Autonomic Ganglia

There are several types of autonomic ganglia. In the sketch of the autonomic reflex arc, two are shown.

The first is labeled "sympathetic trunk ganglion." The sympathetic trunk (or sympathetic chain, as some references call it) is a string of autonomic ganglia lying alongside the spinal cord from the base of the skull to the coccyx (there are two, of course, one on either side). They are connected to the spinal nerves by rami and receive motor fibers of the first ganglion in the motor chain. They contain the somata of the second neurons.

The second type shown in the sketch is a "collateral" ganglion. These are the large ones that usually have their own anatomic names (e.g., the cervicothoracic ganglion). They are located near the organs they serve.

Not shown is the terminal ganglion. These are analogous to the collateral ganglia, but they are usually located within the substance of the organs they serve.

One final special case should be mentioned. The adrenal medulla is formed from neural crest cells in embryonic life. Consequently, since it comes from the neural cell rudiments, its cells have some of the characteristics of neurons. They are secretory, for one thing; more importantly, the materials they secrete (adrenaline and noradrenalin) are known to be neurotransmitters, and in keeping with their neuron-like nature, these cells are capable of responding to a neuronal signal: i.e., they are "excitable" cells. That's exactly what happens. The preganglionic nerve fiber from the CNS enters the adrenal medulla and there it synapses not with another neuron, but with one of the secretory cells. The secretory cell responds to the firing of this preganglionic neuron by secreting its product into the bloodstream. In other words, the secretory cell is the postganglionic "neuron" in this particular situation. It puts out a hormone for which all cells have receptors. Here we see an exquisite interconnection between the nervous system and the endocrine system, the body's two means for responding to changes in the environment.


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