Autonomic Nervous System Anatomy

This picture is worth at least a 1,000 words when it comes to learning ANS anatomy.

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Let’s do a quick review of the somatic motor system first so that we can contrast it with the ANS. If we look at the very first box in the diagram below we can make one important observation immediately. Only one neuron is necessary to carry an action potential to its target. This neuron, called the lower motor neuron or alpha motor neuron, is highly myelinated and its cell body is located in the ventral horn of the spinal cord. This neuron releases Acetylcholine onto its target organ, skeletal muscles.

So we just categorized the peripheral component of the somatic motor system in three ways: Site of origin, structure of the efferent pathway and ganglia (one highly myelinated axon, no ganglia in somatic system), and we identified its effector (skeletal muscle). Lets do the same for the two divisions of the autonomic nervous system and compare.

Parasympathetic Division

Site of Origin:

We sometimes refer to the parasympathetic division as the craniosacral division. We do this because of where its fibers leave the CNS. Specifically, the parasympathetic division’s site of origin (meaning the location of the pre-ganglionic neuron cell bodies) is cranial nerve nuclei in nerves CN 3, 7, 9, 10, and the lateral horns of spinal cord segments s2-s4.

Efferent Pathway and Ganglia:

Unlike the Somatic motor system, both divisions of the ANS utilize a 2 neuron chain to carry action potentials from the spinal cord to their target organs. The parasympathetic division utilizes a long pre-ganglionic neuron, and a short post- ganglionic neuron. Where in the world did we come up with these names you ask? Well this is actually another important feature of the ANS. Since both divisions utilize a two neuron chain this means that we have neuron cell bodies outside of the CNS. Remember that a collection of cell bodies outside of the CNS is called a ganglion. The location of these two neurons relative to the ganglia is how their names are determined. The neuron that carries the nerve impulse from the spinal cord to the second neuron is called the pre-ganglionic neuron, while the 2nd neuron in the chain is called the post ganglionic neuron.

Due to the fact that the parasympathetic nervous system utilizes a long pre-ganglionic neuron and short post, its ganglia are located close to the target organs.

Effectors:

Smooth muscle, cardiac muscle, and glands

Sympathetic Division

Site of Origin:

The symmetric division is sometimes referred to as the thoracolumbar division, because its preganglionic neurons arise from the lateral horns of the thoracic and lumbar spinal cord segments, specifically segments T1-L2.

Efferent Pathway and Ganglia:

Like the parasympathetic division, the sympathetic division utilizes a two neuron chain to reach its targets (effectors). This chain is composed of a relatively short pre-ganglionic neuron with a highly myelinated axon, and a relatively long post-ganglionic neuron with an unmyelinated axon. 

In addition to this slight difference in fiber length, there is a great deal of difference between this and other systems in terms of the actual path this two neuron chain takes to reach its targets. An example of this somewhat unique path is illustrated below:

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Lets break this pathway down into a few different components:

1. The preganglionic neuron cell body is located in the lateral horn of a spinal cord segment in the Thoracolumbar region.

2. The preganglionic neuron’s nerve fiber travels out of the spinal cord through the ventral root of the same spinal cord segment. It then passes through a structure known as the white rams communicans. This structure is white because it contains myelinated fibers.

3. After passing through the white ramus it enters a sympathetic trunk ganglion where it synapses on the postganglionic neuron. Remember, anywhere that many synapses and cell bodies are found outside of the CNSwe find a ganglion.

4. The post-ganglionic nerve fiber then exits the sympathetic trunk ganglion through the gray rams communicans where it re-enters the spinal root and travels to its target organ.

Effectors:

Smooth muscle, cardiac muscle, glands

 

 

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  1. Pingback: Peripheral Nervous System Study Guide | AandPonline.com

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