Neuron lengths, sizes and quantity

[icakeov]

As I understand, there are peripheral neurons that stretch for up to a meter or more from the spine to wherever the muscle is.

I was curious, does this apply to all the neurons that for example, move my right hand? Are all the neurons in my fingers individually that long, starting at the spine and ending in my fingers? Or is it the specific odd one and the rest "chain up" in dozens or hundreds? For example, this image portrays the bunch of peripheral neurons as single neurons, straight to the destination. https://image.slidesharecdn.com/030...spinal-nerves-part-i-21-728.jpg?cb=1311007080

Furthermore, based on all this, how many neurons does it really take to move my index finger approximately starting from the spine to the periphery? I always figured it was thousands of neurons, just like it is dense in the brain itself. But I guess things are a bit more "scarce" and "elongated" in the peripheral world?

Are there similarly long neurons in the brain and the spine as well? I have also found a piece of information that says that a neuron on average starts at around one inch. That is pretty long for a cell.

And finally, if the neuron is that long and yet that thin, does that mean that it is incredibly agile? Wouldn't a nerve that is running to the finger be in danger of sooner or later breaking in it's lifetime?

On that note, this reminded me of the "infamous" sciatica nerve and whether it is the nerve that is causing a hip pain or if it is predominantly the muscles. I watched this video once where this physiotherapist guy says it is a bad idea to use the lacrosse ball on your hip and then he showed the "real size" of the nerve saying, look at how big this is (exact moment of the clip right below @1:44). This totally confused me, I understand that the nerve is super long but how thick is it, or not, and if not, how exposed and damage prone really is a super tiny thin and super long nerve?


I must be missing something here, I haven't really easily found topics on this online, and I know it is a bunch of questions on the topic too. Any thoughts or comments are welcome.

 

[Tom.G]

https://www.google.com/search?source=&q=size+peripheral+nerves+human&oq=size+peripheral+nerves+human

found 3 010 000 matches

 

[icakeov]

Should I erase this thread, now that you have offered me 3 010 000 matches?

 

[Tom.G]

Naw, there are bound to be others that are curious. This is, afterall, an Biology forum.

 

[icakeov]

I don't know why you even bothered posting anything. The whole point of going to a forum is to get some pointers and if lucky to get some kind of a dialogue.
It took me a while to put the above questions together, I have been reading about neural stuff for a while and this always kept being vague for me. And I am not an expert on this topic.

I looked through the link that your provided, the whole first page and none of it addressed any of my questions. There was one article that got close yet had so much science lingo that I didn't know what to make of it. The following pages with 3 010 000 - 10 matches only got more vague and further from the topic. So really, your input was rather worthless. Sounds to me you spent 5 seconds in total entertaining the thread.

Next time, if you see a posting with my account name, please do me a favor and don't even bother clicking on it. Thank you.

 

[Tom.G]

You're right, only a couple minutes. Try this for an admittedly short answer to your first question.
https://www.reference.com/science/size-nerve-cell-6f0a12116861dbe3#
It took about 26 minutes to find.

 

[jim mcnamara]

This post is predicated on clearing up confusion.

https://faculty.washington.edu/chudler/what.html

First off, the sciatic nerve is composed of many neurons. It is the largest, in the human body, and carries "traffic" for both sensory and motor processes.

There are huge number of neurons in the body. The brain has most of the neurons.

The brain has approximately 1 quadrillion synapses (10¹⁵), synapses are connections from axon -> dendrite. One neuron may connect to multiple other neurons. There are approximately 100 billion (100,000,000,000) neurons in the human brain.

Length:
http://bionumbers.hms.harvard.edu/bionumber.aspx?id=104901 <- great site for biological how many and how big questions

Answer for longest single neuron in humans:
per the site above is from the base of the spine to the big toe, circa one-half of your height.

Communication in the brain:
Using those estimated numbers of neurons you can see that communicating from one brain region to another is most likely via a large number of very small neurons "talking" to neighboring neurons. The corpus callosum allows communication between brain hemispheres and is like a trunk circuit in the telephone system.

In all honesty, some questions asked are self-evident, ex: are nerves flexible? Bend your index finger, and touch the very tip of the bent finger, did you feel the touch? What does that tell you about the flexibility of nerve tissue? The sensation "went around the corner" did it not?

 

[BillTre]

Hopefully @jim mcnamara answered a lot of your questions.

