A dorsal root ganglion (or spinal ganglion; also known as a posterior root ganglion) is a cluster of neurons (a ganglion) in a dorsal root of a spinal nerve. The cell bodies of sensory neurons known as first-order neurons are located in the dorsal root ganglia. 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). 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 central nervous system, an action potential 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. 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, which render the distal processes of mechanosensory neurons sensitive to stroking only, and Pacinian corpuscles, which make neurons more sensitive to vibration. Location The dorsal root ganglia lie in the intervertebral foramina. 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 cells, not neural tube. 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 are expressed by DRG sensory neurons and might play a role in acid-induced nociception. 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