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Discussion in 'MCAT Study Question Q&A' started by whoknows2008, Jun 14, 2008.
Can someone explain the difference? Thanks!
Pre refers to the neuron that comes before the synaptic junction, post refers to the neuron that is after the junction. In example, preganglionic CNS neurons synapse with postganglionic neurons that then go to some effector organ. When you use the term ganglion, it generally refers to the PNS while nucleus refers to analogous structures in the CNS.
And I would just like to add one tiny tidbit that I have seen appear more and more frequently and it's the location of the post ganglion for sympathetic vs parasympathetic neurons. Sympathetic is located near the spinal cord vs parasympathetic is near the organ of choice.
With the exception of sympathetic splanchnic nerves.
Another important point is that the sympathetic nervous systems uses ACh preganglion and epinephrine/norepinephrine (aka adrenaline) post ganglion. The parasympathetic system uses ACh for both. The sympathetic nervous system begins in the spinal cord while the parasympathetic begins in the brain.
isn't this backwards? I thought the ganglia for the SNS was near the spinal cord, and it has a long post-ganglionic neuron stretching to the effector, whereas the PNS has its ganglia near the effector, with a short post-ganglionic neuron.
There's a EK pic that just summarizes it perfectly. I'll try to "draw" it... heh. The | | is your spinal cord, and the O inside it is the cell body. < is a adrenergic terminal secreting epinephrine, or adrenaline. < is a cholinergic terminal secreting achetylcholine. (Btw, a group of cell bodies inside the CNS = nucleus, while a group of cell bodies outside the CNS = ganglion.)
The preganglionic neuron has the cell body originating from the spinal cord. The postganglionic neuron has the cell body originating from outside the spinal cord.
Also, notice how in the sympathetic nervous system, the postganglionic neuron originates sorta close to the spinal cord, while in the parasympathetic nervous system, the postganglionic neuron originates close to the effector.
So we can see that acetylcholine (Ach) is the neurotransmitter of choice in all parts of the parasympathetic nervous system to stimulate "rest or digest". Also, it's used by the somatic nervous system to stimulate voluntary muscles. However, the sympathetic nervous system uses epinephrine at the effector to stimulate "fight or flight" responses.
i'm not trying to sound like a douche, but...that is completely wrong. it is the autonomic NS that is only sent to the spinal cord (i.e. reflexes) and somatic NS that sends to the brain. neither of them "start" either places. they technically "start" where their sensory neurons are first receiving sensation.
look it up in a physio book, not EK. EK isn't complete. they always leave things out. honestly, it doesn't really matter for the mcat, though.
the sympathetic nervous system uses both Ach and epi/norepi. it depends on the receptors they are binding, too. for example, the preganglionic neuron releases Ach that binds to Nicotinic Ach receptors on the postganglionic cell. the postganglionic cell then releases catecholamines (norepi/epi/etc) that bind to alpha (either 1 or 2, i forget) adrenergic receptors on smooth muscle for vasoconstriction to decrease digestive activity, to Beta-2 receptors on smooth muscle for vasodilation and increased blood flow to heart, and so on.
the parasympathetic, like the other person said, only uses Ach, though. it uses Ach at the pre+post ganglions.
It is. post-mcat idiocy.
What page is it in the EK books? Thanks for your help everyone!
Symptathetic - Ach secretion (cholinergic), ends not near organ (such as paravertebral)
Parasympathetic - Ach secretion, end near / in organ (such as heart, vagus nerve fibers)
Sympathetic - noreadrenaline secretion (from paravertebral ganglion to organ,etc.), longer fibers
Paraysmpathetic - Ach (usually), in organ, shorter fibers
You guys are awesome! Completely understand it now! Sorry being an idiot! =)
Dude, I mentioned that. Look at the pic I drew Anyway, of course the drawing is a simplification, but I think for MCAT purposes, it's *plenty* good enough.