Corpus: Dorsal root ganglion

A dorsal root ganglion is a collection of nerve cell bodies located at the dorsal root (radix posterior) of each spinal nerve.

Dorsal root ganglia are found on both sides of each spinal cord segment, forming a distinct swelling along the posterior nerve root. These ganglia are situated in or near the intervertebral foramina between adjacent vertebrae. They typically measure 4 to 7 mm in size and have a spindle-shaped appearance.

3.1. Envelope structures

Dorsal root ganglia are encased in a dense capsule of tough connective tissue, rich in fibers. This capsule extends centrally into the dura mater of the spinal cord and peripherally into the epineurium of the spinal nerve. The outer layer of loose connective tissue contains vessels and nerves that penetrate the capsule, providing nourishment to the ganglion. Internally, the perineurium, a protective layer, extends centrally into the dural neurothelium between the dura mater and arachnoid mater, and peripherally into the perineurium of the spinal nerve.

3.2. Stroma

The internal structure of the dorsal root ganglion consists of loose connective tissue, similar to the endoneurium of peripheral nerves, with numerous free cells, particularly mast cells. This stroma environment is comparable to the perineural sheath of peripheral nerves, containing a liquor-like fluid. The stroma is richly supplied with blood capillaries, some of which have fenestrated endothelium.

3.3. Pseudounipolar neurones

The stroma of the dorsal root ganglia houses groups of sensory pseudounipolar neurons, their associated satellite (mantle) cells, and bundles of nerve fibers originating from these neurons. Each dorsal root ganglion contains tens of thousands of neuronal cell bodies (perikarya), which are typically round and measure between 15 and 110 µm in diameter. The nuclei of these neurons are approximately 25 µm in diameter, with prominent nucleoli. The cytoplasm is abundant in Nissl substance, and as the neurons age, they accumulate lipofuscin.

The peripheral projections of these neurons collect sensory information from the body and transmit it to the spinal cord via central projections. There is no synaptic switching within the ganglion. Neurons can be classified into two types based on cytological and functional characteristics:

• A cells: These are large, lightly stained neurons primarily responsible for transmitting afferents from proprioceptors and mechanoreceptors. They are associated with myelinated I and II nerve fibers.
• B cells: These are smaller, darkly stained neurons with densely packed Nissl substance. They are associated with III and IV nerve fibers, which function as nociceptors, thermoreceptors, and visceral receptors.

The neurons in the dorsal root ganglia produce various neuropeptides, such as substance P, CGRP, neuropeptide Y, VIP, glutamate, and galanin, which serve as neurotransmitters at their terminals in the spinal cord. Some of these neuropeptides are also released at peripheral nerve endings, contributing to peripheral sensitization and the axon reflex, which increases blood flow in the surrounding tissue.

3.4. Central termination

The central processes of pseudounipolar neurons in the dorsal root ganglia terminate in different areas of the posterior column of the spinal cord.

• III and IV fibers (primarily from nociceptors and thermoreceptors) enter the spinal cord through the lateral bundle of the posterior root, dividing in the posterolateral tract (Lissauer marginal zone). These fibers ascend and descend a few segments and primarily terminate in laminae I to III.
• I and II fibers (from mechanoreceptors and proprioceptors) enter medially into the posterior horn as part of the medial bundle, with some extending into deeper layers or ascending via the posterior cord to supraspinal centers.

Dorsal root ganglia develop from cells of the neural crest during embryogenesis, not from the neural tube. Initially, these neurons are bipolar, but they become pseudounipolar as development progresses.

The nucleus mesencephalicus nervi trigemini is sometimes considered the highest dorsal root ganglion because it contains pseudounipolar neurons, similar to those found in dorsal root ganglia, despite being a cranial nerve nucleus rather than a terminal nucleus.^[1]