What is the criteria for classifying neurons as small and large? Is this classification based on gross size or the length of axon? Do they have any physiological difference?

For instance it is said that Substantia gelatinosa of Rolando contains small neurons .

What is so special about calling those neurons as such? Aren't they doing the same function of conduction of impulses?

I have googled it but couldn't find any explanation for such naming.

  • $\begingroup$ Spinal cord isn't my forte, but when there are references to "small cells" these are always in my experience related to histological appearance, and though there is no specific criterion for what makes a cell small, it usually means "relative to other tissues" - I would expect these cells to be small relative to others in the spinal cord. I looked for a couple examples, and in the situations I found so far "small cell" always seemed like a description, rather than a designation. $\endgroup$ – Bryan Krause Apr 11 '17 at 15:12
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    $\begingroup$ My guess, though based on knowledge of other areas, is that the Substantia gelatinosa of Rolando is (or was originally) defined by the presence of smaller cell bodies that distinguished it from the surrounding areas before any function was elucidated. $\endgroup$ – Bryan Krause Apr 11 '17 at 15:13
  • $\begingroup$ (note that this terminology goes back to the early 1800s) $\endgroup$ – Bryan Krause Apr 11 '17 at 15:15

Depending on the neuron, small/large could mean one of two things:

1: Axon diameter, larger axons have lower resistance, so transmit action potentials faster see tables here. Important for neurons that cover long distances to convey information quickly.

2: What I think you mean though is the size of the cell body, where some neurons, e.g. Purkinje cells are much larger than nearby golgi cells. In which case the size is related to the function, large cells tend to have large dendritic fields and integrate many inputs into a single output, often relayed to a different area of the brain. Small cells are typically interneurons that only have an effect over a very small area.

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  • $\begingroup$ So, what kind of small neutrons are present in substantial gelatinosa of Rolando? Do they have large cell bodies or axons? $\endgroup$ – JM97 Apr 11 '17 at 14:57
  • $\begingroup$ I pretty much have no idea, I've never heard of it, but a quick look at wikipedia suggests that small slow-conducting sensory neurons synapse with neurons here, so I guess it forms a relay, where information from the sensory neurons is sorted out before being sent up to the brain. For some reason this kind of sensory information is transmitted slowly, so small diameter fibres. I'd guess the cell bodies are pretty small as well, but I can't be sure. $\endgroup$ – Oliver Houston Apr 11 '17 at 15:10
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    $\begingroup$ In my experience, calling a neuron "small/large" is always in reference to the size of the cell body. In some areas there may be a correlation with axon or dendrite length but it is usually based on Nissl stains of tissue so people are just referring to the soma. $\endgroup$ – Bryan Krause Apr 11 '17 at 15:16

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