Classification of peripheral nerves

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Peripheral nervous system box diagram NSdiagram.png
Peripheral nervous system box diagram

The classification of peripheral nerves in the peripheral nervous system (PNS) groups the nerves into two main groups, the somatic and the autonomic nervous systems. [1] Together, these two systems provide information regarding the location and status of the limbs, organs, and the remainder of the body to the central nervous system (CNS) via nerves and ganglia present outside of the spinal cord and brain. [1] The somatic nervous system directs all voluntary movements of the skeletal muscles, and can be sub-divided into afferent and efferent neuronal flow. The autonomic nervous system is divided primarily into the sympathetic and parasympathetic nervous systems with a third system, the enteric nervous system, receiving less recognition. [2]

Contents

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. [3] Previous researchers had utilized different terms such as "the sympathetic nerves" [Winslow et al.] to describe the way in which neurons in one part of the body brought about sympathetic reactions in another part of the body, as well as the "ganglionic nerves and ganglionic nervous system" [Livingstone et al.] for the ganglionic conversion of voluntary to involuntary movements (which supposedly made these peripheral nerves 'ganglionic nerves'), among other classifications. [4] 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 because his hypothesis, while incorporating much previous research, was largely distinct from his predecessors. [4]

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 spinal cord and cranium made the 'ganglionic' connections of the peripheral nerves a pointless, if not 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. [5] Langley also mentions an enteric nervous system in his writing, but this third grouping has largely fallen out of discussion in modern practice.

Langley's Classification Tree from his 1921 book The Autonomic Nervous System Langley's Classification Tree his book "The Autonomic Nervous System" (1921).png
Langley's Classification Tree from his 1921 book The Autonomic Nervous System

Classification techniques

Traditional techniques

Much of the original nerve classification done by Langley, and Gaskell by extension, was based on pharmacological responses of nerves throughout the body as well as the gross anatomical similarities of nerves. [6] For example, adrenaline was demonstrated to elicit the same bodily effects as direct electrical stimulation of the sympathetic neurons, and other classes of drugs likewise elicited responses that were contained to the parasympathetic neurons. [6] The sympathetic nervous system's responsiveness to adrenaline, [7] in particular, lead to the system's association with the 'fight or flight' response of humans, although this is an oversimplification of the roles played by the SNS. [8] These two distinct classifications were further supported by differences in the location of the ganglionic synapses. [4] Since the time of Langley's unified proposal in the 1920s, these divisions have remained relatively constant; [9] [10] the parasympathetic nervous system is known to regulate unconscious activities of the body and maintain homeostasis, the sympathetic nervous system controls responses to external stimuli, but both are involuntary functions. Langley also described an enteric nervous system although it has received minimal attention, and most modern textbooks tend to only mention the sympathetic and parasympathetic pathways in the context of the autonomic nervous system.

Modern techniques

The nerve classifications of the autonomic nervous system created by the traditional methodologies have remained mostly unchanged over the last 100 years. [10] However, modern perspectives have 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, development features, and overall functions of these neurons. The classification of the sacral outflow as parasympathetic, in particular, has come under some scrutiny as of 2016. [11]

Isabel Espinosa-Medina, working in the lab of French researcher Jean-François Brunet at IBENS, identified several transcription factors within pre-ganglionic neurons of the lower lumbar and sacral regions that are essential to neurogenesis. [11] These transcription factors were only identified within other portions of the spinal cord, but they were not co-expressed in the developing cranium. [11] This genomic similarity of the lumbrasacral region to the rest of the spinal cord lead to the conclusion that the sacral region of the spinal cord may actually be part of the sympathetic nervous system. [11] The effect of this revision produces a simple bisection of the autonomic nervous system wherein the cranium is solely parasympathetic, and the spinal cord is solely sympathetic. [11]

Other researchers in the field have challenged this assertion. A publication out of the lab of German researcher Wilfrid Jänig claims that this reclassification would be a 'mistake. [9] ' Among the arguments made by Jänig et al. is a disagreement with the developmental research approach taken by Espinosa-Medina. Jänig et al. argue that many of the measured gene markers from Espinosa-Medina are only expressed transiently in the lumbrasacral region to assist in migration and differentiation of these cells. [9] As such, these nerves necessarily share expression similarities with their neighboring sympathetic spinal nerves, but that alone does not make the nerves anything more than spinal in nature. [9] Furthermore, Jänig et al. state that this reclassification would contradict information on the opposing actions of the pelvic ganglia's parasympathetic and sympathetic pathways. [9]

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. [12] 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. [7] This overlap is a result of some directly opposing functions of the sympathetic and parasympathetic systems in regulating the stomach region. [10] [7] The relationship is further detailed in the diagrams below.

