15. ANATOMY OF AUTONOMIC MOTOR PATHWAYS

15. ANATOMY OF AUTONOMIC
MOTOR PATHWAYS

Anatomical Components

• The first of the two motor neurons in any autonomic motor pathway is called a preganglionic neuron 
• Its cell body is in the brain or spinal cord, 
• and its axon exits the CNS as part of a cranial or spinal nerve. 

The axon of a preganglionic neuron is 

• a small-diameter, 
• myelinated
• type B fiber 
• that usually extends to an autonomic ganglion,
• where it synapses with a postganglionic neuron, the second neuron in the autonomic motor pathway 

• Notice that the postganglionic neuron lies entirely outside the CNS. 
• Its cell body and dendrites are located in an autonomic ganglion, 
• where it forms synapses with one or more preganglionic axons. 

The axon of a postganglionic neuron is 

• a small-diameter, 
• unmyelinated 
• type C fiber
• that terminates in a visceral effector. 

• Thus, preganglionic neurons convey nerve impulses from the CNS to autonomic ganglia, and 
• postganglionic neurons relay the impulses from autonomic ganglia to visceral effectors.

Preganglionic Neurons

• In the sympathetic division, the preganglionic neurons have their cell bodies in the lateral horns of the gray matter in the 12 tho-racic segments and the first two (and sometimes three) lumbarsegments of the spinal cord . 
• For this reason, the sympathetic division is also called the thoracolumbar division (tho¯r -a-ko¯-LUM-bar), 
• and the axons of the sympathetic preganglionic neurons are known as the thoracolumbar outflow.

• Cell bodies of preganglionic neurons of the parasympathetic division are located in the nuclei of four cranial nerves in the brain stem (III, VII, IX, and X) and in the lateral gray matter of the second through fourth sacral segments of the spinal cord. 
• Hence, the parasympathetic division is also known as the craniosacral division (kra¯ -ne¯-o¯-SA¯ K-ral), and
• the axons of the parasympathetic preganglionic neurons are referred to as the craniosacral outflow.

Autonomic Ganglia

There are two major groups of autonomic ganglia: 

(1) sympathetic ganglia,

•  which are components of the sympathetic division of the ANS and 

(2) parasympathetic ganglia, 

• which are components of the parasympathetic division of the ANS.

SYMPATHETIC GANGLIA 

• The sympathetic ganglia are the sites of synapses between sympathetic preganglionic and postganglionic neurons. 

There are two major types of sympathetic ganglia:

1. sympathetic trunk ganglia and 
2. prevertebral ganglia.

Sympathetic trunk ganglia (also called vertebral chain ganglia or paravertebral ganglia) 

• lie in a vertical row on either side of the vertebral column. 
• These ganglia extend from the base of the skull to the coccyx. 
• Postganglionic axons from sympathetic trunk ganglia primarily innervate organs above the diaphragm. 

Sympathetic trunk ganglia in the neck have specific names. 
They are the 

• superior, 
• middle, and
•  inferior cervical ganglia . 

• The remaining sympathetic trunk ganglia do not have individual names. 

Because the sympathetic trunk ganglia are near the spinal cord, 

• most sympathetic preganglionic axons are short and 
• most sympathetic postganglionic axons are long.

Prevertebral (collateral) ganglia

• The second group of sympathetic ganglia, 
• lies anterior to the vertebral column and 
• close to the large abdominal arteries. 

In general, postganglionic axons from prevertebral ganglia 

• innervate organs below the diaphragm.

There are five major prevertebral ganglia 

 (1) The celiac ganglion (SE--le- -ak) 

• is on either side of the celiac trunk, an artery that is just inferior to the diaphragm. 

(2) The superior mesenteric ganglion 

• is near the beginning of the superior mesenteric artery in the upper abdomen. 

(3) The inferior mesenteric ganglion 

• is near the beginning of the inferior mesenteric artery in the middle of the abdomen.

(4) The aorticorenal ganglion and (5) the renal ganglion

• are near the renal artery of each kidney.

PARASYMPATHETIC GANGLIA 

• Preganglionic axons of the parasympathetic division synapse with postganglionic neurons in terminal (intramural) ganglia. 
• Most of these ganglia are located close to or actually within the wall of a visceral organ.

Terminal ganglia in the head have specific names. 

• They are the

1. ciliary ganglion, 
2. pterygopalatine ganglion, 
3. submandibular ganglion, and 
4. otic ganglion 

• The remaining terminal ganglia do not have specific names. 
• Because terminal ganglia are located either close to or in the wall of the visceral organ, 
• parasympathetic preganglionic axons are long, in contrast to parasympathetic postganglionic axons, which are short.

Postganglionic Neurons

• Once axons of sympathetic preganglionic neurons pass to sympathetic trunk ganglia, they may connect with postganglionic neurons in one of the following ways :

1 An axon may synapse with postganglionic neurons in the ganglion it first reaches.

2 An axon may ascend or descend to a higher or lower ganglion before synapsing with postganglionic neurons. 

• The axons of incoming sympathetic preganglionic neurons that pass up or down the sympathetic trunk collectively form the sympathetic chains, the fibers on which the ganglia are strung.

3 An axon may continue, without synapsing, through the sympathetic trunk ganglion to end at a prevertebral ganglion and synapse with postganglionic neurons there.

4 An axon may also pass, without synapsing, through the sympathetic trunk ganglion and a prevertebral ganglion and then extend to chromaffin cells of the adrenal medullae that are functionally similar to sympathetic postganglionic neurons.

• A single sympathetic preganglionic fiber has many axon collaterals (branches) 
• and may synapse with 20 or more postganglionic neurons. 
• This pattern of projection is an example of divergence
• and helps explain why many sympathetic responses affect almost the entire body simultaneously. 

• After exiting their ganglia, the postganglionic axons typically terminate in several visceral effectors.

• Axons of preganglionic neurons of the parasympathetic division pass to terminal ganglia near or within a visceral effector. 

• In the ganglion, the presynaptic neuron usually synapses with only four or five postsynaptic neurons, 
• all of which supply a single visceral effector, 
• allowing parasympathetic responses to be localized to a single effector.