Dorsal root ganglion
|Dorsal root ganglion|
|This is a dorsal root ganglion (DRG) from a chicken embryo (around stage of day 7) after incubation overnight in NGF growth medium stained with anti-neurofilament antibody. Axons growing out of the ganglion are visible.|
|A spinal nerve with its anterior and posterior roots. The dorsal root ganglion is the "spinal ganglion", following the posterior/dorsal root.|
|Latin||ganglion sensorium nervi spinalis|
|Gray's||subject #185 750|
In anatomy and neuroscience, a dorsal root ganglion (or spinal ganglion) is a nodule on a dorsal root of the spine that contains cell bodies of nerve cells (neurons) that carry signals from sensory organs toward the appropriate integration center. Nerves that carry signals toward the central nervous system (brain or spinal cord) are known as afferent nerves.
Unique unipolar structure 
The axons of dorsal root ganglion neurons are known as afferents. In the peripheral nervous system, afferents refer to the axons that relay sensory information into the central nervous system (i.e. the brain and the spinal cord). These neurons are of the pseudo-unipolar type, meaning they have an axon with two branches that act as a single axon, often referred to as a distal process and a proximal process.
Note: the neuron can consist of three parts:
1. Dendrite that receives the information and relays it to the Soma, or cell body.
2. Soma - the cell body of the neuron
3. Axon: which relays information from the soma.
In a neuron, the dendrite receives information from another neuron's axon at the synapse, and the axon sends information to the next neuron's dendrites, even though the dendrite may be covered with myelin.
Unlike the majority of neurons found in the central nervous system, an action potential in dorsal root ganglion neuron may initiate in the distal process in the periphery, bypass the cell body, and continue to propagate along the proximal process until reaching the synaptic terminal in the dorsal horn of the spinal cord.
Distal section 
The distal section of the axon may either be a bare nerve ending or encapsulated by a structure that helps relay specific information to nerve. For example, a Meissner's corpuscle or Pacinian corpuscle may encapsulate the nerve ending, rendering the distal process sensitive to mechanical stimulation, such as stroking or vibration, respectively.
The dorsal root ganglia lie along the vertebral column by the spine.
The dorsal root ganglia develops in the embryo from neural crest cells, not neural tube. Hence, the spinal ganglia can be regarded as gray matter of the spinal cord that became translocated to the periphery.
Mechanosensitive Channels 
The nerve endings of dorsal root ganglion neurons have a variety of sensory receptors that are activated by mechanical, thermal, chemical, and noxious stimuli. In these sensory neurons, a group of ion channels thought to be responsible for somatosensory transduction have been identified. Compression of the dorsal root ganglion by a mechanical stimulus lowers the voltage threshold needed to evoke a response and causes action potentials to be fired. This firing may even persist after the removal of the stimulus.
Two distinct types of mechanosensitive ion channels have been found in the dorsal root ganglion neurons. The two channels are broadly classified as either high-threshold (HT) or low threshold (LT) . As their names suggest, they have different thresholds as well as different sensitivities to pressure. These are cationic channels whose activity appears to be regulated by the proper functioning of the cytoskelton and cytoskeleton associated proteins. The presence of these channels in the dorsal root ganglion gives reason to believe that other sensory neurons may contain them as well.
High-Threshold Mechanosensitive Channels 
HT channels have a possible role in nociception. These channels are found predominantly in smaller sensory neurons in the dorsal root ganglion cells and are activated by higher pressures, two attributes that are characteristic of nociceptors. Also, the threshold of HT channels was lowered in the presence of PGE2 (a compound that sensitizes neurons to mechanical stimuli and mechanical hyperalgesia) which further supports a role for HT channels in the transduction of mechanical stimuli into nociceptive neuronal signals.
See also 
- 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
- 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.
- Cho, H.; Shin, J.; Shin, C. Y.; Lee, S. Y.; Oh, U. (2002). "Mechanosensitive ion channels in cultured sensory neurons of neonatal rats". The Journal of neuroscience : the official journal of the Society for Neuroscience 22 (4): 1238–1247. PMID 11850451.
- Sugawara, O.; Atsuta, Y.; Iwahara, T.; Muramoto, T.; Watakabe, M.; Takemitsu, Y. (1996). "The effects of mechanical compression and hypoxia on nerve root and dorsal root ganglia. An analysis of ectopic firing using an in vitro model". Spine 21 (18): 2089–2094. PMID 8893432.
- Syriatowicz, J. P.; Hu, D.; Walker, J. S.; Tracey, D. J. (1999). "Hyperalgesia due to nerve injury: Role of prostaglandins". Neuroscience 94 (2): 587–594. doi:10.1016/S0306-4522(99)00365-6. PMID 10579219.
Additional images 
- SUNY Figs 02:04-09
- BU Histology Learning System: 04401loa
- Photo of model at Ohio State University
- Diagram at webanatomy.net
- Photo at uwlax.edu