Talk:Neuron

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I added the sentence about size -- useful for non-biologists to orient themselves to the scale being discussed.

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Article Grading: The following comments were left by the quality and importance raters: (edit · refresh) rated top as high school/SAT biology content - tameeria 14:53, 17 February 2007 (UTC) The article seems fairly complete to me, but I'm no expert on neurology. If the B rating seems wrong, please adjust. - tameeria 19:14, 18 February 2007 (UTC)

The first sentence of this entry used to read "The basic cells of the nervous systems of metazoan animals, whose job is to transmit and process signals." I took the liberty of changing it, suggesting as it did that the job of metazoan animals is to transmit and process signals. (That's a notion of God that's rather different from any I had previously thought of: suppose that there is a God and that s/he created other beings merely to experience things that s/he can not, and then to report on the experience?)

This is a neutral, objective science article. Please don't input any personal Abrahamic religious belief into it. -intranetusa

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From the article: "Synapses of nerves may be between two axons, two dendrites or an axon and a dendrite." Do we have any biologists, physiologists, MD or other specialist who can confirm this statement is correct? My understanding of recent reading I have been pursuing has the axon as an emitter and the dendrite as a collector. Recurrent connections would thus require a portion of the neurons in the nerve fiber or neuron cluster to be oriented roughly 180 degrees. In my current understanding, for the organic circuitry to function as the statement above implies would require multiple neurons in complex patterns rather than individual axon-axon or dendrite-dendrite pairs. mirwin 01:14 Aug 24, 2002 (PDT)

Here is an article that talks about electrical synapses between adjacent dendrites [1]. I think dendro-dendritic chemcal synaptic connections are fairly rare, but here is an article claiming the existence of dendro-dendritic chemical synapses [2]. Chemical synapses between two axons are not unusual; here is a recent article involving such connections [3]. Some of the important work by Eric Kandel on leraning and memory in Aplysia involves axo-axonic sysapses. See the yellow "facilitating interneurons" making axo-axonic synapses onto the sensory neurone in this diagram [4]. 68.109.166.14 04:37, 7 Dec 2004 (UTC)

It is my understanding that all synapses only occur at the tips of dendrites.

No - I don't think this is true. There can be different kinds of synapses - for example axonal-dendritic are the obvious ones, but there can also be axonal-somatic, possibly somatic-somatic etc. There is no reason why there can't be multiple synapses on a dendrite, axon or soma. There have been quite a few constraints which various researchers have imagined about neurons and their synapses - most have turned out not to be applicable.DaveM

Axons function as a conducting line that may go long distances while dendrites branch from end of the axon or directly from the nerve cell body. Electrochemical signals can only go in one direction for each type of neuron (away from the cell body); in sensory neurons signals are sent from sensory receptors (which are modified neurons), in motor cells a signal propagates from the motor cell neuron to effector cells in muscle. The quoted statement doesn't make sense to me. --mav

• Not much of the above is true (well, DaveM is right, he just didn't go far enough). Synapses can be made between a wide variety of different cell types, and sub-cellular regions. The "classic" synapse is the neuromuscular junction. It is often referred to as a junction or an end-foot, but it is a synapse between a motorneuron axon and a muscle fiber. The "classic" CNS synapse is the axo-dendritic synapse - between the axon of a presynaptic neuron and the dendrite of a postsynaptic neuron. Axons, dendrites, somata of neurons can all act as presynaptic elements in a synapse. Probably other things can as well. Axons, synaptic terminals, dendrites, somata, skeletal muscles, glands, blood vessels, smooth muscles (gut etc.), skin, receptors, and almost any other biological part can act as a postsynaptic element. Combine those into every possible combination, and you get an idea of the anatomical diversity of synapses. Synaptidude 01:22, 11 January 2006 (UTC)

• Lest we not forget that there is an entire morphological feature that seems to be missing from this discussion - dendritic spines. Spines are largely associated with excitatory connections and are most often axo-dendritic. (Inhibitory connections are very often located on the soma.) Sirshane13

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I am concerned at the notion that neurotransmitters are hormones - I will have to check up on this. My gut feeling is that this is incorrect. DaveM

