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
- 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.
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)
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.
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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Last Updated 09/06/01 08:53:36 PM
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