Sacral Efferent Pathways are Sympathetic, Not Parasympathetic

A recent report in the journal Science proposed a big change in how we understand the sympathetic and parasympathetic pathways of the autonomic nervous system (ANS).

In a nutshell, the new model stipulates that the outflow (efferent pathways) are divided into a cranial division and spinal division—not the craniosacral and thoracolumbar divisions that we learned (and that exist in all A&P textbooks):

Current model:

• Craniosacral division (parasympathetic outflow)
• Thoracolumbar division (sympathetic outflow)

New model:

• Cranial division (parasympathetic outflow)
• Spinal division (sympathetic outflow)

The authors lay out embryological and genetic phenotype evidence to show that the sacral components of the ANS outflow pathways are similar to sympathetic thoracic pathways—not to cranial parasympathetic pathways as we have long supposed. 

But wait, you say, what about the parasympathetic control of the genitals, rectum, bladder? What about, well, all kinds of things that now seem to unravel? I suggest reading the rather brief and plainly written article in Science for the full answer. 

However, a few quick points may reduce your blood pressure a bit—and perhaps pique your interest.

Quick points about the new ANS model

• Thoracic and sacral pathways share common embryologic development by location and when looking at transcriptional markers associated with neurotransmitters that differ from the developmental pattern of cranial pathways.


• Thoracic and sacral pathways have a ventral exit point from the spinal cord; cranial pathways have a dorsal exit point.


• The pelvic ganglion has been considered a "mixed" sympathetic/parasympathetic ganglion because it receives fibers from both the upper lumbar and sacral segments. But if the sacral pathways are sympathetic, the pelvic ganglion is clearly a sympathetic ganglion (not mixed). 


• Analyses of transcription factors show that cells of the pelvic ganglia resemble those sympathetic ganglia and do not resemble cells in cranial ganglia.


• The supposed lumbar vs. sacral antagonism in the urinary bladder's detrusor muscle does not seem to hold up, with the lumbar inhibitory effects either not demonstrable in experiments or of questionable functional relevance.


• The effects on vessel dilation in genitals can be explained as a "continuity of action—rather than antagonism"


• The sacral pathway to the rectum seems to resemble sympathetic structure, not cranial (parasympathetic) structure.

What can we use from this in teaching undergraduate A&P?

• When covering the craniosacral/thoracolumbar scheme, consider mentioning this newly proposed model.


• Consider using this scenario to illustrate the dynamic nature of science. Perhaps discuss that long-held dogma is occasionally challenged using newer methods and ways of thinking.


• Consider discussing pros and cons of adopting the new model. For example, can evidence from mice extend to all vertebrates? Which is stronger, evidence for the current model or the new model? Which model is most useful in understanding principles of ANS regulation? A little critical thinking never hurt anyone (at least not much).

Want to know more?

The sacral autonomic outflow is sympathetic

• I. Espinosa-Medina, O. Saha, F. Boismoreau, Z. Chettouh, F. Rossi, W. D. Richardson, J.-F. Brunet. Science  18 Nov 2016: Vol. 354, Issue 6314, pp. 893-897 DOI: 10.1126/science.aah5454
• Peer-reviewed research report describing this discovery, Includes an updated version of the classic diagram of sympathetic and parasympathetic pathways.
• my-ap.us/2fNdcF3

Neural circuitry gets rewired

• Adameyko, I. Science 18 Nov 2016: Vol. 354, Issue 6314, pp. 833-834 DOI: 10.1126/science.aal2810
• Companion article to the report cited above, stating that "This finding provokes a serious shift in textbook knowledge, and, as with any fundamental discovery, it brings important practical implications..." and goes on to mention of a few of the implications (e.g., how to treat bladder dysfunction).
• my-ap.us/2gg9O8P

The Autonomic Nervous System. Part I.

• John Newport Langley. W. Heffer & Sons Ltd., Cambridge, 1921.80pp.
• Classic "primary source" that codified the modern concept of the ANS. 
• my-ap.us/2fYHt3M

Gray's Anatomy ANS diagram

• Henry Gray. 1918 (online edition at Bartleby)
• Classic diagram by Henry Vandyke Carter of ANS pathways from an early edition of Gray's Anatomy.
• my-ap.us/2fYGMaT or my-ap.us/2gcAmaW

Checking Our Attitudes About A&P Students "These Days"

Ever been part of a conversation among faculty about "students these days" and how unmotivated they are, or how they lack the skills or knowledge that you'd like them to have? Yeah, me too.

Today in my daily Nuzzel newsletter, I shared an excellent article from Faculty Focus that does a great job of exposing the dangers of such conversations. Dangers to students, dangers to our academic institutions, and dangers to ourselves as educators. Although the author, Maryellen Wiemer, admits that occasional venting to a trusted colleagues helps us put things in perspective, she also points out the many harms that outright chronic complaining can do.

I'm not going to summarize that article here—it's best read in it's entirety. However, I'd like to add my two cents. After all, what's the good of having my own blog if I can't do that once in a while, eh?

It took me decades of teaching in high school and college classrooms to fully realize what I think my role as an A&P professor should be. It's not solely to guide well-prepared, self-motivated, highly skilled students to the success that they can easily achieve without me. Sure, that's easy and mostly annoyance-free. But it can be awfully boring. What do they need me for, anyway?  Not much.

I came to discover that what really rocks my boat as a professor is when I can help a struggling student achieve even a very small success. When I can help a learning-disabled student find ways to "get it" when studying those messy histology specimens. When I can help under-prepared students "catch up" and learn some effective study skills to continue keeping up. When I can get through to unfocused, unmotivated, immature students in some small way.

We pay a lot of lip service to making our courses "student centered" and making carefully devised learning outcomes our primary goal, but we often just don't want to do the work—or put up with the frustrations—of really making that happen.

It's when I finally started embracing those challenges and leaving aside my unhelpful judgments of "students these days" that I finally started truly and totally loving teaching my A&P students. I found that the more I connected with "problem students," the closer I got to finding the underlying reasons for their apparent lack of will or ability—and thus able to help them find appropriate strategies to succeed.

Sometimes, sporadic attendance is more about serious family or health issues than it is about their attitude toward my course. Sometimes, their lack of focus in my class is more about neurological issues, personal emergencies outside the classroom, or side effects of an illness or therapy, than it is about them "not caring" about their learning. Sometimes, their lack of reading is more about dyslexia than it is about laziness.

Sure, it's sometimes hard to face challenges. Otherwise, we wouldn't call them challenges, eh? But when I ask myself, "what kind of teacher do I want to be today?" the answer always comes back to, "the kind who is going to help even the most challenging students." And that makes all the difference.

Want to know more?

Ugly Consequences of Complaining about 'Students These Days'

• Maryellen Weimer. Faculty Focus. 16 November 2016.
• This is the article to which I refer in today's blog post.
• my-ap.us/2fY77ZU

Photo: John Wisbey