[adorisz]

A 35-yr old man has a knife wound to the neck that destroys the left C6 dorsal root ganglion. Three months later the dendrites, axons and cell bodies have been completely degenerated. what changes will we see in the spinal cord as a result of this injury.

a. axonal loss in the contralateral spinothalamic tract
b. axonal loss in the ipsilateral dorsal columns
c. axonal loss in the ipsilateral lateral corticospinal tract
d. nerve cell body loss in the ipsilateral dorsal horn

please give an analytical answer, thanks.

 

[heights]

The answer is b. axonal loss in the ipsilateral dorsal columns.

The dorsal root ganglion (DRG) is simply the cell bodies of the axonal tracts that make up the dorsal columns, so if you cut through the cell bodies, the axons will die. These neurons are the 1st order neurons in the dorsal column- medial lemniscus pathway, so they ascend ipsilaterally in the dorsal columns (faciculus cuneatus and gracilis), and synapse in the medulla in the nucleus cuneatus and gracilis. The 2nd order neuron decussates and is called the medial lemniscus, which travels to VPL thalamus. The 3rd order neuron leaves the thalamus and travels via the internal capsule to the postcentral gyrus (somatosensory cortex).

Why I think the others are wrong:

a) the cell bodies of the spinothalamic tract are located in the dorsal horn, so cutting the DRG won't affect these cells.

c) the corticospinal tract is motor and has nothing to do with the DRG; these fibers synapse on the ventral horn, then the lower motor neuron leaves the spinal cord through the ventral roots.

d) pain and temp fibers from the contralateral DRG cross over to synapse here, so these wouldn't be affected at all by cutting the ipsilateral DRG.

 

[adorisz]

The answer is b. axonal loss in the ipsilateral dorsal columns.

The dorsal root ganglion (DRG) is simply the cell bodies of the axonal tracts that make up the dorsal columns, so if you cut through the cell bodies, the axons will die. These neurons are the 1st order neurons in the dorsal column- medial lemniscus pathway, so they ascend ipsilaterally in the dorsal columns (faciculus cuneatus and gracilis), and synapse in the medulla in the nucleus cuneatus and gracilis. The 2nd order neuron decussates and is called the medial lemniscus, which travels to VPL thalamus. The 3rd order neuron leaves the thalamus and travels via the internal capsule to the postcentral gyrus (somatosensory cortex).

Why I think the others are wrong:

a) the cell bodies of the spinothalamic tract are located in the dorsal horn, so cutting the DRG won't affect these cells.

c) the corticospinal tract is motor and has nothing to do with the DRG; these fibers synapse on the ventral horn, then the lower motor neuron leaves the spinal cord through the ventral roots.

d) pain and temp fibers from the contralateral DRG cross over to synapse here, so these wouldn't be affected at all by cutting the ipsilateral DRG.

Very well said, answer is indeed B.

I just had the query because in the kaplan neuroscience notes: the diagram of the spinothalamic tract also shows cell bodies in DRG , so it was a bit fuzzy to me.