Special Senses: somatic vs. visceral distinction

[icakeov]

Hello, I was reading up on different afferent neural input and came across the "special senses" (besides general senses) definition:
https://en.wikipedia.org/wiki/Special_senses

In there, there is a distinction between somatic and visceral senses:
special somatic afferents and special visceral afferents.
I was a bit confused about why vision and hearing got "somatic", whereas smell and taste got "visceral". The explanation is that the latter two "develop in association with the gastrointestinal tract."

Basically, due to association to a visceral organ, such as the gastrointestinal tract, an organ gets considered "visceral"? Is this because the gastrointestinal tract sends "information" to smell and taste organs, making it more "visceral" and less "somatic?

Thanks for any feedback.

 

[jim mcnamara]

visceral = deep inward feelings, not intellect. So, do you think this concept of afferent senses came about by circa 1850, 1950, or 2001?

Association to viscera? No, visceral afferent is inbound. Your viscera are the organs in your abdomen -> afferent is inbound from there to the CNS.

Hint: biology has been around for a loooong time.

Spoiler

in the mid 1800's

 

[BillTre]

The wikipedia entry is kinda on the light side.
As @jim mcnamara said this is an old subject, however, I think it is actually more associated with CNS structure and embryological origins.

Here is a long rather complex explanation for these names (good luck getting through it):

The CNS forms from a neural plate that starts out as a dorsal (top as opposed to belly side) region of ectoderm (skin equivalent of the embryo). It rolls up into a longitudinal running neural tube and sinks into the body with the ectoderm (future skin now) closing over top. Shown well here. The neural crest comes from the edge of the neural tube/plate and makes peripheral nervous system elements among other things.

Within the neural tube/plate, functional columns run longitudinally (from the anterior (head end) to the posterior (tail end)), forming what are called functional columns, in which particular kinds of neurons form and different fiber tracts run. This is most clear in the spinal cord which lacks the diversity of inputs and outputs of the hindbrain. The spinal cord columns form continuous columns. The hindbrain is like (and continuous with) the spinal cord but much wider to pack in all the different bits needed to handle its additional functions. Anterior to the hindbrain, the midbrain and forebrain regions are even more distorted.

The spinal cord and hindbrain both have follow the general rule of motor functions are in the ventral half and sensory functions are dorsal half. in some places anatomical grooves can be found on the brain where these two areas meet. This distinction applies to location of cell bodies, where longitudinal fibers run, and where synapses form (with many exceptions). Molecular markers are expressed in these different regions also.

Within the spinal cord, the somatic columns are found in the dorsal most (in a non-rolled up plate, most lateral) column (sensory) and ventral (most medial (near the midline) in the neural plate) most column (motor). The visceral sensory (dorsal, below the somatic sensory) and visceral motor (ventral but dorsal to the somatic motor column) columns lie between the separated somatic columns This pattern is continuous into the larger hindbrain. These Jim said, the visceral afferents (inputs) and motor outputs go to and from the visceral (organs and GI tract, autonomic nervous system). The somatic columns deal with mostly muscles and sensory feedback from muscles, skin tendons, joints etc. (which together make a bunch of other under appreciated sensory modalities). A lot of the peripheral (outside the CNS) autonomic nervous system is derived from the neural crest which migrates to many places during embryology, thus this has a distinct embryonic origin.

In the hindbrain, the nice continuous columnar organization is broken up into cranial nerve nuclei as the hindbrain has a segmental organization imposed on top of this simple columnar structure. In addition, the rolling up of the neural plate doesn't happen so much in the hindbrain leaving it a kind of curled plate in cross section. Here is a decent picture of spinal cord x-section on top and hindbrain below it. This shows the columns in cross section. http://www.nature.com/nrn/journal/v1/n2/images/nrn1100_116a_f1.gif largely correspond to the branchial (gill) arches that are developing next to it and and are coded for by different combinations of Hox genes. The hindbrain general somatic and visceral motor and sensory columns do similar things to those in the spinal cord. In addition, to the neural crest derived peripheral nervous system elements, the head also has peripheral neural elements derived from placodes (ectodermal thickening) that (often, but not always) form in association with the neural arches. These elements (unique to the head) make a variety of sensory elements that are not found in the non-head body and are therefore called special. The vertebrate eyes are induced by the lens placode (the retinal is actually a CNS derived structure), hair cells for hearing and balance come from the otic (ear) palcode and lateral line cells come from migrating palcode that arise near the otic placode, the taste buds could be argued to be placodes, and the olfactory neurons come from the olfactory placode.

The special somatic and special visceral senses are laid onto this already existing spinal cord columnar organization by forming additional columns. So, more columns and columns broken up into segmental nuclei (clusters of neurons) result in a complex hindbrain with this obscure naming.

Interestingly, many special sense cell receptor structures are modified cilia: photo-receptors (vision), hair cells (hearing, sense of balance, lateral line), taste, and possibly olfactory receptors (smell).

 

[jim mcnamara]

Ontogenesis.

This subject is an example of why back in the early 1960's graduate students were advised to learn German. Most of the research in ontogeny (a micro-fraction thereof, what @BillTre just explained) was developed by German researchers. In fact a lot of the early major developments came from Germany in the 1800's and early 1900's. Students also were expected to have learned Latin and sometimes Greek in secondary school. Nomenclature, like the word afferent, derives almost exclusively from root words in those two languages.
Afferent derives: Latin ad==to or towards, ferre==to carry

I do not know what language proficiencies grad schools require now. If any. Way back it was two romance languages, one classical (or sometimes one romance, two classical).

