Major anatomic differences between the sympathetic and parasympathetic nervous systems

What are the major anatomic differences between the sympathetic and parasympathetic nervous systems? 

The sympathetic neurons are located in the interomediolateral (IML) and interomediomedial (IMM) columns of the thoracic and upper lumbar spinal cord.

Axons of these neurons make synaptic contact with ganglionic neurons in the bilateral paravertebral or largely unpaired prevertebral ganglia.

The parasympathetic neurons lie in the brainstem and the intermediate zone of the sacral spinal cord.

The parasympathetic relay ganglia are located near or in the wall of the effector organs.

Because of the close proximity of sympathetic ganglia to the primary efferent sympathetic neurons (IML and IMM columns), the sympathetic preganglionic fibers are short, whereas the postganglionic fibers may extend a long way to their target organs.

The parasympathetic preganglionic axons, on the other hand, are relatively long myelinated fibers that synapse with parasympathetic relay ganglia located near or within the wall of individual innervated organs.

The postganglionic parasympathetic fibers are, therefore, short (1 mm to several centimeters). 

The number of postganglionic neurons is very close to the number of preganglionic neurons in the parasympathetic system.

On the other hand, the high ratio of postganglionic to preganglionic neurons in the sympathetic system explains the massive sympathetic outflow and the wide range of autonomic effects that occur during strenuous and stressful situations.

Sources

From Nadeau SE, Ferguson TS, Valenstein E, et al.: Medical Neuroscience . Philadelphia, Saunders, 2004, p 504.

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