In the autonomic nervous system (ANS), nerve fibers that connect the central nervous system to ganglia are known as preganglionic fibers. Learning Objectives Key Points
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Preganglionic Neuron PropertiesIn the autonomic nervous system (ANS), fibers from the central nervous system to the ganglion are known as preganglionic fibers. All preganglionic fibers, whether they are in the sympathetic nervous system (SNS) or in the parasympathetic nervous system (PSNS), are cholinergic—that is, these fibers use acetylcholine as their neurotransmitter—and are myelinated. The ANS is unique in that it requires a sequential two-neuron efferent pathway; the preganglionic neuron must first cross a synapse onto a postganglionic neuron before innervating the target organ. The preganglionic, or first neuron will begin at the outflow and will cross a synapse at the postganglionic, or second neuron’s cell body. The postganglionic neuron will then cross a synapse at the target organ. Sympathetic preganglionic fibers tend to be shorter than parasympathetic preganglionic fibers because sympathetic ganglia are often closer to the spinal cord while parasympathetic preganglionic fibers tend to project to and synapse with the postganglionic fiber close to the target organ. Outflow SitesProperties of the SNS and PSNS preganglionic neurons also differ with respect to the spinal cord exit points. The sympathetic division has thoracolumbar outflow, meaning that the neurons begin at the thoracic and lumbar (T1–L2) portions of the spinal cord. The parasympathetic division has craniosacral outflow, meaning that the neurons begin at the cranial nerves (CN3, CN7, CN9, CN10) and sacral (S2–S4) spinal cord. The sympathetic division (thoracolumbar outflow) consists of cell bodies in the lateral horn of the spinal cord (intermediolateral cell columns) from T1 to L2. These cell bodies are GVE (general visceral efferent) neurons and are the preganglionic neurons. There are several locations where preganglionic neurons create synapses with their postganglionic neurons:
The parasympathetic division (craniosacral outflow) consists of cell bodies from one of two locations: the brainstem (cranial nerves III, VII, IX, X) or the sacral spinal cord (S2, S3, S4). These are the preganglionic neurons that synapse with the postganglionic neurons in these locations: Parasympathetic ganglia of the head: The parasympathetic division has craniosacral outflow, meaning that the neurons begin at the cranial nerves (CN3, CN7, CN9, CN10) and the sacral (S2–S4) spinal cord. Pre- and post-ganglionic fibers and targets are depicted.
DivergenceAnother major difference between the two ANS systems is divergence, or the number of postsynaptic fibers a single preganglionic fiber creates a synapse with. Whereas in the parasympathetic division there is a divergence factor of roughly 1:4, in the sympathetic division there can be a divergence of up to 1:20. The site of synapse formation and this divergence for both the sympathetic and parasympathetic preganglionic neurons does, however, occur within ganglia situated within the peripheral nervous system. Where are preganglionic sympathetic neurons located in the spinal cord?The cell bodies of sympathetic preganglionic neurons are located in intermediolateral nuclei of the spinal cord. (A nucleus is a profile of a neuron column.) Each intermediolateral column forms a lateral gray horn in a thoracolumbar spinal cord transection.
Where are the cell bodies of preganglionic neurons?The cell bodies of the preganglionic neurons are in the brainstem or spinal cord of the central nervous system (CNS). The cell bodies of the postganglionic neurons are in autonomic ganglia located peripherally. Axon terminal of preganglionic neurons synapse on dendrites and cell bodies of postganglionic neurons.
Where are the cell bodies of sympathetic postganglionic neurons located?This location of the peripheral parasympathetic ganglia in the visceral organ itself is quite different from the arrangement of the sympathetic ganglia, for the cell bodies of the sympathetic postganglionic neurons are always located in the ganglia of the sympathetic chain or in various other discrete ganglia in the ...
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