Spinal Cord Is Not Hard Wired and Reorganizes After Injury

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Main Category: Neurology / Neuroscience
Article Date: 07 Jan 2008 - 0:00 PDT

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A new US study using laboratory mice found the spinal cord is not hard wired and when injured it reorganizes the way messages are routed, using alternate pathways that circumvent the damaged ones.

The study is published in the advanced online issue of Nature Medicine and is the work of researchers at the University of California, Los Angeles (UCLA). The study was led by Dr Michael V Sofroniew, professor of neurobiology at the David Geffen School of Medicine at UCLA.

Spinal cord injuries often result in disruption or loss of ability to walk because the long axons or nerve fibres that reach from the brain down into all regions of the spinal cord are severed.

Yet humans and laboratory animals with spinal cord injuries often make varying degrees of spontaneous recovery within a few months of injury. One explanation offered is that new long axons grow from the brain down into the injured spinal cord and establish new pathways.

However, Sofroniew and colleagues showed, using mice, that the central nervous system actually reorganizes itself by rerouting limb control messages via alternate pathways.

Sofroniew said:

"Imagine the long nerve fibers that run between the cells in the brain and lower spinal cord as major freeways."

"When there's a traffic accident on the freeway, what do drivers do? They take shorter surface streets. These detours aren't as fast or direct, but still allow drivers to reach their destination," he explained.

He went on to describe that they found something similar in their investigation. When damage to the spinal cord blocks signals from the brain, in some cases the messages can get through using other routes, making a detour around the blockage.

The messages "follow a series of shorter connections to deliver the brain's command to move the legs," explained the professor.

Using laboratory mice, Sofroniew and colleagues blocked half of the long nerve fibres descending from the brain at different places along both sides of the spinal cord. They also did this at different times.

They did not, however, touch the centre of the cord, which has a series of interconnected, much shorter nerve pathways. These are more like "side streets" than freeways, and carry information over short distances, up and down the whole of the spinal cord.

They were amazed and excited when they saw the results.

Most of the mice regained the ability to control their limbs within eight weeks of their injuries.

"They walked more slowly and less confidently than before their injury, but still recovered mobility," explained Sofroniew.

When the scientists then blocked the shorter nerve pathways running up and down the centre of the spinal cord they found the mice lost their regained mobility and became paralysed again.

This showed that the nervous system had indeed used the shorter nerve pathways as an alternative route. These cells were critical to the animals' recovery, said the researchers. It took some getting used to, this new idea, because as Soroniew explained, this is not what a doctor is taught to expect:

"When I was a medical student, my professors taught that the brain and spinal cord were hard-wired at birth and could not adapt to damage. Severe injury to the spinal cord meant permanent paralysis," said Sofroniew.

He went on to explain how he has gradually changed this "pessimistic view" over the course of his life. This study showed that the body can use other pathways to send messages that control walking.

"Our findings add to a growing body of research showing that the nervous system can reorganize after injury," said Sofroniew.

The team will next be exploring ways to encourage nerve cells in the spinal cord to grow and make new pathways that reach out across and around injured sites. Hopefully this will lead to new ways to restore mobility after spinal cord injury, they said.

They have already identified which cells these could be, and they will try to target them.

According to the Christopher and Dana Reeve Foundation who co-sponsored the study, there are approximately quarter of a million Americans suffering from traumatic spinal cord injury, with 10,000 new cases a year.

The higher up the spinal column the injury occurs, the greater the resulting paralysis, which can also include loss of movement of the rest of the body, and loss of control of digestion and breathing.

"Recovery of supraspinal control of stepping via indirect propriospinal relay connections after spinal cord injury."
Gregoire Courtine, Bingbing Song, Roland R Roy, Hui Zhong, Julia E Herrmann, Yan Ao, Jingwei Qi, V Reggie Edgerton & Michael V Sofroniew.
Published online ahead of print, 06 January 2008.
doi:10.1038/nm1682

Click here for Abstract.

Sources: UCLA press release, journal article.

Written by: Catharine Paddock
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