Scientists have made a breakthrough that could help to treat people with spinal cord injuries.

They have discovered a protein that appears to play a crucial role in helping the brain to communicate with the spinal cord.

The breakthrough came while studying a rare condition known as mirror-movement disorder.

The main symptom of the disorder is an involuntary symmetrical movement of limbs.

For example, when people with the disorder move their right hand, their left hand involuntarily moves with it.

Typically, they cannot perform complex activities like tying shoes and typing, and even running and walking can be difficult.

Mirror movement

Mirror movement is natural in infants, but over time people normally develop the motor coordination to control their limbs and digits more independently.

A team from the University of Texas have found that mutated mice bred without the protein - ephrin-B3 - exhibit symptoms of the disorder.

Researcher Dr Mark Henkemeyer said the findings should help researchers understand how the brain forms its nervous connections with the spinal cord.

Such knowledge could someday prove helpful in the regeneration of nerves in people with spinal cord injuries.

More immediately, the research proves clues as to how the brain becomes "hard-wired" to the spinal cord.

Researchers have believed for some time that mirror movement is caused by defects in the connection between a part of the brain called the cortex and nerve cells in the spinal cord.

Nerve cell growth

Ephrins are believed to control the growth of nerve cells necessary to help form proper circuits within the spinal cord.

Dr Henkemeyer said: "Ephrin-B3 is a very important molecule.

"It serves as a repulsive barrier that keeps the left axons [nerve cells] on the left side of the nervous system and the right axons on the right."

The mice used in the study were genetically engineered to completely lack ephrin-B3 protein.

This meant that nervous connections were made to both sides of the spinal cord, rather than just to one.

The mice hopped about like kangaroos, using both their forelimbs and hind limbs.

Dr John Cavanagh, head of research at the International Spinal Research Trust, said: "This research will not have an immediate impact on repair of spinal cord injuries, but serves to emphasise how complex the task is.

"The spinal cord is not merely a bundle of wires passing information to and from the brain but also an important processor and editor of that information.

"For full restoration of movement and sensation after injury these systems will need to be rebuilt.

"This work therefore helps to understand the types of molecular signals involved in directing regrowing nerve fibres to their correct destinations.

"It may also help to explain the mechanisms behind erratic or uncontrolled movements that can arise as a result of spinal cord injury, when signals to and from the brain become disrupted."

The research is published in the journal Neuron.