Why are you worrying about a med school topic for the MCAT? Oh well, here goes:
Ganglions are clusters of neuronal cell bodies found OUTSIDE of the Central Nervous System. In this context, they are part of the Autonomic Nervous System (ANS), which includes the Sympathetic Nervous System (SNS) and Parasympathetic Nervous System (PSNS).
The SNS is thoracolumbar, coming off the lateral horns of the spinal cord from T1 to L2, and synapse either onto the paravertebral (sympathetic) chain ganglion, the cervical ganglia, or the celiac and mesenteric ganglia. The SNS preganglionic fibers synapse onto the dendrites of the neuronal cell bodies at one of these ganglions. That cell body at the ganglion sends an axon, part of he postganglionic fiber, toward the organ which is usually not on top of the ganglion (especially evident when you consider where the paravertebral ganglion is in comparison to the innervation organs). On the body wall, postganglionic innervation include sweat glands, erector pilii muscles (goosebumps), and vascular endothelium in blood vessels. Inside the body, postganglionic innervation include eye, nose, mouth, glands in the head, heart, larynx, esophagus, stomach, liver, pancreas, andrenal, intestines, kidneys, bladder, sexual organs, and external genitalia.
In the SNS's paravertebral chain ganglion, the preganglion fiber jumps onto the chain ganglion using white rami comunicantes (myelinated nerve fibers), and jump off the chain ganglion using gray rami comunicantes (unmyelinated nerve fibers). The myelinated fibers conduct signals between the lateral horn and the ganglion much faster than the unmyelinated ones. In the PSNS, all postganglionic fibers are unmyelinated. As for the physiological why, I don't know, and you don't need to know.
Each terminal axon (in a fiber, which is a bundle of axons) typically innervates one dendrite which belongs to one neuronal cell body. A neuronal cell body can have multiple dendrites. HOWEVER, axons can branch, sometimes profusely. This means that one preganglionic axon can innervate multiple postganglionic dendrites of neuronal cell bodies. However, I would not expect one preganglionic neuron to innervate multiple postganglionic neurons in different ganglions (that is, the branching effect is local).
The PSNS is craniosacral, coming off the cranial nerves (CN) III, VII, IX, and X (vagus nerve) and the S2, S3, and S4 spinal nerves (pelvic splachnic nerves) in the sacral area. The PSNS preganglionic fibers synapse onto dendrites of neuronal cell bodies in ganglions either next to an organ or right on top of the organ. In the abdominal area, sometimes they will synapse onto ganglions dispersed around the walls of the tissues or organs they innervate. Examples would be the myenteric plexus of Auerbach or the submucosal plexus of Meissner. These cell bodies send off short axons in postganglionic fibers to the organ right next to them to innervate them.
In short, the SNS preganglionic fibers synapse onto named ganglions far(ther) away from organs. The PSNS preganglionic fibers synapse onto named ganglions either next to the organs or right inside the organs. Thus, the SNS postganglionic fibers are longer, whereas PSNS postganglionic fibers are shorter.
Red is SNS. Blue is PSNS.
http://upload.wikimedia.org/wikipedia/commons/thumb/f/f7/Gray839.png/410px-Gray839.png
