Neuron's location and signal exchange with the rest of the body

[fog37]

TL;DR Summary

where are neurons located inside the body

Hello,

Just checking that my understanding is correct: the nervous system is formed by the central nervous system (CNS), which comprises the brain and the spinal cord, and by the peripheral nervous system (PNS) which bring signals from and to the CNS from and to the rest of the body.

Both CNS and PNS are made out of neurons (and glial cells). Is the PNS just composed of nerves (bundles of axons from the neurons in the spinal cord) branching out to all the various regions of the human body? For example, I don't think there are neurons in our hands under the skin but just nerves, i.e. axons, transmitting signals from the sensory cells in our hand to the neurons which reside in the spinal chord. In essence, neurons are only located in the brain and in the spinal cord and reach out to the rest of the body via nerves...

Axons transfer signal from neuron to neuron or from neurons to other types of cells. Neurons receive signal from their dendrites which interface, through synaptic gaps, with the axons of other neurons. Does that mean a sensory cell in our skin also has an axon on which to load its signal and send it to neurons in the spinal cord? I don't think the neurons in the brain connect directly to other cells types in the body...

Is that correct?

Thanks!

 

[jim mcnamara]

Are you aware of "neural circuits" - like the patellar reflex. It looks to me like you may be confusing some things.
PNS is just handy way to categorize, not like an iron clad set of rules. Proprioception has special nerves for detecting muscle contractions/extensions, for example.

@BillTre can help, I think.

 

[fog37]

Thank you jim mcnamara.

So, for example, there are neurons even in our hands under the skin?

 

[jim mcnamara]

Please define neuron.
To answer your question using my understanding, yes neurons are in your hands. Especially in your fingertips.
My definition of neuron makes any part of a neural branch still a "neuron", so I'm still not clear on how you got your position. Can you cite a reference? Or is this something you derived on your own?

 

[BillTre]

Is that correct?

No, that is incorrect.

Although most neurons are in the CNS, there are many examples that are not.
In addition to the axons going all over the place peripherally, there are also nerve cell bodies in many places in the body.
Most or all of the cranial nerves have peripheral ganglia associated with them.
All of the spinal cord nerve roots have peripheral ganglia associated with them.

Peripheral nerve cell bodies are usually grouped together in ganglia.
Peripheral nervous system wikipedia entry here.
There are dorsal root ganglia (DRGs) that provide a lot of the sensory input to the spinal cord.
There are autonomic ganglia that are involved in neural control of various peripheral non-skeletal muscle functions.
There are large numbers of neurons around the gut (intestines, stomach etc.).
There are also peripheral neurons associated with hair cell inputs from the ears (hearing and balance) and the lateral line in fish and some amphibians.
The neurons of the eye are not considered peripheral because the retina is considered part of the CNS (due to its embryological origins).

So, for example, there are neurons even in our hands under the skin?

The peripheral neurons involved in innervating your hands would be located in the dorsal root ganglia of the nerves that go to the hands. These DRGs however are not in the hands but near the spinal cord at the spinal segments where the hand innervation arises from (shoulder area probably).

In general, the "primitive" state of nervous systems (evolutionarially speaking) is a peripheral nerve net (hydra, jellyfish).
Greater central control was achieved in evolution by grouping large numbers of neurons together in central nervous systems. However, many peripheral neurons remain (even in people) because they worked fine for simple local controls. Connections with the CNS allow their functions to be centrally modulated for adaptive reasons.

 

[fog37]

Please define neuron.
To answer your question using my understanding, yes neurons are in your hands. Especially in your fingertips.
My definition of neuron makes any part of a neural branch still a "neuron", so I'm still not clear on how you got your position. Can you cite a reference? Or is this something you derived on your own?

Well, a neuron is a specialized cell of the nervous system (I think there are 4 different types of neurons). Neurons form neural networks that process information from other connected neurons...

 

[BillTre]

The term neuron has been used to mean either just the cell body or the whole thing including its axons and dendrites.
Unfortunately confusing.

 

[jim mcnamara]

If you took human anatomy, as in med school, a long time ago, the entirety as well as parts of the nerve cell including axons, and dendrites was a neuron. That definition is what I was using earlier.

This is part of the cubby hole problem that scientists face in biology. Example: species
This is really a taxonomic catch-all. It cannot be anything but a sort of holding place. Because too many organisms appear to be a poor fit for a variety of reasons. Example: bacterial species swapping their DNA - horizontal DNA transfer. Or using the term species on a virus, which thing is debatably either non-living or living.

 

[fog37]

Thanks again. Yes, I agree that a neuron should be considered in its entirety with its axon (long or short) and dendrites.

My dilemma was really about the soma (central part of neural cell) be found not just in the CNS but in other place of the body like in our skin, our muscles, etc. The long axons surely reach there to receive the stimuli from other sensory cells...