The usual definition for a hormone in human medicine is a substance which is produced in one tissue and carried through the bloodstream to another tissue, where it has its physiological effect. The part that may be getting confused is that some substances (e.g. norepinephrine) are both hormones and neurotransmitters. I think it's probably best not to refer to such substances as hormones when you are discussing them as neurotransmitters. -- Someone else 22:13 Nov 19, 2002 (UTC)

So shall we remove the reference to hormones then? DaveM

Just did. It didn't add anything anyway<G>. -- Someone else 22:31 Nov 19, 2002 (UTC)

Correct: neurotransmitters are not considered to be hormones. Synaptidude 01:22, 11 January 2006 (UTC)

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I rearranged things a bit and deleted stuff. The stuff about nerves and the nervous system was up too high given the topic, I thought, and not all of it really germane. Also, I've written long articles on synapses and action potentials, so I felt the paragraphs about those could be pared down. I hope people find these changes agreeable. 168...

Contents 1 100 billions neurons in brains 2 100 trillion connections 3 Neural stem cells 4 Brain cell regeneration? 5 Nerve Cell Longevity 6 Cleanup and expansion 7 Other meanings 8 Classification 9 Neuron count by species 10 Wikification of opening paragraph 11 Challenges to the neuron doctrine 12 Obfuscation of Opening Paragraph? 13 Two copyright violations 14 Deleted image 15 Neuron/synaptic memory? 16 Rodent hippocampus? 17 NEED ENK NEURON

[edit] 100 billions neurons in brains

From WordNet: 1. (1) billion, one million million, 1000000000000 -- ((in Britain) the number that is represented as a one followed by 12 zeros) 2. billion, one thousand million, 1000000000 -- ((in the United States) the number that is represented as a one followed by 9 zeros)

Which billion (US or Britain)? : 100 Billion should be avoided for the standard 10^12 representation

I have seen 100 billion, United States. There are a large number of small neurons in the cerebellum, and I have seen 100 billion as an estimate of cerebellar neurons. 68.109.166.14 04:55, 7 Dec 2004 (UTC)

• • • Yes, what I've always heard is that there are 10^16 neurons in the brain..........and 10^17 of them are cerebellar granule cells. Synaptidude 01:14, 11 January 2006 (UTC) : something wrong! its 10^12 neurons and 10^16 synapses

From http://www.nervenet.org/papers/NUMBER_REV_1988.html : "The average human brain (1350 gm) contains about 85 billion neurons. Of these, 12 to 15 billion are telencephalic neurons (Shariff 1953), 70 billion are cerebellar granule cells (Lange 1975), and fewer than 1 billion are brainstem and spinal neurons. A revision: In a beautiful quantitative analysis of human cortex using the optical disector, Pakkenberg and Gundersen (1997) have shown that the number of neocortical neurons ranges from 15 to 31 billion and averages about 21 billion. Other forebrain structures—primarily the hippocampal region, basal ganglia, and thalamus—are likely to contain an additional 5–8 billion neurons. Total neuron number in humans therefore probably averages 95–100 billion. What is perhaps more remarkable is the normal two-fold difference in neocortical neuron number among healthy adults of normal intelligence." Meduz 16:29, 16 November 2006 (UTC)

[edit] 100 trillion connections

Is it certain that there are 100 trillion? Any reference? Multiplying 1000 by 100 billion is not correct, as 100 billion neurons can be connected multiply. It is probably more accurate to say it is possible to have up to 100 billion to the power of 1000 connections or something similar involving factorials. I'll try to find a reference... It seems to vary between 1 trillion and 1000 trillion on the web. 500 trillion is quoted as the norm for adults. I think the number in the article should maybe be changed to 500 trillion? [5]

on the brain page they say each neuron has 25 thousand connections...if that means anything... jess523s

That page is... not necessarily accurate. I'm trying to find good source references for the actual numbers. Semiconscious (talk · home) 07:55, 26 December 2005 (UTC)

• • The rule of thumb I've generally heard is that there are about 10K synapses made on every nerve cell. But this is a number that is hard to pin down Synaptidude 01:12, 11 January 2006 (UTC)

I constantly hear 10^11 neurons, 10^15 synapses. Sirshane13

consistentency would be good here, for instance, under connectivity:

