Learning Objectives
By the end of this section, you will be able to:
- Describe the structures found in the PNS
Ganglia
A ganglion is a group of neuron cell bodies in the periphery (a.k.a. the peripheral nervous system). Ganglia can be categorized, for the most part, as either sensory ganglia or autonomic ganglia, referring to their primary functions. The most common type of sensory ganglion is a dorsal (posterior) root ganglion. These ganglia are the cell bodies of neurons with axons that are associated with sensory endings in the periphery, such as in the skin, and that extend into the CNS through the dorsal nerve root. The ganglion is an enlargement of the nerve root. Note that nerve roots are not surrounded by the pia mater, and as such are part of the peripheral nervous system. Under microscopic inspection, it can be seen to include the cell bodies of the neurons, as well as bundles of fibers that are the dorsal nerve root (Figure 13.2.1). The cells of the dorsal root ganglion are unipolar cells, classifying them by shape. Also, the small round nuclei of satellite cells can be seen surrounding—as if they were orbiting—the neuron cell bodies.
External Website
View the University of Michigan WebScope at http://virtualslides.med.umich.edu/Histology/Basic%20Tissues/Nervous%20Tissue/065-2_HISTO_40X.svs/view.apml to explore the tissue sample in greater detail. If you zoom in on the dorsal root ganglion, you can see smaller satellite glial cells surrounding the large cell bodies of the sensory neurons. From what structure do satellite cells derive during embryologic development?
Another type of sensory ganglion is a cranial nerve ganglion. This is analogous to the dorsal root ganglion, except that it is associated with a cranial nerve (associated with the brain) instead of a spinal nerve (associated with the spinal cord). The roots of cranial nerves are within the cranium, whereas the ganglia are outside the skull. For example, the trigeminal ganglion is superficial to the temporal bone whereas its associated nerve is attached to the mid-pons region of the brain stem. Like the sensory neurons associated with the spinal cord, the sensory neurons of cranial nerve ganglia are unipolar in shape with associated satellite cells.
The other major category of ganglia are those of the autonomic nervous system, which is divided into the sympathetic and parasympathetic nervous systems. The sympathetic chain ganglia constitute a row of ganglia along the vertebral column that receive central input from the lateral horn of the thoracic and upper lumbar spinal cord. At the superior end of the chain ganglia are three paravertebral ganglia in the cervical region. Three other autonomic ganglia that are related to the sympathetic chain are the prevertebral ganglia, which are located outside of the chain but have similar functions. They are referred to as prevertebral because they are anterior to the vertebral column. The neurons of these autonomic ganglia are multipol