Outermost is the bone part, inner is the yellow region divided by white matter (outer, with mylination axons fibers) and gray matter (inside, composed by cell body). The only exception is the sensory neurons (here the peripheral sensory neurons, PSN). Instead of residing inside the cord, PSN bodies are located outside and form a assmbly called dorsal root ganglion (DRG).
Note the enlargement can be only observed in the dorsal root, since motor neuron (ventrally located) nuclei are inside the spinal cord.
Here the green parts are DRG. Note dorsal and ventral bundle converge into one tract and project outward. DRGs are at dorsal root.
DRG neurons are pseudounipolar; a single axon projects from the cell body and bifurcates at the T-junction. The peripheral portion of the axon extends to receptor endings in the periphery and is responsible for afferent signaling. The central part of the axon extends into the central nervous system (CNS). (a T shaped axon, with a long long peripheral side and very short central side, looks like unipolar but not) It shows considerable axonal arborizations into the spinal cord, terminating in synapses at ipsilateral or contralateral wide dynamic range neurons, inhibitory interneuron networks, and other targets in the dorsal horn.
More details are here
https://www.ncbi.nlm.nih.gov/books/NBK532291/
The role of peripheral sensory neurons is to conduct action potentials from peripheral sensory neurons to central terminals for transmission to the central nervous system. Based on the signal transmission speed, it can be divided into 3 types.
Group A Nerve Fibres: Group A nerve fibres are heavily myelinated nerve fibres that are further subdivided into four types: alpha Aα; beta Aβ; gamma Aγ; and delta Aδ (gradullay decrease their radius). The fibres with larger diameters and more myelination tend to transmit the impulses at a faster rate.
Group B Nerve Fibres: Group B nerve fibres are less myelinated than group A, but more myelinated than group C nerve fibres. They include visceral nerves such as the vagus nerve.
Group C Nerve Fibres: Group C nerve fibres are unmyelinated fibres that usually have a smaller diameter and low conduction velocity.
An extra knowledge, DRG stimulation
What is DRG Stimulation?
Applying electrical stimulation to the DRG can interrupt the pain signals and provide relief of focal nerve pain. DRG stimulation is relatively new receiving FDA approval in 2016.
The function of DRG stimulation is very similar to that of spinal cord (or dorsal column) stimulation with several key differences. DRG stimulation is able to provide stimulation to focal areas of pain that have been historically difficult to treat with traditional spinal cord stimulators, such as the hand, chest, abdomen, groin, knee and foot.
How Does it Work?
During the procedure, leads – essentially specialized wires that transmit the electrical stimulation- are placed next to the DRG within the epidural space. The leads are then connected to a pulse generator that delivers small electrical impulses to block the pain. A one week trial is typically performed where temporary leads are placed and attached to an external pulse generator. If effective, the leads can be surgically implanted and attached to an internal pulse generator that gets implanted under the skin.
By targeting these easily accessible nerves, the stimulation can be focused on a certain part of the body, while avoiding stimulation of other, non-painful areas. There is less fluid surrounding these nerves, which means less energy is required to stimulate the nerves leading to longer battery life. This also means the leads are less likely to move or shift and change in body positions will likely not alter the stimulation as seen with traditional spinal cord stimulators.
