Physiology III
Neuroscience
Exam III - Essay #3

That which is in red was updated after input from Dr. Winterson...See below

The parasympathetic nervous system is sometime said to be "trophotropic", that is. energy conserving. Support this notion with a description of three examples of parasympathetic activation.

PNS is essentially the opposite of the SNS. The general construction of this system is to have a preganglionic neuron that has a very long axon that travel to and synapse closer to the effector cells. These receptors are nicotinic and utilize the NT ACh. The preganglionic cell bodies are located in the brainstem for the cranial nerves and the sacral spinal column for the lower gi and pelvic regions. The postganglionic neurons are located close to the effector cell so travel only a short distance to synapse via the NT ACh using muscarinic receptors. There are M1 and M2 receptors. Another different and special feature of the PNS is that it is more specific in its stimulation. The SNS is basically or at least usually an all or none response with the norepi and epi hitting most targets all at once thus a full generallized response. The PNS is more specific to stimulate particular areas and limited to control the overall tone of the effector cells. The PNS will put the brakes on the SNS thus attempt to conserve energy and maintain the body in homeostasis.

PSNS as ‘energy conserving’

Describe 3 examples of PSNS activation

  1. the gastrointestinal system – via increased activity of the intramural plexus, PSNS stimulation increases the overall degree of activity of the GI tract by promoting peristalsis and relaxing the sphincters thus allowing rapid propulsion of contents along the tract. This is mediated by the Xth cranial nerve, the vagal nerve. Also increases rates of secretion by many of the upper GI glands namely the nasal, parotid, submandibular, gastric, and pancreatic glands. These will aid in digestion by increasing the secretion of the enzymes needed for digestion. Blood flow to the gut is increased promoting digestion, and pancreatic insulin and glucagon in the liver are increased. It is these that allow for food digestion and energy storage thus the energy conserving feature. The term "rest and digest" is used to denote the PNS and this is a prime example of energy conservation in the form of food digestion and storage for use later.
  2. the heart – via the vagus nerve, PSNS activation decreases the overall activity of the heart by decreased rate and force of contraction (thus an overall decrease in cardiac demands.) The decreased pumping capability allows the heart some degree of rest between bouts of stenuous activity (energy conserving feature)
  3. most glands of the body are strongly stimulated by PSNS activety, encouraging digestion, endocrine functions, reproductive functions all which are energy conserving and maintenance activities. The lacrimal gland in particular will increase its secretion rate under PNS stimulation. I suppose you could stretch the energy conserving thought in that you can’t move around much and use up energy if you are caught in the middle of a crying jag! All the other glands were mentioned in the gi system above. Of note is that PNS stimulation to the reproductive areas will cause erection.
  4. the eyes – via the occulomotor nerve, PSNS activety contracts the circular muscle of the iris to constrict the pupil. It also contracts the ciliary muscle which releases the tension on the lens and the lens becomes convex allowing focus on near objects. The energy conserving feature here is that you can now see the food that is close in front of you and thus begin to eat and therefore store your energy and conserve for later!

(I’m not sure how we can say that the last two are energy conserving – any ideas???)

The preganglionic NT for the PSNS is ACH, and the receptors are nicotinic. The postganglionic NT for the PSNS is Ach and the receptor is muscarinic. The same mechanism is used as described in essay #2.

The comments in this box are from Dr. Winterson.  They have been integrated into the essay outline above and highlighted in red.
"Your examples should include at least one secretory function, perhaps more and at least on GI function. You could include decreased cardiac demands. You should also make the point that activation of parasympathetic control is often than not relatively discrete, (e.g. mote in eye, tearing) rather than en masse as in the sympathetic system "fight or flight". The parasympathetic innervation of the eye aids identification of food stuffs nearby (e.g. pupillary constriction increases depth of field, lens accommodation focuses eye on objects nearby.

Also, you could mention the anatomic distinctions of the parasympathetic nervous system and neurostransmitter and receptor subtypes as is appropriate."


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