The Five Senses and Beyond: The Encyclopedia of Perception - Jennifer L. Hellier 2017
Within the central nervous system and the peripheral nervous system, there are regions where cell bodies or somas of neurons and their vast dendritic arbors group together. These can make very distinct “bumps” within the nervous systems. In the central nervous system, collections of somas and dendrites are called nuclei (plural; nucleus, singular form). In the peripheral nervous system, these collections are called ganglia (plural; ganglion, singular form). This distinction is used to help scientists and anatomists identify the location of the cells.
The term ganglion is derived from Greek meaning a “cyst-like tumor.” In the nervous system, ganglia may interconnect with other ganglia forming a complex structure called a plexus. The inputs and outputs of a ganglion can form a nerve, a collection of fibers and/or axons that travel relatively together. In general, a ganglion will be the interconnection between the peripheral and central nervous systems. For example, the cranial nerves that supply motor and sensory function to the head, neck, and face are part of the peripheral nervous system. Their sensory ganglia are located within the brainstem, which is part of the central nervous system.
Anatomy and Physiology
In the mammalian nervous system, there are two main types of ganglia: the dorsal root ganglia and the autonomic ganglia. The dorsal root ganglia are located just outside of the spinal cord. They are the collection of cell bodies for the sensory nerves (afferent nerves) and lack any motor neurons. The dorsal root ganglia are sometimes called the spinal ganglia and are located in the intervertebral foramina (openings between two spinal vertebrae). They attach to the spinal cord by the posterior (or dorsal) roots. Along the length of the spinal cord, there are 31 bilaterally symmetrical pairs of spinal nerves with each having two divisions. The dorsal division carries sensory information from the skin, muscles, and visceral organs, while the ventral portion transmits motor information. At each level of the spinal cord—cervical, thoracic, lumbar, and sacral—the dorsal root ganglia contain the cell bodies of the neurons that serve that region of the body.
The axons of the dorsal root ganglia neurons are called afferent nerves or just afferents. This is because they transmit information to the central nervous system. Sensory neurons within the dorsal root ganglia are classified as pseudo-unipolar, where the axon has two branches that act as a single axon. These distinct axon branches are named based on their location: distal process and proximal process. These dorsal root ganglia pseudo-unipolar neurons are unique as they can initiate an action potential in the distal process. These action potentials will then bypass the soma and continue to transmit to the proximal process, where they will enter and synapse in the spinal cord’s dorsal horn.
The autonomic ganglia are part of the autonomic nervous system and are located between the spinal cord and the target organ. The autonomic nervous system has two divisions and hence there are two types of ganglia: the sympathetic ganglia and the parasympathetic ganglia. The sympathetic ganglia house about 20,000 to 30,000 neurons that signal distress or possible danger for the person, providing the “fight-or-flight” response. These ganglia are found next to and on both sides of the spinal cord, forming bilaterally symmetric long chains from the upper neck to the coccyx. However, the coccygeal ganglion is unpaired. The parasympathetic ganglia are small compared to the sympathetic ganglia and are located close to the organs or effectors that they innervate. The four paired parasympathetic ganglia that supply the head and neck (the ciliary, pterygopalatine, submandibular, and otic ganglia), however, do not follow this rule.
Of important note, the basal ganglia are part of the central nervous system and are a collection of deep neurons within the cerebrum that are associated with movement. These nuclei were misnamed when anatomy was beginning to develop as a science. Today, anatomists are using the term basal nuclei more often than the previous name, basal ganglia.
Jennifer L. Hellier
See also: Autonomic Nervous System; Central Nervous System; Peripheral Nervous System
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