Enteric division

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

The division of the autonomic nervous system into sympathetic and parasympathetic pathways is particularly useful from a medical treatment perspective [source]. Jänig et al. warn that changing the classifications of the systems could result in confusion on how to treat some disease, particularly those of the gut and stomach region. [9]

Somatic nervous system

Related Research Articles

Ganglion Clusters of neurons

A ganglion is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system there are both sympathetic and parasympathetic ganglia which contain the cell bodies of postganglionic sympathetic and parasympathetic neurons respectively.

Nerve enclosed, cable-like bundle of axons in the peripheral nervous system

A nerve is an enclosed, cable-like bundle of nerve fibres called axons, in the peripheral nervous system. A nerve transmits electrical impulses and is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the electrochemical nerve impulses called action potentials that are transmitted along each of the axons to peripheral organs or, in the case of sensory nerves, from the periphery back to the central nervous system. Each axon within the nerve is an extension of an individual neuron, along with other supportive cells such as some Schwann cells that coat the axons in myelin.

Peripheral nervous system Part of the nervous system

The peripheral nervous system (PNS) is one of two components that make up the nervous system of bilateral animals, with the other part being the central nervous system (CNS). The PNS consists of the nerves and ganglia outside the brain and spinal cord. The main function of the PNS is to connect the CNS to the limbs and organs, essentially serving as a relay between the brain and spinal cord and the rest of the body. Unlike the CNS, the PNS is not protected by the vertebral column and skull, or by the blood–brain barrier, which leaves it exposed to toxins and mechanical injuries.

Autonomic nervous system Division of the peripheral nervous system supplying smooth muscle and glands

The autonomic nervous system (ANS), formerly the vegetative nervous system, is a division of the peripheral nervous system that supplies smooth muscle and glands, and thus influences the function of internal organs. The autonomic nervous system is a control system that acts largely unconsciously and regulates bodily functions, such as the heart rate, digestion, respiratory rate, pupillary response, urination, and sexual arousal. This system is the primary mechanism in control of the fight-or-flight response.

Parasympathetic nervous system A division of the autonomic nervous system

The parasympathetic nervous system (PSNS) is one of the two divisions of the autonomic nervous system, the other being the sympathetic nervous system. The enteric nervous system (ENS) is now usually referred to as separate from the autonomic nervous system since it has its own independent reflex activity. The autonomic nervous system is responsible for regulating the body's unconscious actions. The parasympathetic system is responsible for stimulation of "rest-and-digest" or "feed and breed" activities that occur when the body is at rest, especially after eating, including sexual arousal, salivation, lacrimation (tears), urination, digestion and defecation. Its action is described as being complementary to that of the sympathetic nervous system, which is responsible for stimulating activities associated with the fight-or-flight response.

Sympathetic nervous system Term in biology

The sympathetic nervous system (SNS) is one of the two main divisions of the autonomic nervous system, the other being the parasympathetic nervous system.

Nervous tissue Main component of the nervous system

Nervous tissue, also called neural tissue, is the main tissue component of the nervous system. The nervous system regulates and controls bodily functions and activity and consists of two parts: the central nervous system (CNS) comprising the brain and spinal cord, and the peripheral nervous system (PNS) comprising the branching peripheral nerves. It is composed of neurons, also known as nerve cells, which receive and transmit impulses, and neuroglia, also known as glial cells or glia, which assist the propagation of the nerve impulse as well as provide nutrients to the neurons.

The baroreflex or baroreceptor reflex is one of the body's homeostatic mechanisms that helps to maintain blood pressure at nearly constant levels. The baroreflex provides a rapid negative feedback loop in which an elevated blood pressure reflexively causes the heart rate to decrease and also causes blood pressure to decrease. Decreased blood pressure decreases baroreflex activation and causes heart rate to increase and to restore blood pressure levels. The baroreflex can begin to act in less than the duration of a cardiac cycle and thus baroreflex adjustments are key factors in dealing with postural hypotension, the tendency for blood pressure to decrease on standing due to gravity.

