Dorsal Root Ganglion

Cross references:  Sensory System     Sensory Input      Pseudounipolar Neuron 
Spinal Cord   
Ganglion   Basal Ganglia      Locomotion Sequence Revision        

 Since my major focus is on locomotion, I'm going to learn about the dorsal root ganglia. 

Dorsal root ganglion (Wiki)   
    "A dorsal root ganglion (or spinal ganglion) (also known as a posterior root ganglion), is a cluster of nerve cell bodies (a ganglion) in a posterior root of a spinal nerve. The dorsal root ganglia lack motor neurons.


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). These neurons are of the pseudo-unipolar type, meaning they have an axon 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.   

My comments

1.  Although it is common practice to refer to the distal process as an axon, it seems to me that this is unnecessarily confusing.  Since it functions as a dendrite, we should call it a dendrite.    

2.  Since I'm especially interested in locomotion, it's the proximal, rather than the distal, process in which I'm interested.  In particular, I'm interested in the neurotransmitter delivered at the terminal end of the proximal process.  Is it excitatory or inhibitory?  Unfortunately, this reference doesn't say.  

1.1 Distal section

Looking for the dorsal root ganglion neurotransmitter

Searching PubMed for "dorsal root ganglion neurotransmitter" found 4,054 references:   

Searching PubMed for "dorsal root ganglion glutamate" found 606 references:   

Searching PubMed for "dorsal root ganglion glutamate ionotropic" found 38 references:   

Searching PubMed for "dorsal root ganglion glutamate metabotropic" found 60 references:  

Since I remember reading somewhere that ionotropic receptors are faster than the metabotropic, I'm going to start with the ionotropic

Searching PubMed for "dorsal root ganglion glutamate ionotropic" found 38 references:   

38<38    1993 
Region-specific expression of subunits of ionotropic glutamate receptors (AMPA-type, KA-type and NMDA receptors) in the rat spinal cord with special reference to nociception.      
    "The present study attempted to explore the gene expression of the subunits (GluR1-4) of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type receptor, subunit (GluR5) of kainic acid (KA)-type receptor, NR1 [a subunit of N-methyl-D-aspartate (NMDA) receptors] and the possible glutamate-binding subunit of an NMDA receptor complex in the dorsal horn of the rat spinal cords using in situ hybridization histochemistry.  
    These results were compared with those of the spinal motor neurons. Expression of the subunits of the AMPA-type receptor was also examined at the protein level using immunocytochemistry, with reference to the motor neurons.  
    Although all the four subunits of the AMPA-type receptor were expressed throughout the dorsal horn, the pattern of expression was different according to the dorsal horn region and to the subunits.  
    GluR2 showed the strongest expression in the dorsal horn. Huge numbers of strongly labelled cells formed a dense collection in lamina II and superficial parts of lamina III.  
    Many neurons in lamina II and superficial parts of lamina III expressed GluR1 moderately.  
    Scattered neurons moderately expressing GluR3 were also seen in these regions, while the expression of GluR4 was very low.  
    Labelling of the dorsal horn neurons by the GluR5 probe was low, and NR1 probe and a glutamate-binding subunit of an NMDA receptor complex probe labelled them diffusely with low to moderate intensity.  
    These findings show a close relationship between the glutamergic nociceptive primary afferent system and AMPA-type receptors in which GluR2 is especially highly expressed. The present study further showed that the expression pattern of the glutamate receptors in the spinal sensory neurons differs considerably from that of spinal motor neurons. Motor neurons very strongly express GluR3 and 4, while the expression of GluR2 and GluR1 is moderate and low, respectively. Expression of GluR5 is also low in the motor neurons. However, expression of NR1 and the glutamate-binding subunit of an NMDA receptor complex is very strong.  
    These findings indicate that the subunit composition of the AMPA-type receptors regulating motor neurons is different from that of the AMPA-type receptors in the spinal sensory neurons, and that there are at least two kinds of glutamergic systems which regulate motor neurons: via AMPA-type receptors and via NMDA receptors."  
    My comments
1.  I would not have guessed that there's a difference between "motor neurons" and "spinal sensory neurons", but now that it's been pointed out to me I can see the difference.    
2.  Are the two types of neurons, above, connected by interneurons? 
3.  Yes!  See: 
Spinal Cord

SubC:  Dorsal Root Ganglion
161009 - 1341