Whilst the structural arrangement of the two divisions of the ANS is based upon the general scheme outlined in the introduction, there are some fairly fundamental differences between the two. These are shown in the diagram opposite and described below.
A. Sympathetic Division
The preganglionic neurones of the sympathetic division (known as sympathetic preganglionic neurones) are located in the lateral horns of the T1 to L2/3 segments of the spinal cord. For this reason the sympathetic division is sometimes referred to as the thoracolumbar division.
Sympathetic postganglionic neurones have their cell bodies in either the sympathetic chain ganglia or collateral ganglia, small diameter unmyelinated axons and axon terminals that form neuroeffector junctions with a variety of peripheral tissues.
B. Parasympathetic Division
The preganglionic neurones of the parasympathetic division are located in the brain stem or in the lateral horns of the S2-S4 segments of the spinal cord. For this reason the parasympathetic division is sometimes referred to as the craniosacral division.
Preganglionic parasympathetic neurones have unmyelinated axons which leave the brain through one of four cranial nerves (if they have their cell bodies in the brain stem) or through the ventral roots (if they have their cell bodies in the sacral spinal cord).
These neurones synapse with parasympathetic postganglionic neurones in terminal ganglia which are located very close to (or actually within) the peripheral tissue they innervate.
Parasympathetic postganglionic neurones have relatively short, small diameter myelinated or unmyelinated axons that form neuroeffector junctions with the adjacent tissues.
From these comparisons another fundamental difference between the two divisions of the ANS should now become apparent.
Because of the proximity of the collateral and sympathetic chain ganglia to the spinal cord, sympathetic preganglionic neurones are relatively short whilst sympathetic postganglionic neurones are comparatively long.
In the parasympathetic division however the terminal ganglia are actually within or close to the peripheral tissue they innervate so the parasympathetic preganglionic neurones are long whilst the postganglionic neurones are short.