Ciliary ganglion parasympathetic ganglion located just behind the eye in the posterior orbit

The ciliary ganglion is a parasympathetic ganglion located just behind the eye in the posterior orbit. It measures 1–2 mm in diameter and in humans contains approximately 2,500 neurons. The ganglion contains postganglionic parasympathetic neurons. These neurons innervate the pupillary sphincter muscle, which constricts the pupil, and the ciliary muscle which contracts to make the lens more convex. Both of these muscles are involuntary since they are controlled by the parasympathetic division of the autonomic nervous system.

Prevertebral ganglia

Prevertebral ganglia are sympathetic ganglia which lie between the paravertebral ganglia and the target organ.

Superior cervical ganglion

The superior cervical ganglion (SCG) is part of the autonomic nervous system (ANS), more specifically it is part of the sympathetic nervous system, a division of the ANS most commonly associated with the fight or flight response. The ANS is composed of pathways that lead to and from ganglia, groups of nerve cells. A ganglion allows a large amount of divergence in a neuronal pathway and also enables a more localized circuitry for control of the innervated targets. The SCG is the only ganglion in the sympathetic nervous system that innervates the head and neck. It is the largest and most rostral (superior) of the three cervical ganglia. The SCG innervates many organs, glands and parts of the carotid system in the head.

Sympathetic ganglion ganglia of the sympathetic nervous system

The sympathetic ganglia, or autonomic ganglia, are the ganglia of the sympathetic nervous system. Ganglia are 20,000 to 30,000 afferent and efferent nerve cell bodies that run along on either side of the spinal cord. Afferent nerve cell bodies bring information from the body to the brain and spinal cord, while efferent nerve cell bodies bring information from the brain and spinal cord to the rest of the body. The cell bodies create long sympathetic chains that are on either side of the spinal cord. They also form para- or pre-vertebral gangalia of gross anatomy.

Lateral grey column

The lateral grey column is one of the three grey columns of the spinal cord ; the others being the anterior and posterior grey columns. The lateral grey column is primarily involved with activity in the sympathetic division of the autonomic motor system. It projects to the side as a triangular field in the thoracic and upper lumbar regions of the postero-lateral part of the anterior grey column.

Autonomic dysreflexia (AD), also previously known as mass reflex, is a potential medical emergency classically characterized by uncontrolled hypertension and bradycardia, although tachycardia is known to commonly occur. AD occurs most often in individuals with spinal cord injuries with lesions at or above the T6 spinal cord level, although it has been reported in patients with lesions as low as T10. Guillain–Barré syndrome may also cause Autonomic Dysreflexia.

Pelvic splanchnic nerves or nervi erigentes are splanchnic nerves that arise from sacral spinal nerves S2, S3, S4 to provide parasympathetic innervation to the hindgut.

General visceral afferent fibers

The general visceral afferent (GVA) fibers conduct sensory impulses from the internal organs, glands, and blood vessels to the central nervous system. They are considered to be part of the visceral nervous system, which is closely related to the autonomic nervous system, but 'visceral nervous system' and 'autonomic nervous system' are not direct synonyms and care should be taken when using these terms. Unlike the efferent fibers of the autonomic nervous system, the afferent fibers are not classified as either sympathetic or parasympathetic.

Lumbar ganglia

The lumbar ganglia are paravertebral ganglia. The lumbar portion of the sympathetic trunk typically has 4 lumbar ganglia. The lumbar splanchnic nerves arise from the ganglia here, and contribute sympathetic efferent fibers to the nearby plexuses. The first two lumbar ganglia have both white and gray rami communicates.

Axon reflex

The axon reflex is the response stimulated by peripheral nerves of the body that travels away from the nerve cell body and branches to stimulate target organs. Reflexes are single reactions that respond to a stimulus making up the building blocks of the overall signaling in the body's nervous system. Neurons are the excitable cells that process and transmit these reflex signals through their axons, dendrites, and cell bodies. Axons directly facilitate intercellular communication projecting from the neuronal cell body to other neurons, local muscle tissue, glands and arterioles. In the axon reflex, signaling starts in the middle of the axon at the stimulation site and transmits signals directly to the effector organ skipping both an integration center and a chemical synapse present in the spinal cord reflex. The impulse is limited to a single bifurcated axon, or a neuron whose axon branches into two divisions and does not cause a general response to surrounding tissue.

