Chapter 73

INTRODUCTION

Primary sensory neurons (PSNs) process and transmit a multitude of sensory information from the periphery to the central nervous system. In this regard, they are a key component of the initial pathway by which pain (as well as a variety of other sensory information) is transmitted from both peripheral and axial structures to secondary and tertiary order neurons in the central nervous system. In an acute sense, pain transduction serves as a safety mechanism to protect tissues from injury. Classic pain reflexes that result in withdrawal from harmful stimuli protect tissues from damage. Unfortunately, in some cases, these same neurologic pathways can be dramatically altered resulting in the development of chronic neuropathic pain. One component of the sensory nervous system shown to exhibit a number of pathophysiologic changes that may contribute to chronic pain is the dorsal root ganglion (DRG), the home of the primary sensory neuron cell bodies.

ANATOMY OF THE DRG

One of the hallmarks of PSNs is that, anatomically, they are fairly unique in structure. They are considered bipolar or pseudounipolar neurons, in that a single axon exits the soma and divides to form two branches, one of which travels to the periphery and the other to the spinal cord. Positioned between the distal and proximal axons is the T junction (Figure 73-1). Studies have shown very little variability in DRG position among healthy individuals.

Figure 73-1.

Illustration of primary sensory neuron cell bodies within the DRG.

• The soma of the primary sensory neurons are housed in the dorsal root ganglion. There can be up to 15,000 primary sensory neuron soma housed in a single DRG depending upon spinal level.

• The soma within the DRG can be large and stretches from distal tissue structures to the spine means it can be meters in length.

• The relative size of the soma compared to the axon is very small.

• Since much of the cell's protein synthesis occurs in the soma, the metabolic demands on this structure are very high.

• The DRG is typically located in the foraminal space between the medial and lateral edges of the pedicles (Figure 73-2).

• There is slight variability in DRG size as DRGs located more caudal tend to be larger.

• Sensory axons maintain a fairly consistent anatomical distribution as they travel to the spinal cord.

• There is an organized and convergent distribution of cellular fibers that eventually synapse in the dorsal horn.

Figure 73-2.

Lumbar DRGs from coronal MIP imaging. The pedicles are dark (white P) and the DRGs are indicated by the white arrows. Notice the relatively uniform position of the DRGs relative to the pedicles. The vast majority of DRGs from healthy volunteers lie between the medial and lateral borders of the pedicles. (Reproduced with permission from Shen J, Wang HY, Chen ...

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