ANS Anatomy

Where is the ANS?

General layout:

Well, most of the ANS is obviously not in the brain or spinal cord, since those are parts of the CNS. Yet, the CNS has to be able to control the ANS. That should make sense, since your emotional state (generated from your brain) can certainly change things like sweat production and heart rate. Then, the neurons that are outside of the CNS do not lie alone, but instead are found in ganglia (clumps of neurons with glial cell support), called autonomic ganglia.

So, there are neurons in the CNS that project out to neurons lying in ganglia outside of the CNS. The neurons in the ganglia then go and affect our body organs. Is this understandable? The scheme is thus:

CNS neurons -----> neurons in autonomic ganglia -----> visceral target

Now let's throw some names on these things. The controlling neurons in the CNS are called preganglionic neurons, since they go to the ganglia but are not within them. The neurons within the autonomic ganglia are called postganglionic neurons, since they send their axons out of the ganglia.

Exactly where are these ANS ganglia located?

They are typically found in one of three places

linked together in a long chain that runs alongside the spinal cord
on their own within the body
lying directly on or beside their target.

This is one way that the sympathetic and parasympathetic nervous systems (the 2 subdivisions of the ANS) differ anatomically. The sympathetic nervous system uses the chain of ganglia (#1 above) and also has some ganglia strewn about the body (#2 above). The parasympathetic nervous system mainly uses ganglia that lie on or beside their target (#3 above).

Where do the ANS cells lie throughout the body axis?

The ANS is found throughout the axial regions of the body, all the way along from the head through the trunk. Although that coverage is thorough, the distribution of the sympathetic and parasympathetic nervous systems is not even throughout. Let me explain:

	The sympathetic nervous system preganglionic neurons and ganglia are found mainly in the thoracic region.
	The parasympathetic nervous system preganglionic neurons are mainly found in the brain/head and sacral regions (remember, the parasympathetic ganglia are found wherever their targets are).

This layout may seem bizarre, but to remember it, think about it like a sandwich... where the bread (top and bottom) is parasympathetic and the meat (middle) is sympathetic. This set-up is depicted in the table below. So our ANS is set up like a sympathetic sandwich. That sounds yucky, but maybe it will help you think about it.

Location	System
cranial	Parasympathetic
thoracic	Sympathetic
sacral	Parasympathetic

For more on the anatomy of each system, see the sympathetic nervous system anatomy or the parasympathetic nervous system anatomy subtopic pages.