Here are some others and some general explanation:

1) Among all vertebrate animals (people are vertebrates; invertebrate animals are varied and can have different rules), the cell bodies of striated (voluntary) muscle motor neurons (motor neurons are the neurons that control muscles) reside in the spinal cord. These muscles are what most people think of as their muscles and do not include heart muscle and the smooth muscles found in the intestine and other places. Non-voluntary muscle innervation in vertebrates can follow different rules. Even these exceptions do not have huge numbers of cells strung together to reach their targets (probably not more than 10). Stringing together many nerve cells would slow down how quickly a signal could be transmitted to the muscle because each synapse going from one cell to another would add a synaptic delay to the progression of the signal. Synaptic delays are about 2-3 milli-seconds per synapse. The rate of movement of an action potential (the signal) down an axons is much faster. Thus it is adaptive for an animal to have fewer synapses in these situations so it can react more quickly to the situations it has to deal with.
The longest axons would be found in large vertebrates with the greatest distance between the spinal cord and the furthest muscle in its body, such as in a giraffe are a whale.

2) The cells do not start out very large, but they can get larger. Generally speaking, the longer the axon, the larger the cell body.
Axons grow out from the cell body. Most of them are quite skinny. It would take a microscope to see them. Their growth is directed where to go by their growth cone, which is kind of like a bit of moving cell that pulls away from the stationary cell body while remaining attached to the cell body by the axon. Here is a movie. The long skinny things extending from the growth cone are often called filopodia. They are both sensory for the growth cone, telling it what is in its environment, and motor in that they exert tension and can pull the growth cone forward.
Most of the supplies for the axon and synapses at its end are transported down the axon from the cell body. Much of it is due to fast axonal transport which due to little protein motors dragging things down along the microtubules that go down the center of the axon. The periferal glial cells can also provide some supplies to the axons/synapses.

3) In the nerve in the periphery (outside of the CNS (brain and spinal cord)), the axons are encased in myelin made from wrappings of periferal glial cells (non-neuronal nervous system cells) around individual axons. All this can then be wrapped in protective connective tissue like fibroblasts, so they are reasonably well cushioned and protected. As Jim said they are flexible and can move around to a degree inside the body. This helps them avoid pinching damage. They tend to run along with blood vessels (also potentially fragile) through the body. They can be damaged, but tend to be in protected areas (with exceptions like the Sciatic nerve, in certain places where it is more exposed).
You should be able to see the sciatic in your turkey today if you get a drumstick. The sciatic nerve is pretty much the same in turkeys, chickens, and people. Nerves are generally located deep in the body near the bone.

4) Each muscle is composed of a huge number of individual muscle cells. In vertebrates, each muscle cell is innervated (controlled by) one motor neuron. Each motor neuron will innervate many muscle cells in a muscle. There are usually a large number of motor neurons innervating a single muscle, making large nerves. As a nerve (composed of many axons) passes muscles on its way further away from the CNS, it will get smaller as more axons branch off from the main body of the nerve to innervate the various muscles it goes by. This is because the number of axons in the nerve is being reduced.

 

[icakeov]

This was super helpful and cleared up many things, thank you @jim mcnamara and @BillTre!

- I realize I ended up mixing the terms "neuron" and "nerve". My bad! A "neuron" means a single neuron, whereas a "nerve" can mean one, but more often, a bunch of neuron cells. I think that's where most of the confusion came from. (And I realized that the confusion was all that bigger with this image, where it shows one or just few neurons per domain doing long stretches. https://image.slidesharecdn.com/030...spinal-nerves-part-i-21-728.jpg?cb=1311007080 )

- This created the distinction between a "long nerve" (which seems to be what nerve is by definition) and a "long neuron" (which seems to apply to certain neurons). I thought neurons in nerve fibres were mostly or all long. I imagine the "long neuron" could be and probably usually is within a "nerve", surrounded by other smaller neurons, right?

- Finally, my question about the "flexibility of nerve tissue", I was actually asking about the flexibility of the "long neurons" and how agile and "flexible" those were. I guess they have some flexibility, but mainly that the long neurons are probably fairly tucked in and protected with all the other body tissues.

Many thanks again!

 

[icakeov]

Hi, I think I managed to narrow down my question, after learning a few specific terminologies. What I meant to ask is:

In long peripheral nerves, how long are on average neuronal axons? I understand that the longest can be meters long and I keep finding that statistic, but is there any statistic that shows the average length of axons in one's nerve? And especially, what the variance would be, are there axons that are super short, ranging to the super long ones, or is the mean somewhere on the long end with a low variance?

And also, if there is any statistics on how many axons would be passing at cross section of a nerve? Orders of hundreds? thousands?

Many thanks again.

 

[BillTre]

Neurons come in in different classes. Each of those classes will have their own characteristics with respect to their axon length.
Two big classes of neurons:

• local neurons: only innervate (make synapses with) cells near to their location. These could be thought of as forming part of a local processor to generate location specific Input/Output relationships.
  
