Dorsal Root Ganglion

A ganglion is the collection of cell bodies of neurons located outside the central nervous system. A dorsal root ganglion is the one associated with the dorsal or posterior root of the nerves originating from the spinal cord.

All the posterior roots of spinal nerves contain a ganglion. As the dorsal or posterior root of a spinal nerve is primarily sensory, the dorsal root ganglion contains cell bodies of these sensory nerve fibers.

In this article, we will talk about the structure,
location, and connections of the dorsal root ganglion. We will also discuss the
functions and clinical conditions associated with the dorsal root ganglion.

Location

As the name indicates, the dorsal root ganglion is associated with the posterior or dorsal root of the spinal nerve. It is located in close proximity to the spinal cord. As the dorsal root of spinal nerve emerges from the intervertebral neural foramen, it expands to form the ganglion. It is present before the dorsal and ventral roots join together to form a single spinal nerve.

Structure

The dorsal root ganglion contains cell bodies of sensory neurons. These are the unipolar neurons, also called pseudo-unipolar neurons. These cell bodies give off a single nerve fiber or axon. The axon splits at a little distance from the cell body to form two fibers.

One of these fibers carries the nerve impulses towards
the cell body and is called dendrite. The other fiber carries the nerve
impulses away from the cell body and is called the axon.

It is important to note that both these fibers resemble the axons in their structure because they are formed by splitting of a single axon emerging from the cell body. They are divided into axon and dendrite depending on the direction of nerve impulses to or away from the cell body.

Histology

Upon histological examination, dorsal root ganglion is seen as a basophilic structure distinct from the surrounding highly eosinophilic nerve fibers. Two types of cells can be seen in a slide of a dorsal root ganglion. These are the neuron cells and the satellite cells.

The neurons cells or cell bodies are the larger cells
having a big nucleus. Upon careful examination, a nucleolus can also be seen
inside each nucleus. The neuronal cell bodies are seen to be highly granular.
The granules represent the RNA present in them.

The satellite cells are the smaller cells seen in the
periphery of the actual neurons. These are the glial cells that serve to
protect the neurons. They contain a very small nucleus. These satellite cells
perform the same function in dorsal root ganglion as performed by the
astrocytes in the CNS.

Embryology

The dorsal root ganglion develops from the neural crest cells. It has the same origin as the origin of sympathetic and parasympathetic ganglia. The precursor cells of the dorsal root ganglion emerge as neural crest cells in the fourth week of pregnancy. T

hese cells migrate ventrally. During the eleventh week, the neural crest cells differentiate into the pseudo-unipolar cells of the dorsal root ganglion.

As these cells develop from the neural crest cells,
the cells in the dorsal root ganglion are also regarded as the grey matter of
the spinal cord. It is considered that this part of the spinal cord’s grey
matter migrated to the periphery during the later phase of pregnancy.

Blood supply

The dorsal root ganglion of the spinal cord is supplied by the branches of the segmental arteries. Contrary to the rest of the nervous system that has little permeability of blood vessels, the blood vessels in the dorsal root ganglion have much greater permeability. This greater permeability of blood vessels is clinically significant.

Physiology

Under this heading, we will discuss important functions of the dorsal root ganglion. As we already know that the dorsal root of the spinal nerve is associated with the sensory functions, all the functions associated with the dorsal root ganglion are also related to different sensations.

Nociception

The sensation of pain is called nociception. The sensory neurons in the dorsal root ganglion express G protein-coupled receptors highly sensitive to protons. They are involved in the perception of pain caused by acids or protons.

Perception of Mechanical Stimuli

The endings of the fibers originating from the dorsal root ganglion have a number of sensory receptors that are activated by different stimuli. Some of these receptors are ion channels that can be activated by the compression of the dorsal root ganglion.

When a mechanical stimulus is applied to the dorsal root ganglion, it causes the opening of these ion channels. It causes the perception of a compressing force that lasts even after the removal of the mechanical stimulus.

These mechanoreceptors present in the dorsal root
ganglion are divided into two types; high-threshold (HT) channels and the
low-threshold (LT) channels. These channels have different threshold for
activation by pressure or mechanical stimuli.

Processing of the Sensory Information

The dorsal root ganglion contains first-order neurons of sensory fibers. They carry sensations of touch, both crude and fine, thermal sensations, sensations of pain, sensation of pressure, and vibration.

