Re: Autonomic nervous system: why are only the preganglionic fibres myelinated?

There are a few possible reasons why postganglionic axons in the autonomic 
nervous system are unmyelinated, but I was unable to find a definite 
answer.  Before going into those reasons, I'll start with the advantages of 
myelination. Myelination of axons increases the conduction velocity of the 
electrical signals called action potentials. Another way to increase 
conduction velocity is to increase the axon diameter. Invertebrates do not 
have myelinated axons, but have far fewer neurons than vertebrates. Thus, 
another advantage of myelination is the ability to increase conduction 
velocity for a large number of neurons within a relatively small volume. 

Concerning your question, myelination of postganglionic axons would be more 
important for sympathetic axons as opposed to parasympathetic axons. 
Parasympathetic nervous control is involved in slowing of the heartbeat, 
increasing digestion, and genital erections. Preganglionic parasympathetic 
axons terminate very close to the target tissue, such as the heart. 
Therefore, postganglionic axons are relatively short and do not require 
myelination for speed. In contrast, sympathetic preganglionic axons 
terminate in sympathetic ganglia located near the spinal cord, but among 
their targets are muscles that may be located relatively far from the the 
ganglia. Sympathetic nervous control is involved in increasing heartbeat, 
increasing blood sugar, and redirecting blood flow to skeletal muscles 
(mainly those that you control voluntarily). Sympathetic control is 
"dramatically expressed during stress and emergency situations (the fight 
or flight response)", which you might imagine would need to occur quickly, 
so it is somewhat surprising that the postganglionic axons are 
unmyelinated. 

However, there are at least two reasons that I can think of why they may 
not be myelinated or need to be. The first is that the postsynaptic actions 
of both the sympathetic and parasympathetic systems are relatively slow, 
meaning that they take place in hundreds of milliseconds instead of a few 
milliseconds. Thus, whether an action potential reaches a sympathetic nerve 
terminal in 10 milliseconds or 100 milliseconds, the response at the target 
won't be that much faster. The second possible reason may be regulatory. 
Myelinated axons can fire at higher rates than unmyelinated ones. The axons 
of the sympathetic branch a lot, so they can act upon many targets 
simultaneously. With a lack of myelination limiting the maximal firing 
rate, it may lessen the chances that the sympathetic system will get out of 
control and overexcite the targets.