I had a botanical dictionary from 1938 that had about 40000 entries in it. With words most modern graduate students/scientists will seldom if ever encounter. Unless they are in plant anatomy or taxonomy. Or enjoy weird word games.

I digress.
Anyway, if you get 'Ontogeny recapitulates phylogeny' that is what Bill is on about. What organizational levels/structures of tissues are derived from.
Example: Cell type A in colonial organism->tissue B in Porifera->organ C in amphibians->complex organ D in mammals.
Great exam questions. And close to impossible if you do not know all of the terms. Also why dissection of frogs and fish is part of the Biology curriculum in High School in the US.

 

[icakeov]

Amazing detail! Thanks so much Jim & Bill, that really clarifies so much and answers my question fully. (Had to spend some time with it)

And I would have not guessed 1800! Makes sense that 200+ years would have generated such a wealthy library of terms.

 

[BillTre]

My thesis involved this kind of stuff, thus, lots of obscuria.

Nomenclature, like the word afferent, derives almost exclusively from root words in those two languages.
Afferent derives: Latin ad==to or towards, ferre==to carry

I do not know what language proficiencies grad schools require now. If any. Way back it was two romance languages, one classical (or sometimes one romance, two classical).

I was all into this as a kid and took Latin when I was younger. And have a dictionary of prefixes and suffixes (Latin and Greek mostly) which certainly helps to understand word meanings.
I am lousy at languages though and took computer languages (mostly forgotten) as my language requirement in grad school.

 

[jim mcnamara]

@BillTre said - 'My thesis involved this kind of stuff, thus, lots of obscuria.'

My condolences.

My PhD involved grasses. Just as bad. Ever see Agnes Chase's ' Manual of the Grasses of the United States' 1954?
The title says A S Hitchcock but everyone knows she did all of the work. A tome and a half. I came to be very conversant with it.

https://www.amazon.com/dp/0486227189/?tag=pfamazon01-20

 

[BillTre]

Never saw that book (not a bad price on it). Plants somewhat confound me. I was in zoology and the Botany Dept. wouldn't let me take the botany courses I wanted (required some stupid prerequisites).

I did run into https://www.jstor.org/stable/20103101?seq=1#page_scan_tab_contents while researching a fiddler crab project.
I have always had a fondness for decapod crustaceans.

 

[icakeov]

And what about where the lines are drawn between "external senses" and "internal senses"? (I think the terms for these are exteroceptive and interoceptive)

For a while I imagined that somatic organs would automatically be organs that "collect external stimuli" and visceral "collect the internal".

Or I suppose a pain or temperature receptor on the skin could detect a pain inflicted by either external or internal process. How strict is the distinction between exteroceptive and interoceptive organs?

 

[BillTre]

The external-internal (or exteroceptive and interoceptive) distinction makes sense (sensory inputs from outside the body vs. sensory inputs from the inside of the body), but it does not match up with the visceral-somatic distinction.

Visceral senses are for sure internal (organs and GI tract), but there are other internal inputs which are functionally associated with the somatic components of the nervous system (which deals largely with muscle control).
For example, muscle spindle receptors are involved in controlling muscle contraction and project to parts of the general somatic sensory column, thus not visceral in the neurological way of considering at things. Similarly, other sensory inputs that are involved in fine tuning motor functions, like tendon strain receptors, would be conveying sensory information generated from the inside of the body. They would therefore be somatic since they are involved in muscle control and they project axons to somatic areas of the nervous system.

 

[icakeov]

What about feelings?
Is there any consensus on where in this complexity of sensory input (somatic or visceral, external or internal) the concept of "feelings" would fall under?

I imagine in the visceral sensory domain?

If yes, this would mean that if one's arm hurts, they wouldn't quite say that they are "feeling" pain? Or at least not in the context of it being "a feeling", but rather that their "hand hurts" or "is in pain" "somatically"?

 

[jim mcnamara]

We are getting to the fuzzy end of Biology. Many 'feelings' are secondary to a variety of very complex inputs that have very little to do with neurobiology per se. Example someone points a gun at you. Your hormones quickly react to a real threat. The 'react' part of this comes derives from cognition and direct influence on hormones, like adrenaline.

Not afferent nerves necessarily. Pain, yes. Extreme pain can alter your basic homeostasis, not to mention your feelings. Pains come in via afferent nerves. Even so-called nerve pain (referred pain) , like sciatica. Sciatica is caused usually near the lower four lumbar vertebrae where inflammation or damaged tissue impinge on the sciatic nerve, creating pain well away from the site of actual problem - like outside of the thigh, hip, outside of the calf.

Becoming depressed or sad or lonely can be a really complex set of circumstances, and completely due to brain activity altering levels of neurotransmitters. External input from afferent nerves absolutely not required. Example: you become sad thinking of your dog - that died last year.

So, let's not go there. Start another thread on that topic if you like. Please cite a reference on how you got to the point of asking your question. It helps us to help you.

 

[icakeov]

Agreed, it's too much of a general question to tackle in this thread that is pretty specific. Thanks for all the thoughts about it nevertheless Jim!