The term ‘ganglion’ means a collection of neuronal cell bodies present in the peripheral nervous system. A ganglion consists of cell bodies of neurons present in the peripheral parts of the body. Hence, a ganglion is a part of the peripheral nervous system.
The peripheral nervous system of humans consists of several types of ganglia, for example, sympathetic ganglia, parasympathetic ganglia, etc. One of the several types of ganglia present in the peripheral nervous system is dorsal root ganglion.
In this article, we will study different aspects of dorsal root ganglion. We will try to understand the location, structure, histology, and functions of the dorsal root ganglion. We will also discuss some diseases associated with the dorsal root ganglion. So, keep reading.
It is evident from the name that a dorsal root ganglion is associated with the dorsal or posterior root of spinal nerves. One dorsal root ganglion is associated with each spinal nerve present in our body.
The neurons present in the dorsal root ganglion gives rise to all the fibers present in the dorsal root of a spinal nerve. It lies suspended via these fibers just lateral to each segment of the spinal cord. It is protected by different processes of a vertebra.
As mentioned earlier, a ganglion is just a collection of nerve cells outside the CNS. Similarly, dorsal root ganglion also consists of cell bodies of neurons. The neurons present in the dorsal root ganglion are called bipolar or pseudo-unipolar neurons. These neurons have a spindle-shaped cell body that gives rise to a single nerve fiber. This nerve fiber soon splits into two processes.
One of the two processes travels towards the spinal cord. It is called the central process. It is also sometimes called an axon, as it carries the nerve impulses away from the cell body.
The other process travels towards the periphery. It is called the peripheral process. Sometimes, this peripheral process is also called s dendritic process because it carries the nerve impulses towards the cell body.
The histological structure of the dorsal root ganglion is quite different from the surrounding tissue. When a spinal nerve along with the dorsal root ganglion is examined under a light microscope, the dorsal root ganglion appears as a collection of highly basophilic cells surrounded by eosinophilic nerve fibers.
The large-sized basophilic cells seen in the ganglion are actually the neuronal cell bodies. These cells have a prominent large nucleus present in the center. A nucleolus is also present within each nucleus.
In addition to the neuronal cell bodies, one can easily demarcate small cells present in the periphery of the large-sized neurons. These small cells appear as small dots present along the cell membrane of actual neurons. These are actually the glial cells associated with the cell bodies called the satellite cells. These cells provide nutrition and support to the actual neurons.
The neural crest cells give rise to the neurons present in the dorsal root ganglia. These neural crest cells originate in the fourth week of pregnancy when the neural folds join to form a neural tube in place of neural plate.
These neural crest cells then migrate laterally and give rise to ganglia of the nervous system, including the dorsal root ganglion.
The cells in the dorsal root ganglia are originally bipolar. However, as the ganglion develops, the two axons of the cell come together and fuse in the midline in a T-shaped manner. As a result, a pseudo-unipolar neuron is formed. Both the process of this pseudo-unipolar neuron have the characteristics of an axon.
The cells in the dorsal root ganglion receive nutrients via the blood vessels supplying them. Just like the surrounding structure, the dorsal root ganglia get their blood supply from the segmental arteries arising from the aorta.
The blood vessels in the dorsal root ganglia are highly permeable allowing the substances to diffuse easily between the two media. This high permeability of blood vessels in dorsal root ganglia is clinically very important as it allows the drugs to easily diffuse and act on the dorsal root ganglia.
The dorsal root ganglia contain the cell bodies of neurons that perceive different sensations from the peripheral body parts. Thus, the sensory function of the spinal nerves is completely dependent on the dorsal root ganglia. Some important functions associated with the dorsal root ganglia are as follows.
Perception of Painful Stimuli
The cells in the dorsal root ganglia act as sites of perception of a painful stimulus. These cells have special receptors called the nociceptors. These are the G-protein coupled receptors that can sense even minute concentrations of protons. Thus, any pain caused by protons is first perceived in the dorsal root ganglia.
Perception of Mechanical Stimuli
Mechanical stimuli are also perceived by the dorsal root ganglia. the peripheral fibers of these ganglia have different kinds of nerve endings. These nerve endings act as independent receptors that can perceive any mechanical stimulus applied to them.
When a mechanical stimulus is applied to a nerve ending, it generates nerve impulses in the associated nerve fiber. The nerve fiber carries these impulses towards the cells present in the dorsal root ganglia. The central process of the dorsal root ganglia then carries this information to the higher centers of the CNS.
Acting as First Order Neurons
The neurons present in the dorsal root ganglia act as first-order neurons for a number of sensations. These include sensations of crude and fine touch, sensations of heat and cold, sensations of pain, sensations of vibration and pressure. Any disease of the dorsal root ganglion will result in the loss of all these sensations at the level of involvement.
The pathway followed by nerve impulses in the case of a reflex action is called reflex arc. It is an involuntary protective response to a painful stimulus that allows the withdrawal of the body away from the source of pain. The dorsal root ganglion forms the sensory component of the reflex arc.
For example, when you accidentally touch a hot object, your hand immediately moves away from that object. This withdrawal of the hand away from the hot object is due to reflex action. In this action, the pain is preserved due to the dorsal root ganglion. If this ganglion is diseased or absent, the reflex action would not occur properly.
The following are the important clinical correlates associated with the dorsal root ganglia.
Source of Viral Infections
This is the most important clinical correlate associated with the dorsal root ganglia. They act as sites of viral infection. There are some viruses that can live inside the dorsal root ganglia such as herpes virus, varicella-zoster virus, and virus causing multiple sclerosis, etc.
These viruses live inside the ganglia for a prolonged period of time without causing a disease. However, in favorable conditions such as weak immunity, they come out of the ganglia and cause infection within the body.
As mentioned earlier, the dorsal root ganglia are responsible for sthe perception of pain. According to the latest pain treatment techniques, dorsal root ganglia can be targeted for the management of chronic pain. The stimulation of dorsal root ganglia by different methods helps in relieving pain.
In some cases, the dorsal root ganglia is removed via ganglionectomy for the permanent treatment of chronic pain.
Ganglia are the collection of neurons within the peripheral nervous system.
One of these ganglia is the dorsal root ganglion attached to the posterior root of spinal nerve.
These ganglia give rise to sensory fibers of each spinal nerve.
The neurons in these ganglia are pseudo-unipolar having a peripheral process and a central process.
The peripheral process acts as a dendrite while the central process acts as an axon.
These dorsal ganglia appear highly basophilic upon histological examinations. The large neurons of the ganglia also have some peripheral supporting cells called the satellite cells.
These neural crest cells are the precursors of pseudo-unipolar cells present in the dorsal root ganglia. Initially, they are bipolar cells. But later on, they become pseudo-unipolar after fusion of the two processes in the midline.
The dorsal root ganglia get nutrients from the blood coming via the segmental arteries.
These dorsal root ganglia perform the functions of;
- Perception of pain stimuli
- Perception of mechanical stimuli
- First-order neurons of a number of sensations
- Sensory component of Reflex Arc
The dorsal root ganglia have important clinical association because they have a role in;
- Some viral infections
- Management of chronic pain
- 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
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- 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.