Outline of the human nervous system Overview of and topical guide to the human nervous system

The following Diagram is provided as an overview of and topical guide to the human nervous system:

Adrenergic system or Adrenergic nervous system (ANS) is a group of organs and nerves in which adrenaline (epinephrine) and/or noradrenaline (norepinephrine) act as neurotransmitters. ANS is counted as one of the main neurohormonal systems that regulate cardiovascular function, including smooth muscle tone.

References

  1. 1 2 Brodal, Per (2004). The Central Nervous System: Structure and Functioning. Oxford University Press US. pp. 369–396. ISBN   0-19-516560-8.
  2. ↑ McCorry, Laurie Kelly (2007-08-15). "Physiology of the Autonomic Nervous System". American Journal of Pharmaceutical Education. 71 (4): 78. doi:10.5688/aj710478. ISSN   0002-9459. PMC   1959222 . PMID   17786266.
  3. ↑ Langley, J. N. (1898-07-26). "On the Union of Cranial Autonomic (Visceral) Fibres with the Nerve Cells of the Superior Cervical Ganglion". The Journal of Physiology. 23 (3): 240–270. doi:10.1113/jphysiol.1898.sp000726. ISSN   0022-3751. PMC   1516595 . PMID   16992456.
  4. 1 2 3 Langley, John Newport (1921). The autonomic nervous system. Gerstein - University of Toronto. Cambridge, Heffer.
  5. ↑ Langley, J. N. (1905-12-30). "On the reaction of cells and of nerve-endings to certain poisons, chiefly as regards the reaction of striated muscle to nicotine and to curari". The Journal of Physiology. 33 (4–5): 374–413. doi:10.1113/jphysiol.1905.sp001128. ISSN   0022-3751. PMC   1465797 . PMID   16992819.
  6. 1 2 MAEHLE, ANDREAS-HOLGER (2004-04-01). ""Receptive Substances": John Newport Langley (1852–1925) and his Path to a Receptor Theory of Drug Action". Medical History. 48 (2): 153–174. doi:10.1017/s0025727300000090. ISSN   0025-7273. PMC   546337 . PMID   15151102.
  7. 1 2 3 Lutz, Brenton R. (1931). "The Innervation of the Stomach and Rectum and the Action of Adrenaline in Elasmobranch Fishes". Biological Bulletin. 61 (1): 93–100. doi:10.2307/1537047. JSTOR   1537047.
  8. ↑ Cannon, Walter (1932). Wisdon of the Body. W.W. Norton and Company. ISBN   0393002055.
  9. 1 2 3 4 5 6 Jänig, Wilfrid; Keast, Janet R.; McLachlan, Elspeth M.; Neuhuber, Winfried L.; Southard-Smith, Michelle (2017-09-01). "Renaming all spinal autonomic outflows as sympathetic is a mistake". Autonomic Neuroscience. 206: 60–62. doi:10.1016/j.autneu.2017.04.003. ISSN   1566-0702. PMID   28566236.
  10. 1 2 3 de Groat, William C.; Saum, William R. (January 1972). "Sympathetic inhibition of the urinary bladder and of pelvic ganglionic transmission in the cat". The Journal of Physiology. 220 (2): 297–314. doi:10.1113/jphysiol.1972.sp009708. ISSN   0022-3751. PMC   1331704 . PMID   5014101.
  11. 1 2 3 4 5 Espinosa-Medina, I.; Saha, O.; Boismoreau, F.; Chettouh, Z.; Rossi, F.; Richardson, W. D.; Brunet, J.-F. (2016-11-18). "The sacral autonomic outflow is sympathetic". Science. 354 (6314): 893–897. doi:10.1126/science.aah5454. ISSN   1095-9203. PMC   6326350 . PMID   27856909.
  12. ↑ "The Vertebral Column and Spinal Cord". www.emory.edu. 1997-08-21. Retrieved 2017-12-06.
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