My professor claims that the autonomic nervous system is only activated by stimuli from organs but I really feel like I've read that it can be activated by outside stimuli, although I'm not sure what receptors that would be.

Is my professor right?

  • $\begingroup$ my bet is that you should examine specific pathway. Saying in general that ANS is excited only from within doesn't mean much, to my taste. ANS actions are mediated by CNS so one can say that environment does have influence on ANS $\endgroup$ Commented Jun 10, 2015 at 23:02

1 Answer 1


Short answer
External stimuli can drive autonomic responses.

The autonomic nervous system is a visceral sensory and motor system. The viscera are the internal organs. Virtually all visceral reflexes are mediated by local circuits in the brain stem or spinal cord (Fig. 1). It is one of two major subdivisions of the nervous system; the other being the somatic (or voluntary) nervous system. The main distinction between the two is that the latter is involved in voluntary and conscious actions (e.g., movement, perception and cognition), while the autonomic nervous system is engaged in involuntary processes, such as regulating heart rate, breathing and bowel function (Fig. 1).

Autonomic nervous system Fig. 1. Autonomic nervous system. Source: Austin Community College

Although by far most stimuli driving autonomous responses come from within (think muscle movements driving bowel movements, increased heart rates during physical exercise), outside stimuli do directly impinge on the autonomous system.

For example, the cilliary muscle in the iris is under the control of light coming into the retina. The pupillary light reflex test is based on this principle, and is routinely used to diagnose damage to the visual system. Also, production of saliva is controlled by external stimuli (smell and taste of food and see the striking example below from @anongoodnurse). And technically, the entire digestive tract can be considered to be outside the body, as it is continuous with the skin and lining of the mouth. Hence, the mechanical stimuli impinging on the intestines by a moving bolus of food that drives the waves of peristaltic movements can, arguably, be considered to be caused by external stimuli too. Lastly, the fight-flight-fright response involving the hypothalamic-pituitary-adrenal gland (HPA) axis is often initiated by external stimuli (loud sudden noises etc.).

The most important characteristic of the autonomic nervous system, as said, is that the autonomous responses are not voluntarily controlled. While you can decide to look or track a visual stimulus, you cannot decide to move the bolus of food back up to your stomach, or voluntarily constrict your pupil. The way the input is received may not be the best way to distinguish it from the voluntary nervous system.

Didactic answer
If you are left with questions, the first place to go to is your professor.

- Kandel et al. Principles of Neural Science 4th ed. (2000). Ch.49 The autonomic nervous system and hypothalamus. p.960-81

  • 2
    $\begingroup$ I wonder if the professor meant it on another level: that the autonomic nerve system does not have sensory neurons connected to the outside world. The examples you cite are ultimately caused by external stimuli, but as far as I remember, visual and olfactory perception is not part of the ANS. This is of course a nitpicking distinction and not meant to make your answer look bad (I liked and upvoted it), but for a student who is trying to find their way through complex material, easily delineated distinctions are good signposts, so it could be an explanation for why the prof used the statement. $\endgroup$
    – rumtscho
    Commented Jun 11, 2015 at 7:54
  • $\begingroup$ @rumtscho - I have edited my answer to accommodate your comments. I agree that questioning your professor's words on SE may not be the best practice. In fact, my first short answer was always listen to your prof. I removed it, because lectures generate questions so it is logical to be left with questions. Further, there are always exceptions to the rule, that is fundamental to biological sciences. Please check my "didactic answer" and I hope that suffices your critique on a student's needs. $\endgroup$
    – AliceD
    Commented Jun 11, 2015 at 10:19
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    $\begingroup$ @AliceD - Hmm... good answer, but professors are often enough wrong that I think SE is a great place to double check. One fun trick (to which you alluded) is to cause saliva to flow by telling a patient to imagine biting into a lemon, because this has become a conditioned response in adults. When there is a parotid duct obstruction, the parotid gland will swell before our very eyes., making the diagnosis obvious (and easy). Sensory nerves in the tongue for sour cause salivary production. There are other examples. $\endgroup$ Commented Jun 11, 2015 at 22:40
  • $\begingroup$ @anongoodnurse - Thanks for your comment and your interesting addition directly from medical experience. I made my answer stronger, and although leaving the didactic answer in place, I made it less binding. BTW: mod nominations at Bio are open :) $\endgroup$
    – AliceD
    Commented Jun 12, 2015 at 4:02
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    $\begingroup$ @anongoodnurse - thank you :) But I was in fact hinting that you should nominate yourself! $\endgroup$
    – AliceD
    Commented Jun 12, 2015 at 4:13

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