• projection neurons: grow axons that go for a long way to targets in other areas. These could be thought of as providing an Output from a local area to other specific areas (with their own processing properties).

Different classes of projection neurons have been characterized and in most cases their cell bodies are found in characteristic places (either ganglia or longitudinal columns in the CNS). Here are some classes:

1. somatic motor neurons
2. branchial motor (special visceral) neurons
   
3. autonomic motor (general visceral) neurons (sympathetic and parasympathetic)
4. autonomic sensory (general visceral) neurons
5. branchial sensory (special visceral) neurons
6. somatic sensory neurons
7. various special sensory neurons (eyes, ears, taste, smell)

Most of the neurons in long peripheral nerves will be going directly, the whole distance through the nerve.
These would include those most people are most familiar with like #'s 1, 2, 6, most of 7 (smell goes only a short distance).

#1 cells are in the CNS (usually in a ventro-medial column) and control muscle movement
#2 cells in the CNS (dorsal to #1), project to their peripheral targets (gill arches or evolutionarially derived from gill arch strutures).
#6 cell bodies are in ganglia just out side of the CNS and project an axon to the periphery to get sensory input and another to the CNS to pass the info on to the CNS
#7 These cells gro axons that go long distances to from sensory organs to their targets located in the CNS
​

Other neurons project their axons shorter distances, indirectly getting only part way to their "final targets".

#3 cells usually innervate a second set of intermediary neurons in peripheral ganglia, located either in a chain of ganglia along the spinal cord or embedded in the a peripheral target organ.
#4 cells should project indirectly to cells found either in local ganglia in peripheral organs or in ganglia near the spinal cord.
#5 cell bodies would be in peripheral ganglia associated with branchial arches (or their evolutionary derivatives)​

Peripheral nerves will have a mix of these classes on nerve cell axons. Numerically, most will be long directly projecting axons of groups # 1,2 and 6. Some other neurons will project their axons indirectly to ganglia which will then innervate the "final target". Most of these will not be chains of lots of cells to span the length of the nerve. Most will only have one or two intervening neurons across this distance and those neurons will be found in particular locations (ganglia).

 

[icakeov]

Thank you so much @BillTre! That really answered it!

 

[icakeov]

Here is a good example of why I was confusing nerves and neurons. This video is really well done and seems to have a lot of information packed into it, but when it makes a reference to how the impulse moves through the body, the video uses the expression "sensory neuron", rather than "sensory nerve", and for all the other domains too (interneuron, motor neuron... ).

Do you think in this video they actually literally mean "neuron" (because it all moves all that faster the less the neurons in the chain), or are they just pairing the complexity down to make it easier for people to understand the overall concept?


Any thoughts appreciated!

 

[BillTre]

I agree it s a nicely done video.

They were talking about the neurons as if they were single cells, not a lot in a bundle. I'm not sure if that's what you are getting at though.
The reason they describe things like that is the single neurons and their connections and functional features are can be understood as the single set of elements, repeated many times. A nerve being a bundle of many (hundreds to thousands) parallel running single nerve cells.
In this way, one could say they are reducing the complexity to better convey the idea of how the signal flows through the system. at a cellular level.

You are right that (generally speaking) the fewer steps of neurons, the faster the signal can go. Synapses take 2 msec for a signal to go less than a micron.

My own annoyance with the video:
I did not like their animation of the dendrites, cell body, and the axon hillock depolarizing.
Little localized flashes in the dendrites makes sense because there would be different little inputs from synapses out there.
However, the whole cell body would have to depolarize to influence the axon hillock's depolarization, on the other side of the cell.
It would be more of a diffuse glow, building to a brighter threshold level for the axon hillock. Not like a glowball moving across the cell.

 

[Ygggdrasil]

My own annoyance with the video:
I did not like their animation of the dendrites, cell body, and the axon hillock depolarizing.
Little localized flashes in the dendrites makes sense because there would be different little inputs from synapses out there.
However, the whole cell body would have to depolarize to influence the axon hillock's depolarization, on the other side of the cell.
It would be more of a diffuse glow, building to a brighter threshold level for the axon hillock. Not like a glowball moving across the cell.

Researchers have been able to take movies of action potentials moving along the dendrites and axons of neurons. They take these movies by using voltage-sensitive proteins that change their fluorescence as the membrane potential of the neuron changes. You can see a movie on the website of one of the scientists who has created some of these voltage-sensing proteins: http://cohenweb.rc.fas.harvard.edu/

Details of how the movies are created can be found in the following paper: https://www.nature.com/articles/nmeth.3000