The human brain has a huge number of synapses. Each of the 1012 neurons (1,000 billion, i.e. 1 trillion) has on average 7,000 synaptic connections to other neurons. Most authors estimate that the brain of a three-year-old child has about 1016 synapses (10,000 trillion). This number declines with age, stabilizing by adulthood. Estimates vary for an adult, ranging from 1015 to 5 x 1015 synapses (1,000 to 5,000 trillion). [3]

and at the bottom of page, "neurons in the brain":

The number of neurons in the brain varies dramatically from species to species. The human brain has about 100 billion (1011) neurons and 100 trillion (1014) synapses. By contrast, the nematode worm (Caenorhabditis elegans) has just 302 neurons making it an ideal experimental subject as scientists have been able to map all of the organism's neurons. Many properties of neurons, from the type of neurotransmitters used to ion channel composition, are maintained across species, allowing scientists to study processes occurring in more complex organisms in much simpler experimental systems.

quite the discrepancy!

[edit] Neural stem cells

Anyone heard of these?

They are quite a revolutionary discovery, so....I don't see much mention of them in here. I'm looking for information on it as well. Not to mention the neurons' flexibility in rerouting.

This textbook might be useful for you [6]. Check this out [7]. Human neural stem cells [8]. 68.109.166.14 04:55, 7 Dec 2004 (UTC)

From the article:

1. The soma, a relatively fat central cell.

I know v. little about biology, but a neuron is a single cell, is it not. The word 'cell' should be replaced (unless the soma is a seperate cell). SgtThroat 22:51, 19 Dec 2004 (UTC)

I tried to give a better description of the "soma". I'd rather say "neuronal cell body". It is rather disorienting when you follow the link for soma from the neuron page. Memenen 01:07, 20 Dec 2004 (UTC)

I've fixed this by adding a stub at soma_(biology), but I don't know what to write there - I'm not a biologist - maybe you could add something? SgtThroat 13:13, 21 Dec 2004 (UTC)

[edit] Brain cell regeneration?

I've heard a few contradicting statements about whether or not brain cells regenerate over time, so I wondered if anyone would be so kind to give me some clarification on it. Also, if they regenerate - do they regenerate faster than they die, or is the amount of brain cells decreasing all the time? Thanks in advance.

• • I think sometimes they can. It depends on the situation. From what I've read, that's a very experimental subject right now. It's also very controversial to try to artificially regernate neurons, because many times the cells used to regenerate neurons are brain stem cells from aborted babies. There is also research to see if hair follicles could be used, which would eliminate the ethical issues. It would be very important if brain cells could regenerate, because so many diseases are a result of destroyed cells that are vital to the brain's existence.

• • • This is a very new discovery but it is confirmed. It should be added

The brain keeps growing throughout childhood and adolescence. The issue is whether adult neurons can regenerate. For the most part, adult neurons cannot regenerate. There are specific types of cells in the olfactory system and in and area of the brain called the dentate gyrus of the hippocampus that do regenerate throughout life. Oddly enough, the rate of regeneration of these cells in the hippocampus regenerate faster with exersice. Scientists are devising strategies to harvest these regenerating neurons and hope to one day use them as neural stem cells to help other brain areas regenerate after injury or illness. Nrets 14:59, 6 January 2006 (UTC)

• • • • Nrets is right. Most of the growth of brain size and weight is not due to an increase in the number of neurons. It can be, ni part, due to the increase in size and volume of the existing neurons. There are some kinds of neurons that do regenerate. But most do not.

• • • • • As far as I know, nerve cells are not derived from aborted babies - unless you want to count rat and mouse babies.Synaptidude 01:10, 11 January 2006 (UTC)

• We should be careful to distinguish between regenerate (as in after injury) or continue growth. The brain show enormous signs of plasticity throughout all stages of life, and even in adults. See Pons 1991, Florence & Kaas, etc. There is an entire body of work especially considering the plasticity of the Somatosensory cortex. The topographic organization of entire cortical regions can be regenerated, though the central debate concerns the precise nature of the growth. It is well accepted that plasticity occurs. Concerning certain types of injury to peripheral nerves, they can also in certain cases exhibit an incredible tendency to regenerate, reforming the original connection (when crushed) and restoring the cortical topography that existed normally (See Florence & Kaas). Sirshane13

• Another interesting point on this topic is to realize that the anonymous comment above is also correct. In fact the number of synaptic connections peaks quite early in life, at which time a process called pruning occurs that keeps connections that are robust and removes neurites (processes of neurons like dendrites and axons) that are not utilized. Sirshane13

[edit] Nerve Cell Longevity

What is the average longevity of a nerve cell? That's what I was looking for. (I seem to remember it being that nerve cells shouldn't die, and so their life span would be the life of their organism, but if that's so it should be mentioned I think.)

