiated in the peripheral branch and conducted towards the spinal cord. Cajal was the first one to describe in detail this process when he observed that bird DRG neurons are indeed bipolar during embryonic development and only become secondarily unipolar Cajal, , acquiring this peculiar morphology where the peripheral dendritic and central axonal branches are directly connected to each other.

While the subject of the subcellular nature of the stem axon is still debated, one hypothesis is that it corresponds to a shrinking and elongation of the somatic membrane that would bring the dendritic and axonal branches in close apposition Nascimento et al.

From a functional point of view, APs are initiated in the peripheral branch close to the peripheral endings where sensory transduction occurs Carr et al. Consistent with this finding, in non-myelinated peripheral branches, Nav1. Figure 5. Specificities of unipolar neurons.

A Schematic representing the simplified morphology of a dorsal root ganglion DRG neuron. B Scatter plot showing the relationship between soma area and stem neurite diameter in neurons with a central soma gray dots and neurons with an externalized soma unipolar neurons, blue dots.

The regression lines showing the correlation between the soma area and stem neurite diameter appear as dotted lines of matching colors.

The point corresponding to cultured rat DRG neurons is indicated with a black arrow. B Adapted from Hesse and Schreiber ; with permission. This organization seems to bear several advantages because: i it allows a clear segregation of a cell body area and a pure neuropil area where contacts are made between pre-synaptic and post-synaptic neurons Rivera-Alba et al.

In fact, two studies suggested that soma size may be one of the main constraints determining whether the soma is externalized leading to a unipolar morphology , independent of the species studied Rivera-Alba et al. More precisely, using computational modeling of passive and active propagation of signals, Hesse and Schreiber demonstrated that soma externalization is beneficial only if the stem neurite is sufficiently resistive in terms of axial resistance compared to soma size.

So far, it is still unclear why unipolar neurons are predominant in arthropods while multipolar neurons represent the vast majority of vertebrates and lower invertebrates Bullock and Horridge, ; Triarhou, Therefore, while invertebrate neurons appear rather dissimilar to vertebrate neurons, this dissimilarity appears to be secondarily acquired during development due to cell body exclusion from the connecting path. Consistent with this, although the presence of a distinctive AIS in invertebrates has long been questioned, recent evidence demonstrated that cultured drosophila neurons express a specific isoform of ankyrin in the proximal axonal region Rolls et al.

In summary, many neurons in the animal kingdom are unipolar, departing from the classical neuronal morphology depicted in most textbooks. On the other side, vertebrate DRG neurons display two neurites behaving like axons, even though their developmental origin suggests that the peripheral and central branches are a dendrite and an axon, respectively Cajal, ; Nascimento et al.

So far, we reviewed examples demonstrating the diversity of morphological and functional properties of dendrites and axons in various neuronal types. Some neurons display a highly excitable axon together with fairly passive dendrites. In other neuronal types, dendrites are highly excitable and can initiate, propagate APs and release neurotransmitters.

In the olfactory bulb and the retina, some interneurons are devoid of an axon, and all pre- and post-synaptic functions are carried out by the dendrites. Finally, some neurons, such as the DRG neurons, possess neurites that all behave more or less like axons. From these observations, one may wonder whether general rules bind variations in dendritic excitability and morphology to variations in axon or AIS excitability and morphology.

In other words, are dendritic and axonal properties balancing each other to ensure optimal neuronal output, such that axonal properties might differ between neurons with passive or active dendrites?

We will see that in some neuronal types, dendritic and axonal properties seem to be co-tuned to optimize neuronal output, while in others axonal and dendritic properties appear fairly independent from each other.

Since many morphological and biophysical parameters can influence both dendritic and axonal properties, these relationships are particularly difficult to demonstrate experimentally. Therefore, computational approaches have proved particularly useful in determining why co-tuning rules might exist or be absent from a specific neuronal type. We already mentioned that, in the bird auditory nucleus laminaris, AIS geometry was correlated with the neuron preferred frequency in a manner consistent with the theoretical predictions Kuba et al.

Since the nucleus laminaris is tonotopically organized, such that neuronal position correlates with the preferred frequency encoded with high-frequency and low-frequency neurons located in the rostromedial and caudolateral regions, respectively; Rubel and Parks, , it means that AIS geometry depends on the neuron position within the nucleus Kuba, ; Kuba et al.

Interestingly, the tonotopic organization of the nucleus is also associated with a gradient in dendritic morphology Smith and Rubel, ; Kuba et al. Specifically, dendritic arborization length seems to be negatively correlated with preferred frequency such that high-, middle- and low-frequency neurons exhibit a small, medium and large dendritic arbor, respectively Smith and Rubel, ; Kuba et al.

While the influence of soma size has been less studied, some observations suggest that the soma surface is also negatively correlated with preferred frequency Kuba et al. This SD scaling seems to favor the integration of fast inputs and improve interaural time-detection sensitivity in high- and middle-frequency neurons as they are more electrotonically compact than low-frequency neurons Kuba et al. Consistently, this tonotopic gradient of dendritic complexity across the nucleus laminaris is also associated with a gradient of EPSC filtering: more filtering occurs in low-frequency neurons due to a more complex and less compact dendritic tree leading to smaller and slower somatic EPSCs.

On the other hand, EPSCs are larger and faster in the compact middle and high-frequency neurons Kuba et al. Every nerve cell has an axon. The short structures that extend from the cell body are called dendrites. A single nerve cell has many dendrites. The main difference between axon and dendrite is that axon carries nerve impulses away from the cell body whereas dendrites carry nerve impulses from synapses to the cell body. What is an Axon — Definition, Characteristics, Function 2.

What is a Dendrite — Definition, Characteristics, Function 3. An axon is single, long projection of a nerve cell. Axons carry nerve impulses away from the cell body. The membrane that covers the axon is called the axolemma. Axoplasm is the cytoplasm of the axon. Axons are branched at their terminal ends. The tips of the branched ends are formed by telodendria. The axon terminals are the swollen ends of the telodendria. The axon terminals form the synaptic connection with a dendron of another neuron or with an effector organ.

The membrane of the axon terminal is linked to the membrane of the target cell. Vesicles that contain neurotransmitters are present in the axon terminals to transmit the nerve impulses by means of chemical signals through the synaptic gap. The axon hillock is the initial segment of an axon.

It initiates the action potential. A cross-section of an axon is shown in figure 1. The two types of axons are myelinated axons and non-myelinated axons. The myelin sheath forms an insulation on the axon to increase the speed of transmission of nerve impulses through the axon. This type of transmission of nerve impulses is called saltatory conduction.