The cell bodies in the dorsal root ganglion serve as the first relay center and the processing center of this information. If the sensation is very small, it is unable to cause the firing of a large number of nerve fibers. As a result, it usually dies out at the level of the ganglion and is not carried to the higher centers of the brain.

Reflex Action

The dorsal root ganglion is an important part of the reflex arc. It forms the sensory arm of the reflex arc required to carry out the reflex action. The sensations of pain, hurt or temperature is carried through the dorsal root ganglion to the spinal cord.

Destruction of the dorsal root ganglion in any part of the body results in loss of reflex action in that part of the body.

Clinical Significance

The clinical significance of dorsal root ganglion are
discussed in this section.

Treatment of Chronic Pain

The gate-control theory of pain has revolutionized the therapeutic approach for the treatment of chronic pain. The dorsal root ganglion is one of the targets for chronic pain management. The methods used to treat chronic pain with dorsal root ganglion involvement include the following:

• Continuous stimulation or
  modulation of dorsal root ganglion by thermal or radiofrequency stimuli
• Using pulsed radiofrequency for
  stimulation of ganglion
• Electric stimulation of dorsal root
  ganglion

In addition to the above-mentioned stimulation techniques, the cellular functions of the dorsal root ganglion can also be modified by using viral vectors in order to reduce pain. The gene silencing technique can also be used to alter the cellular functions.

If all the above-mentioned techniques fail, the dorsal root ganglion may be removed as a final resort. This process is known as ganglionectomy.

Reservoirs of Viral Infections

Some scientific studies have proved that the dorsal root ganglia of the spinal cord act as reservoirs of viral infections. This is especially important in the case of multiple sclerosis.

Other viruses such as varicella-zoster virus and herpes complex virus can also reside in the dorsal root ganglion. They can remain latent in the ganglion for quite some time before causing the disease.

Conclusion/Summary

The dorsal root ganglion is a collection of neuronal cell bodies associated with the sensory root of the spinal nerves. These ganglia are considered to be a part of the grey matter of the spinal cord.

The dorsal root ganglia are present very close to the
spinal cord. The posterior root expands to form the ganglia as soon as it
leaves the spinal cord.

The ganglion contains cell bodies of pseudo-unipolar
neurons. The single fiber that emerges from the cell body immediately splits to
form an afferent root or dendrite, and an efferent root or axon.

Upon histological examination, the ganglia is seen to have prominent basophilia. Two types of cells can be seen; the larger neurons and the smaller satellite cells. The satellite cells are seen in the periphery of the larger neuronal cells.

The pseudo-unipolar cells in the dorsal root ganglia
are derived from the neural crest cells which first appear in the fourth week
after fertilization.

The blood supply is derived from the branches of
segmental arteries.

The functions of dorsal root ganglia are all
associated with the perception of sensations. These include:

• Nociception
• Perception of mechanical stimulus
  upon compression
• First processing center of the
  sensory information
• Role in reflex action

The clinical significance of dorsal root ganglion includes the treatment of chronic pain according to the gate-therapy or via complete resection of the ganglion. The ganglion also serves as a source or reservoir of several viruses such as herpes simplex virus.

References

1. Zerboni L, Ku CC, Jones CD, Zehnder JL, Arvin AM. Varicella-zoster virus infection of human dorsal root ganglia in vivo. Proc Natl Acad Sci U S A. 2005;102(18):6490–6495. doi:10.1073/pnas.0501045102
2. Purves, Dale; Augustine, George J.; Fitzpatrick, David; Katz, Lawrence C.; LaMantia, Anthony-Samuel; McNamara, James O.; Williams, S. Mark (2001). "The Major Afferent Pathway for Mechanosensory Information: The Dorsal Column-Medial Lemniscus System". Neuroscience. 2nd edition. Retrieved 30 May 2018.
3. ^ Kandel ER, Schwartz JH, Jessell TM. Principles of Neural Science, 4th ed., p.431–433. McGraw-Hill, New York (2000). ISBN 0-8385-7701-6
4. ^ Huang CW, Tzeng JN, Chen YJ, Tsai WF, Chen CC, Sun WH (2007). "Nociceptors of dorsal root ganglion express proton-sensing G-protein-coupled receptors". Mol. Cell. Neurosci. 36 (2): 195–210. doi:10.1016/j.mcn.2007.06.010. PMID 17720533.