aside from having different functions the sympathetic and parasympathetic divisions of the autonomic nervous system also contain structural differences in this learning objective we'll compare the structural differences between sympathetic and parasympathetic modor neurons so in both divisions of the autonomic nervous system the motor pathway contains two motor neurons the first mon neuron is called a pre ganglionic neuron its cell body is located in the brain or spinal cord and its axon exits the central nervous system as part of a cranial nerve or a spinal nerve the axon of a preganglionic neuron is melinated and usually extends to an autonomic gangion where it synapses with the postganglionic neuron the second neuron in an autonomic motor pathway is called the post ganglionic neuron this is because it is post or comes after the ganglion the postganglionic neuron lies entirely outside of the central nervous system in our peripheral nervous system its cell body and dendrites are located in the gangan and the axon is an unmated ver fiber that extends to an affector so in an autonomic motor pathway the preg gangling onic neuron conveys nerve impulses from the central nervous system to the autonomic ganglia and postganglionic neurons relay nerve impulses from the autonomic ganglia to the effectors so taking a look at this image we have our pre ganglionic neuron here so going from the spinal cord or the brain stem to our gangon it's a melinated nerve fiber here we have our post ganglionic neuron extending from our gangion to our affectors and this is an unmarinated nerve fiber now taking a look at some of the structural differences and starting with a sympathetic Division and the preganglionic neurons of the sympathetic division of the autonomic nervous system have their cell bodies in the lateral horns of the gray matter in the 12 thoracic and first two Lumber segments of the spinal cord so this is that these cell bodies along here the ganglia of the sympathetic nervous system which remember is where our pre and our postganglionic neurons will syapse are located either in our sympathetic trunk ganglia which is also called our vertebral chain ganglia because it lies all in a row on either side of the vertebral column or the prevertebral ganglia which lie anterior to or in front of the vertebral column so our sympathetic trunk ganglia are these ones that run all the way from the base of the skull right down to the coxic on either side our prevertebral ganglia are these ganglia here now the post ganglionic axons that extend from the sympathetic trunk ganglia primarily innovate organs above the DI uh diaphragm so organs in the head the neck the shoulders and the Heart the postganglionic axons that extend from the prevertebral ganglia tend to innovate organs below the diaphragm so all of these down here now because our sympathetic trunk and our pre vertebral ganglia sit quite close to the spinal cord most of our sympathetic pre ganglionic neurons are quite short and most of our sympathetic postganglionic neurons or on are quite long so you can see that first pre ganglionic neuron here quite short because the gangon sits nice and close to the vertebral column but then we have the long post ganglionic neuron extending all the way to our fector is much longer now once the axons of our sympathetic pre ganglionic neurons reach the sympathetic trunk ganglia they can sign upse with a postganglionic neuron in a number of ways so an axin can syapse with a postganglionic neuron in the first gangon that it reaches so for example it comes straight out of our spinal cord and synapses with the first gangon it reaches the axon May Ascend or descend to either a higher or lower gangion perform synapsing with a post ganglionic neuron so again we might go uh through that spinal nerve and then either up or down to lower ganglion the axons of the pre gangion preganglionic neurons that either go up or down form what we call the sympathetic chain and so that's this part here that kind of connects all of those sympathetic trunk ganglia an axon can then continue without synapsing through the sympathetic trunk gangion and end at a pre vertebral gangion and syapse with the postganglionic neuron there so for example coming out of the spinal cord through the sympathetic trunk ganglia now to our prevertebral gangion and axon may also pass without synapsing through both the sympathetic trunk gangion and a prevertebral gangion and then extend to the chromaffin cells of our adrenal Medela which remember is this endocrine gland which sits on top of the kidneys so here we're going out the spinal nerve through our sympathetic trunk ganglia through our vertebral prevertebral ganglia to our chromaffin cells now the reason why a preganglionic neuron can take one of many Pathways to meet the postganglionic neuron is because a single sympathetic pre ganglionic neuron has many Aon collaterals which are just like axon branches that can syapse with 20 or more postganglionic neurons so one sympathetic preganglionic neuron can syapse with 20 or more post ganglionic neurons now this pattern of projection is what we call Divergence and it helps explain why many of our sympathetic responses affect almost the entire body simultaneously so think about when you're really nervous perhaps you have a presentation you have to give in class your heart races you feel sick in the stomach and you begin to sweat your whole body is feeling those nerves now looking at the parasympathetic division of the autonomic nervous system and the cell bodies of our parasympathetic preganglionic neurons are located either in the nuclei of four cranial nerves in the brain stem or in the lateral gray Horn of the gray matter in the second to Fourth sacral segments of the spinal cord so up in the nuclei in the brain stem up here or down in the spinal cord right at the bottom the autonomic ganglia of the parasympathetic itic division are called terminal ganglia and instead of being nice and close to the spinal cord these are actually either close to or within the wall of a visceral organ so they sit further out or quite close to the organs themselves now because the termal ganglia are located either close to or in the wall of a viseral organ our parasympathetic pre ganglionic axons are long while our parasym sythetic post ganglionic axons are short so we have these long pre ganglionic neurons and quite short post ganglionic neurons now the axons of our pre ganglionic neurons pass directly through the terminal ganglia either near or within that visceral affector in the ganglia the pre synaptic neuron usually only synapses with up to four or five post synaptic neurons all of which will supply single visceral effector so this is what allows our parasympathetic responses to be much more localized than our sympathetic responses now I know that that was a lot of information and a lot of me talking but most of that information that I just gave you can be summarized into a table like this now the only thing that this table doesn't describe is how one sympathetic pre ganglionic neuron can syapse with many post syap neurons which is why we experience the effect of the sympathetic nervous system more widely but that one parasympathetic pre ganglionic neuron synapses with far fewer postganglionic neurons which is why our parasympathetic effects are more localized but taking a look at our table we have our sympathetic division the cell bodies of those preganglionic neurons are located in the thoracic and Lumar spinal cords so remember these parts here the ganglia are found close to the spinal cord so either in this sympathetic trunk ganglia along here or our pre vertebral ganglia which sits anterior to the spinal cord because of the location of the ganglia we have short pre ganglionic fibers and long post ganglionic fibers if we take a look at our parasympathetic division these fibers originate in the brain and the sacral spinal cord so the nuclei in those cranial nerves up here in the brain stem and then also in the sacral spinal cord the ganglia are located either close to or actually within the walls of the affector so you can see some of those here you can see the rest actually Within These organs for that reason we have long pre ganglionic nerve fibers and short postganglionic nerve fibers