ANS Divisions: Sympathetic & Parasympathetic

Based on anatomical, physiological, and pharmacological differences, the Autonomic Nervous System is divided into two divisions: Sympathetic and Parasympathetic. The following table summarizes key differences:

Difference Sympathetic Parasympathetic
Innervtion Distribution: whole body body cavities & head
Preganglionic Origin: thoraco-lumbar spinal cord brainstem & sacral spinal cord
Postganglionic Neuron: usually adrenergic always cholinergic
Target Neurotransmitter: norepinephrine acetylcholine
Functional Role: "fight or flight" routine operations

NOTE: It is sometimes stated that the length ratio of preganglionic to postganglionic axons is a sympathetic vs parasympathetic difference, but aside from the vagus nerve, length ratios are regionally variable and not consistently different.

Although the sympathetic division has routine functions (e.g., maintaining blood pressure), its most noticeable role involves global activation of innervation targets in response to a brain perceived “life-threatening” situation. In contrast, the parasympathetic division operates more locally, to restore routine visceral functions. Although sympathetic postganglionic neurons are generally adrenergic, some of them may be cholinergic (sweat glands & dilation of muscle vessels). To view a detailed list of visceral organ innervation: click here.

The CNS origins of preganglionic neurons and the locations of ganglia which contain postganglionic neurons are shown below schematically for parasympathetic (right) and sympathetic (left) divisions. Notice that parasympathetic preganglionic pathways originate (bilaterally) from four cranial nerve nuclei in the brainstem or from the sacral spinal cord. Whereas sympathetic preganglionic pathways originate (bilaterally) from only thoracolumbar spinal cord segments, even though even though sympathetic innervation extends throughout the body.






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