Classification of Peripheral Nerves

This article discusses the history and methods of classifying the parts of the peripheral nervous system, namely the autonomic nervous system and the somatic nervous system.

PNS Divisions

The peripheral nervous system (PNS) is classified into two broad groups, the somatic and the autonomic nervous systems. Together, these two systems provide information about the limbs, organs, and the rest of the body to the central nervous system (CNS) via nerves and ganglia external to the spinal cord and brain. The somatic nervous system directs all voluntary movements of the skeletal muscles, and can be sub-divided into afferent and efferent neurons. The autonomic nervous system is divided primarily into the sympathetic and parasympathetic nervous systems with a third system, the enteric nervous system, receiving lesser recognition.

Autonomic Nervous System

History

In 1898, British scientist John Newport Langley first coined the term "autonomic" in classifying the connections of nerve fibers to peripheral nerve cells (Langley 1898). Previous researchers had utilized different terms such as "the sympathetic nerves" (Winslow 1732) to describe the way in which neurons in one part of the body brought about sympathetic reactions in another part of the body, and the "ganglionic nerves and ganglionic nervous system" (Johnstone 1764) for the ganglionic conversion of voluntary to involuntary movements (which supposedly made these peripheral nerves 'ganglionic nerves'), among other classifications. Langley stated that his choice of autonomic was not intended to imply a special degree of independence for the cells at hand, but rather to demarcate a clear departure from previous nomenclature (Langley 1921) because his hypothesis, while incorporating much previous research, was largely distinct from his predecessors.

Langley rejected that the sympathetic nerves possessed a particular relationship to the 'sympathies,' and aptly pointed out that the presence of ganglial nerves in both the spine and cranium made the 'ganglionic' connections of the peripheral nerves a misleading term. Instead, he noted that the sympathetic neurons that innervated the entire body tended to have opposing functions to the other autonomic neurons of the tectal and bulbo-sacral regions. The latter two, tectal and bulbo-sacral, were grouped together to form the parasympathetic system because they tended to respond in a like manner to various drugs (Langley 1905). Langley also mentions an enteric nervous system in his writing, but this third grouping has largely fallen out of use in modern practice.

Classification Techniques

Traditional Techniques

Much of the original classification done by Langley, and Gaskell by extension, was based on tests of pharmacological responses of nerves throughout the body and gross anatomical similarities of nerves (Gaskell 1916) (???)(???). For example, adrenaline was demonstrated to elicit the same effects as direct electrical stimulation of the sympathetic neurons, and other classes of drugs likewise elicited responses that were contained to the parasympathetic neurons. The sympathetic nervous system's responsiveness to adrenaline, in particular, lead to its association with the 'fight or flight' response of humans, although this is an oversimplification of the roles played by the SNS. These classifications were further supported by differences in the location of the ganglionic synapses (Jänig 2017). Since the time of Langley's unified proposal in the 1920s, these divisions have remained relatively constant (Groat 1971, Jänig 2017, Brunet 2016); the parasympathetic nervous system is known to innervate (PNS innervates XYZ and SNS innervates PQR--"See the other pages for greater expansion on these topics"). Langley also described an enteric nervous system which has received minimal attention; modern textbooks tend to only mention the sympathetic and parasympathetic pathways [source?? some author mentioned this somewhere]

Modern Techniques

The nerve classifications of the autonomic nervous system created by the traditional methodologies have remained mostly unchanged over the last 100 years (source). However, modern biology has placed more of an emphasis on the developmental and molecular mechanisms of these systems. As such, more attention has been paid to elements such as gene expression in these neurons.

have come under some scrutiny as of late, with eminent challenges coming from Isabel Espinosa-Medina within the lab of French researcher Jean-François Brunet at IBENS.

Counter arguments:

--research was done at development stage so lots of the expression of specific markers are just transient, and presumably should match in spatial relationship thus giving the impression of a strict cranial/spinal division

--integration of spinal and supraspinal nerves?? not entirely sure on this one, but apparently the role of supraspinal circuits are well documented in their relevance to gastrointestinal movements and parasympathetic actions

Sympathetic and Parasympathetic Divisions

Disregarding the current debate of changing the sacral outflow from parasympathetic to sympathetic, the divisions of the two systems are fairly straightforward; the sympathetic system encompasses those peripheral nerves that synapse along the thoracolumbar region of the spine (roughly vertebrae T1-L3) whereas the parasympathetic system covers peripheral nerve synapses in the vertebrae regions of T12-S4 in addition to a number of cranial nerves. As can be noticed from this description, there is a slight overlap of these divisions in the lowest region of the thoracic spine as well as throughout the lumbar spine region. This is further detailed in the diagrams below.

Autonomic spinal cord divisions
Innvervations of the sympathetic nervous system
Innervations of the parasympathetic nervous system

Enteric Nervous System

This section is a place holder for information on classification of the enteric nervous system's nerves. This section may be removed altogether, as there is evidence to indicate the enteric nervous system is independent from the autonomic nervous system.

Classification Implications

[German drs claimed that teaching the sacral region as parasympathetic has implications for how future drs will prescribe treatments for some gastrointestinal issues]

Somatic Nervous System

This section is a place holder for information on classification of the somatic nervous system's nerves.

References

I still don't know how to properly cite a source. Sorry Wikipedia