|07-26-2003, 11:03 AM||#1|
Join Date: Mar 2003
We have been talking many times about different advanced aspects of SCI, but there are some basic questions that I have. Does the spinal cord or the motor axons below the injury survive or do they diminish with time because there are no motor signals going through? and also about the spinal cord or sensory axons above the injury? In the last MRI it's interpretation has gotten me concern. As you allready Know
One of the aspects says that SC is never definetely visualized below T6 to T12. Does the SC survives just with blood flow? If so then why do when talking about recovery you say that the axons have to grow all the way to the targets?
Does that differ from the white and gray matter?
|07-26-2003, 05:30 PM||#2|
Join Date: Jul 2001
Location: New Brunswick, NJ, USA
Let me make sure that we are talking about the same things:
1. Motoneurons are neurons at each segment level of the spinal cord that connect to muscles. They receive sensory information from the muscle. The brain sends axons to these motoneurons and other neurons in the spinal cord that connect to the motoneurons. Spinal cord injury disconnects these descending axons, as well as ascending axons that bring sensory signals from the spinal cord.
2. The spinal cord below the injury site usually is still functioning. You know this because most people have spasticity and reflexes (which indicates that the motoneurons are active and also receiving signals from the limbs).
3. The brain neurons that send axons to the spinal cord also remain. Changes of course occur to the neurons that have been disconnected when they are not used. There is now evidence suggesting that if you do not use neurons for a while, they turn themselves off but intensive repetitive exercise can turn them back on. To get function back, i.e. for your brain to control the motoneurons and to detect sensation, we must have regeneration of axons that have been disrupted. Fegeneration alone may not be sufficient to restore function if the neurons have been turned off. In people who may still have residue axons, exercise may restore substantial function. That is one of the reasons why there has been so much interest and excitement in ambulation training.
Okay, now, to your questions.
• Does the spinal cord or the motor axons below the injury site diminish with time? The motor axons die if they have been disconnected by the injury. Axons are parts of a cell. They cannot survive by themselves. However, the motoneurons that the motor axons connect to usually should remain alive. Yes, the spinal cord below the injury site will diminish with time and aging. We all lose neurons over time. Spinal cord injury may accelerate this aging process. Also, if you have damaged the lower spinal cord, either by direct trauma to the lower spinal cord or by interfering with the blood supply to the lower spinal cord, there may well be neuronal loss from the spinal cord below the injury site.
• Does the spinal cord survive just with blood flow? Yes, if there is adequate blood flow to the lower spinal cord, it should survive even if it is no longer connected to the brain.
• The spinal cord is not visualized form T6-T12. Because some 70% of the thoracic spinal cord is composed of ascending and descending axons that may have been interrupted. These axons die. Because they die, the spinal cord appears to be thinner and atrophic around the injury site. This is very likely to be part of the reason why the spinal cord is never definitively visualized in your lower thoracic cord. But there may be another reason as well. You have a spinal curvature (kyphosis) in the lower cord. This may be compressing the spinal cord in the region and also pushing the spinal cord so that it cannot be seen well on an MRI.
• White versus gray matter. Gray matter occupies only about 10-20% of the tissue in the thoracic spinal cord. The gray matter contains the neurons that supply the dermatimes of the 6th to 12th ribs. Note that part of the lumbar cord is located around the T10-12th segments. This is because the spinal cord is shortened.
I hope that this answers your questions.