The "average" longevity of a nerve cell is shorter than the life of organism. But this is more a quirk of the math than a useful measure. Most nerve cells live until their organsim dies. But some die before, and none die after, so the average will be less than the lifespan of the organism. Synaptidude 01:03, 11 January 2006 (UTC)

[edit] Cleanup and expansion

I've tried to reorganize, clean up and expand this article. I think it looks better now, but there is still some room for expansion and I'm not entirely happy with the organization. If anybody is interested in collaborating to expand this article please let me know. Nrets 16:28, 19 December 2005 (UTC)

Hello, Nrets, thank you for your work on this article. I was hoping to get a gauge concerning how detailed it should be. There are several important features such as dendritic spines and axon collaterals that I was unable to find any reference to in this article. They should certainly be included. Interneurons should probably be discussed explicitly. The drawing of the "structure of a typical neuron" is quite inaccurate in several regards. That is perhaps a typical motor neuron, and the presence of a Schwann cell as the identifier for the myelin sheath makes that picture a typical PNS neuron but NOT the CNS. As pointed out in the article, myelin in the CNS is created by the oligodendrocytes, and many functional differences exist.

Also, the types of support structures inside neurites includes microtubules and microfilaments, not just neurofilaments. Perhaps something could and should be included about Neurofilament Tangle, which is associated with AD.

These are just a few initial thoughts. What do you think? Sirshane13

I definitely think this article could use more detail. By all means, your proposed additions about axon collaterals and dendritic spines sound great, you should definitely have a go at it. You could stick interneurons (inhibitory and excitatory) somewhere in the functional classification. You could also add a section about intracellular structures specific to neurons like microtubules and filaments, post-synaptic densities, etc. I've been meaning to expand this article but have been incredibly busy of late. Nrets 02:40, 17 November 2006 (UTC)

[edit] Other meanings

Neuron is also an unmanned aircraft currently developed by French Dassualt in collaboration with other European partners. See [9]. Perhaps something to add on the page? Filur 07:13, 20 December 2005 (UTC)

[edit] Classification

As part of the Neuroscien project, I thought I'd work on this article a bit. I'd like to take issue with the Neuron Classification scheme that is currently listed in the argument. "Afferent" and "Efferent" are not really useful classifications. Neurons are generally both afferent AND efferent at the same time. The usual classification is "Principal" (those that make longer-range connections) and "Interneurons" locally projecting neurons. The types of neurons listed "bipolar, unipolar" etc. is fairly archaic and incomplete. Nonetheless, I know this article represents considerable work from other authors, so I'm going to lay off for a couple of days to see if anyone wants to defend these classification schemes. Synaptidude 01:00, 11 January 2006 (UTC)

• Go forth and update! That's my vote. :) Semiconscious · talk 01:06, 11 January 2006 (UTC)

• In some cases afferent and efferent can be useful terms, even if the same cell can be both. But this is more relevant when talking in terms of neural circuitry rather than neurons per se. Nevertheless, some mention should remain. As far as principal versus interneurons, I think that's something worth adding, although some locally projecting neurons also make long-range connections. Also, I agree that the morphological characterizations are a bit archaic, but are still preserved in most textbooks and again are useful to someone who has never heard of a neuron before. So again, if you find a better classification scheme which retains some functional aspects as well as morphological aspect, I'm all for it. What scheme are you proposing? Nrets 01:42, 11 January 2006 (UTC)

• • At the moment I'm not proposing anything besides 'it should be improved' and hoping to build a consensus. Everything you say above is true. I would think that something along the lines of "The functional and anatomical properties of neurons can take many forms, and may even fall along a continuum of properties. Nonetheless, it is useful to try to classify neurons according to function and/or shape, even if no perfect classification scheme exists. In general, neurons are classified according to the shape of their dendritic trees or their axonal arborization...."

