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New technique to repair spinal cord injuries
The brain and neurons (nerve cells) throughout our body are connected via the spine. Here, motor neurons — which control muscle movement, and sensory neurons — which relay sensory information such as pain, temperature and touch, connect through the spinal cord.
Where neurons connect to the spinal cord, motor neurons bundle together and form a structure called the motor root, while sensory neurons form the sensory root. Patients with traumatic injuries lose control of these roots if they are torn, causing areas of the body to lose neural control.
Surgeons can implant motor roots at the area from which they are torn, and they will usually successfully reconnect, as motor neurons can regrow out of the spinal cord and into the motor root. However, this does not apply to the more troublesome sensory root, which surgeons have not been able to reconnect accurately — until now.
"Doctors previously considered this type of spinal cord injury impossible to repair. These torn root injuries can cause serious disability and excruciating pain."
Happily, Thomas Carlstedt, also at King's College London, recently helped develop a new surgical technique to reconnect the sensory root. It involves cutting the original sensory nerve cells out of the root and implanting the remaining root directly into a deeper structure within the spinal cord. This area — the dorsal horn — contains secondary sensory neurons that don't normally directly connect to sensory roots. When the team tried the technique in patients, certain spinal reflexes returned, indicating the implanted neuron did integrate with the spine to form a functional neural circuit.
In a new study recently published in Frontiers in Neurology, James, Carlstedt and other collaborators set out to understand how the implanted sensory root was connecting with the spinal cord in the dorsal horn. By understanding the mechanism, they hope to develop new treatments for patients with other types of spinal injuries.
The scientists used a rat model of spinal injury to study the process at a cellular level. During surgery, they produced a similar spinal injury in the rats and then reattached the sensory root using the new technique. After 12-16 weeks following surgery, researchers assessed the spinal repair by passing a small electric current along the neurons to see if they make a complete neural circuit. They then sacrificed the rats to analyze the neural tissue under a microscope.
Electrical tests show the neural circuit was complete — the root had successfully integrated with the spinal cord. When examined, they saw the tissue had small neural offshoots growing from structures called dendrites (branching projections off the end of neurons) growing into the dorsal horn. These thin offshoots extended all the way into the implanted sensory root creating a functioning neural circuit.
So, what does this teach us about spinal cord repair? Researchers hope this type of neural growth can be used to repair other types of spinal cord injury. "The strategy of encouraging new growth from spinal neurons could potentially be of use in other injuries of the nervous system," explains Carlstedt. Scientists might capitalize on this mechanism when designing therapies for injuries where the spinal cord itself is severed, implanting grafts that encourage or facilitate this type of nerve growth.
In a recent clinical report, return of the tendon stretch reflex was demonstrated after spinal cord surgery in a case of total traumatic brachial plexus avulsion injury. Peripheral nerve grafts had been implanted into the spinal cord to reconnect to the peripheral nerves for motor and sensory function. The dorsal root ganglia (DRG) containing the primary sensory nerve cells had been surgically removed in order for secondary or spinal cord sensory neurons to extend into the periphery and replace the deleted DRG neurons. The present experimental study uses a rat injury model first to corroborate the clinical finding of a re-established spinal reflex arch, and second, to elucidate some of the potential mechanisms underlying these findings by means of morphological, immunohistochemical, and electrophysiological assessments. Our findings indicate that, after spinal cord surgery, the central nervous system sensory system could replace the traumatically detached original peripheral sensory connections through new neurite growth from dendrites.
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Schematic drawing of surgical injury to spinal cord in cross section
(A) L3 to L6 dorsal roots cut at L4 and L5 Dorsal Root Ganglions DRGs
and subsequent reimplantation of the L4 spinal nerve and L5 dorsal root.
(B) Dashed red line is site of repair, where L5 dorsal root transitions into the L4 spinal nerve.
Image credit: The Wolfson Centre for Age-Related Diseases, King’s College London, United Kingdom.