Inhibitory post synaptic effects in the PNS?

January 29, 2006 4:33 PM

Biopsychology extra credit question and I have no idea... "Why do you suppose activation of the first synapse of the parasympathetic nervous system produces excitatory post synaptic effects, but activation at some target organs produce inhibitory post synaptic effects (i.e. slowing down organ activity)?"
posted by j-urb to science & nature (10 comments total)

If we're talking fight or flight, stimulation of the parasympathetic NS resulting in certain inhibitory effects is beneficial. For example, you want your heartbeat to increase and pump blood out to your muscles and brain, but who wants to worry about digesting their latest meal when they're facing down a bear?
posted by moira at 4:51 PM on January 29, 2006

Only I've got it backwards. Oops.
posted by moira at 4:53 PM on January 29, 2006

In fact, I didn't read the post well at all, it looks like. In which case, I can be of no help; please ignore me.
posted by moira at 5:01 PM on January 29, 2006

The parasympathetic system is a 2-neuron system. The first neuron exits your spinal column and synapses at a ganglion. This is where it meets up with the second neuron. Neuron A's effect on Neuron B is to excite it, in other words, to transmit the message on down the chain. (I am sure you already know this but, for clarity, "post synaptic effect" means "what the neuron does to the thing it's synapsing onto.")

Then, Neuron B synapses onto the target organ. So Neuron B's post synaptic effect is often to slow down the target organ. Just cuz that's what the parasympathetic nervous system does.

Hope I am reading the question correctly and this helps.
posted by selfmedicating at 5:19 PM on January 29, 2006

Selfmedicating wrote the answer I was just composing in my head. Probably more clearly than I would have done, too.
posted by ikkyu2 at 7:52 PM on January 29, 2006

You have to remember your receptors. First order PNS neurons release ACh which acts on nicotinic receptors, triggering an EPSP in the target, but second-order uses ACh released onto G-protein coupled muscarinic receptors, triggering an IPSP in the target.
posted by caution live frogs at 8:33 PM on January 29, 2006

Oh this question was on my Neurobio final two years ago. The type of nervous system doesn't really matter. Both ParaSymp and Symp NS can have EPSPs and IPSPs (excitatory and inhibitory postsynaptic potentials.)

What DOES matter however is the receptor that the neuron is acting on. (Acting on via the PNS's NT of choice acetylcholine - ACh)

For example, ACh can act on two types of receptors, Nicotinic and Muscarinic (named for famous drugs that block those receptors). The M2 Receptors are present in the myocardium (heart muscle), some smooth muscle around blood vessels, and other locations. M3 Receptors are located in exocrine glands, and some blood vessel smooth muscles. Anyway, ACh activity on M2 receptors opens K+ channels which hyperpolarize the target cell, and can inhibit adenyl cyclase (through G-proteins etc) and decrease intracellular cAMP concentration (generally seen as a "slowing down" effect).

However, M3 receptors (through another G-Protein cascade) increases the production of Inositol Triphosphate (IP3) and Diacylglyceride (DAG) which in turn release Ca++ from intracellular stores and generally "speeds up/turns on" a cell. (Actually in smooth muscle it'll increase contractility etc, but I hope you get my meaning).

So the ultimate issue is not about the ParaSymp NS, but rather about the NT receptor that it acts upon, and that receptors ultimate effect on the cell.

Hope this helps. (On Preview: damn you caution live frogs, that'll teach me to spend time on a response.)
posted by ruwan at 8:35 PM on January 29, 2006

Why are we helping someone cheat on their homework, again?
posted by matildaben at 9:07 PM on January 29, 2006

Because AxMe is often more succinct than a textbook and lets people who know stuff crosscheck it with others.
posted by flabdablet at 10:55 PM on January 29, 2006

What makes you think it's cheating? Do you know what class this is for, matildaben? Do you have the professor's syllabus in hand? Does it say, "you may not seek assistance from anyone else on your homework assignments?"

I took many classes in college where we were encouraged to seek help from others on homework, both other students in the class as well as outside help.
posted by DevilsAdvocate at 1:19 PM on January 30, 2006