A dorsal root ganglion (or spinal ganglion; also known as a posterior root ganglion) is a cluster of

neurons A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...

ganglion 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 sympatheti ...

) in a

dorsal root The dorsal root of spinal nerve (or posterior root of spinal nerve or sensory root) is one of two "roots" which emerge from the spinal cord. It emerges directly from the spinal cord, and travels to the dorsal root ganglion. Nerve fibres with the v ...

of a

spinal nerve A spinal nerve is a mixed nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body. In the human body there are 31 pairs of spinal nerves, one on each side of the vertebral column. These are grouped into th ...

. The cell bodies of sensory neurons known as first-order neurons are located in the dorsal root ganglia. The

axons An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, th ...

of dorsal root ganglion neurons are known as afferents. In the

peripheral 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 nerves and ganglia, which lie outside the brain ...

, afferents refer to the axons that relay sensory information into the

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all par ...

(i.e. the

brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...

and the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spi ...

Structure

The neurons comprising the dorsal root ganglion are of the pseudo-unipolar type, meaning they have a cell body (soma) with two branches that act as a single axon, often referred to as a ''distal process'' and a ''proximal process''. Unlike the majority of neurons found in the

, an

action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...

in posterior 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

posterior horn of spinal cord The posterior grey column (posterior cornu, dorsal horn, spinal dorsal horn, posterior horn, sensory horn) of the spinal cord is one of the three grey columns of the spinal cord. It receives several types of sensory information from the body, inc ...

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. Two examples where the nerve ending of the ''distal process'' is encapsulated as such are,

Meissner's corpuscles Tactile corpuscles or Meissner's corpuscles are a type of mechanoreceptor discovered by anatomist Georg Meissner (1829–1905) and Rudolf Wagner. This corpuscle is a type of nerve ending in the skin that is responsible for sensitivity to pressu ...

, which render the ''distal processes'' of mechanosensory neurons sensitive to stroking only, and

Pacinian corpuscles Pacinian corpuscle or lamellar corpuscle or Vater-Pacini corpuscle; is one of the four major types of mechanoreceptors (specialized nerve ending with adventitious tissue for mechanical sensation) found in mammalian skin. This type of mechanorecept ...

, which make neurons more sensitive to vibration.

Location

The dorsal root ganglia lie in the

intervertebral foramina The intervertebral foramen (also called neural foramen, and often abbreviated as IV foramen or IVF) is a :wikt:foramen, foramen between two spinal vertebrae. Cervical vertebrae, Cervical, thoracic vertebrae, thoracic, and lumbar vertebrae all ...

. The anterior and posterior spinal nerve roots join just beyond (lateral) to the location of the dorsal root ganglion.

Development

The dorsal root ganglia develop in the embryo from

neural crest Neural crest cells are a temporary group of cells unique to vertebrates that arise from the embryonic ectoderm germ layer, and in turn give rise to a diverse cell lineage—including melanocytes, craniofacial cartilage and bone, smooth muscle, per ...

cells, not

neural tube In the developing chordate (including vertebrates), the neural tube is the embryonic precursor to the central nervous system, which is made up of the brain and spinal cord. The neural groove gradually deepens as the neural fold become elevated, a ...

. Hence, the spinal ganglia can be regarded as gray matter of the spinal cord that became translocated to the periphery.

Function

Nociception

Proton-sensing G protein-coupled receptors Proton-sensing G protein-coupled receptors are transmembrane receptors which sense acidic pH and include GPR132 (G2A), GPR4, GPR68 (OGR1) and GPR65 (TDAG8). These G protein-coupled receptors are activated when extracellular pH falls into the ra ...

are expressed by DRG sensory neurons and might play a role in acid-induced

nociception Nociception (also nocioception, from Latin ''nocere'' 'to harm or hurt') is the sensory nervous system's process of encoding noxious stimuli. It deals with a series of events and processes required for an organism to receive a painful stimulus, co ...

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 channel Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by gating the flow of io ...

s thought to be responsible for

somatosensory In physiology, the somatosensory system is the network of neural structures in the brain and body that produce the perception of touch (haptic perception), as well as temperature (thermoception), body position (proprioception), and pain. It i ...

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

s to be fired. This firing may even persist after the removal of the stimulus. Two distinct types of

mechanosensitive ion channel Mechanosensitive channels, mechanosensitive ion channels or stretch-gated ion channels (not to be confused with mechanoreceptors). They are present in the membranes of organisms from the three domains of life: bacteria, archaea, and eukarya. They ...

s have been found in the posterior 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 cytoskeleton and cytoskeleton associated proteins. The presence of these channels in the posterior root ganglion gives reason to believe that other sensory neurons may contain them as well.

High-threshold mechanosensitive channels

High-threshold channels have a possible role in

. 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.

Presynaptic control

The presynaptic regulation of the dorsal nerve ending discharge in the spinal cord can occur through certain types of GABA_A receptors but not through the activation of

glycine receptor The glycine receptor (abbreviated as GlyR or GLR) is the receptor of the amino acid neurotransmitter glycine. GlyR is an ionotropic receptor that produces its effects through chloride current. It is one of the most widely distributed inhibitory ...

s which are absent from these types of terminals. Thus GABA_A receptors but not

s can presynaptically control

and

pain Pain is a distressing feeling often caused by intense or damaging stimuli. The International Association for the Study of Pain defines pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, ...

transmission.

References

Additional images

File:Medulla spinalis - Section - English.svg, Medulla spinalis File:Spinal nerve.svg, The formation of the spinal nerve from the posterior and anterior roots File:Gray799.svg, Scheme showing structure of a typical spinal nerve.

External links

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