Homeostasis and the Autonomic Nervous System

Homeostasis, remember, is a word that Walter Cannon coined, and what does the autonomic nervous system have to do with homeostasis?  In general, there are two ways you can think of things sort of staying the same.  One way is by an integrative physiology point of view. You’ve got a homeostat, like a thermostat for instance, and it’s comparing the information about what’s going on with some set point for responding and when there’s a discrepancy then the effector gets turned on and the regulated variable is controlled.  There’s a lot of teleology involved here – that means, teleology basically is a school of philosophy based on the ends, the goals.  What is the goal of a thermostat?  The goal of a thermostat is to keep your temperature.  Well who says?  The inside the body no one has ever found evidence for a comparator, including a thermostat, even though of course your temperature is very closely regulated.  The alternative view is a systems biological point of view which in essence says, you know why your temperature is held in constant?  Because that’s the way it is.  That’s the way it is.  All the organisms that didn’t do this died out in evolution. And don’t give me this story about the purpose of a particular reflex or whatever.  The reflex is there because it’s there.  So that is the systems biology point of view.

But I am going to be going with the much more easy to understand integrative physiological point of view.  The reason that a monitored variable is held, the level is held in check, is because of negative feedback.  There’s a homeostat of some sort, so this is a classic picture of the way a blood pressure’s supposed to be regulated.  There is a barostatic system.  When the blood pressure goes up, there are these receptors that are stretched because of the increase in blood pressure that causes a nerve traffic to go to some barostat in the brainstem (you remember the brainstem is the chewy chocolate center of the Tootsie Roll Pop) and then there’s outflow, autonomic outflow.  When the blood pressure goes up, the heart rate goes down, and when the blood pressure goes up, the sympathetic nervous system is turned off and so the blood pressure tends to come back down.  So that’s a kind of a simple but often sufficient explanation of a barostatic system.

Now in humans a substantial number of the baroreceptors, which are stressed receptors, are in the wall of the carotid sinus…I’m sorry, the wall of the carotid artery, where the carotid artery splits into the internal and external carotid.  Right there is carotid sinus. And there are a sort of hybrid cells that are sensitive to stretch and when stimulated, when they’re stretched, there’s an increase in afferent traffic to the brain by the carotid sinus nerve.

If you’ve ever watched professional wresting you may have come across the sleeper hold where somebody quickly goes around the opponent’s back and sort of massages this part of the person’s neck, doesn’t choke him, that would be a disqualification, but just sort of massages.  And then the person falls asleep, just faints and collapses to the tarp, and that’s the sleeper hold.  When I was a kid, I remember there was a professional wrestler named Argentina Apollo I think, I think that was the guy, who used the sleeper hold a lot.  I’m not sure that professional wrestling is really like a competitive sport, I’m not sure, but I do like the idea of the sleeper hold, because it tells you that when you mechanically distort the carotid sinus, the baroreceptors there kind of think that the blood pressure’s gone up and so causes that reflex to occur where vagal outflow goes up, sympathetic outflow goes down, and the blood pressure goes down and then the person loses consciousness.

I think of the baroreflex, or the barostat, as kind of metaphor.  This is the way it actually looks in the brainstem and higher up in the central nervous system.  There’re all sorts of the interactions between all sorts of centers, but conceptually you have a barostat.  A barostat doesn’t exist, it’s a metaphor, but I think it’s very handy because this is tough to understand.