• • I still don't think that afferent and efferent are useful terms when describing single neurons. As you say, they are useful in circuits, but if you are considering a single neuron, it is neither afferent nor efferent. It is, as you know a relative term.Synaptidude 01:23, 13 January 2006 (UTC)

• Granted, efferent and afferent don't make sense in this article. Maybe we can fit these concepts somewhere in brain? Nrets 02:13, 13 January 2006 (UTC)

• I completely agree that afferent and efferent are not standard terms for use in the context of neurons. Especially considering the dendritic arborization and local neural activity that makes those terms almost meaningless. Afferent and efferent have an obvious place in nerves and pathways, which can certainly be made up of long axonal projections. Sirshane13

[edit] Neuron count by species

I am looking for a partial list of species by neuron count, neuron density or neurons per kg of body mass. Somegeek 13:17, 21 August 2006 (UTC)

I don't have a ref for what you are asking, but based on your request, you might find the work of Sam Wang at Princeton relevant to what you are looking for. Here's a link to one fo his papers. Nrets 13:48, 21 August 2006 (UTC)

[edit] Wikification of opening paragraph

I've done some copy editing aiming at wikifying the opening paragraph, the idea of being to summarise what a neuron is rather than stacking facts on top of each other. I'd welcome comments whether this is a step in the right direction. I dropped the word "morphologically" simply because it's unlikely to be understood by a lay audience. It may be redundant too because the sentence is talking about structure rather than function. I think this paragraph still needs plenty of work. For example what is meant by "classic view". Does this mean what the text books teach? A commonly agreed view? An out of date view? etc.--Saganaki- 03:32, 13 October 2006 (UTC)

[edit] Challenges to the neuron doctrine

The contents of this section are a reworded expansion of a section of the same name in the neural doctrine article. I think this section in the neuron article should be moved to the neural doctrine article and replaced with language that points there. Wdfarmer 04:42, 28 November 2006 (UTC)

[edit] Obfuscation of Opening Paragraph?

At the time I write this, the opening paragraph begins: "Neurons ... are not electrically excitable cells in the digestive system that function to process and transmit information." I don't know what to say about this. To take it seriously, I'd say, the digestive system is entirely irrelevant to the topic, and should not be in the opening gloss paragraph, although something general could be said distinguishing nerve cells from cerebral neurons. But the opening paragraph as it stands is hard to take seriously. Is this vandalism, or am I missing something? As vandalism it seems less than silly. Peter H. St.John, M.S. 14:35, 28 November 2006 (UTC)

Thank you for alerting us to the vandal edit. I have reverted it. You can too, as with experience you will recognize it quickly. The great majority of vandalism is done by anonymous IP address editors. Hu 14:43, 28 November 2006 (UTC)

[edit] Two copyright violations

The images NisslHippo2.jpg and NeuronGolgi.png are copyrighted. They are mislabled as NIH on their wikimedia pages. They are in fact propertiy of U.C.Davis and useable for personal or academic but not commercial use and therefore incompatable with the GFDL or CreativeCommons.

If we are to bring this article up to Featured status, this must be addressed.

--Selket 23:53, 25 January 2007 (UTC)

I've replaced one of them with an equivalent with compatible license. Nrets 17:55, 2 February 2007 (UTC)

[edit] Deleted image

The images at Brainmaps.org are copyrighted. Just because you zoomed in on it and took a screen shot does not make you the creator of the image. We need to be very carefull on Wikipedia not to inadvertantly infringe on someone's copyright. --Selket 22:50, 3 February 2007 (UTC)

This image is a screenshot and thus in the public domain according to their terms of use. Medlat 23:31, 3 February 2007 (UTC)

[edit] Neuron/synaptic memory?

What is the current understanding on how information in the brain is stored? Do neurons themselves have a capacity to store information or is information stored by the pattern of synaptic links? Where can I find out more? Thanks.

Solicitous 06:48, 3 March 2007 (UTC)

[edit] Rodent hippocampus?

The caption for the picture here states: "Golgi-stained neurons in the rodent hippocampus." The information given with the article that houses the actual image states: "Image of Golgi stained neurons in the dentate gyrus of an epilepsy patient. 40 times magnification." Are we to assume that the rodent in question suffered from epilepsy, or has there been some kind of error? --Mal 12:54, 24 March 2007 (UTC)

[edit] NEED ENK NEURON

Need ENK Neuron information. endogenous inhibitor of pain. enkephalinergic neuron