Dorsal root ganglion
Spinal nerves are mixed nerves that transmit motor, sensory, and autonomic signals between the central nervous system and the periphery. Each spinal nerve carries afferent (sensory) fibers and efferent (motor) fibers to and from the spinal cord, the former of which comprise the posterior/dorsal roots. Each posterior root presents a ganglion as it emerges from the intervertebral foramen.
The dorsal root ganglion, more recently referred to as the spinal ganglion, is a collection of neuronal cell bodies of sensory neurons. It is the most common type of sensory ganglion in the human body. Each cell body in the ganglion belongs to what is considered to be a pseudounipolar neuron. A pseudounipolar neuron consists of a cell body bearing one short axon which bifurcates into two processes. The main function of the spinal ganglion cells is to transmit the sensory neural signals from the peripheral to the central nervous system.
This article will discuss the anatomy and function of the spinal/dorsal root ganglion.
|Definition||Collection of neuronal cell bodies of sensory neurons that transmit sensory impulses|
|Structure||Cell bodies of pseudounipolar neurons, satellite (glial) cells|
|Blood supply||Radicular arteries (branches of vertebral artery (C1-C7), deep cervical artery (C7-T1), posterior intercostal arteries (T1-T11), subcostal artery (T12) and lumbar arteries)|
|Function||Transmission of sensory neural signals to the central nervous system from the peripheral nervous system|
The term ganglion refers to a cluster of the neuronal cell bodies in the peripheral nervous system. The spinal ganglion contains the cell bodies of sensory neurons situated in the posterior root of each spinal nerve (except for spinal nerve C1). These neurons are known as first-order neurons of the somatosensory system and carry sensations related to touch, vibration, proprioception, pain and temperature. Their cell bodies are typically variable in size but all are encapsulated by satellite glial cells which prevent the impulse transmission between them.
All cell bodies contained in the spinal ganglion are considered to belong to pseudounipolar neurons, which are a variation of bipolar neurons. These neurons have specialized structures: cell bodies that extend a short axon that quickly terminates by bifurcating into two processes, central and peripheral. The peripheral process carries information from the periphery through the spinal nerve (together with motor fibers), while the shorter central process extends towards the posterior horns of the gray matter of the spinal cord.
Because of this specific ‘T-shaped’ structure of pseudounipolar neurons, impulses generated in the periphery bypass the cell bodies in the spinal l root ganglion and continue directly to the spinal cord through the central process of the neuron.
Spinal ganglia are ensheathed by extensions of the spinal meninges, which are replaced by a connective tissue covering known as epineurium at the level of the spinal nerve.
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The spinal ganglion contains the cell bodies of all sensory nerve fibers in the spinal nerve, , thus it has a crucial role in sensory transmission in the human body. More specifically, its main function is to relay the sensory nerve impulses from the periphery to the peripheral nervous system. The cell bodies contain genetic information and organelles which direct and drive cellular activity, as well as maintain the structure of the neuron.
The sensory impulses generated in the periphery (pain, touch, temperature changes) activate the peripheral processes of the spinal ganglion neurons. These impulses then travel to the spinal cord either directly (bypassing the cell bodies) or through the neuronal cell body in the dorsal root ganglion. It has been long believed that these cell bodies acted as storage "helpers" in transmitting impulses, such as pain impulses (nociception). However, it has been shown that these neural bodies are indeed active participants in the regulation of this signaling process.
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